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     1.4 +\chapter{Finding and fixing your mistakes}
     1.5 +\label{chap:undo}
     1.6 +
     1.7 +To err might be human, but to really handle the consequences well
     1.8 +takes a top-notch revision control system.  In this chapter, we'll
     1.9 +discuss some of the techniques you can use when you find that a
    1.10 +problem has crept into your project.  Mercurial has some highly
    1.11 +capable features that will help you to isolate the sources of
    1.12 +problems, and to handle them appropriately.
    1.13 +
    1.14 +\section{Easily recovered errors}
    1.15 +
    1.16 +\subsection{The accidental commit}
    1.17 +
    1.18 +I have the occasional but persistent problem of typing rather more
    1.19 +quickly than I can think, which sometimes results in me committing a
    1.20 +changeset that is either incomplete or plain wrong.  In my case, the
    1.21 +usual kind of incomplete changeset is one in which I've created a new
    1.22 +source file, but forgotten to \hgcmd{add} it.  A ``plain wrong''
    1.23 +changeset is not as common, but no less annoying.
    1.24 +
    1.25 +\subsection{Rolling back a transaction}
    1.26 +
    1.27 +In section~\ref{sec:concepts:txn}, I mentioned that Mercurial treats
    1.28 +each modification of a repository as a \emph{transaction}.  Every time
    1.29 +you commit a changeset or pull changes from another repository,
    1.30 +Mercurial remembers what you did.  You can undo, or \emph{roll back},
    1.31 +exactly one of these actions using the \hgcmd{rollback} command.
    1.32 +
    1.33 +Here's a mistake that I often find myself making: committing a change
    1.34 +in which I've created a new file, but forgotten to \hgcmd{add} it.
    1.35 +\interaction{rollback.commit}
    1.36 +Looking at the output of \hgcmd{status} after the commit immediately
    1.37 +confirms the error.
    1.38 +\interaction{rollback.status}
    1.39 +The commit captured the changes to the file \filename{a}, but not the
    1.40 +new file \filename{b}.  If I were to push this changeset to a
    1.41 +repository that I shared with a colleague, the chances are high that
    1.42 +something in \filename{a} would refer to \filename{b}, which would not
    1.43 +be present in their repository when they pulled my changes.  I would
    1.44 +thus become the object of some indignation.
    1.45 +
    1.46 +However, luck is with me---I've caught my error before I pushed the
    1.47 +changeset.  I use the \hgcmd{rollback} command, and Mercurial makes
    1.48 +that last changeset vanish.
    1.49 +\interaction{rollback.rollback}
    1.50 +Notice that the changeset is no longer present in the repository's
    1.51 +history, and the working directory once again thinks that the file
    1.52 +\filename{a} is modified.  The changeset has been completely erased.
    1.53 +I can now safely \hgcmd{add} the file \filename{b}, and rerun my
    1.54 +commit.
    1.55 +\interaction{rollback.add}
    1.56 +
    1.57 +\subsection{The erroneous pull}
    1.58 +
    1.59 +It's common practice with Mercurial to maintain separate development
    1.60 +branches of a project in different repositories.  Your development
    1.61 +team might have one shared repository for your project's ``0.9''
    1.62 +release, and another, containing different changes, for the ``1.0''
    1.63 +release.
    1.64 +
    1.65 +Given this, you can imagine that the consequences could be messy if
    1.66 +you had a local ``0.9'' repository, and accidentally pulled changes
    1.67 +from the shared ``1.0'' repository into it.  At worst, you could be
    1.68 +paying insufficient attention, and push those changes into the shared
    1.69 +``0.9'' tree, confusing your entire team (but don't worry, we'll
    1.70 +return to this horror scenario later).  However, it's more likely that
    1.71 +you'll notice immediately, because Mercurial will display the URL it's
    1.72 +pulling from, or you will see it pull a suspiciously large number of
    1.73 +changes into the repository.
    1.74 +
    1.75 +The \hgcmd{rollback} command will work nicely to expunge all of the
    1.76 +changesets that you just pulled.  Mercurial groups all changes from
    1.77 +one \hgcmd{pull} into a single transaction, so one \hgcmd{rollback} is
    1.78 +all you need to undo this mistake.
    1.79 +
    1.80 +\subsection{Rolling back is useless once you've pushed}
    1.81 +
    1.82 +The value of the \hgcmd{rollback} command drops to zero once you've
    1.83 +pushed your changes to another repository.  Rolling back a change
    1.84 +makes it disappear entirely, but \emph{only} in the repository in
    1.85 +which you perform the \hgcmd{rollback}.  Because a rollback eliminates
    1.86 +history, there's no way for the disappearance of a change to propagate
    1.87 +between repositories.
    1.88 +
    1.89 +If you've pushed a change to another repository---particularly if it's
    1.90 +a shared repository---it has essentially ``escaped into the wild,''
    1.91 +and you'll have to recover from your mistake in a different way.  What
    1.92 +will happen if you push a changeset somewhere, then roll it back, then
    1.93 +pull from the repository you pushed to, is that the changeset will
    1.94 +reappear in your repository.
    1.95 +
    1.96 +(If you absolutely know for sure that the change you want to roll back
    1.97 +is the most recent change in the repository that you pushed to,
    1.98 +\emph{and} you know that nobody else could have pulled it from that
    1.99 +repository, you can roll back the changeset there, too, but you really
   1.100 +should really not rely on this working reliably.  If you do this,
   1.101 +sooner or later a change really will make it into a repository that
   1.102 +you don't directly control (or have forgotten about), and come back to
   1.103 +bite you.)
   1.104 +
   1.105 +\subsection{You can only roll back once}
   1.106 +
   1.107 +Mercurial stores exactly one transaction in its transaction log; that
   1.108 +transaction is the most recent one that occurred in the repository.
   1.109 +This means that you can only roll back one transaction.  If you expect
   1.110 +to be able to roll back one transaction, then its predecessor, this is
   1.111 +not the behaviour you will get.
   1.112 +\interaction{rollback.twice}
   1.113 +Once you've rolled back one transaction in a repository, you can't
   1.114 +roll back again in that repository until you perform another commit or
   1.115 +pull.
   1.116 +
   1.117 +%%% Local Variables: 
   1.118 +%%% mode: latex
   1.119 +%%% TeX-master: "00book"
   1.120 +%%% End: