
bos@121: \chapter{Finding and fixing your mistakes}
bos@121: \label{chap:undo}
bos@121: 
bos@121: To err might be human, but to really handle the consequences well
bos@121: takes a top-notch revision control system.  In this chapter, we'll
bos@121: discuss some of the techniques you can use when you find that a
bos@121: problem has crept into your project.  Mercurial has some highly
bos@121: capable features that will help you to isolate the sources of
bos@121: problems, and to handle them appropriately.
bos@121: 
bos@122: \section{Erasing local history}
bos@121: 
bos@121: \subsection{The accidental commit}
bos@121: 
bos@121: I have the occasional but persistent problem of typing rather more
bos@121: quickly than I can think, which sometimes results in me committing a
bos@121: changeset that is either incomplete or plain wrong.  In my case, the
bos@121: usual kind of incomplete changeset is one in which I've created a new
bos@121: source file, but forgotten to \hgcmd{add} it.  A ``plain wrong''
bos@121: changeset is not as common, but no less annoying.
bos@121: 
bos@121: \subsection{Rolling back a transaction}
bos@121: 
bos@121: In section~\ref{sec:concepts:txn}, I mentioned that Mercurial treats
bos@121: each modification of a repository as a \emph{transaction}.  Every time
bos@121: you commit a changeset or pull changes from another repository,
bos@121: Mercurial remembers what you did.  You can undo, or \emph{roll back},
bos@121: exactly one of these actions using the \hgcmd{rollback} command.
bos@121: 
bos@121: Here's a mistake that I often find myself making: committing a change
bos@121: in which I've created a new file, but forgotten to \hgcmd{add} it.
bos@121: \interaction{rollback.commit}
bos@121: Looking at the output of \hgcmd{status} after the commit immediately
bos@121: confirms the error.
bos@121: \interaction{rollback.status}
bos@121: The commit captured the changes to the file \filename{a}, but not the
bos@121: new file \filename{b}.  If I were to push this changeset to a
bos@121: repository that I shared with a colleague, the chances are high that
bos@121: something in \filename{a} would refer to \filename{b}, which would not
bos@121: be present in their repository when they pulled my changes.  I would
bos@121: thus become the object of some indignation.
bos@121: 
bos@121: However, luck is with me---I've caught my error before I pushed the
bos@121: changeset.  I use the \hgcmd{rollback} command, and Mercurial makes
bos@121: that last changeset vanish.
bos@121: \interaction{rollback.rollback}
bos@121: Notice that the changeset is no longer present in the repository's
bos@121: history, and the working directory once again thinks that the file
bos@122: \filename{a} is modified.  The commit and rollback have left the
bos@122: working directory exactly as it was prior to the commit; the changeset
bos@122: has been completely erased.  I can now safely \hgcmd{add} the file
bos@122: \filename{b}, and rerun my commit.
bos@121: \interaction{rollback.add}
bos@121: 
bos@121: \subsection{The erroneous pull}
bos@121: 
bos@121: It's common practice with Mercurial to maintain separate development
bos@121: branches of a project in different repositories.  Your development
bos@121: team might have one shared repository for your project's ``0.9''
bos@121: release, and another, containing different changes, for the ``1.0''
bos@121: release.
bos@121: 
bos@121: Given this, you can imagine that the consequences could be messy if
bos@121: you had a local ``0.9'' repository, and accidentally pulled changes
bos@121: from the shared ``1.0'' repository into it.  At worst, you could be
bos@121: paying insufficient attention, and push those changes into the shared
bos@121: ``0.9'' tree, confusing your entire team (but don't worry, we'll
bos@121: return to this horror scenario later).  However, it's more likely that
bos@121: you'll notice immediately, because Mercurial will display the URL it's
bos@121: pulling from, or you will see it pull a suspiciously large number of
bos@121: changes into the repository.
bos@121: 
bos@121: The \hgcmd{rollback} command will work nicely to expunge all of the
bos@121: changesets that you just pulled.  Mercurial groups all changes from
bos@121: one \hgcmd{pull} into a single transaction, so one \hgcmd{rollback} is
bos@121: all you need to undo this mistake.
bos@121: 
bos@121: \subsection{Rolling back is useless once you've pushed}
bos@121: 
bos@121: The value of the \hgcmd{rollback} command drops to zero once you've
bos@121: pushed your changes to another repository.  Rolling back a change
bos@121: makes it disappear entirely, but \emph{only} in the repository in
bos@121: which you perform the \hgcmd{rollback}.  Because a rollback eliminates
bos@121: history, there's no way for the disappearance of a change to propagate
bos@121: between repositories.
bos@121: 
bos@121: If you've pushed a change to another repository---particularly if it's
bos@121: a shared repository---it has essentially ``escaped into the wild,''
bos@121: and you'll have to recover from your mistake in a different way.  What
bos@121: will happen if you push a changeset somewhere, then roll it back, then
bos@121: pull from the repository you pushed to, is that the changeset will
bos@121: reappear in your repository.
bos@121: 
bos@121: (If you absolutely know for sure that the change you want to roll back
bos@121: is the most recent change in the repository that you pushed to,
bos@121: \emph{and} you know that nobody else could have pulled it from that
bos@121: repository, you can roll back the changeset there, too, but you really
bos@121: should really not rely on this working reliably.  If you do this,
bos@121: sooner or later a change really will make it into a repository that
bos@121: you don't directly control (or have forgotten about), and come back to
bos@121: bite you.)
bos@121: 
bos@121: \subsection{You can only roll back once}
bos@121: 
bos@121: Mercurial stores exactly one transaction in its transaction log; that
bos@121: transaction is the most recent one that occurred in the repository.
bos@121: This means that you can only roll back one transaction.  If you expect
bos@121: to be able to roll back one transaction, then its predecessor, this is
bos@121: not the behaviour you will get.
bos@121: \interaction{rollback.twice}
bos@121: Once you've rolled back one transaction in a repository, you can't
bos@121: roll back again in that repository until you perform another commit or
bos@121: pull.
bos@121: 
bos@122: \section{Reverting the mistaken change}
bos@122: 
bos@122: If you make a modification to a file, and decide that you really
bos@122: didn't want to change the file at all, the \hgcmd{revert} command is
bos@122: the one you'll need.  It looks at the changeset that's the parent of
bos@122: the working directory, and restores the contents of the file to their
bos@122: state as of that changeset.  (That's a long-winded way of saying that,
bos@122: in the normal case, it undoes your modifications.)
bos@122: 
bos@122: Let's illustrate how the \hgcmd{revert} command works with yet another
bos@122: small example.  We'll begin by modifying a file that Mercurial is
bos@122: already tracking.
bos@122: \interaction{daily.revert.modify}
bos@122: If we don't want that change, we can simply \hgcmd{revert} the file.
bos@122: \interaction{daily.revert.unmodify}
bos@122: The \hgcmd{revert} command provides us with an extra degree of safety
bos@122: by saving our modified file with a \filename{.orig} extension.
bos@122: \interaction{daily.revert.status}
bos@122: 
bos@122: \subsection{File management errors}
bos@122: \label{sec:undo:mgmt}
bos@122: 
bos@122: The \hgcmd{revert} command is useful for more than just modified
bos@122: files.  It lets you reverse the results of all of Mercurial's file
bos@122: management commands---\hgcmd{add}, \hgcmd{remove}, and so on.
bos@122: 
bos@122: If you \hgcmd{add} a file, then decide that in fact you don't want
bos@122: Mercurial to track it, use \hgcmd{revert} to undo the add.  Don't
bos@122: worry; Mercurial will not modify the file in any way.  It will just
bos@122: ``unmark'' the file.
bos@122: \interaction{daily.revert.add}
bos@122: 
bos@122: Similarly, if you ask Mercurial to \hgcmd{remove} a file, you can use
bos@122: \hgcmd{revert} to restore it to the contents it had as of the parent
bos@122: of the working directory.
bos@122: \interaction{daily.revert.remove}
bos@122: This works just as well for a file that you deleted by hand, without
bos@122: telling Mercurial (recall that in Mercurial terminology, this kind of
bos@122: file is called ``missing'').
bos@122: \interaction{daily.revert.missing}
bos@122: 
bos@122: If you revert a \hgcmd{copy}, the copied-to file remains in your
bos@122: working directory afterwards, unmodified.
bos@122: \interaction{daily.revert.copy}
bos@122: 
bos@122: \subsubsection{A slightly special case: reverting a rename}
bos@122: 
bos@122: If you \hgcmd{rename} a file, there is one small detail that
bos@122: you should remember.  When you \hgcmd{revert} a rename, it's not
bos@122: enough to provide the name of the renamed-to file, as you can see
bos@122: here.
bos@122: \interaction{daily.revert.rename}
bos@122: As you can see from the output of \hgcmd{status}, the renamed-to file
bos@122: is no longer identified as added, but the renamed-\emph{from} file is
bos@122: still removed!  This is counter-intuitive (at least to me), but at
bos@122: least it's easy to deal with.
bos@122: \interaction{daily.revert.rename-orig}
bos@122: So remember, to revert a \hgcmd{rename}, you must provide \emph{both}
bos@122: the source and destination names.  
bos@122: 
bos@122: (By the way, if you rename a file, then modify the renamed-to file,
bos@122: then revert both components of the rename, when Mercurial restores the
bos@122: file that was removed as part of the rename, it will be unmodified.
bos@122: If you need the modifications in the renamed-to file to show up in the
bos@122: renamed-from file, don't forget to copy them over.)
bos@122: 
bos@122: These fiddly aspects of reverting a rename arguably consitute a small
bos@122: bug in Mercurial.
bos@122: 
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