hgbook / en / ch12-mq.xml

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

<chapter id="chap:mq">
  <?dbhtml filename="managing-change-with-mercurial-queues.html"?>
  <title>Managing change with Mercurial Queues</title>

  <sect1 id="sec:mq:patch-mgmt">
    <title>The patch management problem</title>

    <para id="x_3ac">Here is a common scenario: you need to install a software
      package from source, but you find a bug that you must fix in the
      source before you can start using the package.  You make your
      changes, forget about the package for a while, and a few months
      later you need to upgrade to a newer version of the package.  If
      the newer version of the package still has the bug, you must
      extract your fix from the older source tree and apply it against
      the newer version.  This is a tedious task, and it's easy to
      make mistakes.</para>

    <para id="x_3ad">This is a simple case of the <quote>patch management</quote>
      problem.  You have an <quote>upstream</quote> source tree that
      you can't change; you need to make some local changes on top of
      the upstream tree; and you'd like to be able to keep those
      changes separate, so that you can apply them to newer versions
      of the upstream source.</para>

    <para id="x_3ae">The patch management problem arises in many situations.
      Probably the most visible is that a user of an open source
      software project will contribute a bug fix or new feature to the
      project's maintainers in the form of a patch.</para>

    <para id="x_3af">Distributors of operating systems that include open source
      software often need to make changes to the packages they
      distribute so that they will build properly in their
      environments.</para>

    <para id="x_3b0">When you have few changes to maintain, it is easy to manage
      a single patch using the standard <command>diff</command> and
      <command>patch</command> programs (see <xref
	linkend="sec:mq:patch"/> for a discussion of these
      tools). Once the number of changes grows, it starts to make
      sense to maintain patches as discrete <quote>chunks of
	work,</quote> so that for example a single patch will contain
      only one bug fix (the patch might modify several files, but it's
      doing <quote>only one thing</quote>), and you may have a number
      of such patches for different bugs you need fixed and local
      changes you require.  In this situation, if you submit a bug fix
      patch to the upstream maintainers of a package and they include
      your fix in a subsequent release, you can simply drop that
      single patch when you're updating to the newer release.</para>

    <para id="x_3b1">Maintaining a single patch against an upstream tree is a
      little tedious and error-prone, but not difficult.  However, the
      complexity of the problem grows rapidly as the number of patches
      you have to maintain increases.  With more than a tiny number of
      patches in hand, understanding which ones you have applied and
      maintaining them moves from messy to overwhelming.</para>

    <para id="x_3b2">Fortunately, Mercurial includes a powerful extension,
      Mercurial Queues (or simply <quote>MQ</quote>), that massively
      simplifies the patch management problem.</para>

  </sect1>
  <sect1 id="sec:mq:history">
    <title>The prehistory of Mercurial Queues</title>

    <para id="x_3b3">During the late 1990s, several Linux kernel developers
      started to maintain <quote>patch series</quote> that modified
      the behavior of the Linux kernel.  Some of these series were
      focused on stability, some on feature coverage, and others were
      more speculative.</para>

    <para id="x_3b4">The sizes of these patch series grew rapidly.  In 2002,
      Andrew Morton published some shell scripts he had been using to
      automate the task of managing his patch queues.  Andrew was
      successfully using these scripts to manage hundreds (sometimes
      thousands) of patches on top of the Linux kernel.</para>

    <sect2 id="sec:mq:quilt">
      <title>A patchwork quilt</title>

      <para id="x_3b5">In early 2003, Andreas Gruenbacher and Martin Quinson
	borrowed the approach of Andrew's scripts and published a tool
	called <quote>patchwork quilt</quote>
	<citation>web:quilt</citation>, or simply <quote>quilt</quote>
	(see <citation>gruenbacher:2005</citation> for a paper
	describing it).  Because quilt substantially automated patch
	management, it rapidly gained a large following among open
	source software developers.</para>

      <para id="x_3b6">Quilt manages a <emphasis>stack of patches</emphasis> on
	top of a directory tree. To begin, you tell quilt to manage a
	directory tree, and tell it which files you want to manage; it
	stores away the names and contents of those files.  To fix a
	bug, you create a new patch (using a single command), edit the
	files you need to fix, then <quote>refresh</quote> the
	patch.</para>

      <para id="x_3b7">The refresh step causes quilt to scan the directory tree;
	it updates the patch with all of the changes you have made.
	You can create another patch on top of the first, which will
	track the changes required to modify the tree from <quote>tree
	  with one patch applied</quote> to <quote>tree with two
	  patches applied</quote>.</para>

      <para id="x_3b8">You can <emphasis>change</emphasis> which patches are
	applied to the tree.  If you <quote>pop</quote> a patch, the
	changes made by that patch will vanish from the directory
	tree.  Quilt remembers which patches you have popped, though,
	so you can <quote>push</quote> a popped patch again, and the
	directory tree will be restored to contain the modifications
	in the patch.  Most importantly, you can run the
	<quote>refresh</quote> command at any time, and the topmost
	applied patch will be updated.  This means that you can, at
	any time, change both which patches are applied and what
	modifications those patches make.</para>

      <para id="x_3b9">Quilt knows nothing about revision control tools, so it
	works equally well on top of an unpacked tarball or a
	Subversion working copy.</para>
    </sect2>

    <sect2 id="sec:mq:quilt-mq">
      <title>From patchwork quilt to Mercurial Queues</title>

      <para id="x_3ba">In mid-2005, Chris Mason took the features of quilt and
	wrote an extension that he called Mercurial Queues, which
	added quilt-like behavior to Mercurial.</para>

      <para id="x_3bb">The key difference between quilt and MQ is that quilt
	knows nothing about revision control systems, while MQ is
	<emphasis>integrated</emphasis> into Mercurial.  Each patch
	that you push is represented as a Mercurial changeset.  Pop a
	patch, and the changeset goes away.</para>

      <para id="x_3bc">Because quilt does not care about revision control tools,
	it is still a tremendously useful piece of software to know
	about for situations where you cannot use Mercurial and
	MQ.</para>

    </sect2>
  </sect1>
  <sect1>
    <title>The huge advantage of MQ</title>

    <para id="x_3bd">I cannot overstate the value that MQ offers through the
      unification of patches and revision control.</para>

    <para id="x_3be">A major reason that patches have persisted in the free
      software and open source world&emdash;in spite of the
      availability of increasingly capable revision control tools over
      the years&emdash;is the <emphasis>agility</emphasis> they
      offer.</para>

    <para id="x_3bf">Traditional revision control tools make a permanent,
      irreversible record of everything that you do.  While this has
      great value, it's also somewhat stifling.  If you want to
      perform a wild-eyed experiment, you have to be careful in how
      you go about it, or you risk leaving unneeded&emdash;or worse,
      misleading or destabilising&emdash;traces of your missteps and
      errors in the permanent revision record.</para>

    <para id="x_3c0">By contrast, MQ's marriage of distributed revision control
      with patches makes it much easier to isolate your work.  Your
      patches live on top of normal revision history, and you can make
      them disappear or reappear at will.  If you don't like a patch,
      you can drop it.  If a patch isn't quite as you want it to be,
      simply fix it&emdash;as many times as you need to, until you
      have refined it into the form you desire.</para>

    <para id="x_3c1">As an example, the integration of patches with revision
      control makes understanding patches and debugging their
      effects&emdash;and their interplay with the code they're based
      on&emdash;<emphasis>enormously</emphasis> easier. Since every
      applied patch has an associated changeset, you can give <command
	role="hg-cmd">hg log</command> a file name to see which
      changesets and patches affected the file.  You can use the
      <command role="hg-cmd">hg bisect</command> command to
      binary-search through all changesets and applied patches to see
      where a bug got introduced or fixed.  You can use the <command
	role="hg-cmd">hg annotate</command> command to see which
      changeset or patch modified a particular line of a source file.
      And so on.</para>
  </sect1>

  <sect1 id="sec:mq:patch">
    <title>Understanding patches</title>

    <para id="x_3c2">Because MQ doesn't hide its patch-oriented nature, it is
      helpful to understand what patches are, and a little about the
      tools that work with them.</para>

    <para id="x_3c3">The traditional Unix <command>diff</command> command
      compares two files, and prints a list of differences between
      them. The <command>patch</command> command understands these
      differences as <emphasis>modifications</emphasis> to make to a
      file.  Take a look below for a simple example of these commands
      in action.</para>

      &interaction.mq.dodiff.diff;

    <para id="x_3c4">The type of file that <command>diff</command> generates (and
      <command>patch</command> takes as input) is called a
      <quote>patch</quote> or a <quote>diff</quote>; there is no
      difference between a patch and a diff.  (We'll use the term
      <quote>patch</quote>, since it's more commonly used.)</para>

    <para id="x_3c5">A patch file can start with arbitrary text; the
      <command>patch</command> command ignores this text, but MQ uses
      it as the commit message when creating changesets.  To find the
      beginning of the patch content, <command>patch</command>
      searches for the first line that starts with the string
      <quote><literal>diff -</literal></quote>.</para>

    <para id="x_3c6">MQ works with <emphasis>unified</emphasis> diffs
      (<command>patch</command> can accept several other diff formats,
      but MQ doesn't).  A unified diff contains two kinds of header.
      The <emphasis>file header</emphasis> describes the file being
      modified; it contains the name of the file to modify.  When
      <command>patch</command> sees a new file header, it looks for a
      file with that name to start modifying.</para>

    <para id="x_3c7">After the file header comes a series of
      <emphasis>hunks</emphasis>.  Each hunk starts with a header;
      this identifies the range of line numbers within the file that
      the hunk should modify.  Following the header, a hunk starts and
      ends with a few (usually three) lines of text from the
      unmodified file; these are called the
      <emphasis>context</emphasis> for the hunk.  If there's only a
      small amount of context between successive hunks,
      <command>diff</command> doesn't print a new hunk header; it just
      runs the hunks together, with a few lines of context between
      modifications.</para>

    <para id="x_3c8">Each line of context begins with a space character.  Within
      the hunk, a line that begins with
      <quote><literal>-</literal></quote> means <quote>remove this
	line,</quote> while a line that begins with
      <quote><literal>+</literal></quote> means <quote>insert this
	line.</quote>  For example, a line that is modified is
      represented by one deletion and one insertion.</para>

    <para id="x_3c9">We will return to some of the more subtle aspects of patches
      later (in <xref linkend="sec:mq:adv-patch"/>), but you
      should have
      enough information now to use MQ.</para>
  </sect1>

  <sect1 id="sec:mq:start">
    <title>Getting started with Mercurial Queues</title>

    <para id="x_3ca">Because MQ is implemented as an extension, you must
      explicitly enable before you can use it.  (You don't need to
      download anything; MQ ships with the standard Mercurial
      distribution.)  To enable MQ, edit your <filename
	role="home">~/.hgrc</filename> file, and add the lines
      below.</para>

    <programlisting>[extensions]
hgext.mq =</programlisting>

    <para id="x_3cb">Once the extension is enabled, it will make a number of new
      commands available.  To verify that the extension is working,
      you can use <command role="hg-cmd">hg help</command> to see if
      the <command role="hg-ext-mq">qinit</command> command is now
      available.</para>

    &interaction.mq.qinit-help.help;

    <para id="x_3cc">You can use MQ with <emphasis>any</emphasis> Mercurial
      repository, and its commands only operate within that
      repository.  To get started, simply prepare the repository using
      the <command role="hg-ext-mq">qinit</command> command.</para>

    &interaction.mq.tutorial.qinit;

    <para id="x_3cd">This command creates an empty directory called <filename
	role="special" class="directory">.hg/patches</filename>, where
      MQ will keep its metadata.  As with many Mercurial commands, the
      <command role="hg-ext-mq">qinit</command> command prints nothing
      if it succeeds.</para>

    <sect2>
      <title>Creating a new patch</title>

      <para id="x_3ce">To begin work on a new patch, use the <command
	  role="hg-ext-mq">qnew</command> command.  This command takes
	one argument, the name of the patch to create.</para>

      <para id="x_3cf">MQ will use this as the name of an actual file in the
	<filename role="special"
	  class="directory">.hg/patches</filename> directory, as you
	can see below.</para>

      &interaction.mq.tutorial.qnew;

      <para id="x_3d0">Also newly present in the <filename role="special"
	  class="directory">.hg/patches</filename> directory are two
	other files, <filename role="special">series</filename> and
	<filename role="special">status</filename>.  The <filename
	  role="special">series</filename> file lists all of the
	patches that MQ knows about for this repository, with one
	patch per line.  Mercurial uses the <filename
	  role="special">status</filename> file for internal
	book-keeping; it tracks all of the patches that MQ has
	<emphasis>applied</emphasis> in this repository.</para>

      <note>
	<para id="x_3d1">  You may sometimes want to edit the <filename
	    role="special">series</filename> file by hand; for
	  example, to change the sequence in which some patches are
	  applied.  However, manually editing the <filename
	    role="special">status</filename> file is almost always a
	  bad idea, as it's easy to corrupt MQ's idea of what is
	  happening.</para>
      </note>

      <para id="x_3d2">Once you have created your new patch, you can edit files
	in the working directory as you usually would.  All of the
	normal Mercurial commands, such as <command role="hg-cmd">hg
	  diff</command> and <command role="hg-cmd">hg
	  annotate</command>, work exactly as they did before.</para>
    </sect2>

    <sect2>
      <title>Refreshing a patch</title>

      <para id="x_3d3">When you reach a point where you want to save your work,
	use the <command role="hg-ext-mq">qrefresh</command> command
	to update the patch you are working on.</para>

      &interaction.mq.tutorial.qrefresh;

      <para id="x_3d4">This command folds the changes you have made in the
	working directory into your patch, and updates its
	corresponding changeset to contain those changes.</para>

      <para id="x_3d5">You can run <command role="hg-ext-mq">qrefresh</command>
	as often as you like, so it's a good way to
	<quote>checkpoint</quote> your work.  Refresh your patch at an
	opportune time; try an experiment; and if the experiment
	doesn't work out, <command role="hg-cmd">hg revert</command>
	your modifications back to the last time you refreshed.</para>

      &interaction.mq.tutorial.qrefresh2;
    </sect2>

    <sect2>
      <title>Stacking and tracking patches</title>

      <para id="x_3d6">Once you have finished working on a patch, or need to work
	on another, you can use the <command
	  role="hg-ext-mq">qnew</command> command again to create a
	new patch. Mercurial will apply this patch on top of your
	existing patch.</para>

      &interaction.mq.tutorial.qnew2;

      <para id="x_3d7">Notice that the patch contains the changes in our prior
	patch as part of its context (you can see this more clearly in
	the output of <command role="hg-cmd">hg
	  annotate</command>).</para>

      <para id="x_3d8">So far, with the exception of <command
	  role="hg-ext-mq">qnew</command> and <command
	  role="hg-ext-mq">qrefresh</command>, we've been careful to
	only use regular Mercurial commands.  However, MQ provides
	many commands that are easier to use when you are thinking
	about patches, as illustrated below.</para>

      &interaction.mq.tutorial.qseries;

      <itemizedlist>
	<listitem><para id="x_3d9">The <command
	      role="hg-ext-mq">qseries</command> command lists every
	    patch that MQ knows about in this repository, from oldest
	    to newest (most recently
	    <emphasis>created</emphasis>).</para>
	</listitem>
	<listitem><para id="x_3da">The <command
	      role="hg-ext-mq">qapplied</command> command lists every
	    patch that MQ has <emphasis>applied</emphasis> in this
	    repository, again from oldest to newest (most recently
	    applied).</para>
	</listitem></itemizedlist>
    </sect2>

    <sect2>
      <title>Manipulating the patch stack</title>

      <para id="x_3db">The previous discussion implied that there must be a
	difference between <quote>known</quote> and
	<quote>applied</quote> patches, and there is.  MQ can manage a
	patch without it being applied in the repository.</para>

      <para id="x_3dc">An <emphasis>applied</emphasis> patch has a corresponding
	changeset in the repository, and the effects of the patch and
	changeset are visible in the working directory.  You can undo
	the application of a patch using the <command
	  role="hg-ext-mq">qpop</command> command.  MQ still
	<emphasis>knows about</emphasis>, or manages, a popped patch,
	but the patch no longer has a corresponding changeset in the
	repository, and the working directory does not contain the
	changes made by the patch.  <xref
	  linkend="fig:mq:stack"/> illustrates
	the difference between applied and tracked patches.</para>

      <figure id="fig:mq:stack">
	<title>Applied and unapplied patches in the MQ patch
	  stack</title>
	<mediaobject>
	  <imageobject><imagedata fileref="figs/mq-stack.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_3de">You can reapply an unapplied, or popped, patch using the
	<command role="hg-ext-mq">qpush</command> command.  This
	creates a new changeset to correspond to the patch, and the
	patch's changes once again become present in the working
	directory.  See below for examples of <command
	  role="hg-ext-mq">qpop</command> and <command
	  role="hg-ext-mq">qpush</command> in action.</para>

      &interaction.mq.tutorial.qpop;

      <para id="x_3df">Notice that once we have popped a patch or two patches,
	the output of <command role="hg-ext-mq">qseries</command>
	remains the same, while that of <command
	  role="hg-ext-mq">qapplied</command> has changed.</para>

    </sect2>

    <sect2>
      <title>Pushing and popping many patches</title>

      <para id="x_3e0">While <command role="hg-ext-mq">qpush</command> and
	<command role="hg-ext-mq">qpop</command> each operate on a
	single patch at a time by default, you can push and pop many
	patches in one go.  The <option
	  role="hg-ext-mq-cmd-qpush-opt">hg -a</option> option to
	<command role="hg-ext-mq">qpush</command> causes it to push
	all unapplied patches, while the <option
	  role="hg-ext-mq-cmd-qpop-opt">-a</option> option to <command
	  role="hg-ext-mq">qpop</command> causes it to pop all applied
	patches.  (For some more ways to push and pop many patches,
	see <xref linkend="sec:mq:perf"/> below.)</para>

      &interaction.mq.tutorial.qpush-a;
    </sect2>

    <sect2>
      <title>Safety checks, and overriding them</title>

      <para id="x_3e1">Several MQ commands check the working directory before
	they do anything, and fail if they find any modifications.
	They do this to ensure that you won't lose any changes that
	you have made, but not yet incorporated into a patch.  The
	example below illustrates this; the <command
	  role="hg-ext-mq">qnew</command> command will not create a
	new patch if there are outstanding changes, caused in this
	case by the <command role="hg-cmd">hg add</command> of
	<filename>file3</filename>.</para>

      &interaction.mq.tutorial.add;

      <para id="x_3e2">Commands that check the working directory all take an
	<quote>I know what I'm doing</quote> option, which is always
	named <option>-f</option>.  The exact meaning of
	<option>-f</option> depends on the command.  For example,
	<command role="hg-cmd">hg qnew <option
	    role="hg-ext-mq-cmd-qnew-opt">hg -f</option></command>
	will incorporate any outstanding changes into the new patch it
	creates, but <command role="hg-cmd">hg qpop <option
	    role="hg-ext-mq-cmd-qpop-opt">hg -f</option></command>
	will revert modifications to any files affected by the patch
	that it is popping.  Be sure to read the documentation for a
	command's <option>-f</option> option before you use it!</para>
    </sect2>

    <sect2>
      <title>Working on several patches at once</title>

      <para id="x_3e3">The <command role="hg-ext-mq">qrefresh</command> command
	always refreshes the <emphasis>topmost</emphasis> applied
	patch.  This means that you can suspend work on one patch (by
	refreshing it), pop or push to make a different patch the top,
	and work on <emphasis>that</emphasis> patch for a
	while.</para>

      <para id="x_3e4">Here's an example that illustrates how you can use this
	ability. Let's say you're developing a new feature as two
	patches.  The first is a change to the core of your software,
	and the second&emdash;layered on top of the
	first&emdash;changes the user interface to use the code you
	just added to the core.  If you notice a bug in the core while
	you're working on the UI patch, it's easy to fix the core.
	Simply <command role="hg-ext-mq">qrefresh</command> the UI
	patch to save your in-progress changes, and <command
	  role="hg-ext-mq">qpop</command> down to the core patch.  Fix
	the core bug, <command role="hg-ext-mq">qrefresh</command> the
	core patch, and <command role="hg-ext-mq">qpush</command> back
	to the UI patch to continue where you left off.</para>
    </sect2>
  </sect1>

  <sect1 id="sec:mq:adv-patch">
    <title>More about patches</title>

    <para id="x_3e5">MQ uses the GNU <command>patch</command> command to apply
      patches, so it's helpful to know a few more detailed aspects of
      how <command>patch</command> works, and about patches
      themselves.</para>

    <sect2>
      <title>The strip count</title>

      <para id="x_3e6">If you look at the file headers in a patch, you will
	notice that the pathnames usually have an extra component on
	the front that isn't present in the actual path name.  This is
	a holdover from the way that people used to generate patches
	(people still do this, but it's somewhat rare with modern
	revision control tools).</para>

      <para id="x_3e7">Alice would unpack a tarball, edit her files, then decide
	that she wanted to create a patch.  So she'd rename her
	working directory, unpack the tarball again (hence the need
	for the rename), and use the <option
	  role="cmd-opt-diff">-r</option> and <option
	  role="cmd-opt-diff">-N</option> options to
	<command>diff</command> to recursively generate a patch
	between the unmodified directory and the modified one.  The
	result would be that the name of the unmodified directory
	would be at the front of the left-hand path in every file
	header, and the name of the modified directory would be at the
	front of the right-hand path.</para>

      <para id="x_3e8">Since someone receiving a patch from the Alices of the net
	would be unlikely to have unmodified and modified directories
	with exactly the same names, the <command>patch</command>
	command has a <option role="cmd-opt-patch">-p</option> option
	that indicates the number of leading path name components to
	strip when trying to apply a patch.  This number is called the
	<emphasis>strip count</emphasis>.</para>

      <para id="x_3e9">An option of <quote><literal>-p1</literal></quote> means
	<quote>use a strip count of one</quote>.  If
	<command>patch</command> sees a file name
	<filename>foo/bar/baz</filename> in a file header, it will
	strip <filename>foo</filename> and try to patch a file named
	<filename>bar/baz</filename>.  (Strictly speaking, the strip
	count refers to the number of <emphasis>path
	  separators</emphasis> (and the components that go with them
	) to strip.  A strip count of one will turn
	<filename>foo/bar</filename> into <filename>bar</filename>,
	but <filename>/foo/bar</filename> (notice the extra leading
	slash) into <filename>foo/bar</filename>.)</para>

      <para id="x_3ea">The <quote>standard</quote> strip count for patches is
	one; almost all patches contain one leading path name
	component that needs to be stripped. Mercurial's <command
	  role="hg-cmd">hg diff</command> command generates path names
	in this form, and the <command role="hg-cmd">hg
	  import</command> command and MQ expect patches to have a
	strip count of one.</para>

      <para id="x_3eb">If you receive a patch from someone that you want to add
	to your patch queue, and the patch needs a strip count other
	than one, you cannot just <command
	  role="hg-ext-mq">qimport</command> the patch, because
	<command role="hg-ext-mq">qimport</command> does not yet have
	a <literal>-p</literal> option (see <ulink role="hg-bug"
	  url="http://www.selenic.com/mercurial/bts/issue311">issue
	  311</ulink>).  Your best bet is to <command
	  role="hg-ext-mq">qnew</command> a patch of your own, then
	use <command>patch -pN</command> to apply their patch,
	followed by <command role="hg-cmd">hg addremove</command> to
	pick up any files added or removed by the patch, followed by
	<command role="hg-ext-mq">hg qrefresh</command>. This
	complexity may become unnecessary; see <ulink role="hg-bug"
	  url="http://www.selenic.com/mercurial/bts/issue311">issue
	  311</ulink> for details.
      </para>
    </sect2>

    <sect2>
      <title>Strategies for applying a patch</title>

      <para id="x_3ec">When <command>patch</command> applies a hunk, it tries a
	handful of successively less accurate strategies to try to
	make the hunk apply. This falling-back technique often makes
	it possible to take a patch that was generated against an old
	version of a file, and apply it against a newer version of
	that file.</para>

      <para id="x_3ed">First, <command>patch</command> tries an exact match,
	where the line numbers, the context, and the text to be
	modified must apply exactly.  If it cannot make an exact
	match, it tries to find an exact match for the context,
	without honouring the line numbering information.  If this
	succeeds, it prints a line of output saying that the hunk was
	applied, but at some <emphasis>offset</emphasis> from the
	original line number.</para>

      <para id="x_3ee">If a context-only match fails, <command>patch</command>
	removes the first and last lines of the context, and tries a
	<emphasis>reduced</emphasis> context-only match.  If the hunk
	with reduced context succeeds, it prints a message saying that
	it applied the hunk with a <emphasis>fuzz factor</emphasis>
	(the number after the fuzz factor indicates how many lines of
	context <command>patch</command> had to trim before the patch
	applied).</para>

      <para id="x_3ef">When neither of these techniques works,
	<command>patch</command> prints a message saying that the hunk
	in question was rejected.  It saves rejected hunks (also
	simply called <quote>rejects</quote>) to a file with the same
	name, and an added <filename role="special">.rej</filename>
	extension.  It also saves an unmodified copy of the file with
	a <filename role="special">.orig</filename> extension; the
	copy of the file without any extensions will contain any
	changes made by hunks that <emphasis>did</emphasis> apply
	cleanly.  If you have a patch that modifies
	<filename>foo</filename> with six hunks, and one of them fails
	to apply, you will have: an unmodified
	<filename>foo.orig</filename>, a <filename>foo.rej</filename>
	containing one hunk, and <filename>foo</filename>, containing
	the changes made by the five successful hunks.</para>
    </sect2>

    <sect2>
      <title>Some quirks of patch representation</title>

      <para id="x_3f0">There are a few useful things to know about how
	<command>patch</command> works with files.</para>
      <itemizedlist>
	<listitem><para id="x_3f1">This should already be obvious, but
	    <command>patch</command> cannot handle binary
	    files.</para>
	</listitem>
	<listitem><para id="x_3f2">Neither does it care about the executable bit;
	    it creates new files as readable, but not
	    executable.</para>
	</listitem>
	<listitem><para id="x_3f3"><command>patch</command> treats the removal of
	    a file as a diff between the file to be removed and the
	    empty file.  So your idea of <quote>I deleted this
	      file</quote> looks like <quote>every line of this file
	      was deleted</quote> in a patch.</para>
	</listitem>
	<listitem><para id="x_3f4">It treats the addition of a file as a diff
	    between the empty file and the file to be added.  So in a
	    patch, your idea of <quote>I added this file</quote> looks
	    like <quote>every line of this file was
	      added</quote>.</para>
	</listitem>
	<listitem><para id="x_3f5">It treats a renamed file as the removal of the
	    old name, and the addition of the new name.  This means
	    that renamed files have a big footprint in patches.  (Note
	    also that Mercurial does not currently try to infer when
	    files have been renamed or copied in a patch.)</para>
	</listitem>
	<listitem><para id="x_3f6"><command>patch</command> cannot represent
	    empty files, so you cannot use a patch to represent the
	    notion <quote>I added this empty file to the
	      tree</quote>.</para>
	</listitem></itemizedlist>
    </sect2>

    <sect2>
      <title>Beware the fuzz</title>

      <para id="x_3f7">While applying a hunk at an offset, or with a fuzz factor,
	will often be completely successful, these inexact techniques
	naturally leave open the possibility of corrupting the patched
	file.  The most common cases typically involve applying a
	patch twice, or at an incorrect location in the file.  If
	<command>patch</command> or <command
	  role="hg-ext-mq">qpush</command> ever mentions an offset or
	fuzz factor, you should make sure that the modified files are
	correct afterwards.</para>

      <para id="x_3f8">It's often a good idea to refresh a patch that has applied
	with an offset or fuzz factor; refreshing the patch generates
	new context information that will make it apply cleanly.  I
	say <quote>often,</quote> not <quote>always,</quote> because
	sometimes refreshing a patch will make it fail to apply
	against a different revision of the underlying files.  In some
	cases, such as when you're maintaining a patch that must sit
	on top of multiple versions of a source tree, it's acceptable
	to have a patch apply with some fuzz, provided you've verified
	the results of the patching process in such cases.</para>
    </sect2>

    <sect2>
      <title>Handling rejection</title>

      <para id="x_3f9">If <command role="hg-ext-mq">qpush</command> fails to
	apply a patch, it will print an error message and exit.  If it
	has left <filename role="special">.rej</filename> files
	behind, it is usually best to fix up the rejected hunks before
	you push more patches or do any further work.</para>

      <para id="x_3fa">If your patch <emphasis>used to</emphasis> apply cleanly,
	and no longer does because you've changed the underlying code
	that your patches are based on, Mercurial Queues can help; see
	<xref linkend="sec:mq:merge"/> for details.</para>

      <para id="x_3fb">Unfortunately, there aren't any great techniques for
	dealing with rejected hunks.  Most often, you'll need to view
	the <filename role="special">.rej</filename> file and edit the
	target file, applying the rejected hunks by hand.</para>

      <para id="x_3fd">A Linux kernel hacker, Chris Mason (the author
	of Mercurial Queues), wrote a tool called
	<command>mpatch</command> (<ulink
	  url="http://oss.oracle.com/~mason/mpatch/">http://oss.oracle.com/~mason/mpatch/</ulink>), 
	which takes a simple approach to automating the application of
	hunks rejected by <command>patch</command>.  The
	<command>mpatch</command> command can help with four common
	reasons that a hunk may be rejected:</para>

      <itemizedlist>
	<listitem><para id="x_3fe">The context in the middle of a hunk has
	    changed.</para>
	</listitem>
	<listitem><para id="x_3ff">A hunk is missing some context at the
	    beginning or end.</para>
	</listitem>
	<listitem><para id="x_400">A large hunk might apply better&emdash;either
	    entirely or in part&emdash;if it was broken up into
	    smaller hunks.</para>
	</listitem>
	<listitem><para id="x_401">A hunk removes lines with slightly different
	    content than those currently present in the file.</para>
	</listitem></itemizedlist>

      <para id="x_402">If you use <command>mpatch</command>, you
	should be doubly careful to check your results when you're
	done.  In fact, <command>mpatch</command> enforces this method
	of double-checking the tool's output, by automatically
	dropping you into a merge program when it has done its job, so
	that you can verify its work and finish off any remaining
	merges.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>More on patch management</title>

    <para id="x_6db">As you grow familiar with MQ, you will find yourself wanting
      to perform other kinds of patch management operations.</para>

    <sect2>
      <title>Deleting unwanted patches</title>

      <para id="x_6dc">If you want to get rid of a patch, use the <command
	  role="hg-ext-mq">hg qdelete</command> command to delete the
	patch file and remove its entry from the patch series.  If you
	try to delete a patch that is still applied, <command
	  role="hg-ext-mq">hg qdelete</command> will refuse.</para>

      &interaction.ch11-qdelete.go;
    </sect2>

    <sect2>
      <title>Converting to and from permanent revisions</title>

      <para id="x_6dd">Once you're done working on a patch and want to
      turn it into a permanent changeset, use the <command
      role="hg-ext-mq">hg qfinish</command> command. Pass a revision
      to the command to identify the patch that you want to turn into
      a regular changeset; this patch must already be applied.</para>

      &interaction.ch11-qdelete.convert;

      <para id="x_6e0">The <command role="hg-ext-mq">hg qfinish</command> command
        accepts an <option>--all</option> or <option>-a</option>
        option, which turns all applied patches into regular
        changesets.</para>

      <para id="x_6de">It is also possible to turn an existing changeset into a
	patch, by passing the <option>-r</option> option to <command
	  role="hg-ext-mq">hg qimport</command>.</para>

      &interaction.ch11-qdelete.import;

      <para id="x_6df">Note that it only makes sense to convert a changeset into
	a patch if you have not propagated that changeset into any
	other repositories.  The imported changeset's ID will change
	every time you refresh the patch, which will make Mercurial
	treat it as unrelated to the original changeset if you have
	pushed it somewhere else.</para>
    </sect2>
  </sect1>

  <sect1 id="sec:mq:perf">
    <title>Getting the best performance out of MQ</title>

    <para id="x_403">MQ is very efficient at handling a large number
      of patches. I ran some performance experiments in mid-2006 for a
      talk that I gave at the 2006 EuroPython conference (on modern
      hardware, you should expect better performance than you'll see
      below).  I used as my data set the Linux 2.6.17-mm1 patch
      series, which consists of 1,738 patches. I applied these on top
      of a Linux kernel repository containing all 27,472 revisions
      between Linux 2.6.12-rc2 and Linux 2.6.17.</para>

    <para id="x_404">On my old, slow laptop, I was able to <command
	role="hg-cmd">hg qpush <option
	  role="hg-ext-mq-cmd-qpush-opt">hg -a</option></command> all
      1,738 patches in 3.5 minutes, and <command role="hg-cmd">hg qpop
	<option role="hg-ext-mq-cmd-qpop-opt">hg -a</option></command>
      them all in 30 seconds.  (On a newer laptop, the time to push
      all patches dropped to two minutes.)  I could <command
	role="hg-ext-mq">qrefresh</command> one of the biggest patches
      (which made 22,779 lines of changes to 287 files) in 6.6
      seconds.</para>

    <para id="x_405">Clearly, MQ is well suited to working in large trees, but
      there are a few tricks you can use to get the best performance
      of it.</para>

    <para id="x_406">First of all, try to <quote>batch</quote> operations
      together.  Every time you run <command
	role="hg-ext-mq">qpush</command> or <command
	role="hg-ext-mq">qpop</command>, these commands scan the
      working directory once to make sure you haven't made some
      changes and then forgotten to run <command
	role="hg-ext-mq">qrefresh</command>.  On a small tree, the
      time that this scan takes is unnoticeable.  However, on a
      medium-sized tree (containing tens of thousands of files), it
      can take a second or more.</para>

    <para id="x_407">The <command role="hg-ext-mq">qpush</command> and <command
	role="hg-ext-mq">qpop</command> commands allow you to push and
      pop multiple patches at a time.  You can identify the
      <quote>destination patch</quote> that you want to end up at.
      When you <command role="hg-ext-mq">qpush</command> with a
      destination specified, it will push patches until that patch is
      at the top of the applied stack.  When you <command
	role="hg-ext-mq">qpop</command> to a destination, MQ will pop
      patches until the destination patch is at the top.</para>

    <para id="x_408">You can identify a destination patch using either the name
      of the patch, or by number.  If you use numeric addressing,
      patches are counted from zero; this means that the first patch
      is zero, the second is one, and so on.</para>
  </sect1>

  <sect1 id="sec:mq:merge">
    <title>Updating your patches when the underlying code
      changes</title>

    <para id="x_409">It's common to have a stack of patches on top of an
      underlying repository that you don't modify directly.  If you're
      working on changes to third-party code, or on a feature that is
      taking longer to develop than the rate of change of the code
      beneath, you will often need to sync up with the underlying
      code, and fix up any hunks in your patches that no longer apply.
      This is called <emphasis>rebasing</emphasis> your patch
      series.</para>

    <para id="x_40a">The simplest way to do this is to <command role="hg-cmd">hg
	qpop <option role="hg-ext-mq-cmd-qpop-opt">hg
	  -a</option></command> your patches, then <command
	role="hg-cmd">hg pull</command> changes into the underlying
      repository, and finally <command role="hg-cmd">hg qpush <option
	  role="hg-ext-mq-cmd-qpop-opt">hg -a</option></command> your
      patches again.  MQ will stop pushing any time it runs across a
      patch that fails to apply during conflicts, allowing you to fix
      your conflicts, <command role="hg-ext-mq">qrefresh</command> the
      affected patch, and continue pushing until you have fixed your
      entire stack.</para>

    <para id="x_40b">This approach is easy to use and works well if you don't
      expect changes to the underlying code to affect how well your
      patches apply. If your patch stack touches code that is modified
      frequently or invasively in the underlying repository, however,
      fixing up rejected hunks by hand quickly becomes
      tiresome.</para>

    <para id="x_40c">It's possible to partially automate the rebasing process.
      If your patches apply cleanly against some revision of the
      underlying repo, MQ can use this information to help you to
      resolve conflicts between your patches and a different
      revision.</para>

    <para id="x_40d">The process is a little involved.</para>
    <orderedlist>
      <listitem><para id="x_40e">To begin, <command role="hg-cmd">hg qpush
	    -a</command> all of your patches on top of the revision
	  where you know that they apply cleanly.</para>
      </listitem>
      <listitem><para id="x_40f">Save a backup copy of your patch directory using
	  <command role="hg-cmd">hg qsave <option
	      role="hg-ext-mq-cmd-qsave-opt">hg -e</option> <option
	      role="hg-ext-mq-cmd-qsave-opt">hg -c</option></command>.
	  This prints the name of the directory that it has saved the
	  patches in.  It will save the patches to a directory called
	  <filename role="special"
	    class="directory">.hg/patches.N</filename>, where
	  <literal>N</literal> is a small integer.  It also commits a
	  <quote>save changeset</quote> on top of your applied
	  patches; this is for internal book-keeping, and records the
	  states of the <filename role="special">series</filename> and
	  <filename role="special">status</filename> files.</para>
      </listitem>
      <listitem><para id="x_410">Use <command role="hg-cmd">hg pull</command> to
	  bring new changes into the underlying repository.  (Don't
	  run <command role="hg-cmd">hg pull -u</command>; see below
	  for why.)</para>
      </listitem>
      <listitem><para id="x_411">Update to the new tip revision, using <command
	    role="hg-cmd">hg update <option
	      role="hg-opt-update">-C</option></command> to override
	  the patches you have pushed.</para>
      </listitem>
      <listitem><para id="x_412">Merge all patches using <command>hg qpush -m
	    -a</command>.  The <option
	    role="hg-ext-mq-cmd-qpush-opt">-m</option> option to
	  <command role="hg-ext-mq">qpush</command> tells MQ to
	  perform a three-way merge if the patch fails to
	  apply.</para>
      </listitem></orderedlist>

    <para id="x_413">During the <command role="hg-cmd">hg qpush <option
	  role="hg-ext-mq-cmd-qpush-opt">hg -m</option></command>,
      each patch in the <filename role="special">series</filename>
      file is applied normally.  If a patch applies with fuzz or
      rejects, MQ looks at the queue you <command
	role="hg-ext-mq">qsave</command>d, and performs a three-way
      merge with the corresponding changeset.  This merge uses
      Mercurial's normal merge machinery, so it may pop up a GUI merge
      tool to help you to resolve problems.</para>

    <para id="x_414">When you finish resolving the effects of a patch, MQ
      refreshes your patch based on the result of the merge.</para>

    <para id="x_415">At the end of this process, your repository will have one
      extra head from the old patch queue, and a copy of the old patch
      queue will be in <filename role="special"
	class="directory">.hg/patches.N</filename>. You can remove the
      extra head using <command role="hg-cmd">hg qpop -a -n
	patches.N</command> or <command role="hg-cmd">hg
	strip</command>.  You can delete <filename role="special"
	class="directory">.hg/patches.N</filename> once you are sure
      that you no longer need it as a backup.</para>
  </sect1>

  <sect1>
    <title>Identifying patches</title>

    <para id="x_416">MQ commands that work with patches let you refer to a patch
      either by using its name or by a number.  By name is obvious
      enough; pass the name <filename>foo.patch</filename> to <command
	role="hg-ext-mq">qpush</command>, for example, and it will
      push patches until <filename>foo.patch</filename> is
      applied.</para>

    <para id="x_417">As a shortcut, you can refer to a patch using both a name
      and a numeric offset; <literal>foo.patch-2</literal> means
      <quote>two patches before <literal>foo.patch</literal></quote>,
      while <literal>bar.patch+4</literal> means <quote>four patches
	after <literal>bar.patch</literal></quote>.</para>

    <para id="x_418">Referring to a patch by index isn't much different.  The
      first patch printed in the output of <command
	role="hg-ext-mq">qseries</command> is patch zero (yes, it's
      one of those start-at-zero counting systems); the second is
      patch one; and so on.</para>

    <para id="x_419">MQ also makes it easy to work with patches when you are
      using normal Mercurial commands.  Every command that accepts a
      changeset ID will also accept the name of an applied patch.  MQ
      augments the tags normally in the repository with an eponymous
      one for each applied patch.  In addition, the special tags
      <literal role="tag">qbase</literal> and
      <literal role="tag">qtip</literal> identify
      the <quote>bottom-most</quote> and topmost applied patches,
      respectively.</para>

    <para id="x_41a">These additions to Mercurial's normal tagging capabilities
      make dealing with patches even more of a breeze.</para>
    <itemizedlist>
      <listitem><para id="x_41b">Want to patchbomb a mailing list with your
	  latest series of changes?</para>
	<programlisting>hg email qbase:qtip</programlisting>
	<para id="x_41c">  (Don't know what <quote>patchbombing</quote> is?  See
	  <xref linkend="sec:hgext:patchbomb"/>.)</para>
      </listitem>
      <listitem><para id="x_41d">Need to see all of the patches since
	  <literal>foo.patch</literal> that have touched files in a
	  subdirectory of your tree?</para>
	<programlisting>hg log -r foo.patch:qtip subdir</programlisting>
      </listitem>
    </itemizedlist>

    <para id="x_41e">Because MQ makes the names of patches available to the rest
      of Mercurial through its normal internal tag machinery, you
      don't need to type in the entire name of a patch when you want
      to identify it by name.</para>

    <para id="x_41f">Another nice consequence of representing patch names as tags
      is that when you run the <command role="hg-cmd">hg log</command>
      command, it will display a patch's name as a tag, simply as part
      of its normal output.  This makes it easy to visually
      distinguish applied patches from underlying
      <quote>normal</quote> revisions.  The following example shows a
      few normal Mercurial commands in use with applied
      patches.</para>

    &interaction.mq.id.output;
  </sect1>

  <sect1>
    <title>Useful things to know about</title>

    <para id="x_420">There are a number of aspects of MQ usage that don't fit
      tidily into sections of their own, but that are good to know.
      Here they are, in one place.</para>

    <itemizedlist>
      <listitem><para id="x_421">Normally, when you <command
	    role="hg-ext-mq">qpop</command> a patch and <command
	    role="hg-ext-mq">qpush</command> it again, the changeset
	  that represents the patch after the pop/push will have a
	  <emphasis>different identity</emphasis> than the changeset
	  that represented the hash beforehand.  See <xref
	    linkend="sec:mqref:cmd:qpush"/> for
	  information as to why this is.</para>
      </listitem>
      <listitem><para id="x_422">It's not a good idea to <command
	    role="hg-cmd">hg merge</command> changes from another
	  branch with a patch changeset, at least if you want to
	  maintain the <quote>patchiness</quote> of that changeset and
	  changesets below it on the patch stack.  If you try to do
	  this, it will appear to succeed, but MQ will become
	  confused.</para>
      </listitem></itemizedlist>
  </sect1>

  <sect1 id="sec:mq:repo">
    <title>Managing patches in a repository</title>

    <para id="x_423">Because MQ's <filename role="special"
	class="directory">.hg/patches</filename> directory resides
      outside a Mercurial repository's working directory, the
      <quote>underlying</quote> Mercurial repository knows nothing
      about the management or presence of patches.</para>

    <para id="x_424">This presents the interesting possibility of managing the
      contents of the patch directory as a Mercurial repository in its
      own right.  This can be a useful way to work.  For example, you
      can work on a patch for a while, <command
	role="hg-ext-mq">qrefresh</command> it, then <command
	role="hg-cmd">hg commit</command> the current state of the
      patch.  This lets you <quote>roll back</quote> to that version
      of the patch later on.</para>

    <para id="x_425">You can then share different versions of the same patch
      stack among multiple underlying repositories.  I use this when I
      am developing a Linux kernel feature.  I have a pristine copy of
      my kernel sources for each of several CPU architectures, and a
      cloned repository under each that contains the patches I am
      working on.  When I want to test a change on a different
      architecture, I push my current patches to the patch repository
      associated with that kernel tree, pop and push all of my
      patches, and build and test that kernel.</para>

    <para id="x_426">Managing patches in a repository makes it possible for
      multiple developers to work on the same patch series without
      colliding with each other, all on top of an underlying source
      base that they may or may not control.</para>

    <sect2>
      <title>MQ support for patch repositories</title>

      <para id="x_427">MQ helps you to work with the <filename role="special"
	  class="directory">.hg/patches</filename> directory as a
	repository; when you prepare a repository for working with
	patches using <command role="hg-ext-mq">qinit</command>, you
	can pass the <option role="hg-ext-mq-cmd-qinit-opt">hg
	  -c</option> option to create the <filename role="special"
	  class="directory">.hg/patches</filename> directory as a
	Mercurial repository.</para>

      <note>
	<para id="x_428">  If you forget to use the <option
	    role="hg-ext-mq-cmd-qinit-opt">hg -c</option> option, you
	  can simply go into the <filename role="special"
	    class="directory">.hg/patches</filename> directory at any
	  time and run <command role="hg-cmd">hg init</command>.
	  Don't forget to add an entry for the <filename
	    role="special">status</filename> file to the <filename
	    role="special">.hgignore</filename> file, though</para>

	<para id="x_429">  (<command role="hg-cmd">hg qinit <option
	      role="hg-ext-mq-cmd-qinit-opt">hg -c</option></command>
	  does this for you automatically); you
	  <emphasis>really</emphasis> don't want to manage the
	  <filename role="special">status</filename> file.</para>
      </note>

      <para id="x_42a">As a convenience, if MQ notices that the <filename
	  class="directory">.hg/patches</filename> directory is a
	repository, it will automatically <command role="hg-cmd">hg
	  add</command> every patch that you create and import.</para>

      <para id="x_42b">MQ provides a shortcut command, <command
	  role="hg-ext-mq">qcommit</command>, that runs <command
	  role="hg-cmd">hg commit</command> in the <filename
	  role="special" class="directory">.hg/patches</filename>
	directory.  This saves some bothersome typing.</para>

      <para id="x_42c">Finally, as a convenience to manage the patch directory,
	you can define the alias <command>mq</command> on Unix
	systems. For example, on Linux systems using the
	<command>bash</command> shell, you can include the following
	snippet in your <filename
	  role="home">~/.bashrc</filename>.</para>

      <programlisting>alias mq=`hg -R $(hg root)/.hg/patches'</programlisting>

      <para id="x_42d">You can then issue commands of the form <command>mq
	  pull</command> from the main repository.</para>
    </sect2>

    <sect2>
      <title>A few things to watch out for</title>

      <para id="x_42e">MQ's support for working with a repository full of patches
	is limited in a few small respects.</para>

      <para id="x_42f">MQ cannot automatically detect changes that you make to
	the patch directory.  If you <command role="hg-cmd">hg
	  pull</command>, manually edit, or <command role="hg-cmd">hg
	  update</command> changes to patches or the <filename
	  role="special">series</filename> file, you will have to
	<command role="hg-cmd">hg qpop <option
	    role="hg-ext-mq-cmd-qpop-opt">hg -a</option></command> and
	then <command role="hg-cmd">hg qpush <option
	    role="hg-ext-mq-cmd-qpush-opt">hg -a</option></command> in
	the underlying repository to see those changes show up there.
	If you forget to do this, you can confuse MQ's idea of which
	patches are applied.</para>

    </sect2>
  </sect1>
  <sect1 id="sec:mq:tools">
    <title>Third party tools for working with patches</title>

    <para id="x_430">Once you've been working with patches for a while, you'll
      find yourself hungry for tools that will help you to understand
      and manipulate the patches you're dealing with.</para>

    <para id="x_431">The <command>diffstat</command> command
      <citation>web:diffstat</citation> generates a histogram of the
      modifications made to each file in a patch.  It provides a good
      way to <quote>get a sense of</quote> a patch&emdash;which files
      it affects, and how much change it introduces to each file and
      as a whole.  (I find that it's a good idea to use
      <command>diffstat</command>'s <option
	role="cmd-opt-diffstat">-p</option> option as a matter of
      course, as otherwise it will try to do clever things with
      prefixes of file names that inevitably confuse at least
      me.)</para>

&interaction.mq.tools.tools;

    <para id="x_432">The <literal role="package">patchutils</literal> package
      <citation>web:patchutils</citation> is invaluable. It provides a
      set of small utilities that follow the <quote>Unix
	philosophy;</quote> each does one useful thing with a patch.
      The <literal role="package">patchutils</literal> command I use
      most is <command>filterdiff</command>, which extracts subsets
      from a patch file.  For example, given a patch that modifies
      hundreds of files across dozens of directories, a single
      invocation of <command>filterdiff</command> can generate a
      smaller patch that only touches files whose names match a
      particular glob pattern.  See <xref
	linkend="mq-collab:tips:interdiff"/> for another
      example.</para>

  </sect1>
  <sect1>
    <title>Good ways to work with patches</title>

    <para id="x_433">Whether you are working on a patch series to submit to a
      free software or open source project, or a series that you
      intend to treat as a sequence of regular changesets when you're
      done, you can use some simple techniques to keep your work well
      organized.</para>

    <para id="x_434">Give your patches descriptive names.  A good name for a
      patch might be <filename>rework-device-alloc.patch</filename>,
      because it will immediately give you a hint what the purpose of
      the patch is.  Long names shouldn't be a problem; you won't be
      typing the names often, but you <emphasis>will</emphasis> be
      running commands like <command
	role="hg-ext-mq">qapplied</command> and <command
	role="hg-ext-mq">qtop</command> over and over. Good naming
      becomes especially important when you have a number of patches
      to work with, or if you are juggling a number of different tasks
      and your patches only get a fraction of your attention.</para>

    <para id="x_435">Be aware of what patch you're working on.  Use the <command
	role="hg-ext-mq">qtop</command> command and skim over the text
      of your patches frequently&emdash;for example, using <command
	role="hg-cmd">hg tip <option
	  role="hg-opt-tip">-p</option></command>)&emdash;to be sure
      of where you stand.  I have several times worked on and <command
	role="hg-ext-mq">qrefresh</command>ed a patch other than the
      one I intended, and it's often tricky to migrate changes into
      the right patch after making them in the wrong one.</para>

    <para id="x_436">For this reason, it is very much worth investing a little
      time to learn how to use some of the third-party tools I
      described in <xref linkend="sec:mq:tools"/>,
      particularly
      <command>diffstat</command> and <command>filterdiff</command>.
      The former will give you a quick idea of what changes your patch
      is making, while the latter makes it easy to splice hunks
      selectively out of one patch and into another.</para>

  </sect1>
  <sect1>
    <title>MQ cookbook</title>

    <sect2>
      <title>Manage <quote>trivial</quote> patches</title>

      <para id="x_437">Because the overhead of dropping files into a new
	Mercurial repository is so low, it makes a lot of sense to
	manage patches this way even if you simply want to make a few
	changes to a source tarball that you downloaded.</para>

      <para id="x_438">Begin by downloading and unpacking the source tarball, and
	turning it into a Mercurial repository.</para>

      &interaction.mq.tarball.download;

      <para id="x_439">Continue by creating a patch stack and making your
	changes.</para>

      &interaction.mq.tarball.qinit;

      <para id="x_43a">Let's say a few weeks or months pass, and your package
	author releases a new version.  First, bring their changes
	into the repository.</para>

      &interaction.mq.tarball.newsource;

      <para id="x_43b">The pipeline starting with <command role="hg-cmd">hg
	  locate</command> above deletes all files in the working
	directory, so that <command role="hg-cmd">hg
	  commit</command>'s <option
	  role="hg-opt-commit">--addremove</option> option can
	actually tell which files have really been removed in the
	newer version of the source.</para>

      <para id="x_43c">Finally, you can apply your patches on top of the new
	tree.</para>

      &interaction.mq.tarball.repush;
    </sect2>

    <sect2 id="sec:mq:combine">
      <title>Combining entire patches</title>

      <para id="x_43d">MQ provides a command, <command
	  role="hg-ext-mq">qfold</command> that lets you combine
	entire patches.  This <quote>folds</quote> the patches you
	name, in the order you name them, into the topmost applied
	patch, and concatenates their descriptions onto the end of its
	description.  The patches that you fold must be unapplied
	before you fold them.</para>

      <para id="x_43e">The order in which you fold patches matters.  If your
	topmost applied patch is <literal>foo</literal>, and you
	<command role="hg-ext-mq">qfold</command>
	<literal>bar</literal> and <literal>quux</literal> into it,
	you will end up with a patch that has the same effect as if
	you applied first <literal>foo</literal>, then
	<literal>bar</literal>, followed by
	<literal>quux</literal>.</para>
    </sect2>

    <sect2>
      <title>Merging part of one patch into another</title>

      <para id="x_43f">Merging <emphasis>part</emphasis> of one patch into
	another is more difficult than combining entire
	patches.</para>

      <para id="x_440">If you want to move changes to entire files, you can use
	<command>filterdiff</command>'s <option
	  role="cmd-opt-filterdiff">-i</option> and <option
	  role="cmd-opt-filterdiff">-x</option> options to choose the
	modifications to snip out of one patch, concatenating its
	output onto the end of the patch you want to merge into.  You
	usually won't need to modify the patch you've merged the
	changes from.  Instead, MQ will report some rejected hunks
	when you <command role="hg-ext-mq">qpush</command> it (from
	the hunks you moved into the other patch), and you can simply
	<command role="hg-ext-mq">qrefresh</command> the patch to drop
	the duplicate hunks.</para>

      <para id="x_441">If you have a patch that has multiple hunks modifying a
	file, and you only want to move a few of those hunks, the job
	becomes more messy, but you can still partly automate it.  Use
	<command>lsdiff -nvv</command> to print some metadata about
	the patch.</para>

      &interaction.mq.tools.lsdiff;

      <para id="x_442">This command prints three different kinds of
	number:</para>
      <itemizedlist>
	<listitem><para id="x_443">(in the first column) a <emphasis>file
	      number</emphasis> to identify each file modified in the
	    patch;</para>
	</listitem>
	<listitem><para id="x_444">(on the next line, indented) the line number
	    within a modified file where a hunk starts; and</para>
	</listitem>
	<listitem><para id="x_445">(on the same line) a <emphasis>hunk
	      number</emphasis> to identify that hunk.</para>
	</listitem></itemizedlist>

      <para id="x_446">You'll have to use some visual inspection, and reading of
	the patch, to identify the file and hunk numbers you'll want,
	but you can then pass them to to
	<command>filterdiff</command>'s <option
	  role="cmd-opt-filterdiff">--files</option> and <option
	  role="cmd-opt-filterdiff">--hunks</option> options, to
	select exactly the file and hunk you want to extract.</para>

      <para id="x_447">Once you have this hunk, you can concatenate it onto the
	end of your destination patch and continue with the remainder
	of <xref linkend="sec:mq:combine"/>.</para>

    </sect2>
  </sect1>
  <sect1>
    <title>Differences between quilt and MQ</title>

    <para id="x_448">If you are already familiar with quilt, MQ provides a
      similar command set.  There are a few differences in the way
      that it works.</para>

    <para id="x_449">You will already have noticed that most quilt commands have
      MQ counterparts that simply begin with a
      <quote><literal>q</literal></quote>.  The exceptions are quilt's
      <literal>add</literal> and <literal>remove</literal> commands,
      the counterparts for which are the normal Mercurial <command
	role="hg-cmd">hg add</command> and <command role="hg-cmd">hg
	remove</command> commands.  There is no MQ equivalent of the
      quilt <literal>edit</literal> command.</para>

  </sect1>
</chapter>

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