1.1 --- a/fr/ch09-undo.xml	Sun Aug 16 04:58:01 2009 +0200
     1.2 +++ b/fr/ch09-undo.xml	Tue Nov 24 11:44:49 2009 +0100
     1.3 @@ -1,963 +1,1201 @@
     1.4  <!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->
     1.5  
     1.6 -<chapter>
     1.7 -<title>Finding and fixing your mistakes</title>
     1.8 -<para>\label{chap:undo}</para>
     1.9 -
    1.10 -<para>To err might be human, but to really handle the consequences well
    1.11 -takes a top-notch revision control system.  In this chapter, we'll
    1.12 -discuss some of the techniques you can use when you find that a
    1.13 -problem has crept into your project.  Mercurial has some highly
    1.14 -capable features that will help you to isolate the sources of
    1.15 -problems, and to handle them appropriately.</para>
    1.16 -
    1.17 -<sect1>
    1.18 -<title>Erasing local history</title>
    1.19 -
    1.20 -<sect2>
    1.21 -<title>The accidental commit</title>
    1.22 -
    1.23 -<para>I have the occasional but persistent problem of typing rather more
    1.24 -quickly than I can think, which sometimes results in me committing a
    1.25 -changeset that is either incomplete or plain wrong.  In my case, the
    1.26 -usual kind of incomplete changeset is one in which I've created a new
    1.27 -source file, but forgotten to <command role="hg-cmd">hg add</command> it.  A <quote>plain wrong</quote>
    1.28 -changeset is not as common, but no less annoying.</para>
    1.29 -
    1.30 -</sect2>
    1.31 -<sect2>
    1.32 -<title>Rolling back a transaction</title>
    1.33 -<para>\label{sec:undo:rollback}</para>
    1.34 -
    1.35 -<para>In section <xref linkend="sec:concepts:txn"/>, I mentioned that Mercurial treats
    1.36 -each modification of a repository as a <emphasis>transaction</emphasis>.  Every time
    1.37 -you commit a changeset or pull changes from another repository,
    1.38 -Mercurial remembers what you did.  You can undo, or <emphasis>roll back</emphasis>,
    1.39 -exactly one of these actions using the <command role="hg-cmd">hg rollback</command> command.  (See
    1.40 -section <xref linkend="sec:undo:rollback-after-push"/> for an important caveat
    1.41 -about the use of this command.)</para>
    1.42 -
    1.43 -<para>Here's a mistake that I often find myself making: committing a change
    1.44 -in which I've created a new file, but forgotten to <command role="hg-cmd">hg add</command> it.
    1.45 -<!-- &interaction.rollback.commit; -->
    1.46 -Looking at the output of <command role="hg-cmd">hg status</command> after the commit immediately
    1.47 -confirms the error.
    1.48 -<!-- &interaction.rollback.status; -->
    1.49 -The commit captured the changes to the file <filename>a</filename>, but not the
    1.50 -new file <filename>b</filename>.  If I were to push this changeset to a
    1.51 -repository that I shared with a colleague, the chances are high that
    1.52 -something in <filename>a</filename> would refer to <filename>b</filename>, which would not
    1.53 -be present in their repository when they pulled my changes.  I would
    1.54 -thus become the object of some indignation.</para>
    1.55 -
    1.56 -<para>However, luck is with me&emdash;I've caught my error before I pushed the
    1.57 -changeset.  I use the <command role="hg-cmd">hg rollback</command> command, and Mercurial makes
    1.58 -that last changeset vanish.
    1.59 -<!-- &interaction.rollback.rollback; -->
    1.60 -Notice that the changeset is no longer present in the repository's
    1.61 -history, and the working directory once again thinks that the file
    1.62 -<filename>a</filename> is modified.  The commit and rollback have left the
    1.63 -working directory exactly as it was prior to the commit; the changeset
    1.64 -has been completely erased.  I can now safely <command role="hg-cmd">hg add</command> the file
    1.65 -<filename>b</filename>, and rerun my commit.
    1.66 -<!-- &interaction.rollback.add; --></para>
    1.67 -
    1.68 -</sect2>
    1.69 -<sect2>
    1.70 -<title>The erroneous pull</title>
    1.71 -
    1.72 -<para>It's common practice with Mercurial to maintain separate development
    1.73 -branches of a project in different repositories.  Your development
    1.74 -team might have one shared repository for your project's <quote>0.9</quote>
    1.75 -release, and another, containing different changes, for the <quote>1.0</quote>
    1.76 -release.</para>
    1.77 -
    1.78 -<para>Given this, you can imagine that the consequences could be messy if
    1.79 -you had a local <quote>0.9</quote> repository, and accidentally pulled changes
    1.80 -from the shared <quote>1.0</quote> repository into it.  At worst, you could be
    1.81 -paying insufficient attention, and push those changes into the shared
    1.82 -<quote>0.9</quote> tree, confusing your entire team (but don't worry, we'll
    1.83 -return to this horror scenario later).  However, it's more likely that
    1.84 -you'll notice immediately, because Mercurial will display the URL it's
    1.85 -pulling from, or you will see it pull a suspiciously large number of
    1.86 -changes into the repository.
    1.87 -</para>
    1.88 -
    1.89 -<para>The <command role="hg-cmd">hg rollback</command> command will work nicely to expunge all of the
    1.90 -changesets that you just pulled.  Mercurial groups all changes from
    1.91 -one <command role="hg-cmd">hg pull</command> into a single transaction, so one <command role="hg-cmd">hg rollback</command> is
    1.92 -all you need to undo this mistake.
    1.93 -</para>
    1.94 -
    1.95 -</sect2>
    1.96 -<sect2>
    1.97 -<title>Rolling back is useless once you've pushed</title>
    1.98 -<para>\label{sec:undo:rollback-after-push}
    1.99 -</para>
   1.100 -
   1.101 -<para>The value of the <command role="hg-cmd">hg rollback</command> command drops to zero once you've
   1.102 -pushed your changes to another repository.  Rolling back a change
   1.103 -makes it disappear entirely, but <emphasis>only</emphasis> in the repository in
   1.104 -which you perform the <command role="hg-cmd">hg rollback</command>.  Because a rollback eliminates
   1.105 -history, there's no way for the disappearance of a change to propagate
   1.106 -between repositories.
   1.107 -</para>
   1.108 -
   1.109 -<para>If you've pushed a change to another repository&emdash;particularly if it's
   1.110 -a shared repository&emdash;it has essentially <quote>escaped into the wild,</quote>
   1.111 -and you'll have to recover from your mistake in a different way.  What
   1.112 -will happen if you push a changeset somewhere, then roll it back, then
   1.113 -pull from the repository you pushed to, is that the changeset will
   1.114 -reappear in your repository.
   1.115 -</para>
   1.116 -
   1.117 -<para>(If you absolutely know for sure that the change you want to roll back
   1.118 -is the most recent change in the repository that you pushed to,
   1.119 -<emphasis>and</emphasis> you know that nobody else could have pulled it from that
   1.120 -repository, you can roll back the changeset there, too, but you really
   1.121 -should really not rely on this working reliably.  If you do this,
   1.122 -sooner or later a change really will make it into a repository that
   1.123 -you don't directly control (or have forgotten about), and come back to
   1.124 -bite you.)
   1.125 -</para>
   1.126 -
   1.127 -</sect2>
   1.128 -<sect2>
   1.129 -<title>You can only roll back once</title>
   1.130 -
   1.131 -<para>Mercurial stores exactly one transaction in its transaction log; that
   1.132 -transaction is the most recent one that occurred in the repository.
   1.133 -This means that you can only roll back one transaction.  If you expect
   1.134 -to be able to roll back one transaction, then its predecessor, this is
   1.135 -not the behaviour you will get.
   1.136 -<!-- &interaction.rollback.twice; -->
   1.137 -Once you've rolled back one transaction in a repository, you can't
   1.138 -roll back again in that repository until you perform another commit or
   1.139 -pull.
   1.140 -</para>
   1.141 -
   1.142 -</sect2>
   1.143 -</sect1>
   1.144 -<sect1>
   1.145 -<title>Reverting the mistaken change</title>
   1.146 -
   1.147 -<para>If you make a modification to a file, and decide that you really
   1.148 -didn't want to change the file at all, and you haven't yet committed
   1.149 -your changes, the <command role="hg-cmd">hg revert</command> command is the one you'll need.  It
   1.150 -looks at the changeset that's the parent of the working directory, and
   1.151 -restores the contents of the file to their state as of that changeset.
   1.152 -(That's a long-winded way of saying that, in the normal case, it
   1.153 -undoes your modifications.)
   1.154 -</para>
   1.155 -
   1.156 -<para>Let's illustrate how the <command role="hg-cmd">hg revert</command> command works with yet another
   1.157 -small example.  We'll begin by modifying a file that Mercurial is
   1.158 -already tracking.
   1.159 -<!-- &interaction.daily.revert.modify; -->
   1.160 -If we don't want that change, we can simply <command role="hg-cmd">hg revert</command> the file.
   1.161 -<!-- &interaction.daily.revert.unmodify; -->
   1.162 -The <command role="hg-cmd">hg revert</command> command provides us with an extra degree of safety
   1.163 -by saving our modified file with a <filename>.orig</filename> extension.
   1.164 -<!-- &interaction.daily.revert.status; -->
   1.165 -</para>
   1.166 -
   1.167 -<para>Here is a summary of the cases that the <command role="hg-cmd">hg revert</command> command can
   1.168 -deal with.  We will describe each of these in more detail in the
   1.169 -section that follows.
   1.170 -</para>
   1.171 -<itemizedlist>
   1.172 -<listitem><para>If you modify a file, it will restore the file to its unmodified
   1.173 -  state.
   1.174 -</para>
   1.175 -</listitem>
   1.176 -<listitem><para>If you <command role="hg-cmd">hg add</command> a file, it will undo the <quote>added</quote> state of
   1.177 -  the file, but leave the file itself untouched.
   1.178 -</para>
   1.179 -</listitem>
   1.180 -<listitem><para>If you delete a file without telling Mercurial, it will restore
   1.181 -  the file to its unmodified contents.
   1.182 -</para>
   1.183 -</listitem>
   1.184 -<listitem><para>If you use the <command role="hg-cmd">hg remove</command> command to remove a file, it will
   1.185 -  undo the <quote>removed</quote> state of the file, and restore the file to its
   1.186 -  unmodified contents.
   1.187 -</para>
   1.188 -</listitem></itemizedlist>
   1.189 -
   1.190 -<sect2>
   1.191 -<title>File management errors</title>
   1.192 -<para>\label{sec:undo:mgmt}
   1.193 -</para>
   1.194 -
   1.195 -<para>The <command role="hg-cmd">hg revert</command> command is useful for more than just modified
   1.196 -files.  It lets you reverse the results of all of Mercurial's file
   1.197 -management commands&emdash;<command role="hg-cmd">hg add</command>, <command role="hg-cmd">hg remove</command>, and so on.
   1.198 -</para>
   1.199 -
   1.200 -<para>If you <command role="hg-cmd">hg add</command> a file, then decide that in fact you don't want
   1.201 -Mercurial to track it, use <command role="hg-cmd">hg revert</command> to undo the add.  Don't
   1.202 -worry; Mercurial will not modify the file in any way.  It will just
   1.203 -<quote>unmark</quote> the file.
   1.204 -<!-- &interaction.daily.revert.add; -->
   1.205 -</para>
   1.206 -
   1.207 -<para>Similarly, if you ask Mercurial to <command role="hg-cmd">hg remove</command> a file, you can use
   1.208 -<command role="hg-cmd">hg revert</command> to restore it to the contents it had as of the parent
   1.209 -of the working directory.
   1.210 -<!-- &interaction.daily.revert.remove; -->
   1.211 -This works just as well for a file that you deleted by hand, without
   1.212 -telling Mercurial (recall that in Mercurial terminology, this kind of
   1.213 -file is called <quote>missing</quote>).
   1.214 -<!-- &interaction.daily.revert.missing; -->
   1.215 -</para>
   1.216 -
   1.217 -<para>If you revert a <command role="hg-cmd">hg copy</command>, the copied-to file remains in your
   1.218 -working directory afterwards, untracked.  Since a copy doesn't affect
   1.219 -the copied-from file in any way, Mercurial doesn't do anything with
   1.220 -the copied-from file.
   1.221 -<!-- &interaction.daily.revert.copy; -->
   1.222 -</para>
   1.223 -
   1.224 -<sect3>
   1.225 -<title>A slightly special case: reverting a rename</title>
   1.226 -
   1.227 -<para>If you <command role="hg-cmd">hg rename</command> a file, there is one small detail that
   1.228 -you should remember.  When you <command role="hg-cmd">hg revert</command> a rename, it's not
   1.229 -enough to provide the name of the renamed-to file, as you can see
   1.230 -here.
   1.231 -<!-- &interaction.daily.revert.rename; -->
   1.232 -As you can see from the output of <command role="hg-cmd">hg status</command>, the renamed-to file
   1.233 -is no longer identified as added, but the renamed-<emphasis>from</emphasis> file is
   1.234 -still removed!  This is counter-intuitive (at least to me), but at
   1.235 -least it's easy to deal with.
   1.236 -<!-- &interaction.daily.revert.rename-orig; -->
   1.237 -So remember, to revert a <command role="hg-cmd">hg rename</command>, you must provide <emphasis>both</emphasis>
   1.238 -the source and destination names.
   1.239 -</para>
   1.240 -
   1.241 -<para>% TODO: the output doesn't look like it will be removed!
   1.242 -</para>
   1.243 -
   1.244 -<para>(By the way, if you rename a file, then modify the renamed-to file,
   1.245 -then revert both components of the rename, when Mercurial restores the
   1.246 -file that was removed as part of the rename, it will be unmodified.
   1.247 -If you need the modifications in the renamed-to file to show up in the
   1.248 -renamed-from file, don't forget to copy them over.)
   1.249 -</para>
   1.250 -
   1.251 -<para>These fiddly aspects of reverting a rename arguably constitute a small
   1.252 -bug in Mercurial.
   1.253 -</para>
   1.254 -
   1.255 -</sect3>
   1.256 -</sect2>
   1.257 -</sect1>
   1.258 -<sect1>
   1.259 -<title>Dealing with committed changes</title>
   1.260 -
   1.261 -<para>Consider a case where you have committed a change $a$, and another
   1.262 -change $b$ on top of it; you then realise that change $a$ was
   1.263 -incorrect.  Mercurial lets you <quote>back out</quote> an entire changeset
   1.264 -automatically, and building blocks that let you reverse part of a
   1.265 -changeset by hand.
   1.266 -</para>
   1.267 -
   1.268 -<para>Before you read this section, here's something to keep in mind: the
   1.269 -<command role="hg-cmd">hg backout</command> command undoes changes by <emphasis>adding</emphasis> history, not
   1.270 -by modifying or erasing it.  It's the right tool to use if you're
   1.271 -fixing bugs, but not if you're trying to undo some change that has
   1.272 -catastrophic consequences.  To deal with those, see
   1.273 -section <xref linkend="sec:undo:aaaiiieee"/>.
   1.274 -</para>
   1.275 -
   1.276 -<sect2>
   1.277 -<title>Backing out a changeset</title>
   1.278 -
   1.279 -<para>The <command role="hg-cmd">hg backout</command> command lets you <quote>undo</quote> the effects of an entire
   1.280 -changeset in an automated fashion.  Because Mercurial's history is
   1.281 -immutable, this command <emphasis>does not</emphasis> get rid of the changeset you
   1.282 -want to undo.  Instead, it creates a new changeset that
   1.283 -<emphasis>reverses</emphasis> the effect of the to-be-undone changeset.
   1.284 -</para>
   1.285 -
   1.286 -<para>The operation of the <command role="hg-cmd">hg backout</command> command is a little intricate, so
   1.287 -let's illustrate it with some examples.  First, we'll create a
   1.288 -repository with some simple changes.
   1.289 -<!-- &interaction.backout.init; -->
   1.290 -</para>
   1.291 -
   1.292 -<para>The <command role="hg-cmd">hg backout</command> command takes a single changeset ID as its
   1.293 -argument; this is the changeset to back out.  Normally,
   1.294 -<command role="hg-cmd">hg backout</command> will drop you into a text editor to write a commit
   1.295 -message, so you can record why you're backing the change out.  In this
   1.296 -example, we provide a commit message on the command line using the
   1.297 -<option role="hg-opt-backout">-m</option> option.
   1.298 -</para>
   1.299 -
   1.300 -</sect2>
   1.301 -<sect2>
   1.302 -<title>Backing out the tip changeset</title>
   1.303 -
   1.304 -<para>We're going to start by backing out the last changeset we committed.
   1.305 -<!-- &interaction.backout.simple; -->
   1.306 -You can see that the second line from <filename>myfile</filename> is no longer
   1.307 -present.  Taking a look at the output of <command role="hg-cmd">hg log</command> gives us an idea
   1.308 -of what the <command role="hg-cmd">hg backout</command> command has done.
   1.309 -<!-- &interaction.backout.simple.log; -->
   1.310 -Notice that the new changeset that <command role="hg-cmd">hg backout</command> has created is a
   1.311 -child of the changeset we backed out.  It's easier to see this in
   1.312 -figure <xref linkend="fig:undo:backout"/>, which presents a graphical view of the
   1.313 -change history.  As you can see, the history is nice and linear.
   1.314 -</para>
   1.315 -
   1.316 -<informalfigure>
   1.317 -
   1.318 -<para>  <mediaobject><imageobject><imagedata fileref="undo-simple"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
   1.319 -  <caption><para>Backing out a change using the <command role="hg-cmd">hg backout</command> command</para></caption>
   1.320 -  \label{fig:undo:backout}
   1.321 -</para>
   1.322 -</informalfigure>
   1.323 -
   1.324 -</sect2>
   1.325 -<sect2>
   1.326 -<title>Backing out a non-tip change</title>
   1.327 -
   1.328 -<para>If you want to back out a change other than the last one you
   1.329 -committed, pass the <option role="hg-opt-backout">--merge</option> option to the
   1.330 -<command role="hg-cmd">hg backout</command> command.
   1.331 -<!-- &interaction.backout.non-tip.clone; -->
   1.332 -This makes backing out any changeset a <quote>one-shot</quote> operation that's
   1.333 -usually simple and fast.
   1.334 -<!-- &interaction.backout.non-tip.backout; -->
   1.335 -</para>
   1.336 -
   1.337 -<para>If you take a look at the contents of <filename>myfile</filename> after the
   1.338 -backout finishes, you'll see that the first and third changes are
   1.339 -present, but not the second.
   1.340 -<!-- &interaction.backout.non-tip.cat; -->
   1.341 -</para>
   1.342 -
   1.343 -<para>As the graphical history in figure <xref linkend="fig:undo:backout-non-tip"/>
   1.344 -illustrates, Mercurial actually commits <emphasis>two</emphasis> changes in this
   1.345 -kind of situation (the box-shaped nodes are the ones that Mercurial
   1.346 -commits automatically).  Before Mercurial begins the backout process,
   1.347 -it first remembers what the current parent of the working directory
   1.348 -is.  It then backs out the target changeset, and commits that as a
   1.349 -changeset.  Finally, it merges back to the previous parent of the
   1.350 -working directory, and commits the result of the merge.
   1.351 -</para>
   1.352 -
   1.353 -<para>% TODO: to me it looks like mercurial doesn't commit the second merge automatically!
   1.354 -</para>
   1.355 -
   1.356 -<informalfigure>
   1.357 -
   1.358 -<para>  <mediaobject><imageobject><imagedata fileref="undo-non-tip"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
   1.359 -  <caption><para>Automated backout of a non-tip change using the <command role="hg-cmd">hg backout</command> command</para></caption>
   1.360 -  \label{fig:undo:backout-non-tip}
   1.361 -</para>
   1.362 -</informalfigure>
   1.363 -
   1.364 -<para>The result is that you end up <quote>back where you were</quote>, only with some
   1.365 -extra history that undoes the effect of the changeset you wanted to
   1.366 -back out.
   1.367 -</para>
   1.368 -
   1.369 -<sect3>
   1.370 -<title>Always use the <option role="hg-opt-backout">--merge</option> option</title>
   1.371 -
   1.372 -<para>In fact, since the <option role="hg-opt-backout">--merge</option> option will do the <quote>right
   1.373 -thing</quote> whether or not the changeset you're backing out is the tip
   1.374 -(i.e. it won't try to merge if it's backing out the tip, since there's
   1.375 -no need), you should <emphasis>always</emphasis> use this option when you run the
   1.376 -<command role="hg-cmd">hg backout</command> command.
   1.377 -</para>
   1.378 -
   1.379 -</sect3>
   1.380 -</sect2>
   1.381 -<sect2>
   1.382 -<title>Gaining more control of the backout process</title>
   1.383 -
   1.384 -<para>While I've recommended that you always use the
   1.385 -<option role="hg-opt-backout">--merge</option> option when backing out a change, the
   1.386 -<command role="hg-cmd">hg backout</command> command lets you decide how to merge a backout
   1.387 -changeset.  Taking control of the backout process by hand is something
   1.388 -you will rarely need to do, but it can be useful to understand what
   1.389 -the <command role="hg-cmd">hg backout</command> command is doing for you automatically.  To
   1.390 -illustrate this, let's clone our first repository, but omit the
   1.391 -backout change that it contains.
   1.392 -</para>
   1.393 -
   1.394 -<para><!-- &interaction.backout.manual.clone; -->
   1.395 -As with our earlier example, We'll commit a third changeset, then back
   1.396 -out its parent, and see what happens.
   1.397 -<!-- &interaction.backout.manual.backout; -->
   1.398 -Our new changeset is again a descendant of the changeset we backout
   1.399 -out; it's thus a new head, <emphasis>not</emphasis> a descendant of the changeset
   1.400 -that was the tip.  The <command role="hg-cmd">hg backout</command> command was quite explicit in
   1.401 -telling us this.
   1.402 -<!-- &interaction.backout.manual.log; -->
   1.403 -</para>
   1.404 -
   1.405 -<para>Again, it's easier to see what has happened by looking at a graph of
   1.406 -the revision history, in figure <xref linkend="fig:undo:backout-manual"/>.  This
   1.407 -makes it clear that when we use <command role="hg-cmd">hg backout</command> to back out a change
   1.408 -other than the tip, Mercurial adds a new head to the repository (the
   1.409 -change it committed is box-shaped).
   1.410 -</para>
   1.411 -
   1.412 -<informalfigure>
   1.413 -
   1.414 -<para>  <mediaobject><imageobject><imagedata fileref="undo-manual"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
   1.415 -  <caption><para>Backing out a change using the <command role="hg-cmd">hg backout</command> command</para></caption>
   1.416 -  \label{fig:undo:backout-manual}
   1.417 -</para>
   1.418 -</informalfigure>
   1.419 -
   1.420 -<para>After the <command role="hg-cmd">hg backout</command> command has completed, it leaves the new
   1.421 -<quote>backout</quote> changeset as the parent of the working directory.
   1.422 -<!-- &interaction.backout.manual.parents; -->
   1.423 -Now we have two isolated sets of changes.
   1.424 -<!-- &interaction.backout.manual.heads; -->
   1.425 -</para>
   1.426 -
   1.427 -<para>Let's think about what we expect to see as the contents of
   1.428 -<filename>myfile</filename> now.  The first change should be present, because
   1.429 -we've never backed it out.  The second change should be missing, as
   1.430 -that's the change we backed out.  Since the history graph shows the
   1.431 -third change as a separate head, we <emphasis>don't</emphasis> expect to see the
   1.432 -third change present in <filename>myfile</filename>.
   1.433 -<!-- &interaction.backout.manual.cat; -->
   1.434 -To get the third change back into the file, we just do a normal merge
   1.435 -of our two heads.
   1.436 -<!-- &interaction.backout.manual.merge; -->
   1.437 -Afterwards, the graphical history of our repository looks like
   1.438 -figure <xref linkend="fig:undo:backout-manual-merge"/>.
   1.439 -</para>
   1.440 -
   1.441 -<informalfigure>
   1.442 -
   1.443 -<para>  <mediaobject><imageobject><imagedata fileref="undo-manual-merge"/></imageobject><textobject><phrase>XXX add text</phrase></textobject></mediaobject>
   1.444 -  <caption><para>Manually merging a backout change</para></caption>
   1.445 -  \label{fig:undo:backout-manual-merge}
   1.446 -</para>
   1.447 -</informalfigure>
   1.448 -
   1.449 -</sect2>
   1.450 -<sect2>
   1.451 -<title>Why <command role="hg-cmd">hg backout</command> works as it does</title>
   1.452 -
   1.453 -<para>Here's a brief description of how the <command role="hg-cmd">hg backout</command> command works.
   1.454 -</para>
   1.455 -<orderedlist>
   1.456 -<listitem><para>It ensures that the working directory is <quote>clean</quote>, i.e. that
   1.457 -  the output of <command role="hg-cmd">hg status</command> would be empty.
   1.458 -</para>
   1.459 -</listitem>
   1.460 -<listitem><para>It remembers the current parent of the working directory.  Let's
   1.461 -  call this changeset <literal>orig</literal>
   1.462 -</para>
   1.463 -</listitem>
   1.464 -<listitem><para>It does the equivalent of a <command role="hg-cmd">hg update</command> to sync the working
   1.465 -  directory to the changeset you want to back out.  Let's call this
   1.466 -  changeset <literal>backout</literal>
   1.467 -</para>
   1.468 -</listitem>
   1.469 -<listitem><para>It finds the parent of that changeset.  Let's call that
   1.470 -  changeset <literal>parent</literal>.
   1.471 -</para>
   1.472 -</listitem>
   1.473 -<listitem><para>For each file that the <literal>backout</literal> changeset affected, it
   1.474 -  does the equivalent of a <command role="hg-cmd">hg revert -r parent</command> on that file,
   1.475 -  to restore it to the contents it had before that changeset was
   1.476 -  committed.
   1.477 -</para>
   1.478 -</listitem>
   1.479 -<listitem><para>It commits the result as a new changeset.  This changeset has
   1.480 -  <literal>backout</literal> as its parent.
   1.481 -</para>
   1.482 -</listitem>
   1.483 -<listitem><para>If you specify <option role="hg-opt-backout">--merge</option> on the command line, it
   1.484 -  merges with <literal>orig</literal>, and commits the result of the merge.
   1.485 -</para>
   1.486 -</listitem></orderedlist>
   1.487 -
   1.488 -<para>An alternative way to implement the <command role="hg-cmd">hg backout</command> command would be
   1.489 -to <command role="hg-cmd">hg export</command> the to-be-backed-out changeset as a diff, then use
   1.490 -the <option role="cmd-opt-patch">--reverse</option> option to the <command>patch</command> command to
   1.491 -reverse the effect of the change without fiddling with the working
   1.492 -directory.  This sounds much simpler, but it would not work nearly as
   1.493 -well.
   1.494 -</para>
   1.495 -
   1.496 -<para>The reason that <command role="hg-cmd">hg backout</command> does an update, a commit, a merge, and
   1.497 -another commit is to give the merge machinery the best chance to do a
   1.498 -good job when dealing with all the changes <emphasis>between</emphasis> the change
   1.499 -you're backing out and the current tip.
   1.500 -</para>
   1.501 -
   1.502 -<para>If you're backing out a changeset that's 100 revisions back in your
   1.503 -project's history, the chances that the <command>patch</command> command will
   1.504 -be able to apply a reverse diff cleanly are not good, because
   1.505 -intervening changes are likely to have <quote>broken the context</quote> that
   1.506 -<command>patch</command> uses to determine whether it can apply a patch (if
   1.507 -this sounds like gibberish, see <xref linkend="sec:mq:patch"/> for a
   1.508 -discussion of the <command>patch</command> command).  Also, Mercurial's merge
   1.509 -machinery will handle files and directories being renamed, permission
   1.510 -changes, and modifications to binary files, none of which
   1.511 -<command>patch</command> can deal with.
   1.512 -</para>
   1.513 -
   1.514 -</sect2>
   1.515 -</sect1>
   1.516 -<sect1>
   1.517 -<title>Changes that should never have been</title>
   1.518 -<para>\label{sec:undo:aaaiiieee}
   1.519 -</para>
   1.520 -
   1.521 -<para>Most of the time, the <command role="hg-cmd">hg backout</command> command is exactly what you need
   1.522 -if you want to undo the effects of a change.  It leaves a permanent
   1.523 -record of exactly what you did, both when committing the original
   1.524 -changeset and when you cleaned up after it.
   1.525 -</para>
   1.526 -
   1.527 -<para>On rare occasions, though, you may find that you've committed a change
   1.528 -that really should not be present in the repository at all.  For
   1.529 -example, it would be very unusual, and usually considered a mistake,
   1.530 -to commit a software project's object files as well as its source
   1.531 -files.  Object files have almost no intrinsic value, and they're
   1.532 -<emphasis>big</emphasis>, so they increase the size of the repository and the amount
   1.533 -of time it takes to clone or pull changes.
   1.534 -</para>
   1.535 -
   1.536 -<para>Before I discuss the options that you have if you commit a <quote>brown
   1.537 -paper bag</quote> change (the kind that's so bad that you want to pull a
   1.538 -brown paper bag over your head), let me first discuss some approaches
   1.539 -that probably won't work.
   1.540 -</para>
   1.541 -
   1.542 -<para>Since Mercurial treats history as accumulative&emdash;every change builds
   1.543 -on top of all changes that preceded it&emdash;you generally can't just make
   1.544 -disastrous changes disappear.  The one exception is when you've just
   1.545 -committed a change, and it hasn't been pushed or pulled into another
   1.546 -repository.  That's when you can safely use the <command role="hg-cmd">hg rollback</command>
   1.547 -command, as I detailed in section <xref linkend="sec:undo:rollback"/>.
   1.548 -</para>
   1.549 -
   1.550 -<para>After you've pushed a bad change to another repository, you
   1.551 -<emphasis>could</emphasis> still use <command role="hg-cmd">hg rollback</command> to make your local copy of the
   1.552 -change disappear, but it won't have the consequences you want.  The
   1.553 -change will still be present in the remote repository, so it will
   1.554 -reappear in your local repository the next time you pull.
   1.555 -</para>
   1.556 -
   1.557 -<para>If a situation like this arises, and you know which repositories your
   1.558 -bad change has propagated into, you can <emphasis>try</emphasis> to get rid of the
   1.559 -changeefrom <emphasis>every</emphasis> one of those repositories.  This is, of
   1.560 -course, not a satisfactory solution: if you miss even a single
   1.561 -repository while you're expunging, the change is still <quote>in the
   1.562 -wild</quote>, and could propagate further.
   1.563 -</para>
   1.564 -
   1.565 -<para>If you've committed one or more changes <emphasis>after</emphasis> the change that
   1.566 -you'd like to see disappear, your options are further reduced.
   1.567 -Mercurial doesn't provide a way to <quote>punch a hole</quote> in history,
   1.568 -leaving changesets intact.
   1.569 -</para>
   1.570 -
   1.571 -<para>XXX This needs filling out.  The <literal>hg-replay</literal> script in the
   1.572 -<literal>examples</literal> directory works, but doesn't handle merge
   1.573 -changesets.  Kind of an important omission.
   1.574 -</para>
   1.575 -
   1.576 -<sect2>
   1.577 -<title>Protect yourself from <quote>escaped</quote> changes</title>
   1.578 -
   1.579 -<para>If you've committed some changes to your local repository and they've
   1.580 -been pushed or pulled somewhere else, this isn't necessarily a
   1.581 -disaster.  You can protect yourself ahead of time against some classes
   1.582 -of bad changeset.  This is particularly easy if your team usually
   1.583 -pulls changes from a central repository.
   1.584 -</para>
   1.585 -
   1.586 -<para>By configuring some hooks on that repository to validate incoming
   1.587 -changesets (see chapter <xref linkend="chap:hook"/>), you can automatically
   1.588 -prevent some kinds of bad changeset from being pushed to the central
   1.589 -repository at all.  With such a configuration in place, some kinds of
   1.590 -bad changeset will naturally tend to <quote>die out</quote> because they can't
   1.591 -propagate into the central repository.  Better yet, this happens
   1.592 -without any need for explicit intervention.
   1.593 -</para>
   1.594 -
   1.595 -<para>For instance, an incoming change hook that verifies that a changeset
   1.596 -will actually compile can prevent people from inadvertantly <quote>breaking
   1.597 -the build</quote>.
   1.598 -</para>
   1.599 -
   1.600 -</sect2>
   1.601 -</sect1>
   1.602 -<sect1>
   1.603 -<title>Finding the source of a bug</title>
   1.604 -<para>\label{sec:undo:bisect}
   1.605 -</para>
   1.606 -
   1.607 -<para>While it's all very well to be able to back out a changeset that
   1.608 -introduced a bug, this requires that you know which changeset to back
   1.609 -out.  Mercurial provides an invaluable command, called
   1.610 -<command role="hg-cmd">hg bisect</command>, that helps you to automate this process and accomplish
   1.611 -it very efficiently.
   1.612 -</para>
   1.613 -
   1.614 -<para>The idea behind the <command role="hg-cmd">hg bisect</command> command is that a changeset has
   1.615 -introduced some change of behaviour that you can identify with a
   1.616 -simple binary test.  You don't know which piece of code introduced the
   1.617 -change, but you know how to test for the presence of the bug.  The
   1.618 -<command role="hg-cmd">hg bisect</command> command uses your test to direct its search for the
   1.619 -changeset that introduced the code that caused the bug.
   1.620 -</para>
   1.621 -
   1.622 -<para>Here are a few scenarios to help you understand how you might apply
   1.623 -this command.
   1.624 -</para>
   1.625 -<itemizedlist>
   1.626 -<listitem><para>The most recent version of your software has a bug that you
   1.627 -  remember wasn't present a few weeks ago, but you don't know when it
   1.628 -  was introduced.  Here, your binary test checks for the presence of
   1.629 -  that bug.
   1.630 -</para>
   1.631 -</listitem>
   1.632 -<listitem><para>You fixed a bug in a rush, and now it's time to close the entry
   1.633 -  in your team's bug database.  The bug database requires a changeset
   1.634 -  ID when you close an entry, but you don't remember which changeset
   1.635 -  you fixed the bug in.  Once again, your binary test checks for the
   1.636 -  presence of the bug.
   1.637 -</para>
   1.638 -</listitem>
   1.639 -<listitem><para>Your software works correctly, but runs 15% slower than the
   1.640 -  last time you measured it.  You want to know which changeset
   1.641 -  introduced the performance regression.  In this case, your binary
   1.642 -  test measures the performance of your software, to see whether it's
   1.643 -  <quote>fast</quote> or <quote>slow</quote>.
   1.644 -</para>
   1.645 -</listitem>
   1.646 -<listitem><para>The sizes of the components of your project that you ship
   1.647 -  exploded recently, and you suspect that something changed in the way
   1.648 -  you build your project.
   1.649 -</para>
   1.650 -</listitem></itemizedlist>
   1.651 -
   1.652 -<para>From these examples, it should be clear that the <command role="hg-cmd">hg bisect</command>
   1.653 -command is not useful only for finding the sources of bugs.  You can
   1.654 -use it to find any <quote>emergent property</quote> of a repository (anything
   1.655 -that you can't find from a simple text search of the files in the
   1.656 -tree) for which you can write a binary test.
   1.657 -</para>
   1.658 -
   1.659 -<para>We'll introduce a little bit of terminology here, just to make it
   1.660 -clear which parts of the search process are your responsibility, and
   1.661 -which are Mercurial's.  A <emphasis>test</emphasis> is something that <emphasis>you</emphasis> run
   1.662 -when <command role="hg-cmd">hg bisect</command> chooses a changeset.  A <emphasis>probe</emphasis> is what
   1.663 -<command role="hg-cmd">hg bisect</command> runs to tell whether a revision is good.  Finally,
   1.664 -we'll use the word <quote>bisect</quote>, as both a noun and a verb, to stand in
   1.665 -for the phrase <quote>search using the <command role="hg-cmd">hg bisect</command> command.
   1.666 -</para>
   1.667 -
   1.668 -<para>One simple way to automate the searching process would be simply to
   1.669 -probe every changeset.  However, this scales poorly.  If it took ten
   1.670 -minutes to test a single changeset, and you had 10,000 changesets in
   1.671 -your repository, the exhaustive approach would take on average 35
   1.672 -<emphasis>days</emphasis> to find the changeset that introduced a bug.  Even if you
   1.673 -knew that the bug was introduced by one of the last 500 changesets,
   1.674 -and limited your search to those, you'd still be looking at over 40
   1.675 -hours to find the changeset that introduced your bug.
   1.676 -</para>
   1.677 -
   1.678 -<para>What the <command role="hg-cmd">hg bisect</command> command does is use its knowledge of the
   1.679 -<quote>shape</quote> of your project's revision history to perform a search in
   1.680 -time proportional to the <emphasis>logarithm</emphasis> of the number of changesets
   1.681 -to check (the kind of search it performs is called a dichotomic
   1.682 -search).  With this approach, searching through 10,000 changesets will
   1.683 -take less than three hours, even at ten minutes per test (the search
   1.684 -will require about 14 tests).  Limit your search to the last hundred
   1.685 -changesets, and it will take only about an hour (roughly seven tests).
   1.686 -</para>
   1.687 -
   1.688 -<para>The <command role="hg-cmd">hg bisect</command> command is aware of the <quote>branchy</quote> nature of a
   1.689 -Mercurial project's revision history, so it has no problems dealing
   1.690 -with branches, merges, or multiple heads in a repository.  It can
   1.691 -prune entire branches of history with a single probe, which is how it
   1.692 -operates so efficiently.
   1.693 -</para>
   1.694 -
   1.695 -<sect2>
   1.696 -<title>Using the <command role="hg-cmd">hg bisect</command> command</title>
   1.697 -
   1.698 -<para>Here's an example of <command role="hg-cmd">hg bisect</command> in action.
   1.699 -</para>
   1.700 -
   1.701 -<note>
   1.702 -<para>  In versions 0.9.5 and earlier of Mercurial, <command role="hg-cmd">hg bisect</command> was not a
   1.703 -  core command: it was distributed with Mercurial as an extension.
   1.704 -  This section describes the built-in command, not the old extension.
   1.705 -</para>
   1.706 -</note>
   1.707 -
   1.708 -<para>Now let's create a repository, so that we can try out the
   1.709 -<command role="hg-cmd">hg bisect</command> command in isolation.
   1.710 -<!-- &interaction.bisect.init; -->
   1.711 -We'll simulate a project that has a bug in it in a simple-minded way:
   1.712 -create trivial changes in a loop, and nominate one specific change
   1.713 -that will have the <quote>bug</quote>.  This loop creates 35 changesets, each
   1.714 -adding a single file to the repository.  We'll represent our <quote>bug</quote>
   1.715 -with a file that contains the text <quote>i have a gub</quote>.
   1.716 -<!-- &interaction.bisect.commits; -->
   1.717 -</para>
   1.718 -
   1.719 -<para>The next thing that we'd like to do is figure out how to use the
   1.720 -<command role="hg-cmd">hg bisect</command> command.  We can use Mercurial's normal built-in help
   1.721 -mechanism for this.
   1.722 -<!-- &interaction.bisect.help; -->
   1.723 -</para>
   1.724 -
   1.725 -<para>The <command role="hg-cmd">hg bisect</command> command works in steps.  Each step proceeds as follows.
   1.726 -</para>
   1.727 -<orderedlist>
   1.728 -<listitem><para>You run your binary test.
   1.729 -</para>
   1.730 -</listitem><itemizedlist>
   1.731 -<listitem><para>  \item If the test succeeded, you tell <command role="hg-cmd">hg bisect</command> by running the
   1.732 -    <command role="hg-cmd">hg bisect good</command> command.
   1.733 -  \item If it failed, run the <command role="hg-cmd">hg bisect --bad</command> command.
   1.734 -</para>
   1.735 -</listitem></itemizedlist>
   1.736 -<listitem><para>The command uses your information to decide which changeset to
   1.737 -  test next.
   1.738 -</para>
   1.739 -</listitem>
   1.740 -<listitem><para>It updates the working directory to that changeset, and the
   1.741 -  process begins again.
   1.742 -</para>
   1.743 -</listitem></orderedlist>
   1.744 -<para>The process ends when <command role="hg-cmd">hg bisect</command> identifies a unique changeset
   1.745 -that marks the point where your test transitioned from <quote>succeeding</quote>
   1.746 -to <quote>failing</quote>.
   1.747 -</para>
   1.748 -
   1.749 -<para>To start the search, we must run the <command role="hg-cmd">hg bisect --reset</command> command.
   1.750 -<!-- &interaction.bisect.search.init; -->
   1.751 -</para>
   1.752 -
   1.753 -<para>In our case, the binary test we use is simple: we check to see if any
   1.754 -file in the repository contains the string <quote>i have a gub</quote>.  If it
   1.755 -does, this changeset contains the change that <quote>caused the bug</quote>.  By
   1.756 -convention, a changeset that has the property we're searching for is
   1.757 -<quote>bad</quote>, while one that doesn't is <quote>good</quote>.
   1.758 -</para>
   1.759 -
   1.760 -<para>Most of the time, the revision to which the working directory is
   1.761 -synced (usually the tip) already exhibits the problem introduced by
   1.762 -the buggy change, so we'll mark it as <quote>bad</quote>.
   1.763 -<!-- &interaction.bisect.search.bad-init; -->
   1.764 -</para>
   1.765 -
   1.766 -<para>Our next task is to nominate a changeset that we know <emphasis>doesn't</emphasis>
   1.767 -have the bug; the <command role="hg-cmd">hg bisect</command> command will <quote>bracket</quote> its search
   1.768 -between the first pair of good and bad changesets.  In our case, we
   1.769 -know that revision 10 didn't have the bug.  (I'll have more words
   1.770 -about choosing the first <quote>good</quote> changeset later.)
   1.771 -<!-- &interaction.bisect.search.good-init; -->
   1.772 -</para>
   1.773 -
   1.774 -<para>Notice that this command printed some output.
   1.775 -</para>
   1.776 -<itemizedlist>
   1.777 -<listitem><para>It told us how many changesets it must consider before it can
   1.778 -  identify the one that introduced the bug, and how many tests that
   1.779 -  will require.
   1.780 -</para>
   1.781 -</listitem>
   1.782 -<listitem><para>It updated the working directory to the next changeset to test,
   1.783 -  and told us which changeset it's testing.
   1.784 -</para>
   1.785 -</listitem></itemizedlist>
   1.786 -
   1.787 -<para>We now run our test in the working directory.  We use the
   1.788 -<command>grep</command> command to see if our <quote>bad</quote> file is present in the
   1.789 -working directory.  If it is, this revision is bad; if not, this
   1.790 -revision is good.
   1.791 -<!-- &interaction.bisect.search.step1; -->
   1.792 -</para>
   1.793 -
   1.794 -<para>This test looks like a perfect candidate for automation, so let's turn
   1.795 -it into a shell function.
   1.796 -<!-- &interaction.bisect.search.mytest; -->
   1.797 -We can now run an entire test step with a single command,
   1.798 -<literal>mytest</literal>.
   1.799 -<!-- &interaction.bisect.search.step2; -->
   1.800 -A few more invocations of our canned test step command, and we're
   1.801 -done.
   1.802 -<!-- &interaction.bisect.search.rest; -->
   1.803 -</para>
   1.804 -
   1.805 -<para>Even though we had 40 changesets to search through, the <command role="hg-cmd">hg bisect</command>
   1.806 -command let us find the changeset that introduced our <quote>bug</quote> with
   1.807 -only five tests.  Because the number of tests that the <command role="hg-cmd">hg bisect</command>
   1.808 -command performs grows logarithmically with the number of changesets to
   1.809 -search, the advantage that it has over the <quote>brute force</quote> search
   1.810 -approach increases with every changeset you add.
   1.811 -</para>
   1.812 -
   1.813 -</sect2>
   1.814 -<sect2>
   1.815 -<title>Cleaning up after your search</title>
   1.816 -
   1.817 -<para>When you're finished using the <command role="hg-cmd">hg bisect</command> command in a
   1.818 -repository, you can use the <command role="hg-cmd">hg bisect reset</command> command to drop
   1.819 -the information it was using to drive your search.  The command
   1.820 -doesn't use much space, so it doesn't matter if you forget to run this
   1.821 -command.  However, <command role="hg-cmd">hg bisect</command> won't let you start a new search in
   1.822 -that repository until you do a <command role="hg-cmd">hg bisect reset</command>.
   1.823 -<!-- &interaction.bisect.search.reset; -->
   1.824 -</para>
   1.825 -
   1.826 -</sect2>
   1.827 -</sect1>
   1.828 -<sect1>
   1.829 -<title>Tips for finding bugs effectively</title>
   1.830 -
   1.831 -<sect2>
   1.832 -<title>Give consistent input</title>
   1.833 -
   1.834 -<para>The <command role="hg-cmd">hg bisect</command> command requires that you correctly report the
   1.835 -result of every test you perform.  If you tell it that a test failed
   1.836 -when it really succeeded, it <emphasis>might</emphasis> be able to detect the
   1.837 -inconsistency.  If it can identify an inconsistency in your reports,
   1.838 -it will tell you that a particular changeset is both good and bad.
   1.839 -However, it can't do this perfectly; it's about as likely to report
   1.840 -the wrong changeset as the source of the bug.
   1.841 -</para>
   1.842 -
   1.843 -</sect2>
   1.844 -<sect2>
   1.845 -<title>Automate as much as possible</title>
   1.846 -
   1.847 -<para>When I started using the <command role="hg-cmd">hg bisect</command> command, I tried a few times
   1.848 -to run my tests by hand, on the command line.  This is an approach
   1.849 -that I, at least, am not suited to.  After a few tries, I found that I
   1.850 -was making enough mistakes that I was having to restart my searches
   1.851 -several times before finally getting correct results.
   1.852 -</para>
   1.853 -
   1.854 -<para>My initial problems with driving the <command role="hg-cmd">hg bisect</command> command by hand
   1.855 -occurred even with simple searches on small repositories; if the
   1.856 -problem you're looking for is more subtle, or the number of tests that
   1.857 -<command role="hg-cmd">hg bisect</command> must perform increases, the likelihood of operator
   1.858 -error ruining the search is much higher.  Once I started automating my
   1.859 -tests, I had much better results.
   1.860 -</para>
   1.861 -
   1.862 -<para>The key to automated testing is twofold:
   1.863 -</para>
   1.864 -<itemizedlist>
   1.865 -<listitem><para>always test for the same symptom, and
   1.866 -</para>
   1.867 -</listitem>
   1.868 -<listitem><para>always feed consistent input to the <command role="hg-cmd">hg bisect</command> command.
   1.869 -</para>
   1.870 -</listitem></itemizedlist>
   1.871 -<para>In my tutorial example above, the <command>grep</command> command tests for the
   1.872 -symptom, and the <literal>if</literal> statement takes the result of this check
   1.873 -and ensures that we always feed the same input to the <command role="hg-cmd">hg bisect</command>
   1.874 -command.  The <literal>mytest</literal> function marries these together in a
   1.875 -reproducible way, so that every test is uniform and consistent.
   1.876 -</para>
   1.877 -
   1.878 -</sect2>
   1.879 -<sect2>
   1.880 -<title>Check your results</title>
   1.881 -
   1.882 -<para>Because the output of a <command role="hg-cmd">hg bisect</command> search is only as good as the
   1.883 -input you give it, don't take the changeset it reports as the
   1.884 -absolute truth.  A simple way to cross-check its report is to manually
   1.885 -run your test at each of the following changesets:
   1.886 -</para>
   1.887 -<itemizedlist>
   1.888 -<listitem><para>The changeset that it reports as the first bad revision.  Your
   1.889 -  test should still report this as bad.
   1.890 -</para>
   1.891 -</listitem>
   1.892 -<listitem><para>The parent of that changeset (either parent, if it's a merge).
   1.893 -  Your test should report this changeset as good.
   1.894 -</para>
   1.895 -</listitem>
   1.896 -<listitem><para>A child of that changeset.  Your test should report this
   1.897 -  changeset as bad.
   1.898 -</para>
   1.899 -</listitem></itemizedlist>
   1.900 -
   1.901 -</sect2>
   1.902 -<sect2>
   1.903 -<title>Beware interference between bugs</title>
   1.904 -
   1.905 -<para>It's possible that your search for one bug could be disrupted by the
   1.906 -presence of another.  For example, let's say your software crashes at
   1.907 -revision 100, and worked correctly at revision 50.  Unknown to you,
   1.908 -someone else introduced a different crashing bug at revision 60, and
   1.909 -fixed it at revision 80.  This could distort your results in one of
   1.910 -several ways.
   1.911 -</para>
   1.912 -
   1.913 -<para>It is possible that this other bug completely <quote>masks</quote> yours, which
   1.914 -is to say that it occurs before your bug has a chance to manifest
   1.915 -itself.  If you can't avoid that other bug (for example, it prevents
   1.916 -your project from building), and so can't tell whether your bug is
   1.917 -present in a particular changeset, the <command role="hg-cmd">hg bisect</command> command cannot
   1.918 -help you directly.  Instead, you can mark a changeset as untested by
   1.919 -running <command role="hg-cmd">hg bisect --skip</command>.
   1.920 -</para>
   1.921 -
   1.922 -<para>A different problem could arise if your test for a bug's presence is
   1.923 -not specific enough.  If you check for <quote>my program crashes</quote>, then
   1.924 -both your crashing bug and an unrelated crashing bug that masks it
   1.925 -will look like the same thing, and mislead <command role="hg-cmd">hg bisect</command>.
   1.926 -</para>
   1.927 -
   1.928 -<para>Another useful situation in which to use <command role="hg-cmd">hg bisect --skip</command> is
   1.929 -if you can't test a revision because your project was in a broken and
   1.930 -hence untestable state at that revision, perhaps because someone
   1.931 -checked in a change that prevented the project from building.
   1.932 -</para>
   1.933 -
   1.934 -</sect2>
   1.935 -<sect2>
   1.936 -<title>Bracket your search lazily</title>
   1.937 -
   1.938 -<para>Choosing the first <quote>good</quote> and <quote>bad</quote> changesets that will mark the
   1.939 -end points of your search is often easy, but it bears a little
   1.940 -discussion nevertheless.  From the perspective of <command role="hg-cmd">hg bisect</command>, the
   1.941 -<quote>newest</quote> changeset is conventionally <quote>bad</quote>, and the older
   1.942 -changeset is <quote>good</quote>.
   1.943 -</para>
   1.944 -
   1.945 -<para>If you're having trouble remembering when a suitable <quote>good</quote> change
   1.946 -was, so that you can tell <command role="hg-cmd">hg bisect</command>, you could do worse than
   1.947 -testing changesets at random.  Just remember to eliminate contenders
   1.948 -that can't possibly exhibit the bug (perhaps because the feature with
   1.949 -the bug isn't present yet) and those where another problem masks the
   1.950 -bug (as I discussed above).
   1.951 -</para>
   1.952 -
   1.953 -<para>Even if you end up <quote>early</quote> by thousands of changesets or months of
   1.954 -history, you will only add a handful of tests to the total number that
   1.955 -<command role="hg-cmd">hg bisect</command> must perform, thanks to its logarithmic behaviour.
   1.956 -</para>
   1.957 -
   1.958 -</sect2>
   1.959 -</sect1>
   1.960 +<chapter id="chap:undo">
   1.961 +  <?dbhtml filename="finding-and-fixing-mistakes.html"?>
   1.962 +  <title>Finding and fixing mistakes</title>
   1.963 +
   1.964 +  <para id="x_d2">To err might be human, but to really handle the consequences
   1.965 +    well takes a top-notch revision control system.  In this chapter,
   1.966 +    we'll discuss some of the techniques you can use when you find
   1.967 +    that a problem has crept into your project.  Mercurial has some
   1.968 +    highly capable features that will help you to isolate the sources
   1.969 +    of problems, and to handle them appropriately.</para>
   1.970 +
   1.971 +  <sect1>
   1.972 +    <title>Erasing local history</title>
   1.973 +
   1.974 +    <sect2>
   1.975 +      <title>The accidental commit</title>
   1.976 +
   1.977 +      <para id="x_d3">I have the occasional but persistent problem of typing
   1.978 +	rather more quickly than I can think, which sometimes results
   1.979 +	in me committing a changeset that is either incomplete or
   1.980 +	plain wrong.  In my case, the usual kind of incomplete
   1.981 +	changeset is one in which I've created a new source file, but
   1.982 +	forgotten to <command role="hg-cmd">hg add</command> it.  A
   1.983 +	<quote>plain wrong</quote> changeset is not as common, but no
   1.984 +	less annoying.</para>
   1.985 +
   1.986 +    </sect2>
   1.987 +    <sect2 id="sec:undo:rollback">
   1.988 +      <title>Rolling back a transaction</title>
   1.989 +
   1.990 +      <para id="x_d4">In <xref linkend="sec:concepts:txn"/>, I
   1.991 +	mentioned that Mercurial treats each modification of a
   1.992 +	repository as a <emphasis>transaction</emphasis>.  Every time
   1.993 +	you commit a changeset or pull changes from another
   1.994 +	repository, Mercurial remembers what you did.  You can undo,
   1.995 +	or <emphasis>roll back</emphasis>, exactly one of these
   1.996 +	actions using the <command role="hg-cmd">hg rollback</command>
   1.997 +	command.  (See <xref linkend="sec:undo:rollback-after-push"/>
   1.998 +	for an important caveat about the use of this command.)</para>
   1.999 +
  1.1000 +      <para id="x_d5">Here's a mistake that I often find myself making:
  1.1001 +	committing a change in which I've created a new file, but
  1.1002 +	forgotten to <command role="hg-cmd">hg add</command>
  1.1003 +	it.</para>
  1.1004 +
  1.1005 +      &interaction.rollback.commit;
  1.1006 +
  1.1007 +      <para id="x_d6">Looking at the output of <command role="hg-cmd">hg
  1.1008 +	  status</command> after the commit immediately confirms the
  1.1009 +	error.</para>
  1.1010 +
  1.1011 +      &interaction.rollback.status;
  1.1012 +
  1.1013 +      <para id="x_d7">The commit captured the changes to the file
  1.1014 +	<filename>a</filename>, but not the new file
  1.1015 +	<filename>b</filename>.  If I were to push this changeset to a
  1.1016 +	repository that I shared with a colleague, the chances are
  1.1017 +	high that something in <filename>a</filename> would refer to
  1.1018 +	<filename>b</filename>, which would not be present in their
  1.1019 +	repository when they pulled my changes.  I would thus become
  1.1020 +	the object of some indignation.</para>
  1.1021 +
  1.1022 +      <para id="x_d8">However, luck is with me&emdash;I've caught my error
  1.1023 +	before I pushed the changeset.  I use the <command
  1.1024 +	  role="hg-cmd">hg rollback</command> command, and Mercurial
  1.1025 +	makes that last changeset vanish.</para>
  1.1026 +
  1.1027 +      &interaction.rollback.rollback;
  1.1028 +
  1.1029 +      <para id="x_d9">Notice that the changeset is no longer present in the
  1.1030 +	repository's history, and the working directory once again
  1.1031 +	thinks that the file <filename>a</filename> is modified.  The
  1.1032 +	commit and rollback have left the working directory exactly as
  1.1033 +	it was prior to the commit; the changeset has been completely
  1.1034 +	erased.  I can now safely <command role="hg-cmd">hg
  1.1035 +	  add</command> the file <filename>b</filename>, and rerun my
  1.1036 +	commit.</para>
  1.1037 +
  1.1038 +      &interaction.rollback.add;
  1.1039 +
  1.1040 +    </sect2>
  1.1041 +    <sect2>
  1.1042 +      <title>The erroneous pull</title>
  1.1043 +
  1.1044 +      <para id="x_da">It's common practice with Mercurial to maintain separate
  1.1045 +	development branches of a project in different repositories.
  1.1046 +	Your development team might have one shared repository for
  1.1047 +	your project's <quote>0.9</quote> release, and another,
  1.1048 +	containing different changes, for the <quote>1.0</quote>
  1.1049 +	release.</para>
  1.1050 +
  1.1051 +      <para id="x_db">Given this, you can imagine that the consequences could be
  1.1052 +	messy if you had a local <quote>0.9</quote> repository, and
  1.1053 +	accidentally pulled changes from the shared <quote>1.0</quote>
  1.1054 +	repository into it.  At worst, you could be paying
  1.1055 +	insufficient attention, and push those changes into the shared
  1.1056 +	<quote>0.9</quote> tree, confusing your entire team (but don't
  1.1057 +	worry, we'll return to this horror scenario later).  However,
  1.1058 +	it's more likely that you'll notice immediately, because
  1.1059 +	Mercurial will display the URL it's pulling from, or you will
  1.1060 +	see it pull a suspiciously large number of changes into the
  1.1061 +	repository.</para>
  1.1062 +
  1.1063 +      <para id="x_dc">The <command role="hg-cmd">hg rollback</command> command
  1.1064 +	will work nicely to expunge all of the changesets that you
  1.1065 +	just pulled.  Mercurial groups all changes from one <command
  1.1066 +	  role="hg-cmd">hg pull</command> into a single transaction,
  1.1067 +	so one <command role="hg-cmd">hg rollback</command> is all you
  1.1068 +	need to undo this mistake.</para>
  1.1069 +
  1.1070 +    </sect2>
  1.1071 +    <sect2 id="sec:undo:rollback-after-push">
  1.1072 +      <title>Rolling back is useless once you've pushed</title>
  1.1073 +
  1.1074 +      <para id="x_dd">The value of the <command role="hg-cmd">hg
  1.1075 +	  rollback</command> command drops to zero once you've pushed
  1.1076 +	your changes to another repository.  Rolling back a change
  1.1077 +	makes it disappear entirely, but <emphasis>only</emphasis> in
  1.1078 +	the repository in which you perform the <command
  1.1079 +	  role="hg-cmd">hg rollback</command>.  Because a rollback
  1.1080 +	eliminates history, there's no way for the disappearance of a
  1.1081 +	change to propagate between repositories.</para>
  1.1082 +
  1.1083 +      <para id="x_de">If you've pushed a change to another
  1.1084 +	repository&emdash;particularly if it's a shared
  1.1085 +	repository&emdash;it has essentially <quote>escaped into the
  1.1086 +	  wild,</quote> and you'll have to recover from your mistake
  1.1087 +	in a different way.  If you push a changeset somewhere, then
  1.1088 +	roll it back, then pull from the repository you pushed to, the
  1.1089 +	changeset you thought you'd gotten rid of will simply reappear
  1.1090 +	in your repository.</para>
  1.1091 +
  1.1092 +      <para id="x_df">(If you absolutely know for sure that the change
  1.1093 +	you want to roll back is the most recent change in the
  1.1094 +	repository that you pushed to, <emphasis>and</emphasis> you
  1.1095 +	know that nobody else could have pulled it from that
  1.1096 +	repository, you can roll back the changeset there, too, but
  1.1097 +	you really should not expect this to work reliably.  Sooner or
  1.1098 +	later a change really will make it into a repository that you
  1.1099 +	don't directly control (or have forgotten about), and come
  1.1100 +	back to bite you.)</para>
  1.1101 +
  1.1102 +    </sect2>
  1.1103 +    <sect2>
  1.1104 +      <title>You can only roll back once</title>
  1.1105 +
  1.1106 +      <para id="x_e0">Mercurial stores exactly one transaction in its
  1.1107 +	transaction log; that transaction is the most recent one that
  1.1108 +	occurred in the repository. This means that you can only roll
  1.1109 +	back one transaction.  If you expect to be able to roll back
  1.1110 +	one transaction, then its predecessor, this is not the
  1.1111 +	behavior you will get.</para>
  1.1112 +
  1.1113 +      &interaction.rollback.twice;
  1.1114 +
  1.1115 +      <para id="x_e1">Once you've rolled back one transaction in a repository,
  1.1116 +	you can't roll back again in that repository until you perform
  1.1117 +	another commit or pull.</para>
  1.1118 +
  1.1119 +    </sect2>
  1.1120 +  </sect1>
  1.1121 +  <sect1>
  1.1122 +    <title>Reverting the mistaken change</title>
  1.1123 +
  1.1124 +    <para id="x_e2">If you make a modification to a file, and decide that you
  1.1125 +      really didn't want to change the file at all, and you haven't
  1.1126 +      yet committed your changes, the <command role="hg-cmd">hg
  1.1127 +	revert</command> command is the one you'll need.  It looks at
  1.1128 +      the changeset that's the parent of the working directory, and
  1.1129 +      restores the contents of the file to their state as of that
  1.1130 +      changeset. (That's a long-winded way of saying that, in the
  1.1131 +      normal case, it undoes your modifications.)</para>
  1.1132 +
  1.1133 +    <para id="x_e3">Let's illustrate how the <command role="hg-cmd">hg
  1.1134 +	revert</command> command works with yet another small example.
  1.1135 +      We'll begin by modifying a file that Mercurial is already
  1.1136 +      tracking.</para>
  1.1137 +
  1.1138 +    &interaction.daily.revert.modify;
  1.1139 +
  1.1140 +    <para id="x_e4">If we don't
  1.1141 +      want that change, we can simply <command role="hg-cmd">hg
  1.1142 +	revert</command> the file.</para>
  1.1143 +
  1.1144 +      &interaction.daily.revert.unmodify;
  1.1145 +
  1.1146 +    <para id="x_e5">The <command role="hg-cmd">hg revert</command> command
  1.1147 +      provides us with an extra degree of safety by saving our
  1.1148 +      modified file with a <filename>.orig</filename>
  1.1149 +      extension.</para>
  1.1150 +
  1.1151 +    &interaction.daily.revert.status;
  1.1152 +
  1.1153 +    <tip>
  1.1154 +      <title>Be careful with <filename>.orig</filename> files</title>
  1.1155 +
  1.1156 +      <para id="x_6b8">It's extremely unlikely that you are either using
  1.1157 +	Mercurial to manage files with <filename>.orig</filename>
  1.1158 +	extensions or that you even care about the contents of such
  1.1159 +	files.  Just in case, though, it's useful to remember that
  1.1160 +	<command role="hg-cmd">hg revert</command> will
  1.1161 +	unconditionally overwrite an existing file with a
  1.1162 +	<filename>.orig</filename> extension. For instance, if you
  1.1163 +	already have a file named <filename>foo.orig</filename> when
  1.1164 +	you revert <filename>foo</filename>, the contents of
  1.1165 +	<filename>foo.orig</filename> will be clobbered.</para>
  1.1166 +    </tip>
  1.1167 +
  1.1168 +    <para id="x_e6">Here is a summary of the cases that the <command
  1.1169 +	role="hg-cmd">hg revert</command> command can deal with.  We
  1.1170 +      will describe each of these in more detail in the section that
  1.1171 +      follows.</para>
  1.1172 +    <itemizedlist>
  1.1173 +      <listitem><para id="x_e7">If you modify a file, it will restore the file
  1.1174 +	  to its unmodified state.</para>
  1.1175 +      </listitem>
  1.1176 +      <listitem><para id="x_e8">If you <command role="hg-cmd">hg add</command> a
  1.1177 +	  file, it will undo the <quote>added</quote> state of the
  1.1178 +	  file, but leave the file itself untouched.</para>
  1.1179 +      </listitem>
  1.1180 +      <listitem><para id="x_e9">If you delete a file without telling Mercurial,
  1.1181 +	  it will restore the file to its unmodified contents.</para>
  1.1182 +      </listitem>
  1.1183 +      <listitem><para id="x_ea">If you use the <command role="hg-cmd">hg
  1.1184 +	    remove</command> command to remove a file, it will undo
  1.1185 +	  the <quote>removed</quote> state of the file, and restore
  1.1186 +	  the file to its unmodified contents.</para>
  1.1187 +      </listitem></itemizedlist>
  1.1188 +
  1.1189 +    <sect2 id="sec:undo:mgmt">
  1.1190 +      <title>File management errors</title>
  1.1191 +
  1.1192 +      <para id="x_eb">The <command role="hg-cmd">hg revert</command> command is
  1.1193 +	useful for more than just modified files.  It lets you reverse
  1.1194 +	the results of all of Mercurial's file management
  1.1195 +	commands&emdash;<command role="hg-cmd">hg add</command>,
  1.1196 +	<command role="hg-cmd">hg remove</command>, and so on.</para>
  1.1197 +
  1.1198 +      <para id="x_ec">If you <command role="hg-cmd">hg add</command> a file,
  1.1199 +	then decide that in fact you don't want Mercurial to track it,
  1.1200 +	use <command role="hg-cmd">hg revert</command> to undo the
  1.1201 +	add.  Don't worry; Mercurial will not modify the file in any
  1.1202 +	way.  It will just <quote>unmark</quote> the file.</para>
  1.1203 +
  1.1204 +      &interaction.daily.revert.add;
  1.1205 +
  1.1206 +      <para id="x_ed">Similarly, if you ask Mercurial to <command
  1.1207 +	  role="hg-cmd">hg remove</command> a file, you can use
  1.1208 +	<command role="hg-cmd">hg revert</command> to restore it to
  1.1209 +	the contents it had as of the parent of the working directory.
  1.1210 +	&interaction.daily.revert.remove; This works just as
  1.1211 +	well for a file that you deleted by hand, without telling
  1.1212 +	Mercurial (recall that in Mercurial terminology, this kind of
  1.1213 +	file is called <quote>missing</quote>).</para>
  1.1214 +
  1.1215 +      &interaction.daily.revert.missing;
  1.1216 +
  1.1217 +      <para id="x_ee">If you revert a <command role="hg-cmd">hg copy</command>,
  1.1218 +	the copied-to file remains in your working directory
  1.1219 +	afterwards, untracked.  Since a copy doesn't affect the
  1.1220 +	copied-from file in any way, Mercurial doesn't do anything
  1.1221 +	with the copied-from file.</para>
  1.1222 +
  1.1223 +      &interaction.daily.revert.copy;
  1.1224 +    </sect2>
  1.1225 +  </sect1>
  1.1226 +
  1.1227 +  <sect1>
  1.1228 +    <title>Dealing with committed changes</title>
  1.1229 +
  1.1230 +    <para id="x_f5">Consider a case where you have committed a change
  1.1231 +      <emphasis>a</emphasis>, and another change
  1.1232 +      <emphasis>b</emphasis> on top of it; you then realise that
  1.1233 +      change <emphasis>a</emphasis> was incorrect.  Mercurial lets you
  1.1234 +      <quote>back out</quote> an entire changeset automatically, and
  1.1235 +      building blocks that let you reverse part of a changeset by
  1.1236 +      hand.</para>
  1.1237 +
  1.1238 +    <para id="x_f6">Before you read this section, here's something to
  1.1239 +      keep in mind: the <command role="hg-cmd">hg backout</command>
  1.1240 +      command undoes the effect of a change by
  1.1241 +      <emphasis>adding</emphasis> to your repository's history, not by
  1.1242 +      modifying or erasing it.  It's the right tool to use if you're
  1.1243 +      fixing bugs, but not if you're trying to undo some change that
  1.1244 +      has catastrophic consequences.  To deal with those, see
  1.1245 +      <xref linkend="sec:undo:aaaiiieee"/>.</para>
  1.1246 +
  1.1247 +    <sect2>
  1.1248 +      <title>Backing out a changeset</title>
  1.1249 +
  1.1250 +      <para id="x_f7">The <command role="hg-cmd">hg backout</command> command
  1.1251 +	lets you <quote>undo</quote> the effects of an entire
  1.1252 +	changeset in an automated fashion.  Because Mercurial's
  1.1253 +	history is immutable, this command <emphasis>does
  1.1254 +	  not</emphasis> get rid of the changeset you want to undo.
  1.1255 +	Instead, it creates a new changeset that
  1.1256 +	<emphasis>reverses</emphasis> the effect of the to-be-undone
  1.1257 +	changeset.</para>
  1.1258 +
  1.1259 +      <para id="x_f8">The operation of the <command role="hg-cmd">hg
  1.1260 +	  backout</command> command is a little intricate, so let's
  1.1261 +	illustrate it with some examples.  First, we'll create a
  1.1262 +	repository with some simple changes.</para>
  1.1263 +
  1.1264 +      &interaction.backout.init;
  1.1265 +
  1.1266 +      <para id="x_f9">The <command role="hg-cmd">hg backout</command> command
  1.1267 +	takes a single changeset ID as its argument; this is the
  1.1268 +	changeset to back out.  Normally, <command role="hg-cmd">hg
  1.1269 +	  backout</command> will drop you into a text editor to write
  1.1270 +	a commit message, so you can record why you're backing the
  1.1271 +	change out.  In this example, we provide a commit message on
  1.1272 +	the command line using the <option
  1.1273 +	  role="hg-opt-backout">-m</option> option.</para>
  1.1274 +
  1.1275 +    </sect2>
  1.1276 +    <sect2>
  1.1277 +      <title>Backing out the tip changeset</title>
  1.1278 +
  1.1279 +      <para id="x_fa">We're going to start by backing out the last changeset we
  1.1280 +	committed.</para>
  1.1281 +
  1.1282 +      &interaction.backout.simple;
  1.1283 +
  1.1284 +      <para id="x_fb">You can see that the second line from
  1.1285 +	<filename>myfile</filename> is no longer present.  Taking a
  1.1286 +	look at the output of <command role="hg-cmd">hg log</command>
  1.1287 +	gives us an idea of what the <command role="hg-cmd">hg
  1.1288 +	  backout</command> command has done.
  1.1289 +	&interaction.backout.simple.log; Notice that the new changeset
  1.1290 +	that <command role="hg-cmd">hg backout</command> has created
  1.1291 +	is a child of the changeset we backed out.  It's easier to see
  1.1292 +	this in <xref linkend="fig:undo:backout"/>, which presents a
  1.1293 +	graphical view of the change history.  As you can see, the
  1.1294 +	history is nice and linear.</para>
  1.1295 +
  1.1296 +      <figure id="fig:undo:backout">
  1.1297 +	<title>Backing out a change using the <command
  1.1298 +	    role="hg-cmd">hg backout</command> command</title>
  1.1299 +	<mediaobject>
  1.1300 +	  <imageobject><imagedata fileref="figs/undo-simple.png"/></imageobject>
  1.1301 +	  <textobject><phrase>XXX add text</phrase></textobject>
  1.1302 +	</mediaobject>
  1.1303 +      </figure>
  1.1304 +
  1.1305 +    </sect2>
  1.1306 +    <sect2>
  1.1307 +      <title>Backing out a non-tip change</title>
  1.1308 +
  1.1309 +      <para id="x_fd">If you want to back out a change other than the last one
  1.1310 +	you committed, pass the <option
  1.1311 +	  role="hg-opt-backout">--merge</option> option to the
  1.1312 +	<command role="hg-cmd">hg backout</command> command.</para>
  1.1313 +
  1.1314 +      &interaction.backout.non-tip.clone;
  1.1315 +
  1.1316 +      <para id="x_fe">This makes backing out any changeset a
  1.1317 +	<quote>one-shot</quote> operation that's usually simple and
  1.1318 +	fast.</para>
  1.1319 +
  1.1320 +      &interaction.backout.non-tip.backout;
  1.1321 +
  1.1322 +      <para id="x_ff">If you take a look at the contents of
  1.1323 +	<filename>myfile</filename> after the backout finishes, you'll
  1.1324 +	see that the first and third changes are present, but not the
  1.1325 +	second.</para>
  1.1326 +
  1.1327 +      &interaction.backout.non-tip.cat;
  1.1328 +
  1.1329 +      <para id="x_100">As the graphical history in <xref
  1.1330 +	  linkend="fig:undo:backout-non-tip"/> illustrates, Mercurial
  1.1331 +	still commits one change in this kind of situation (the
  1.1332 +	box-shaped node is the ones that Mercurial commits
  1.1333 +	automatically), but the revision graph now looks different.
  1.1334 +	Before Mercurial begins the backout process, it first
  1.1335 +	remembers what the current parent of the working directory is.
  1.1336 +	It then backs out the target changeset, and commits that as a
  1.1337 +	changeset.  Finally, it merges back to the previous parent of
  1.1338 +	the working directory, but notice that it <emphasis>does not
  1.1339 +	  commit</emphasis> the result of the merge.  The repository
  1.1340 +	now contains two heads, and the working directory is in a
  1.1341 +	merge state.</para>
  1.1342 +
  1.1343 +      <figure id="fig:undo:backout-non-tip">
  1.1344 +	<title>Automated backout of a non-tip change using the
  1.1345 +	  <command role="hg-cmd">hg backout</command> command</title>
  1.1346 +	<mediaobject>
  1.1347 +	  <imageobject><imagedata fileref="figs/undo-non-tip.png"/></imageobject>
  1.1348 +	  <textobject><phrase>XXX add text</phrase></textobject>
  1.1349 +	</mediaobject>
  1.1350 +      </figure>
  1.1351 +
  1.1352 +      <para id="x_103">The result is that you end up <quote>back where you
  1.1353 +	  were</quote>, only with some extra history that undoes the
  1.1354 +	effect of the changeset you wanted to back out.</para>
  1.1355 +
  1.1356 +      <para id="x_6b9">You might wonder why Mercurial does not commit the result
  1.1357 +	of the merge that it performed.  The reason lies in Mercurial
  1.1358 +	behaving conservatively: a merge naturally has more scope for
  1.1359 +	error than simply undoing the effect of the tip changeset,
  1.1360 +	so your work will be safest if you first inspect (and test!)
  1.1361 +	the result of the merge, <emphasis>then</emphasis> commit
  1.1362 +	it.</para>
  1.1363 +
  1.1364 +      <sect3>
  1.1365 +	<title>Always use the <option
  1.1366 +	    role="hg-opt-backout">--merge</option> option</title>
  1.1367 +
  1.1368 +	<para id="x_104">In fact, since the <option
  1.1369 +	    role="hg-opt-backout">--merge</option> option will do the
  1.1370 +	  <quote>right thing</quote> whether or not the changeset
  1.1371 +	  you're backing out is the tip (i.e. it won't try to merge if
  1.1372 +	  it's backing out the tip, since there's no need), you should
  1.1373 +	  <emphasis>always</emphasis> use this option when you run the
  1.1374 +	  <command role="hg-cmd">hg backout</command> command.</para>
  1.1375 +
  1.1376 +      </sect3>
  1.1377 +    </sect2>
  1.1378 +    <sect2>
  1.1379 +      <title>Gaining more control of the backout process</title>
  1.1380 +
  1.1381 +      <para id="x_105">While I've recommended that you always use the <option
  1.1382 +	  role="hg-opt-backout">--merge</option> option when backing
  1.1383 +	out a change, the <command role="hg-cmd">hg backout</command>
  1.1384 +	command lets you decide how to merge a backout changeset.
  1.1385 +	Taking control of the backout process by hand is something you
  1.1386 +	will rarely need to do, but it can be useful to understand
  1.1387 +	what the <command role="hg-cmd">hg backout</command> command
  1.1388 +	is doing for you automatically.  To illustrate this, let's
  1.1389 +	clone our first repository, but omit the backout change that
  1.1390 +	it contains.</para>
  1.1391 +
  1.1392 +      &interaction.backout.manual.clone;
  1.1393 +
  1.1394 +      <para id="x_106">As with our
  1.1395 +	earlier example, We'll commit a third changeset, then back out
  1.1396 +	its parent, and see what happens.</para>
  1.1397 +
  1.1398 +      &interaction.backout.manual.backout;
  1.1399 +
  1.1400 +      <para id="x_107">Our new changeset is again a descendant of the changeset
  1.1401 +	we backout out; it's thus a new head, <emphasis>not</emphasis>
  1.1402 +	a descendant of the changeset that was the tip.  The <command
  1.1403 +	  role="hg-cmd">hg backout</command> command was quite
  1.1404 +	explicit in telling us this.</para>
  1.1405 +
  1.1406 +      &interaction.backout.manual.log;
  1.1407 +
  1.1408 +      <para id="x_108">Again, it's easier to see what has happened by looking at
  1.1409 +	a graph of the revision history, in <xref
  1.1410 +	  linkend="fig:undo:backout-manual"/>.  This makes it clear
  1.1411 +	that when we use <command role="hg-cmd">hg backout</command>
  1.1412 +	to back out a change other than the tip, Mercurial adds a new
  1.1413 +	head to the repository (the change it committed is
  1.1414 +	box-shaped).</para>
  1.1415 +
  1.1416 +      <figure id="fig:undo:backout-manual">
  1.1417 +	<title>Backing out a change using the <command
  1.1418 +	    role="hg-cmd">hg backout</command> command</title>
  1.1419 +	<mediaobject>
  1.1420 +	  <imageobject><imagedata fileref="figs/undo-manual.png"/></imageobject>
  1.1421 +	  <textobject><phrase>XXX add text</phrase></textobject>
  1.1422 +	</mediaobject>
  1.1423 +      </figure>
  1.1424 +
  1.1425 +      <para id="x_10a">After the <command role="hg-cmd">hg backout</command>
  1.1426 +	command has completed, it leaves the new
  1.1427 +	<quote>backout</quote> changeset as the parent of the working
  1.1428 +	directory.</para>
  1.1429 +
  1.1430 +      &interaction.backout.manual.parents;
  1.1431 +
  1.1432 +      <para id="x_10b">Now we have two isolated sets of changes.</para>
  1.1433 +
  1.1434 +      &interaction.backout.manual.heads;
  1.1435 +
  1.1436 +      <para id="x_10c">Let's think about what we expect to see as the contents of
  1.1437 +	<filename>myfile</filename> now.  The first change should be
  1.1438 +	present, because we've never backed it out.  The second change
  1.1439 +	should be missing, as that's the change we backed out.  Since
  1.1440 +	the history graph shows the third change as a separate head,
  1.1441 +	we <emphasis>don't</emphasis> expect to see the third change
  1.1442 +	present in <filename>myfile</filename>.</para>
  1.1443 +
  1.1444 +      &interaction.backout.manual.cat;
  1.1445 +
  1.1446 +      <para id="x_10d">To get the third change back into the file, we just do a
  1.1447 +	normal merge of our two heads.</para>
  1.1448 +
  1.1449 +      &interaction.backout.manual.merge;
  1.1450 +
  1.1451 +      <para id="x_10e">Afterwards, the graphical history of our
  1.1452 +	repository looks like
  1.1453 +	<xref linkend="fig:undo:backout-manual-merge"/>.</para>
  1.1454 +
  1.1455 +      <figure id="fig:undo:backout-manual-merge">
  1.1456 +	<title>Manually merging a backout change</title>
  1.1457 +	<mediaobject>
  1.1458 +	  <imageobject><imagedata fileref="figs/undo-manual-merge.png"/></imageobject>
  1.1459 +	  <textobject><phrase>XXX add text</phrase></textobject>
  1.1460 +	</mediaobject>
  1.1461 +      </figure>
  1.1462 +
  1.1463 +    </sect2>
  1.1464 +    <sect2>
  1.1465 +      <title>Why <command role="hg-cmd">hg backout</command> works as
  1.1466 +	it does</title>
  1.1467 +
  1.1468 +      <para id="x_110">Here's a brief description of how the <command
  1.1469 +	  role="hg-cmd">hg backout</command> command works.</para>
  1.1470 +      <orderedlist>
  1.1471 +	<listitem><para id="x_111">It ensures that the working directory is
  1.1472 +	    <quote>clean</quote>, i.e. that the output of <command
  1.1473 +	      role="hg-cmd">hg status</command> would be empty.</para>
  1.1474 +	</listitem>
  1.1475 +	<listitem><para id="x_112">It remembers the current parent of the working
  1.1476 +	    directory.  Let's call this changeset
  1.1477 +	    <literal>orig</literal>.</para>
  1.1478 +	</listitem>
  1.1479 +	<listitem><para id="x_113">It does the equivalent of a <command
  1.1480 +	      role="hg-cmd">hg update</command> to sync the working
  1.1481 +	    directory to the changeset you want to back out.  Let's
  1.1482 +	    call this changeset <literal>backout</literal>.</para>
  1.1483 +	</listitem>
  1.1484 +	<listitem><para id="x_114">It finds the parent of that changeset.  Let's
  1.1485 +	    call that changeset <literal>parent</literal>.</para>
  1.1486 +	</listitem>
  1.1487 +	<listitem><para id="x_115">For each file that the
  1.1488 +	    <literal>backout</literal> changeset affected, it does the
  1.1489 +	    equivalent of a <command role="hg-cmd">hg revert -r
  1.1490 +	      parent</command> on that file, to restore it to the
  1.1491 +	    contents it had before that changeset was
  1.1492 +	    committed.</para>
  1.1493 +	</listitem>
  1.1494 +	<listitem><para id="x_116">It commits the result as a new changeset.
  1.1495 +	    This changeset has <literal>backout</literal> as its
  1.1496 +	    parent.</para>
  1.1497 +	</listitem>
  1.1498 +	<listitem><para id="x_117">If you specify <option
  1.1499 +	      role="hg-opt-backout">--merge</option> on the command
  1.1500 +	    line, it merges with <literal>orig</literal>, and commits
  1.1501 +	    the result of the merge.</para>
  1.1502 +	</listitem></orderedlist>
  1.1503 +
  1.1504 +      <para id="x_118">An alternative way to implement the <command
  1.1505 +	  role="hg-cmd">hg backout</command> command would be to
  1.1506 +	<command role="hg-cmd">hg export</command> the
  1.1507 +	to-be-backed-out changeset as a diff, then use the <option
  1.1508 +	  role="cmd-opt-patch">--reverse</option> option to the
  1.1509 +	<command>patch</command> command to reverse the effect of the
  1.1510 +	change without fiddling with the working directory.  This
  1.1511 +	sounds much simpler, but it would not work nearly as
  1.1512 +	well.</para>
  1.1513 +
  1.1514 +      <para id="x_119">The reason that <command role="hg-cmd">hg
  1.1515 +	  backout</command> does an update, a commit, a merge, and
  1.1516 +	another commit is to give the merge machinery the best chance
  1.1517 +	to do a good job when dealing with all the changes
  1.1518 +	<emphasis>between</emphasis> the change you're backing out and
  1.1519 +	the current tip.</para>
  1.1520 +
  1.1521 +      <para id="x_11a">If you're backing out a changeset that's 100 revisions
  1.1522 +	back in your project's history, the chances that the
  1.1523 +	<command>patch</command> command will be able to apply a
  1.1524 +	reverse diff cleanly are not good, because intervening changes
  1.1525 +	are likely to have <quote>broken the context</quote> that
  1.1526 +	<command>patch</command> uses to determine whether it can
  1.1527 +	apply a patch (if this sounds like gibberish, see <xref
  1.1528 +	  linkend="sec:mq:patch"/> for a
  1.1529 +	discussion of the <command>patch</command> command).  Also,
  1.1530 +	Mercurial's merge machinery will handle files and directories
  1.1531 +	being renamed, permission changes, and modifications to binary
  1.1532 +	files, none of which <command>patch</command> can deal
  1.1533 +	with.</para>
  1.1534 +
  1.1535 +    </sect2>
  1.1536 +  </sect1>
  1.1537 +  <sect1 id="sec:undo:aaaiiieee">
  1.1538 +    <title>Changes that should never have been</title>
  1.1539 +
  1.1540 +    <para id="x_11b">Most of the time, the <command role="hg-cmd">hg
  1.1541 +	backout</command> command is exactly what you need if you want
  1.1542 +      to undo the effects of a change.  It leaves a permanent record
  1.1543 +      of exactly what you did, both when committing the original
  1.1544 +      changeset and when you cleaned up after it.</para>
  1.1545 +
  1.1546 +    <para id="x_11c">On rare occasions, though, you may find that you've
  1.1547 +      committed a change that really should not be present in the
  1.1548 +      repository at all.  For example, it would be very unusual, and
  1.1549 +      usually considered a mistake, to commit a software project's
  1.1550 +      object files as well as its source files.  Object files have
  1.1551 +      almost no intrinsic value, and they're <emphasis>big</emphasis>,
  1.1552 +      so they increase the size of the repository and the amount of
  1.1553 +      time it takes to clone or pull changes.</para>
  1.1554 +
  1.1555 +    <para id="x_11d">Before I discuss the options that you have if you commit a
  1.1556 +      <quote>brown paper bag</quote> change (the kind that's so bad
  1.1557 +      that you want to pull a brown paper bag over your head), let me
  1.1558 +      first discuss some approaches that probably won't work.</para>
  1.1559 +
  1.1560 +    <para id="x_11e">Since Mercurial treats history as
  1.1561 +      accumulative&emdash;every change builds on top of all changes
  1.1562 +      that preceded it&emdash;you generally can't just make disastrous
  1.1563 +      changes disappear.  The one exception is when you've just
  1.1564 +      committed a change, and it hasn't been pushed or pulled into
  1.1565 +      another repository.  That's when you can safely use the <command
  1.1566 +	role="hg-cmd">hg rollback</command> command, as I detailed in
  1.1567 +      <xref linkend="sec:undo:rollback"/>.</para>
  1.1568 +
  1.1569 +    <para id="x_11f">After you've pushed a bad change to another repository, you
  1.1570 +      <emphasis>could</emphasis> still use <command role="hg-cmd">hg
  1.1571 +	rollback</command> to make your local copy of the change
  1.1572 +      disappear, but it won't have the consequences you want.  The
  1.1573 +      change will still be present in the remote repository, so it
  1.1574 +      will reappear in your local repository the next time you
  1.1575 +      pull.</para>
  1.1576 +
  1.1577 +    <para id="x_120">If a situation like this arises, and you know which
  1.1578 +      repositories your bad change has propagated into, you can
  1.1579 +      <emphasis>try</emphasis> to get rid of the change from
  1.1580 +      <emphasis>every</emphasis> one of those repositories.  This is,
  1.1581 +      of course, not a satisfactory solution: if you miss even a
  1.1582 +      single repository while you're expunging, the change is still
  1.1583 +      <quote>in the wild</quote>, and could propagate further.</para>
  1.1584 +
  1.1585 +    <para id="x_121">If you've committed one or more changes
  1.1586 +      <emphasis>after</emphasis> the change that you'd like to see
  1.1587 +      disappear, your options are further reduced. Mercurial doesn't
  1.1588 +      provide a way to <quote>punch a hole</quote> in history, leaving
  1.1589 +      changesets intact.</para>
  1.1590 +
  1.1591 +    <sect2>
  1.1592 +      <title>Backing out a merge</title>
  1.1593 +
  1.1594 +      <para id="x_6ba">Since merges are often complicated, it is not unheard of
  1.1595 +	for a merge to be mangled badly, but committed erroneously.
  1.1596 +	Mercurial provides an important safeguard against bad merges
  1.1597 +	by refusing to commit unresolved files, but human ingenuity
  1.1598 +	guarantees that it is still possible to mess a merge up and
  1.1599 +	commit it.</para>
  1.1600 +
  1.1601 +      <para id="x_6bb">Given a bad merge that has been committed, usually the
  1.1602 +	best way to approach it is to simply try to repair the damage
  1.1603 +	by hand.  A complete disaster that cannot be easily fixed up
  1.1604 +	by hand ought to be very rare, but the <command
  1.1605 +	  role="hg-cmd">hg backout</command> command may help in
  1.1606 +	making the cleanup easier. It offers a <option
  1.1607 +	  role="hg-opt-backout">--parent</option> option, which lets
  1.1608 +	you specify which parent to revert to when backing out a
  1.1609 +	merge.</para>
  1.1610 +
  1.1611 +      <figure id="fig:undo:bad-merge-1">
  1.1612 +	<title>A bad merge</title>
  1.1613 +	<mediaobject>
  1.1614 +	  <imageobject><imagedata fileref="figs/bad-merge-1.png"/></imageobject>
  1.1615 +	  <textobject><phrase>XXX add text</phrase></textobject>
  1.1616 +	</mediaobject>
  1.1617 +      </figure>
  1.1618 +
  1.1619 +      <para id="x_6bc">Suppose we have a revision graph like that in <xref
  1.1620 +	  linkend="fig:undo:bad-merge-1"/>.  What we'd like is to
  1.1621 +	<emphasis>redo</emphasis> the merge of revisions 2 and
  1.1622 +	3.</para>
  1.1623 +
  1.1624 +      <para id="x_6bd">One way to do so would be as follows.</para>
  1.1625 +
  1.1626 +      <orderedlist>
  1.1627 +	<listitem>
  1.1628 +	  <para id="x_6be">Call <command role="hg-cmd">hg backout --rev=4
  1.1629 +	      --parent=2</command>.  This tells <command
  1.1630 +	      role="hg-cmd">hg backout</command> to back out revision
  1.1631 +	    4, which is the bad merge, and to when deciding which
  1.1632 +	    revision to prefer, to choose parent 2, one of the parents
  1.1633 +	    of the merge.  The effect can be seen in <xref
  1.1634 +	      linkend="fig:undo:bad-merge-2"/>.</para>
  1.1635 +	  <figure id="fig:undo:bad-merge-2">
  1.1636 +	    <title>Backing out the merge, favoring one parent</title>
  1.1637 +	    <mediaobject>
  1.1638 +	      <imageobject><imagedata fileref="figs/bad-merge-2.png"/></imageobject>
  1.1639 +	      <textobject><phrase>XXX add text</phrase></textobject>
  1.1640 +	    </mediaobject>
  1.1641 +	  </figure>
  1.1642 +	</listitem>
  1.1643 +
  1.1644 +	<listitem>
  1.1645 +	  <para id="x_6bf">Call <command role="hg-cmd">hg backout --rev=4
  1.1646 +	      --parent=3</command>.  This tells <command
  1.1647 +	      role="hg-cmd">hg backout</command> to back out revision
  1.1648 +	    4 again, but this time to choose parent 3, the other
  1.1649 +	    parent of the merge.  The result is visible in <xref
  1.1650 +	    linkend="fig:undo:bad-merge-3"/>, in which the repository
  1.1651 +	    now contains three heads.</para>
  1.1652 +	  <figure id="fig:undo:bad-merge-3">
  1.1653 +	    <title>Backing out the merge, favoring the other
  1.1654 +	      parent</title>
  1.1655 +	    <mediaobject>
  1.1656 +	      <imageobject><imagedata fileref="figs/bad-merge-3.png"/></imageobject>
  1.1657 +	      <textobject><phrase>XXX add text</phrase></textobject>
  1.1658 +	    </mediaobject>
  1.1659 +	  </figure>
  1.1660 +	</listitem>
  1.1661 +
  1.1662 +	<listitem>
  1.1663 +	  <para id="x_6c0">Redo the bad merge by merging the two backout heads,
  1.1664 +	    which reduces the number of heads in the repository to
  1.1665 +	    two, as can be seen in <xref
  1.1666 +	      linkend="fig:undo:bad-merge-4"/>.</para>
  1.1667 +	  <figure id="fig:undo:bad-merge-4">
  1.1668 +	    <title>Merging the backouts</title>
  1.1669 +	    <mediaobject>
  1.1670 +	      <imageobject><imagedata fileref="figs/bad-merge-4.png"/></imageobject>
  1.1671 +	      <textobject><phrase>XXX add text</phrase></textobject>
  1.1672 +	    </mediaobject>
  1.1673 +	  </figure>
  1.1674 +	</listitem>
  1.1675 +
  1.1676 +	<listitem>
  1.1677 +	  <para id="x_6c1">Merge with the commit that was made after the bad
  1.1678 +	    merge, as shown in <xref
  1.1679 +	      linkend="fig:undo:bad-merge-5"/>.</para>
  1.1680 +	  <figure id="fig:undo:bad-merge-5">
  1.1681 +	    <title>Merging the backouts</title>
  1.1682 +	    <mediaobject>
  1.1683 +	      <imageobject><imagedata fileref="figs/bad-merge-5.png"/></imageobject>
  1.1684 +	      <textobject><phrase>XXX add text</phrase></textobject>
  1.1685 +	    </mediaobject>
  1.1686 +	  </figure>
  1.1687 +	</listitem>
  1.1688 +      </orderedlist>
  1.1689 +    </sect2>
  1.1690 +
  1.1691 +    <sect2>
  1.1692 +      <title>Protect yourself from <quote>escaped</quote>
  1.1693 +	changes</title>
  1.1694 +
  1.1695 +      <para id="x_123">If you've committed some changes to your local repository
  1.1696 +	and they've been pushed or pulled somewhere else, this isn't
  1.1697 +	necessarily a disaster.  You can protect yourself ahead of
  1.1698 +	time against some classes of bad changeset.  This is
  1.1699 +	particularly easy if your team usually pulls changes from a
  1.1700 +	central repository.</para>
  1.1701 +
  1.1702 +      <para id="x_124">By configuring some hooks on that repository to validate
  1.1703 +	incoming changesets (see chapter <xref linkend="chap:hook"/>),
  1.1704 +	you can
  1.1705 +	automatically prevent some kinds of bad changeset from being
  1.1706 +	pushed to the central repository at all.  With such a
  1.1707 +	configuration in place, some kinds of bad changeset will
  1.1708 +	naturally tend to <quote>die out</quote> because they can't
  1.1709 +	propagate into the central repository.  Better yet, this
  1.1710 +	happens without any need for explicit intervention.</para>
  1.1711 +
  1.1712 +      <para id="x_125">For instance, an incoming change hook that
  1.1713 +	verifies that a changeset will actually compile can prevent
  1.1714 +	people from inadvertently <quote>breaking the
  1.1715 +	  build</quote>.</para>
  1.1716 +    </sect2>
  1.1717 +
  1.1718 +    <sect2>
  1.1719 +      <title>What to do about sensitive changes that escape</title>
  1.1720 +
  1.1721 +      <para id="x_6c2">Even a carefully run project can suffer an unfortunate
  1.1722 +	event such as the committing and uncontrolled propagation of a
  1.1723 +	file that contains important passwords.</para>
  1.1724 +
  1.1725 +      <para id="x_6c3">If something like this happens to you, and the information
  1.1726 +	that gets accidentally propagated is truly sensitive, your
  1.1727 +	first step should be to mitigate the effect of the leak
  1.1728 +	without trying to control the leak itself. If you are not 100%
  1.1729 +	certain that you know exactly who could have seen the changes,
  1.1730 +	you should immediately change passwords, cancel credit cards,
  1.1731 +	or find some other way to make sure that the information that
  1.1732 +	has leaked is no longer useful.  In other words, assume that
  1.1733 +	the change has propagated far and wide, and that there's
  1.1734 +	nothing more you can do.</para>
  1.1735 +
  1.1736 +      <para id="x_6c4">You might hope that there would be mechanisms you could
  1.1737 +	use to either figure out who has seen a change or to erase the
  1.1738 +	change permanently everywhere, but there are good reasons why
  1.1739 +	these are not possible.</para>
  1.1740 +
  1.1741 +      <para id="x_6c5">Mercurial does not provide an audit trail of who has
  1.1742 +	pulled changes from a repository, because it is usually either
  1.1743 +	impossible to record such information or trivial to spoof it.
  1.1744 +	In a multi-user or networked environment, you should thus be
  1.1745 +	extremely skeptical of yourself if you think that you have
  1.1746 +	identified every place that a sensitive changeset has
  1.1747 +	propagated to.  Don't forget that people can and will send
  1.1748 +	bundles by email, have their backup software save data
  1.1749 +	offsite, carry repositories on USB sticks, and find other
  1.1750 +	completely innocent ways to confound your attempts to track
  1.1751 +	down every copy of a problematic change.</para>
  1.1752 +
  1.1753 +      <para id="x_6c6">Mercurial also does not provide a way to make a file or
  1.1754 +	changeset completely disappear from history, because there is
  1.1755 +	no way to enforce its disappearance; someone could easily
  1.1756 +	modify their copy of Mercurial to ignore such directives. In
  1.1757 +	addition, even if Mercurial provided such a capability,
  1.1758 +	someone who simply hadn't pulled a <quote>make this file
  1.1759 +	  disappear</quote> changeset wouldn't be affected by it, nor
  1.1760 +	would web crawlers visiting at the wrong time, disk backups,
  1.1761 +	or other mechanisms.  Indeed, no distributed revision control
  1.1762 +	system can make data reliably vanish. Providing the illusion
  1.1763 +	of such control could easily give a false sense of security,
  1.1764 +	and be worse than not providing it at all.</para>
  1.1765 +    </sect2>
  1.1766 +  </sect1>
  1.1767 +
  1.1768 +  <sect1 id="sec:undo:bisect">
  1.1769 +    <title>Finding the source of a bug</title>
  1.1770 +
  1.1771 +    <para id="x_126">While it's all very well to be able to back out a changeset
  1.1772 +      that introduced a bug, this requires that you know which
  1.1773 +      changeset to back out.  Mercurial provides an invaluable
  1.1774 +      command, called <command role="hg-cmd">hg bisect</command>, that
  1.1775 +      helps you to automate this process and accomplish it very
  1.1776 +      efficiently.</para>
  1.1777 +
  1.1778 +    <para id="x_127">The idea behind the <command role="hg-cmd">hg
  1.1779 +	bisect</command> command is that a changeset has introduced
  1.1780 +      some change of behavior that you can identify with a simple
  1.1781 +      pass/fail test.  You don't know which piece of code introduced the
  1.1782 +      change, but you know how to test for the presence of the bug.
  1.1783 +      The <command role="hg-cmd">hg bisect</command> command uses your
  1.1784 +      test to direct its search for the changeset that introduced the
  1.1785 +      code that caused the bug.</para>
  1.1786 +
  1.1787 +    <para id="x_128">Here are a few scenarios to help you understand how you
  1.1788 +      might apply this command.</para>
  1.1789 +    <itemizedlist>
  1.1790 +      <listitem><para id="x_129">The most recent version of your software has a
  1.1791 +	  bug that you remember wasn't present a few weeks ago, but
  1.1792 +	  you don't know when it was introduced.  Here, your binary
  1.1793 +	  test checks for the presence of that bug.</para>
  1.1794 +      </listitem>
  1.1795 +      <listitem><para id="x_12a">You fixed a bug in a rush, and now it's time to
  1.1796 +	  close the entry in your team's bug database.  The bug
  1.1797 +	  database requires a changeset ID when you close an entry,
  1.1798 +	  but you don't remember which changeset you fixed the bug in.
  1.1799 +	  Once again, your binary test checks for the presence of the
  1.1800 +	  bug.</para>
  1.1801 +      </listitem>
  1.1802 +      <listitem><para id="x_12b">Your software works correctly, but runs 15%
  1.1803 +	  slower than the last time you measured it.  You want to know
  1.1804 +	  which changeset introduced the performance regression.  In
  1.1805 +	  this case, your binary test measures the performance of your
  1.1806 +	  software, to see whether it's <quote>fast</quote> or
  1.1807 +	  <quote>slow</quote>.</para>
  1.1808 +      </listitem>
  1.1809 +      <listitem><para id="x_12c">The sizes of the components of your project that
  1.1810 +	  you ship exploded recently, and you suspect that something
  1.1811 +	  changed in the way you build your project.</para>
  1.1812 +      </listitem></itemizedlist>
  1.1813 +
  1.1814 +    <para id="x_12d">From these examples, it should be clear that the <command
  1.1815 +	role="hg-cmd">hg bisect</command> command is not useful only
  1.1816 +      for finding the sources of bugs.  You can use it to find any
  1.1817 +      <quote>emergent property</quote> of a repository (anything that
  1.1818 +      you can't find from a simple text search of the files in the
  1.1819 +      tree) for which you can write a binary test.</para>
  1.1820 +
  1.1821 +    <para id="x_12e">We'll introduce a little bit of terminology here, just to
  1.1822 +      make it clear which parts of the search process are your
  1.1823 +      responsibility, and which are Mercurial's.  A
  1.1824 +      <emphasis>test</emphasis> is something that
  1.1825 +      <emphasis>you</emphasis> run when <command role="hg-cmd">hg
  1.1826 +	bisect</command> chooses a changeset.  A
  1.1827 +      <emphasis>probe</emphasis> is what <command role="hg-cmd">hg
  1.1828 +	bisect</command> runs to tell whether a revision is good.
  1.1829 +      Finally, we'll use the word <quote>bisect</quote>, as both a
  1.1830 +      noun and a verb, to stand in for the phrase <quote>search using
  1.1831 +	the <command role="hg-cmd">hg bisect</command>
  1.1832 +	command</quote>.</para>
  1.1833 +
  1.1834 +    <para id="x_12f">One simple way to automate the searching process would be
  1.1835 +      simply to probe every changeset.  However, this scales poorly.
  1.1836 +      If it took ten minutes to test a single changeset, and you had
  1.1837 +      10,000 changesets in your repository, the exhaustive approach
  1.1838 +      would take on average 35 <emphasis>days</emphasis> to find the
  1.1839 +      changeset that introduced a bug.  Even if you knew that the bug
  1.1840 +      was introduced by one of the last 500 changesets, and limited
  1.1841 +      your search to those, you'd still be looking at over 40 hours to
  1.1842 +      find the changeset that introduced your bug.</para>
  1.1843 +
  1.1844 +    <para id="x_130">What the <command role="hg-cmd">hg bisect</command> command
  1.1845 +      does is use its knowledge of the <quote>shape</quote> of your
  1.1846 +      project's revision history to perform a search in time
  1.1847 +      proportional to the <emphasis>logarithm</emphasis> of the number
  1.1848 +      of changesets to check (the kind of search it performs is called
  1.1849 +      a dichotomic search).  With this approach, searching through
  1.1850 +      10,000 changesets will take less than three hours, even at ten
  1.1851 +      minutes per test (the search will require about 14 tests).
  1.1852 +      Limit your search to the last hundred changesets, and it will
  1.1853 +      take only about an hour (roughly seven tests).</para>
  1.1854 +
  1.1855 +    <para id="x_131">The <command role="hg-cmd">hg bisect</command> command is
  1.1856 +      aware of the <quote>branchy</quote> nature of a Mercurial
  1.1857 +      project's revision history, so it has no problems dealing with
  1.1858 +      branches, merges, or multiple heads in a repository.  It can
  1.1859 +      prune entire branches of history with a single probe, which is
  1.1860 +      how it operates so efficiently.</para>
  1.1861 +
  1.1862 +    <sect2>
  1.1863 +      <title>Using the <command role="hg-cmd">hg bisect</command>
  1.1864 +	command</title>
  1.1865 +
  1.1866 +      <para id="x_132">Here's an example of <command role="hg-cmd">hg
  1.1867 +	  bisect</command> in action.</para>
  1.1868 +
  1.1869 +      <note>
  1.1870 +	<para id="x_133">  In versions 0.9.5 and earlier of Mercurial, <command
  1.1871 +	    role="hg-cmd">hg bisect</command> was not a core command:
  1.1872 +	  it was distributed with Mercurial as an extension. This
  1.1873 +	  section describes the built-in command, not the old
  1.1874 +	  extension.</para>
  1.1875 +      </note>
  1.1876 +
  1.1877 +      <para id="x_134">Now let's create a repository, so that we can try out the
  1.1878 +	<command role="hg-cmd">hg bisect</command> command in
  1.1879 +	isolation.</para>
  1.1880 +
  1.1881 +      &interaction.bisect.init;
  1.1882 +
  1.1883 +      <para id="x_135">We'll simulate a project that has a bug in it in a
  1.1884 +	simple-minded way: create trivial changes in a loop, and
  1.1885 +	nominate one specific change that will have the
  1.1886 +	<quote>bug</quote>.  This loop creates 35 changesets, each
  1.1887 +	adding a single file to the repository. We'll represent our
  1.1888 +	<quote>bug</quote> with a file that contains the text <quote>i
  1.1889 +	  have a gub</quote>.</para>
  1.1890 +
  1.1891 +      &interaction.bisect.commits;
  1.1892 +
  1.1893 +      <para id="x_136">The next thing that we'd like to do is figure out how to
  1.1894 +	use the <command role="hg-cmd">hg bisect</command> command.
  1.1895 +	We can use Mercurial's normal built-in help mechanism for
  1.1896 +	this.</para>
  1.1897 +
  1.1898 +      &interaction.bisect.help;
  1.1899 +
  1.1900 +      <para id="x_137">The <command role="hg-cmd">hg bisect</command> command
  1.1901 +	works in steps.  Each step proceeds as follows.</para>
  1.1902 +      <orderedlist>
  1.1903 +	<listitem><para id="x_138">You run your binary test.</para>
  1.1904 +	  <itemizedlist>
  1.1905 +	    <listitem><para id="x_139">If the test succeeded, you tell <command
  1.1906 +		  role="hg-cmd">hg bisect</command> by running the
  1.1907 +		<command role="hg-cmd">hg bisect --good</command>
  1.1908 +		command.</para>
  1.1909 +	    </listitem>
  1.1910 +	    <listitem><para id="x_13a">If it failed, run the <command
  1.1911 +		  role="hg-cmd">hg bisect --bad</command>
  1.1912 +		command.</para></listitem></itemizedlist>
  1.1913 +	</listitem>
  1.1914 +	<listitem><para id="x_13b">The command uses your information to decide
  1.1915 +	    which changeset to test next.</para>
  1.1916 +	</listitem>
  1.1917 +	<listitem><para id="x_13c">It updates the working directory to that
  1.1918 +	    changeset, and the process begins again.</para>
  1.1919 +	</listitem></orderedlist>
  1.1920 +      <para id="x_13d">The process ends when <command role="hg-cmd">hg
  1.1921 +	  bisect</command> identifies a unique changeset that marks
  1.1922 +	the point where your test transitioned from
  1.1923 +	<quote>succeeding</quote> to <quote>failing</quote>.</para>
  1.1924 +
  1.1925 +      <para id="x_13e">To start the search, we must run the <command
  1.1926 +	  role="hg-cmd">hg bisect --reset</command> command.</para>
  1.1927 +
  1.1928 +      &interaction.bisect.search.init;
  1.1929 +
  1.1930 +      <para id="x_13f">In our case, the binary test we use is simple: we check to
  1.1931 +	see if any file in the repository contains the string <quote>i
  1.1932 +	  have a gub</quote>.  If it does, this changeset contains the
  1.1933 +	change that <quote>caused the bug</quote>.  By convention, a
  1.1934 +	changeset that has the property we're searching for is
  1.1935 +	<quote>bad</quote>, while one that doesn't is
  1.1936 +	<quote>good</quote>.</para>
  1.1937 +
  1.1938 +      <para id="x_140">Most of the time, the revision to which the working
  1.1939 +	directory is synced (usually the tip) already exhibits the
  1.1940 +	problem introduced by the buggy change, so we'll mark it as
  1.1941 +	<quote>bad</quote>.</para>
  1.1942 +
  1.1943 +      &interaction.bisect.search.bad-init;
  1.1944 +
  1.1945 +      <para id="x_141">Our next task is to nominate a changeset that we know
  1.1946 +	<emphasis>doesn't</emphasis> have the bug; the <command
  1.1947 +	  role="hg-cmd">hg bisect</command> command will
  1.1948 +	<quote>bracket</quote> its search between the first pair of
  1.1949 +	good and bad changesets.  In our case, we know that revision
  1.1950 +	10 didn't have the bug.  (I'll have more words about choosing
  1.1951 +	the first <quote>good</quote> changeset later.)</para>
  1.1952 +
  1.1953 +      &interaction.bisect.search.good-init;
  1.1954 +
  1.1955 +      <para id="x_142">Notice that this command printed some output.</para>
  1.1956 +      <itemizedlist>
  1.1957 +	<listitem><para id="x_143">It told us how many changesets it must
  1.1958 +	    consider before it can identify the one that introduced
  1.1959 +	    the bug, and how many tests that will require.</para>
  1.1960 +	</listitem>
  1.1961 +	<listitem><para id="x_144">It updated the working directory to the next
  1.1962 +	    changeset to test, and told us which changeset it's
  1.1963 +	    testing.</para>
  1.1964 +	</listitem></itemizedlist>
  1.1965 +
  1.1966 +      <para id="x_145">We now run our test in the working directory.  We use the
  1.1967 +	<command>grep</command> command to see if our
  1.1968 +	<quote>bad</quote> file is present in the working directory.
  1.1969 +	If it is, this revision is bad; if not, this revision is good.
  1.1970 +	&interaction.bisect.search.step1;</para>
  1.1971 +
  1.1972 +      <para id="x_146">This test looks like a perfect candidate for automation,
  1.1973 +	so let's turn it into a shell function.</para>
  1.1974 +      &interaction.bisect.search.mytest;
  1.1975 +
  1.1976 +      <para id="x_147">We can now run an entire test step with a single command,
  1.1977 +	<literal>mytest</literal>.</para>
  1.1978 +
  1.1979 +      &interaction.bisect.search.step2;
  1.1980 +
  1.1981 +      <para id="x_148">A few more invocations of our canned test step command,
  1.1982 +	and we're done.</para>
  1.1983 +
  1.1984 +      &interaction.bisect.search.rest;
  1.1985 +
  1.1986 +      <para id="x_149">Even though we had 40 changesets to search through, the
  1.1987 +	<command role="hg-cmd">hg bisect</command> command let us find
  1.1988 +	the changeset that introduced our <quote>bug</quote> with only
  1.1989 +	five tests.  Because the number of tests that the <command
  1.1990 +	  role="hg-cmd">hg bisect</command> command performs grows
  1.1991 +	logarithmically with the number of changesets to search, the
  1.1992 +	advantage that it has over the <quote>brute force</quote>
  1.1993 +	search approach increases with every changeset you add.</para>
  1.1994 +
  1.1995 +    </sect2>
  1.1996 +    <sect2>
  1.1997 +      <title>Cleaning up after your search</title>
  1.1998 +
  1.1999 +      <para id="x_14a">When you're finished using the <command role="hg-cmd">hg
  1.2000 +	  bisect</command> command in a repository, you can use the
  1.2001 +	<command role="hg-cmd">hg bisect --reset</command> command to
  1.2002 +	drop the information it was using to drive your search.  The
  1.2003 +	command doesn't use much space, so it doesn't matter if you
  1.2004 +	forget to run this command.  However, <command
  1.2005 +	  role="hg-cmd">hg bisect</command> won't let you start a new
  1.2006 +	search in that repository until you do a <command
  1.2007 +	  role="hg-cmd">hg bisect --reset</command>.</para>
  1.2008 +
  1.2009 +      &interaction.bisect.search.reset;
  1.2010 +
  1.2011 +    </sect2>
  1.2012 +  </sect1>
  1.2013 +  <sect1>
  1.2014 +    <title>Tips for finding bugs effectively</title>
  1.2015 +
  1.2016 +    <sect2>
  1.2017 +      <title>Give consistent input</title>
  1.2018 +
  1.2019 +      <para id="x_14b">The <command role="hg-cmd">hg bisect</command> command
  1.2020 +	requires that you correctly report the result of every test
  1.2021 +	you perform.  If you tell it that a test failed when it really
  1.2022 +	succeeded, it <emphasis>might</emphasis> be able to detect the
  1.2023 +	inconsistency.  If it can identify an inconsistency in your
  1.2024 +	reports, it will tell you that a particular changeset is both
  1.2025 +	good and bad. However, it can't do this perfectly; it's about
  1.2026 +	as likely to report the wrong changeset as the source of the
  1.2027 +	bug.</para>
  1.2028 +
  1.2029 +    </sect2>
  1.2030 +    <sect2>
  1.2031 +      <title>Automate as much as possible</title>
  1.2032 +
  1.2033 +      <para id="x_14c">When I started using the <command role="hg-cmd">hg
  1.2034 +	  bisect</command> command, I tried a few times to run my
  1.2035 +	tests by hand, on the command line.  This is an approach that
  1.2036 +	I, at least, am not suited to.  After a few tries, I found
  1.2037 +	that I was making enough mistakes that I was having to restart
  1.2038 +	my searches several times before finally getting correct
  1.2039 +	results.</para>
  1.2040 +
  1.2041 +      <para id="x_14d">My initial problems with driving the <command
  1.2042 +	  role="hg-cmd">hg bisect</command> command by hand occurred
  1.2043 +	even with simple searches on small repositories; if the
  1.2044 +	problem you're looking for is more subtle, or the number of
  1.2045 +	tests that <command role="hg-cmd">hg bisect</command> must
  1.2046 +	perform increases, the likelihood of operator error ruining
  1.2047 +	the search is much higher.  Once I started automating my
  1.2048 +	tests, I had much better results.</para>
  1.2049 +
  1.2050 +      <para id="x_14e">The key to automated testing is twofold:</para>
  1.2051 +      <itemizedlist>
  1.2052 +	<listitem><para id="x_14f">always test for the same symptom, and</para>
  1.2053 +	</listitem>
  1.2054 +	<listitem><para id="x_150">always feed consistent input to the <command
  1.2055 +	      role="hg-cmd">hg bisect</command> command.</para>
  1.2056 +	</listitem></itemizedlist>
  1.2057 +      <para id="x_151">In my tutorial example above, the <command>grep</command>
  1.2058 +	command tests for the symptom, and the <literal>if</literal>
  1.2059 +	statement takes the result of this check and ensures that we
  1.2060 +	always feed the same input to the <command role="hg-cmd">hg
  1.2061 +	  bisect</command> command.  The <literal>mytest</literal>
  1.2062 +	function marries these together in a reproducible way, so that
  1.2063 +	every test is uniform and consistent.</para>
  1.2064 +
  1.2065 +    </sect2>
  1.2066 +    <sect2>
  1.2067 +      <title>Check your results</title>
  1.2068 +
  1.2069 +      <para id="x_152">Because the output of a <command role="hg-cmd">hg
  1.2070 +	  bisect</command> search is only as good as the input you
  1.2071 +	give it, don't take the changeset it reports as the absolute
  1.2072 +	truth.  A simple way to cross-check its report is to manually
  1.2073 +	run your test at each of the following changesets:</para>
  1.2074 +      <itemizedlist>
  1.2075 +	<listitem><para id="x_153">The changeset that it reports as the first bad
  1.2076 +	    revision.  Your test should still report this as
  1.2077 +	    bad.</para>
  1.2078 +	</listitem>
  1.2079 +	<listitem><para id="x_154">The parent of that changeset (either parent,
  1.2080 +	    if it's a merge). Your test should report this changeset
  1.2081 +	    as good.</para>
  1.2082 +	</listitem>
  1.2083 +	<listitem><para id="x_155">A child of that changeset.  Your test should
  1.2084 +	    report this changeset as bad.</para>
  1.2085 +	</listitem></itemizedlist>
  1.2086 +
  1.2087 +    </sect2>
  1.2088 +    <sect2>
  1.2089 +      <title>Beware interference between bugs</title>
  1.2090 +
  1.2091 +      <para id="x_156">It's possible that your search for one bug could be
  1.2092 +	disrupted by the presence of another.  For example, let's say
  1.2093 +	your software crashes at revision 100, and worked correctly at
  1.2094 +	revision 50.  Unknown to you, someone else introduced a
  1.2095 +	different crashing bug at revision 60, and fixed it at
  1.2096 +	revision 80.  This could distort your results in one of
  1.2097 +	several ways.</para>
  1.2098 +
  1.2099 +      <para id="x_157">It is possible that this other bug completely
  1.2100 +	<quote>masks</quote> yours, which is to say that it occurs
  1.2101 +	before your bug has a chance to manifest itself.  If you can't
  1.2102 +	avoid that other bug (for example, it prevents your project
  1.2103 +	from building), and so can't tell whether your bug is present
  1.2104 +	in a particular changeset, the <command role="hg-cmd">hg
  1.2105 +	  bisect</command> command cannot help you directly.  Instead,
  1.2106 +	you can mark a changeset as untested by running <command
  1.2107 +	  role="hg-cmd">hg bisect --skip</command>.</para>
  1.2108 +
  1.2109 +      <para id="x_158">A different problem could arise if your test for a bug's
  1.2110 +	presence is not specific enough.  If you check for <quote>my
  1.2111 +	  program crashes</quote>, then both your crashing bug and an
  1.2112 +	unrelated crashing bug that masks it will look like the same
  1.2113 +	thing, and mislead <command role="hg-cmd">hg
  1.2114 +	  bisect</command>.</para>
  1.2115 +
  1.2116 +      <para id="x_159">Another useful situation in which to use <command
  1.2117 +	  role="hg-cmd">hg bisect --skip</command> is if you can't
  1.2118 +	test a revision because your project was in a broken and hence
  1.2119 +	untestable state at that revision, perhaps because someone
  1.2120 +	checked in a change that prevented the project from
  1.2121 +	building.</para>
  1.2122 +
  1.2123 +    </sect2>
  1.2124 +    <sect2>
  1.2125 +      <title>Bracket your search lazily</title>
  1.2126 +
  1.2127 +      <para id="x_15a">Choosing the first <quote>good</quote> and
  1.2128 +	<quote>bad</quote> changesets that will mark the end points of
  1.2129 +	your search is often easy, but it bears a little discussion
  1.2130 +	nevertheless.  From the perspective of <command
  1.2131 +	  role="hg-cmd">hg bisect</command>, the <quote>newest</quote>
  1.2132 +	changeset is conventionally <quote>bad</quote>, and the older
  1.2133 +	changeset is <quote>good</quote>.</para>
  1.2134 +
  1.2135 +      <para id="x_15b">If you're having trouble remembering when a suitable
  1.2136 +	<quote>good</quote> change was, so that you can tell <command
  1.2137 +	  role="hg-cmd">hg bisect</command>, you could do worse than
  1.2138 +	testing changesets at random.  Just remember to eliminate
  1.2139 +	contenders that can't possibly exhibit the bug (perhaps
  1.2140 +	because the feature with the bug isn't present yet) and those
  1.2141 +	where another problem masks the bug (as I discussed
  1.2142 +	above).</para>
  1.2143 +
  1.2144 +      <para id="x_15c">Even if you end up <quote>early</quote> by thousands of
  1.2145 +	changesets or months of history, you will only add a handful
  1.2146 +	of tests to the total number that <command role="hg-cmd">hg
  1.2147 +	  bisect</command> must perform, thanks to its logarithmic
  1.2148 +	behavior.</para>
  1.2149 +
  1.2150 +    </sect2>
  1.2151 +  </sect1>
  1.2152  </chapter>
  1.2153  
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