Source

hgbook / fr / ch08-branch.xml

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

<chapter id="chap:branch">
  <?dbhtml filename="managing-releases-and-branchy-development.html"?>
  <title>Managing releases and branchy development</title>

  <para id="x_369">Mercurial provides several mechanisms for you to manage a
    project that is making progress on multiple fronts at once.  To
    understand these mechanisms, let's first take a brief look at a
    fairly normal software project structure.</para>

  <para id="x_36a">Many software projects issue periodic <quote>major</quote>
    releases that contain substantial new features.  In parallel, they
    may issue <quote>minor</quote> releases.  These are usually
    identical to the major releases off which they're based, but with
    a few bugs fixed.</para>

  <para id="x_36b">In this chapter, we'll start by talking about how to keep
    records of project milestones such as releases.  We'll then
    continue on to talk about the flow of work between different
    phases of a project, and how Mercurial can help you to isolate and
    manage this work.</para>

  <sect1>
    <title>Giving a persistent name to a revision</title>

    <para id="x_36c">Once you decide that you'd like to call a particular
      revision a <quote>release</quote>, it's a good idea to record
      the identity of that revision. This will let you reproduce that
      release at a later date, for whatever purpose you might need at
      the time (reproducing a bug, porting to a new platform, etc).
      &interaction.tag.init;</para>

    <para id="x_36d">Mercurial lets you give a permanent name to any revision
      using the <command role="hg-cmd">hg tag</command> command.  Not
      surprisingly, these names are called <quote>tags</quote>.</para>

    &interaction.tag.tag;

    <para id="x_36e">A tag is nothing more than a <quote>symbolic name</quote>
      for a revision.  Tags exist purely for your convenience, so that
      you have a handy permanent way to refer to a revision; Mercurial
      doesn't interpret the tag names you use in any way.  Neither
      does Mercurial place any restrictions on the name of a tag,
      beyond a few that are necessary to ensure that a tag can be
      parsed unambiguously.  A tag name cannot contain any of the
      following characters:</para>
    <itemizedlist>
      <listitem><para id="x_36f">Colon (ASCII 58,
	  <quote><literal>:</literal></quote>)</para>
      </listitem>
      <listitem><para id="x_370">Carriage return (ASCII 13,
	  <quote><literal>\r</literal></quote>)</para>
      </listitem>
      <listitem><para id="x_371">Newline (ASCII 10,
	  <quote><literal>\n</literal></quote>)</para>
      </listitem></itemizedlist>

    <para id="x_372">You can use the <command role="hg-cmd">hg tags</command>
      command to display the tags present in your repository.  In the
      output, each tagged revision is identified first by its name,
      then by revision number, and finally by the unique hash of the
      revision.</para>

    &interaction.tag.tags;

    <para id="x_373">Notice that <literal>tip</literal> is listed in the output
      of <command role="hg-cmd">hg tags</command>.  The
      <literal>tip</literal> tag is a special <quote>floating</quote>
      tag, which always identifies the newest revision in the
      repository.</para>

    <para id="x_374">In the output of the <command role="hg-cmd">hg
	tags</command> command, tags are listed in reverse order, by
      revision number.  This usually means that recent tags are listed
      before older tags.  It also means that <literal>tip</literal> is
      always going to be the first tag listed in the output of
      <command role="hg-cmd">hg tags</command>.</para>

    <para id="x_375">When you run <command role="hg-cmd">hg log</command>, if it
      displays a revision that has tags associated with it, it will
      print those tags.</para>

    &interaction.tag.log;

    <para id="x_376">Any time you need to provide a revision ID to a Mercurial
      command, the command will accept a tag name in its place.
      Internally, Mercurial will translate your tag name into the
      corresponding revision ID, then use that.</para>

    &interaction.tag.log.v1.0;

    <para id="x_377">There's no limit on the number of tags you can have in a
      repository, or on the number of tags that a single revision can
      have.  As a practical matter, it's not a great idea to have
      <quote>too many</quote> (a number which will vary from project
      to project), simply because tags are supposed to help you to
      find revisions.  If you have lots of tags, the ease of using
      them to identify revisions diminishes rapidly.</para>

    <para id="x_378">For example, if your project has milestones as frequent as
      every few days, it's perfectly reasonable to tag each one of
      those.  But if you have a continuous build system that makes
      sure every revision can be built cleanly, you'd be introducing a
      lot of noise if you were to tag every clean build.  Instead, you
      could tag failed builds (on the assumption that they're rare!),
      or simply not use tags to track buildability.</para>

    <para id="x_379">If you want to remove a tag that you no longer want, use
      <command role="hg-cmd">hg tag --remove</command>.</para>

    &interaction.tag.remove;

    <para id="x_37a">You can also modify a tag at any time, so that it identifies
      a different revision, by simply issuing a new <command
	role="hg-cmd">hg tag</command> command. You'll have to use the
      <option role="hg-opt-tag">-f</option> option to tell Mercurial
      that you <emphasis>really</emphasis> want to update the
      tag.</para>

    &interaction.tag.replace;

    <para id="x_37b">There will still be a permanent record of the previous
      identity of the tag, but Mercurial will no longer use it.
      There's thus no penalty to tagging the wrong revision; all you
      have to do is turn around and tag the correct revision once you
      discover your error.</para>

    <para id="x_37c">Mercurial stores tags in a normal revision-controlled file
      in your repository.  If you've created any tags, you'll find
      them in a file in the root of your repository named <filename
	role="special">.hgtags</filename>.  When you run the <command
	role="hg-cmd">hg tag</command> command, Mercurial modifies
      this file, then automatically commits the change to it.  This
      means that every time you run <command role="hg-cmd">hg
	tag</command>, you'll see a corresponding changeset in the
      output of <command role="hg-cmd">hg log</command>.</para>

    &interaction.tag.tip;

    <sect2>
      <title>Handling tag conflicts during a merge</title>

      <para id="x_37d">You won't often need to care about the <filename
	  role="special">.hgtags</filename> file, but it sometimes
	makes its presence known during a merge.  The format of the
	file is simple: it consists of a series of lines.  Each line
	starts with a changeset hash, followed by a space, followed by
	the name of a tag.</para>

      <para id="x_37e">If you're resolving a conflict in the <filename
	  role="special">.hgtags</filename> file during a merge,
	there's one twist to modifying the <filename
	  role="special">.hgtags</filename> file: when Mercurial is
	parsing the tags in a repository, it
	<emphasis>never</emphasis> reads the working copy of the
	<filename role="special">.hgtags</filename> file.  Instead, it
	reads the <emphasis>most recently committed</emphasis>
	revision of the file.</para>

      <para id="x_37f">An unfortunate consequence of this design is that you
	can't actually verify that your merged <filename
	  role="special">.hgtags</filename> file is correct until
	<emphasis>after</emphasis> you've committed a change.  So if
	you find yourself resolving a conflict on <filename
	  role="special">.hgtags</filename> during a merge, be sure to
	run <command role="hg-cmd">hg tags</command> after you commit.
	If it finds an error in the <filename
	  role="special">.hgtags</filename> file, it will report the
	location of the error, which you can then fix and commit.  You
	should then run <command role="hg-cmd">hg tags</command>
	again, just to be sure that your fix is correct.</para>
    </sect2>

    <sect2>
      <title>Tags and cloning</title>

      <para id="x_380">You may have noticed that the <command role="hg-cmd">hg
	  clone</command> command has a <option
	  role="hg-opt-clone">-r</option> option that lets you clone
	an exact copy of the repository as of a particular changeset.
	The new clone will not contain any project history that comes
	after the revision you specified.  This has an interaction
	with tags that can surprise the unwary.</para>

      <para id="x_381">Recall that a tag is stored as a revision to
	the <filename role="special">.hgtags</filename> file. When you
	create a tag, the changeset in which its recorded refers to an
	older changeset.  When you run <command role="hg-cmd">hg clone
	  -r foo</command> to clone a repository as of tag
	<literal>foo</literal>, the new clone <emphasis>will not
	  contain any revision newer than the one the tag refers to,
	  including the revision where the tag was created</emphasis>.
	The result is that you'll get exactly the right subset of the
	project's history in the new repository, but
	<emphasis>not</emphasis> the tag you might have
	expected.</para>
    </sect2>

    <sect2>
      <title>When permanent tags are too much</title>

      <para id="x_382">Since Mercurial's tags are revision controlled and carried
	around with a project's history, everyone you work with will
	see the tags you create.  But giving names to revisions has
	uses beyond simply noting that revision
	<literal>4237e45506ee</literal> is really
	<literal>v2.0.2</literal>.  If you're trying to track down a
	subtle bug, you might want a tag to remind you of something
	like <quote>Anne saw the symptoms with this
	  revision</quote>.</para>

      <para id="x_383">For cases like this, what you might want to use are
	<emphasis>local</emphasis> tags. You can create a local tag
	with the <option role="hg-opt-tag">-l</option> option to the
	<command role="hg-cmd">hg tag</command> command.  This will
	store the tag in a file called <filename
	  role="special">.hg/localtags</filename>.  Unlike <filename
	  role="special">.hgtags</filename>, <filename
	  role="special">.hg/localtags</filename> is not revision
	controlled.  Any tags you create using <option
	  role="hg-opt-tag">-l</option> remain strictly local to the
	repository you're currently working in.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>The flow of changes&emdash;big picture vs. little</title>

    <para id="x_384">To return to the outline I sketched at the
      beginning of the chapter, let's think about a project that has
      multiple concurrent pieces of work under development at
      once.</para>

    <para id="x_385">There might be a push for a new <quote>main</quote> release;
      a new minor bugfix release to the last main release; and an
      unexpected <quote>hot fix</quote> to an old release that is now
      in maintenance mode.</para>

    <para id="x_386">The usual way people refer to these different concurrent
      directions of development is as <quote>branches</quote>.
      However, we've already seen numerous times that Mercurial treats
      <emphasis>all of history</emphasis> as a series of branches and
      merges.  Really, what we have here is two ideas that are
      peripherally related, but which happen to share a name.</para>
    <itemizedlist>
      <listitem><para id="x_387"><quote>Big picture</quote> branches represent
	  the sweep of a project's evolution; people give them names,
	  and talk about them in conversation.</para>
      </listitem>
      <listitem><para id="x_388"><quote>Little picture</quote> branches are
	  artefacts of the day-to-day activity of developing and
	  merging changes.  They expose the narrative of how the code
	  was developed.</para>
      </listitem></itemizedlist>
  </sect1>

  <sect1>
    <title>Managing big-picture branches in repositories</title>

    <para id="x_389">The easiest way to isolate a <quote>big picture</quote>
      branch in Mercurial is in a dedicated repository.  If you have
      an existing shared repository&emdash;let's call it
      <literal>myproject</literal>&emdash;that reaches a
      <quote>1.0</quote> milestone, you can start to prepare for
      future maintenance releases on top of version 1.0 by tagging the
      revision from which you prepared the 1.0 release.</para>

    &interaction.branch-repo.tag;

    <para id="x_38a">You can then clone a new shared
      <literal>myproject-1.0.1</literal> repository as of that
      tag.</para>

    &interaction.branch-repo.clone;

    <para id="x_38b">Afterwards, if someone needs to work on a bug fix that ought
      to go into an upcoming 1.0.1 minor release, they clone the
      <literal>myproject-1.0.1</literal> repository, make their
      changes, and push them back.</para>

    &interaction.branch-repo.bugfix;

    <para id="x_38c">Meanwhile, development for
      the next major release can continue, isolated and unabated, in
      the <literal>myproject</literal> repository.</para>

    &interaction.branch-repo.new;
  </sect1>

  <sect1>
    <title>Don't repeat yourself: merging across branches</title>

    <para id="x_38d">In many cases, if you have a bug to fix on a maintenance
      branch, the chances are good that the bug exists on your
      project's main branch (and possibly other maintenance branches,
      too).  It's a rare developer who wants to fix the same bug
      multiple times, so let's look at a few ways that Mercurial can
      help you to manage these bugfixes without duplicating your
      work.</para>

    <para id="x_38e">In the simplest instance, all you need to do is pull changes
      from your maintenance branch into your local clone of the target
      branch.</para>

    &interaction.branch-repo.pull;

    <para id="x_38f">You'll then need to merge the heads of the two branches, and
      push back to the main branch.</para>

    &interaction.branch-repo.merge;
  </sect1>

  <sect1>
    <title>Naming branches within one repository</title>

    <para id="x_390">In most instances, isolating branches in repositories is the
      right approach.  Its simplicity makes it easy to understand; and
      so it's hard to make mistakes.  There's a one-to-one
      relationship between branches you're working in and directories
      on your system.  This lets you use normal (non-Mercurial-aware)
      tools to work on files within a branch/repository.</para>

    <para id="x_391">If you're more in the <quote>power user</quote> category
      (<emphasis>and</emphasis> your collaborators are too), there is
      an alternative way of handling branches that you can consider.
      I've already mentioned the human-level distinction between
      <quote>small picture</quote> and <quote>big picture</quote>
      branches.  While Mercurial works with multiple <quote>small
	picture</quote> branches in a repository all the time (for
      example after you pull changes in, but before you merge them),
      it can <emphasis>also</emphasis> work with multiple <quote>big
	picture</quote> branches.</para>

    <para id="x_392">The key to working this way is that Mercurial lets you
      assign a persistent <emphasis>name</emphasis> to a branch.
      There always exists a branch named <literal>default</literal>.
      Even before you start naming branches yourself, you can find
      traces of the <literal>default</literal> branch if you look for
      them.</para>

    <para id="x_393">As an example, when you run the <command role="hg-cmd">hg
	commit</command> command, and it pops up your editor so that
      you can enter a commit message, look for a line that contains
      the text <quote><literal>HG: branch default</literal></quote> at
      the bottom. This is telling you that your commit will occur on
      the branch named <literal>default</literal>.</para>

    <para id="x_394">To start working with named branches, use the <command
	role="hg-cmd">hg branches</command> command.  This command
      lists the named branches already present in your repository,
      telling you which changeset is the tip of each.</para>

    &interaction.branch-named.branches;

    <para id="x_395">Since you haven't created any named branches yet, the only
      one that exists is <literal>default</literal>.</para>

    <para id="x_396">To find out what the <quote>current</quote> branch is, run
      the <command role="hg-cmd">hg branch</command> command, giving
      it no arguments.  This tells you what branch the parent of the
      current changeset is on.</para>

    &interaction.branch-named.branch;

    <para id="x_397">To create a new branch, run the <command role="hg-cmd">hg
	branch</command> command again.  This time, give it one
      argument: the name of the branch you want to create.</para>

    &interaction.branch-named.create;

    <para id="x_398">After you've created a branch, you might wonder what effect
      the <command role="hg-cmd">hg branch</command> command has had.
      What do the <command role="hg-cmd">hg status</command> and
      <command role="hg-cmd">hg tip</command> commands report?</para>

    &interaction.branch-named.status;

    <para id="x_399">Nothing has changed in the
      working directory, and there's been no new history created.  As
      this suggests, running the <command role="hg-cmd">hg
	branch</command> command has no permanent effect; it only
      tells Mercurial what branch name to use the
      <emphasis>next</emphasis> time you commit a changeset.</para>

    <para id="x_39a">When you commit a change, Mercurial records the name of the
      branch on which you committed.  Once you've switched from the
      <literal>default</literal> branch to another and committed,
      you'll see the name of the new branch show up in the output of
      <command role="hg-cmd">hg log</command>, <command
	role="hg-cmd">hg tip</command>, and other commands that
      display the same kind of output.</para>

    &interaction.branch-named.commit;

    <para id="x_39b">The <command role="hg-cmd">hg log</command>-like commands
      will print the branch name of every changeset that's not on the
      <literal>default</literal> branch.  As a result, if you never
      use named branches, you'll never see this information.</para>

    <para id="x_39c">Once you've named a branch and committed a change with that
      name, every subsequent commit that descends from that change
      will inherit the same branch name.  You can change the name of a
      branch at any time, using the <command role="hg-cmd">hg
	branch</command> command.</para>

    &interaction.branch-named.rebranch;

    <para id="x_39d">In practice, this is something you won't do very often, as
      branch names tend to have fairly long lifetimes.  (This isn't a
      rule, just an observation.)</para>
  </sect1>

  <sect1>
    <title>Dealing with multiple named branches in a
      repository</title>

    <para id="x_39e">If you have more than one named branch in a repository,
      Mercurial will remember the branch that your working directory
      is on when you start a command like <command role="hg-cmd">hg
	update</command> or <command role="hg-cmd">hg pull
	-u</command>.  It will update the working directory to the tip
      of this branch, no matter what the <quote>repo-wide</quote> tip
      is.  To update to a revision that's on a different named branch,
      you may need to use the <option role="hg-opt-update">-C</option>
      option to <command role="hg-cmd">hg update</command>.</para>

    <para id="x_39f">This behavior is a little subtle, so let's see it in
      action.  First, let's remind ourselves what branch we're
      currently on, and what branches are in our repository.</para>

    &interaction.branch-named.parents;

    <para id="x_3a0">We're on the <literal>bar</literal> branch, but there also
      exists an older <command role="hg-cmd">hg foo</command>
      branch.</para>

    <para id="x_3a1">We can <command role="hg-cmd">hg update</command> back and
      forth between the tips of the <literal>foo</literal> and
      <literal>bar</literal> branches without needing to use the
      <option role="hg-opt-update">-C</option> option, because this
      only involves going backwards and forwards linearly through our
      change history.</para>

    &interaction.branch-named.update-switchy;

    <para id="x_3a2">If we go back to the <literal>foo</literal> branch and then
      run <command role="hg-cmd">hg update</command>, it will keep us
      on <literal>foo</literal>, not move us to the tip of
      <literal>bar</literal>.</para>

    &interaction.branch-named.update-nothing;

    <para id="x_3a3">Committing a new change on the <literal>foo</literal> branch
      introduces a new head.</para>

    &interaction.branch-named.foo-commit;
  </sect1>

  <sect1>
    <title>Branch names and merging</title>

    <para id="x_3a4">As you've probably noticed, merges in Mercurial are not
      symmetrical. Let's say our repository has two heads, 17 and 23.
      If I <command role="hg-cmd">hg update</command> to 17 and then
      <command role="hg-cmd">hg merge</command> with 23, Mercurial
      records 17 as the first parent of the merge, and 23 as the
      second.  Whereas if I <command role="hg-cmd">hg update</command>
      to 23 and then <command role="hg-cmd">hg merge</command> with
      17, it records 23 as the first parent, and 17 as the
      second.</para>

    <para id="x_3a5">This affects Mercurial's choice of branch name when you
      merge.  After a merge, Mercurial will retain the branch name of
      the first parent when you commit the result of the merge.  If
      your first parent's branch name is <literal>foo</literal>, and
      you merge with <literal>bar</literal>, the branch name will
      still be <literal>foo</literal> after you merge.</para>

    <para id="x_3a6">It's not unusual for a repository to contain multiple heads,
      each with the same branch name.  Let's say I'm working on the
      <literal>foo</literal> branch, and so are you.  We commit
      different changes; I pull your changes; I now have two heads,
      each claiming to be on the <literal>foo</literal> branch.  The
      result of a merge will be a single head on the
      <literal>foo</literal> branch, as you might hope.</para>

    <para id="x_3a7">But if I'm working on the <literal>bar</literal> branch, and
      I merge work from the <literal>foo</literal> branch, the result
      will remain on the <literal>bar</literal> branch.</para>

    &interaction.branch-named.merge;

    <para id="x_3a8">To give a more concrete example, if I'm working on the
      <literal>bleeding-edge</literal> branch, and I want to bring in
      the latest fixes from the <literal>stable</literal> branch,
      Mercurial will choose the <quote>right</quote>
      (<literal>bleeding-edge</literal>) branch name when I pull and
      merge from <literal>stable</literal>.</para>
  </sect1>

  <sect1>
    <title>Branch naming is generally useful</title>

    <para id="x_3a9">You shouldn't think of named branches as applicable only to
      situations where you have multiple long-lived branches
      cohabiting in a single repository.  They're very useful even in
      the one-branch-per-repository case.</para>

    <para id="x_3aa">In the simplest case, giving a name to each branch gives you
      a permanent record of which branch a changeset originated on.
      This gives you more context when you're trying to follow the
      history of a long-lived branchy project.</para>

    <para id="x_3ab">If you're working with shared repositories, you can set up a
      <literal role="hook">pretxnchangegroup</literal> hook on each
      that will block incoming changes that have the
      <quote>wrong</quote> branch name.  This provides a simple, but
      effective, defence against people accidentally pushing changes
      from a <quote>bleeding edge</quote> branch to a
      <quote>stable</quote> branch.  Such a hook might look like this
      inside the shared repo's <filename role="special">
	/.hgrc</filename>.</para>
    <programlisting>[hooks]
pretxnchangegroup.branch = hg heads --template '{branches} ' | grep mybranch</programlisting>
  </sect1>
</chapter>

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