hgbook-untested / en / ch02-tour-merge.xml

<!-- vim: set filetype=docbkxml shiftwidth=2 autoindent expandtab tw=77 : -->

<chapter id="chap:tour-merge">
  <?dbhtml filename="a-tour-of-mercurial-merging-work.html"?>
  <title>A tour of Mercurial: merging work</title>

  <para id="x_338">We've now covered cloning a repository, making changes in a
    repository, and pulling or pushing changes from one repository
    into another.  Our next step is <emphasis>merging</emphasis>
    changes from separate repositories.</para>

  <sect1>
    <title>Merging streams of work</title>

    <para id="x_339">Merging is a fundamental part of working with a distributed
      revision control tool.</para>
    <itemizedlist>
      <listitem><para id="x_33a">Alice and Bob each have a personal copy of a
	  repository for a project they're collaborating on.  Alice
	  fixes a bug in her repository; Bob adds a new feature in
	  his.  They want the shared repository to contain both the
	  bug fix and the new feature.</para>
      </listitem>
      <listitem><para id="x_33b">I frequently work on several different tasks for
	  a single project at once, each safely isolated in its own
	  repository. Working this way means that I often need to
	  merge one piece of my own work with another.</para>
      </listitem></itemizedlist>

    <para id="x_33c">Because merging is such a common thing to need to do,
      Mercurial makes it easy.  Let's walk through the process.  We'll
      begin by cloning yet another repository (see how often they
      spring up?) and making a change in it.</para>

    &interaction.tour.merge.clone;

    <para id="x_33d">We should now have two copies of
      <filename>hello.c</filename> with different contents.  The
      histories of the two repositories have also diverged, as
      illustrated in <xref
	linkend="fig:tour-merge:sep-repos"/>.</para>

    &interaction.tour.merge.cat;

    <figure id="fig:tour-merge:sep-repos">
      <title>Divergent recent histories of the <filename
	  class="directory">my-hello</filename> and <filename
	  class="directory">my-new-hello</filename>
	repositories</title>
      <mediaobject>
	<imageobject><imagedata fileref="figs/tour-merge-sep-repos.png"/></imageobject>
	<textobject><phrase>XXX add text</phrase></textobject>
      </mediaobject>
    </figure>

    <para id="x_33f">We already know that pulling changes from our <filename
	class="directory">my-hello</filename> repository will have no
      effect on the working directory.</para>

    &interaction.tour.merge.pull;

    <para id="x_340">However, the <command role="hg-cmd">hg pull</command>
      command says something about <quote>heads</quote>.</para>

    <sect2>
      <title>Head changesets</title>

      <para id="x_341">A head is a change that has no descendants, or children,
	as they're also known.  The tip revision is thus a head,
	because the newest revision in a repository doesn't have any
	children, but a repository can contain more than one
	head.</para>

      <figure id="fig:tour-merge:pull">
	<title>Repository contents after pulling from <filename
	    class="directory">my-hello</filename> into <filename
	    class="directory">my-new-hello</filename></title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="figs/tour-merge-pull.png"/>
	  </imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_343">In <xref linkend="fig:tour-merge:pull"/>, you can
	see the effect of the pull from <filename
	  class="directory">my-hello</filename> into <filename
	  class="directory">my-new-hello</filename>.  The history that
	was already present in <filename
	  class="directory">my-new-hello</filename> is untouched, but
	a new revision has been added.  By referring to <xref
	  linkend="fig:tour-merge:sep-repos"/>, we can see that the
	<emphasis>changeset ID</emphasis> remains the same in the new
	repository, but the <emphasis>revision number</emphasis> has
	changed.  (This, incidentally, is a fine example of why it's
	not safe to use revision numbers when discussing changesets.)
	We can view the heads in a repository using the <command
	  role="hg-cmd">hg heads</command> command.</para>

      &interaction.tour.merge.heads;

    </sect2>
    <sect2>
      <title>Performing the merge</title>

      <para id="x_344">What happens if we try to use the normal <command
	  role="hg-cmd">hg update</command> command to update to the
	new tip?</para>

      &interaction.tour.merge.update;

      <para id="x_345">Mercurial is telling us that the <command role="hg-cmd">hg
	  update</command> command won't do a merge; it won't update
	the working directory when it thinks we might want to do
	a merge, unless we force it to do so.  Instead, we use the
	<command role="hg-cmd">hg merge</command> command to merge the
	two heads.</para>

      &interaction.tour.merge.merge;

      <para id="x_347">This updates the working directory so that it contains
	changes from <emphasis>both</emphasis> heads, which is
	reflected in both the output of <command role="hg-cmd">hg
	  parents</command> and the contents of
	<filename>hello.c</filename>.</para>

      &interaction.tour.merge.parents;

    </sect2>
    <sect2>
      <title>Committing the results of the merge</title>

      <para id="x_348">Whenever we've done a merge, <command role="hg-cmd">hg
	  parents</command> will display two parents until we <command
	  role="hg-cmd">hg commit</command> the results of the
	  merge.</para>

	&interaction.tour.merge.commit;

      <para id="x_349">We now have a new tip revision; notice that it has
	<emphasis>both</emphasis> of our former heads as its parents.
	These are the same revisions that were previously displayed by
	<command role="hg-cmd">hg parents</command>.</para>

      &interaction.tour.merge.tip;

      <para id="x_34a">In <xref
	  linkend="fig:tour-merge:merge"/>, you can see a
	representation of what happens to the working directory during
	the merge, and how this affects the repository when the commit
	happens.  During the merge, the working directory has two
	parent changesets, and these become the parents of the new
	changeset.</para>

      <figure id="fig:tour-merge:merge">
	<title>Working directory and repository during merge, and
	  following commit</title>
	<mediaobject>
	  <imageobject>
	    <imagedata fileref="figs/tour-merge-merge.png"/>
	  </imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_69c">We sometimes talk about a merge having
	<emphasis>sides</emphasis>: the left side is the first parent
	in the output of <command role="hg-cmd">hg parents</command>,
	and the right side is the second.  If the working directory
	was at e.g. revision 5 before we began a merge, that revision
	will become the left side of the merge.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>Merging conflicting changes</title>

    <para id="x_34b">Most merges are simple affairs, but sometimes you'll find
      yourself merging changes where each side modifies the same portions
      of the same files.  Unless both modifications are identical,
      this results in a <emphasis>conflict</emphasis>, where you have
      to decide how to reconcile the different changes into something
      coherent.</para>

    <figure id="fig:tour-merge:conflict">
      <title>Conflicting changes to a document</title>
      <mediaobject>
	<imageobject><imagedata fileref="figs/tour-merge-conflict.png"/></imageobject>
	<textobject><phrase>XXX add text</phrase></textobject>
      </mediaobject>
    </figure>

    <para id="x_34d"><xref linkend="fig:tour-merge:conflict"/> illustrates
      an instance of two conflicting changes to a document.  We
      started with a single version of the file; then we made some
      changes; while someone else made different changes to the same
      text.  Our task in resolving the conflicting changes is to
      decide what the file should look like.</para>

    <para id="x_34e">Mercurial doesn't have a built-in facility for handling
      conflicts. Instead, it runs an external program, usually one
      that displays some kind of graphical conflict resolution
      interface.  By default, Mercurial tries to find one of several
      different merging tools that are likely to be installed on your
      system.  It first tries a few fully automatic merging tools; if
      these don't succeed (because the resolution process requires
      human guidance) or aren't present, it tries a few
      different graphical merging tools.</para>

    <para id="x_34f">It's also possible to get Mercurial to run another program
      or script instead of <command>hgmerge</command>, by setting the
      <envar>HGMERGE</envar> environment variable to the name of your
      preferred program.</para>

    <sect2>
      <title>Using a graphical merge tool</title>

      <para id="x_350">My preferred graphical merge tool is
	<command>kdiff3</command>, which I'll use to describe the
	features that are common to graphical file merging tools.  You
	can see a screenshot of <command>kdiff3</command> in action in
	<xref linkend="fig:tour-merge:kdiff3"/>.  The kind of
	merge it is performing is called a <emphasis>three-way
	  merge</emphasis>, because there are three different versions
	of the file of interest to us.  The tool thus splits the upper
	portion of the window into three panes:</para>
      <itemizedlist>
	<listitem><para id="x_351">At the left is the <emphasis>base</emphasis>
	    version of the file, i.e. the most recent version from
	    which the two versions we're trying to merge are
	    descended.</para>
	</listitem>
	<listitem><para id="x_352">In the middle is <quote>our</quote> version of
	    the file, with the contents that we modified.</para>
	</listitem>
	<listitem><para id="x_353">On the right is <quote>their</quote> version
	    of the file, the one that from the changeset that we're
	    trying to merge with.</para>
	</listitem></itemizedlist>
      <para id="x_354">In the pane below these is the current
	<emphasis>result</emphasis> of the merge. Our task is to
	replace all of the red text, which indicates unresolved
	conflicts, with some sensible merger of the
	<quote>ours</quote> and <quote>theirs</quote> versions of the
	file.</para>

      <para id="x_355">All four of these panes are <emphasis>locked
	  together</emphasis>; if we scroll vertically or horizontally
	in any of them, the others are updated to display the
	corresponding sections of their respective files.</para>

      <figure id="fig:tour-merge:kdiff3">
	<title>Using <command>kdiff3</command> to merge versions of a
	  file</title>
	<mediaobject>
	  <imageobject>
	    <imagedata width="100%" fileref="figs/kdiff3.png"/></imageobject>
	  <textobject>
	    <phrase>XXX add text</phrase>
	  </textobject>
	</mediaobject>
      </figure>

      <para id="x_357">For each conflicting portion of the file, we can choose to
	resolve the conflict using some combination of text from the
	base version, ours, or theirs.  We can also manually edit the
	merged file at any time, in case we need to make further
	modifications.</para>

      <para id="x_358">There are <emphasis>many</emphasis> file merging tools
	available, too many to cover here.  They vary in which
	platforms they are available for, and in their particular
	strengths and weaknesses.  Most are tuned for merging files
	containing plain text, while a few are aimed at specialised
	file formats (generally XML).</para>

    </sect2>
    <sect2>
      <title>A worked example</title>

      <para id="x_359">In this example, we will reproduce the file modification
	history of <xref linkend="fig:tour-merge:conflict"/>
	above.  Let's begin by creating a repository with a base
	version of our document.</para>

      &interaction.tour-merge-conflict.wife;

      <para id="x_35a">We'll clone the repository and make a change to the
	file.</para>

      &interaction.tour-merge-conflict.cousin;

      <para id="x_35b">And another clone, to simulate someone else making a
	change to the file. (This hints at the idea that it's not all
	that unusual to merge with yourself when you isolate tasks in
	separate repositories, and indeed to find and resolve
	conflicts while doing so.)</para>

      &interaction.tour-merge-conflict.son;

      <para id="x_35c">Having created two
	different versions of the file, we'll set up an environment
	suitable for running our merge.</para>

      &interaction.tour-merge-conflict.pull;

      <para id="x_35d">In this example, I'll set
	<envar>HGMERGE</envar> to tell Mercurial to use the
	non-interactive <command>merge</command> command.  This is
	bundled with many Unix-like systems. (If you're following this
	example on your computer, don't bother setting
	<envar>HGMERGE</envar>.)</para>

      &interaction.tour-merge-conflict.merge;

      <para id="x_35f">Because <command>merge</command> can't resolve the
	conflicting changes, it leaves <emphasis>merge
	  markers</emphasis> inside the file that has conflicts,
	indicating which lines have conflicts, and whether they came
	from our version of the file or theirs.</para>

      <para id="x_360">Mercurial can tell from the way <command>merge</command>
	exits that it wasn't able to merge successfully, so it tells
	us what commands we'll need to run if we want to redo the
	merging operation.  This could be useful if, for example, we
	were running a graphical merge tool and quit because we were
	confused or realised we had made a mistake.</para>

      <para id="x_361">If automatic or manual merges fail, there's nothing to
	prevent us from <quote>fixing up</quote> the affected files
	ourselves, and committing the results of our merge:</para>

      &interaction.tour-merge-conflict.commit;

    </sect2>
  </sect1>
  <sect1 id="sec:tour-merge:fetch">
    <title>Simplifying the pull-merge-commit sequence</title>

    <para id="x_362">The process of merging changes as outlined above is
      straightforward, but requires running three commands in
      sequence.</para>
    <programlisting>hg pull -u
hg merge
hg commit -m 'Merged remote changes'</programlisting>
    <para id="x_363">In the case of the final commit, you also need to enter a
      commit message, which is almost always going to be a piece of
      uninteresting <quote>boilerplate</quote> text.</para>

    <para id="x_364">It would be nice to reduce the number of steps needed, if
      this were possible.  Indeed, Mercurial is distributed with an
      extension called <literal role="hg-ext">fetch</literal> that
      does just this.</para>

    <para id="x_365">Mercurial provides a flexible extension mechanism that lets
      people extend its functionality, while keeping the core of
      Mercurial small and easy to deal with.  Some extensions add new
      commands that you can use from the command line, while others
      work <quote>behind the scenes,</quote> for example adding
      capabilities to the server.</para>

    <para id="x_366">The <literal role="hg-ext">fetch</literal>
      extension adds a new command called, not surprisingly, <command
	role="hg-cmd">hg fetch</command>.  This extension acts as a
      combination of <command role="hg-cmd">hg pull -u</command>,
      <command role="hg-cmd">hg merge</command> and <command
	role="hg-cmd">hg commit</command>.  It begins by pulling
      changes from another repository into the current repository.  If
      it finds that the changes added a new head to the repository, it
      begins a merge, then (if the merge succeeded) commits the result
      of the merge with an automatically-generated commit message.  If
      no new heads were added, it updates the working directory to the
      new tip changeset.</para>

    <para id="x_367">Enabling the <literal
	role="hg-ext">fetch</literal> extension is easy.  Edit the
      <filename role="special">.hgrc</filename> file in your home
      directory, and either go to the <literal
	role="rc-extensions">extensions</literal> section or create an
      <literal role="rc-extensions">extensions</literal> section. Then
      add a line that simply reads
      <quote><literal>fetch=</literal></quote>.</para>

    <programlisting>[extensions]
fetch =</programlisting>

    <para id="x_368">(Normally, the right-hand side of the
      <quote><literal>=</literal></quote> would indicate where to find
      the extension, but since the <literal
	role="hg-ext">fetch</literal> extension is in the standard
      distribution, Mercurial knows where to search for it.)</para>

  </sect1>
</chapter>

<!--
local variables: 
sgml-parent-document: ("00book.xml" "book" "chapter")
end:
-->
Tip: Filter by directory path e.g. /media app.js to search for public/media/app.js.
Tip: Use camelCasing e.g. ProjME to search for ProjectModifiedEvent.java.
Tip: Filter by extension type e.g. /repo .js to search for all .js files in the /repo directory.
Tip: Separate your search with spaces e.g. /ssh pom.xml to search for src/ssh/pom.xml.
Tip: Use ↑ and ↓ arrow keys to navigate and return to view the file.
Tip: You can also navigate files with Ctrl+j (next) and Ctrl+k (previous) and view the file with Ctrl+o.
Tip: You can also navigate files with Alt+j (next) and Alt+k (previous) and view the file with Alt+o.