
bos@95: \chapter{A tour of Mercurial: merging work}
bos@95: \label{chap:tour-merge}
bos@94: 
bos@94: We've now covered cloning a repository, making changes in a
bos@94: repository, and pulling or pushing changes from one repository into
bos@94: another.  Our next step is \emph{merging} changes from separate
bos@94: repositories.
bos@94: 
bos@95: \section{Merging streams of work}
bos@95: 
bos@94: Merging is a fundamental part of working with a distributed revision
bos@94: control tool.
bos@94: \begin{itemize}
bos@94: \item Alice and Bob each have a personal copy of a repository for a
bos@94:   project they're collaborating on.  Alice fixes a bug in her
bos@94:   repository; Bob adds a new feature in his.  They want the shared
bos@94:   repository to contain both the bug fix and the new feature.
bos@94: \item I frequently work on several different tasks for a single
bos@94:   project at once, each safely isolated in its own repository.
bos@94:   Working this way means that I often need to merge one piece of my
bos@94:   own work with another.
bos@94: \end{itemize}
bos@94: 
bos@94: Because merging is such a common thing to need to do, Mercurial makes
bos@94: it easy.  Let's walk through the process.  We'll begin by cloning yet
bos@94: another repository (see how often they spring up?) and making a change
bos@94: in it.
bos@94: \interaction{tour.merge.clone}
bos@94: We should now have two copies of \filename{hello.c} with different
bos@99: contents.  The histories of the two repositories have also diverged,
bos@99: as illustrated in figure~\ref{fig:tour-merge:sep-repos}.
bos@94: \interaction{tour.merge.cat}
bos@94: 
bos@99: \begin{figure}[ht]
bos@99:   \centering
bos@99:   \grafix{tour-merge-sep-repos}
bos@99:   \caption{Divergent recent histories of the \dirname{my-hello} and
bos@99:     \dirname{my-new-hello} repositories}
bos@99:   \label{fig:tour-merge:sep-repos}
bos@99: \end{figure}
bos@99: 
bos@94: We already know that pulling changes from our \dirname{my-hello}
bos@94: repository will have no effect on the working directory.
bos@94: \interaction{tour.merge.pull}
bos@94: However, the \hgcmd{pull} command says something about ``heads''.  
bos@94: 
bos@102: \subsection{Head changesets}
bos@102: 
bos@99: A head is a change that has no descendants, or children, as they're
bos@99: also known.  The tip revision is thus a head, because the newest
bos@99: revision in a repository doesn't have any children, but a repository
bos@99: can contain more than one head.
bos@99: 
bos@99: \begin{figure}[ht]
bos@99:   \centering
bos@99:   \grafix{tour-merge-pull}
bos@99:   \caption{Repository contents after pulling from \dirname{my-hello} into
bos@99:     \dirname{my-new-hello}}
bos@99:   \label{fig:tour-merge:pull}
bos@99: \end{figure}
bos@99: 
bos@99: In figure~\ref{fig:tour-merge:pull}, you can see the effect of the
bos@99: pull from \dirname{my-hello} into \dirname{my-new-hello}.  The history
bos@99: that was already present in \dirname{my-new-hello} is untouched, but a
bos@99: new revision has been added.  By referring to
bos@99: figure~\ref{fig:tour-merge:sep-repos}, we can see that the
bos@99: \emph{changeset ID} remains the same in the new repository, but the
bos@99: \emph{revision number} has changed.  (This, incidentally, is a fine
bos@99: example of why it's not safe to use revision numbers when discussing
bos@99: changesets.)  We can view the heads in a repository using the
bos@99: \hgcmd{heads} command.
bos@94: \interaction{tour.merge.heads}
bos@102: 
bos@102: \subsection{Performing the merge}
bos@102: 
bos@94: What happens if we try to use the normal \hgcmd{update} command to
bos@94: update to the new tip?
bos@94: \interaction{tour.merge.update}
bos@94: Mercurial is telling us that the \hgcmd{update} command won't do a
bos@100: merge; it won't update the working directory when it thinks we might
bos@100: be wanting to do a merge, unless we force it to do so.  Instead, we
bos@100: use the \hgcmd{merge} command to merge the two heads.
bos@94: \interaction{tour.merge.merge}
bos@100: 
bos@100: \begin{figure}[ht]
bos@100:   \centering
bos@100:   \grafix{tour-merge-merge}
bos@100:   \caption{Working directory and repository during merge, and
bos@100:     following commit}
bos@100:   \label{fig:tour-merge:merge}
bos@100: \end{figure}
bos@100: 
bos@94: This updates the working directory so that it contains changes from
bos@100: \emph{both} heads, which is reflected in both the output of
bos@100: \hgcmd{parents} and the contents of \filename{hello.c}.
bos@94: \interaction{tour.merge.parents}
bos@102: 
bos@102: \subsection{Committing the results of the merge}
bos@102: 
bos@94: Whenever we've done a merge, \hgcmd{parents} will display two parents
bos@94: until we \hgcmd{commit} the results of the merge.
bos@94: \interaction{tour.merge.commit}
bos@94: We now have a new tip revision; notice that it has \emph{both} of
bos@94: our former heads as its parents.  These are the same revisions that
bos@94: were previously displayed by \hgcmd{parents}.
bos@94: \interaction{tour.merge.tip}
bos@101: In figure~\ref{fig:tour-merge:merge}, you can see a representation of
bos@101: what happens to the working directory during the merge, and how this
bos@101: affects the repository when the commit happens.  During the merge, the
bos@101: working directory has two parent changesets, and these become the
bos@101: parents of the new changeset.
bos@94: 
bos@102: \section{Merging conflicting changes}
bos@102: 
bos@102: Most merges are simple affairs, but sometimes you'll find yourself
bos@103: merging changes where each modifies the same portions of the same
bos@103: files.  Unless both modifications are identical, this results in a
bos@103: \emph{conflict}, where you have to decide how to reconcile the
bos@103: different changes into something coherent.
bos@103: 
bos@103: \begin{figure}[ht]
bos@103:   \centering
bos@103:   \grafix{tour-merge-conflict}
bos@103:   \caption{Conflicting changes to a document}
bos@103:   \label{fig:tour-merge:conflict}
bos@103: \end{figure}
bos@103: 
bos@103: Figure~\ref{fig:tour-merge:conflict} illustrates an instance of two
bos@103: conflicting changes to a document.  We started with a single version
bos@103: of the file; then we made some changes; while someone else made
bos@103: different changes to the same text.  Our task in resolving the
bos@103: conflicting changes is to decide what the file should look like.
bos@103: 
bos@103: Mercurial doesn't have a built-in facility for handling conflicts.
bos@103: Instead, it runs an external program called \command{hgmerge}.  This
bos@103: is a shell script that is bundled with Mercurial; you can change it to
bos@103: behave however you please.  What it does by default is try to find one
bos@103: of several different merging tools that are likely to be installed on
bos@103: your system.  It first tries a few fully automatic merging tools; if
bos@103: these don't succeed (because the resolution process requires human
bos@103: guidance) or aren't present, the script tries a few different
bos@103: graphical merging tools.
bos@103: 
bos@103: It's also possible to get Mercurial to run another program or script
bos@103: instead of \command{hgmerge}, by setting the \envar{HGMERGE}
bos@103: environment variable to the name of your preferred program.
bos@103: 
bos@103: \subsection{Using a graphical merge tool}
bos@103: 
bos@103: My preferred graphical merge tool is \command{kdiff3}, which I'll use
bos@103: to describe the features that are common to graphical file merging
bos@103: tools.  You can see a screenshot of \command{kdiff3} in action in
bos@103: figure~\ref{fig:tour-merge:kdiff3}.  The kind of merge it is
bos@103: performing is called a \emph{three-way merge}, because there are three
bos@103: different versions of the file of interest to us.  The tool thus
bos@103: splits the upper portion of the window into three panes:
bos@103: \begin{itemize}
bos@103: \item At the left is the \emph{base} version of the file, i.e.~the
bos@103:   most recent version from which the two versions we're trying to
bos@103:   merge are descended.
bos@103: \item In the middle is ``our'' version of the file, with the contents
bos@103:   that we modified.
bos@103: \item On the right is ``their'' version of the file, the one that
bos@103:   from the changeset that we're trying to merge with.
bos@103: \end{itemize}
bos@103: In the pane below these is the current \emph{result} of the merge.
bos@103: Our task is to replace all of the red text, which indicates unresolved
bos@103: conflicts, with some sensible merger of the ``ours'' and ``theirs''
bos@103: versions of the file.
bos@103: 
bos@103: All four of these panes are \emph{locked together}; if we scroll
bos@103: vertically or horizontally in any of them, the others are updated to
bos@103: display the corresponding sections of their respective files.
bos@103: 
bos@103: \begin{figure}[ht]
bos@103:   \centering
bos@103:   \grafix{kdiff3}
bos@103:   \caption{Using \command{kdiff3} to merge versions of a file}
bos@103:   \label{fig:tour-merge:kdiff3}
bos@103: \end{figure}
bos@103: 
bos@103: For each conflicting portion of the file, we can choose to resolve
mrowe@257: the conflict using some combination of text from the base version,
bos@103: ours, or theirs.  We can also manually edit the merged file at any
bos@103: time, in case we need to make further modifications.
bos@103: 
bos@103: There are \emph{many} file merging tools available, too many to cover
bos@103: here.  They vary in which platforms they are available for, and in
bos@103: their particular strengths and weaknesses.  Most are tuned for merging
bos@103: files containing plain text, while a few are aimed at specialised file
bos@103: formats (generally XML).
bos@103: 
bos@103: \subsection{A worked example}
bos@103: 
bos@103: In this example, we will reproduce the file modification history of
bos@103: figure~\ref{fig:tour-merge:conflict} above.  Let's begin by creating a
bos@103: repository with a base version of our document.
bos@103: \interaction{tour-merge-conflict.wife}
bos@103: We'll clone the repository and make a change to the file.
bos@103: \interaction{tour-merge-conflict.cousin}
bos@103: And another clone, to simulate someone else making a change to the
bos@103: file.  (This hints at the idea that it's not all that unusual to merge
bos@103: with yourself when you isolate tasks in separate repositories, and
bos@103: indeed to find and resolve conflicts while doing so.)
bos@103: \interaction{tour-merge-conflict.son}
bos@103: Having created two different versions of the file, we'll set up an
bos@103: environment suitable for running our merge.
bos@103: \interaction{tour-merge-conflict.pull}
bos@103: 
bos@103: In this example, I won't use Mercurial's normal \command{hgmerge}
bos@103: program to do the merge, because it would drop my nice automated
bos@103: example-running tool into a graphical user interface.  Instead, I'll
bos@103: set \envar{HGMERGE} to tell Mercurial to use the non-interactive
bos@103: \command{merge} command.  This is bundled with many Unix-like systems.
bos@103: If you're following this example on your computer, don't bother
bos@103: setting \envar{HGMERGE}.
bos@103: \interaction{tour-merge-conflict.merge}
bos@103: Because \command{merge} can't resolve the conflicting changes, it
bos@103: leaves \emph{merge markers} inside the file that has conflicts,
bos@103: indicating which lines have conflicts, and whether they came from our
bos@103: version of the file or theirs.
bos@103: 
bos@103: Mercurial can tell from the way \command{merge} exits that it wasn't
bos@103: able to merge successfully, so it tells us what commands we'll need to
bos@103: run if we want to redo the merging operation.  This could be useful
bos@103: if, for example, we were running a graphical merge tool and quit
bos@103: because we were confused or realised we had made a mistake.
bos@103: 
bos@103: If automatic or manual merges fail, there's nothing to prevent us from
bos@103: ``fixing up'' the affected files ourselves, and committing the results
bos@103: of our merge:
bos@103: \interaction{tour-merge-conflict.commit}
bos@103: 
bos@224: \section{Simplifying the pull-merge-commit sequence}
bos@224: \label{sec:tour-merge:fetch}
bos@102: 
bos@102: The process of merging changes as outlined above is straightforward,
bos@102: but requires running three commands in sequence.
bos@102: \begin{codesample2}
bos@102:   hg pull
bos@102:   hg merge
bos@102:   hg commit -m 'Merged remote changes'
bos@102: \end{codesample2}
bos@103: In the case of the final commit, you also need to enter a commit
bos@103: message, which is almost always going to be a piece of uninteresting
bos@103: ``boilerplate'' text.
bos@102: 
bos@102: It would be nice to reduce the number of steps needed, if this were
bos@102: possible.  Indeed, Mercurial is distributed with an extension called
bos@102: \hgext{fetch} that does just this.
bos@102: 
bos@102: Mercurial provides a flexible extension mechanism that lets people
bos@102: extend its functionality, while keeping the core of Mercurial small
bos@102: and easy to deal with.  Some extensions add new commands that you can
bos@102: use from the command line, while others work ``behind the scenes,''
bos@102: for example adding capabilities to the server.
bos@102: 
bos@102: The \hgext{fetch} extension adds a new command called, not
bos@102: surprisingly, \hgcmd{fetch}.  This extension acts as a combination of
bos@102: \hgcmd{pull}, \hgcmd{update} and \hgcmd{merge}.  It begins by pulling
bos@102: changes from another repository into the current repository.  If it
bos@102: finds that the changes added a new head to the repository, it begins a
bos@102: merge, then commits the result of the merge with an
bos@102: automatically-generated commit message.  If no new heads were added,
bos@102: it updates the working directory to the new tip changeset.
bos@102: 
bos@102: Enabling the \hgext{fetch} extension is easy.  Edit your
bos@102: \sfilename{.hgrc}, and either go to the \rcsection{extensions} section
bos@102: or create an \rcsection{extensions} section.  Then add a line that
bos@102: simply reads ``\Verb+fetch +''.
bos@102: \begin{codesample2}
bos@102:   [extensions]
bos@102:   fetch =
bos@102: \end{codesample2}
bos@102: (Normally, on the right-hand side of the ``\texttt{=}'' would appear
bos@102: the location of the extension, but since the \hgext{fetch} extension
bos@102: is in the standard distribution, Mercurial knows where to search for
bos@102: it.)
bos@102: 
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