hgbook / en / ch06-collab.xml

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

<chapter id="cha:collab">
  <?dbhtml filename="collaborating-with-other-people.html"?>
  <title>Collaborating with other people</title>

  <para id="x_44a">As a completely decentralised tool, Mercurial doesn't impose
    any policy on how people ought to work with each other.  However,
    if you're new to distributed revision control, it helps to have
    some tools and examples in mind when you're thinking about
    possible workflow models.</para>

  <sect1>
    <title>Mercurial's web interface</title>

    <para id="x_44b">Mercurial has a powerful web interface that provides several
      useful capabilities.</para>

    <para id="x_44c">For interactive use, the web interface lets you browse a
      single repository or a collection of repositories.  You can view
      the history of a repository, examine each change (comments and
      diffs), and view the contents of each directory and file.  You
      can even get a view of history that gives a graphical view of
      the relationships between individual changes and merges.</para>

    <para id="x_44d">Also for human consumption, the web interface provides
      Atom and RSS feeds of the changes in a repository.  This lets you
      <quote>subscribe</quote> to a repository using your favorite
      feed reader, and be automatically notified of activity in that
      repository as soon as it happens.  I find this capability much
      more convenient than the model of subscribing to a mailing list
      to which notifications are sent, as it requires no additional
      configuration on the part of whoever is serving the
      repository.</para>

    <para id="x_44e">The web interface also lets remote users clone a repository,
      pull changes from it, and (when the server is configured to
      permit it) push changes back to it.  Mercurial's HTTP tunneling
      protocol aggressively compresses data, so that it works
      efficiently even over low-bandwidth network connections.</para>

    <para id="x_44f">The easiest way to get started with the web interface is to
      use your web browser to visit an existing repository, such as
      the master Mercurial repository at <ulink
	url="http://www.selenic.com/repo/hg">http://www.selenic.com/repo/hg</ulink>.</para>

    <para id="x_450">If you're interested in providing a web interface
      to your own repositories, there are several good ways to do
      this.</para>

    <para id="x_69d">The easiest and fastest way to get started in an informal
      environment is to use the <command role="hg-cmd">hg
	serve</command> command, which is best suited to short-term
      <quote>lightweight</quote> serving.  See <xref
	linkend="sec:collab:serve"/> below for details of how to use
      this command.</para>

    <para id="x_69e">For longer-lived repositories that you'd like to
      have permanently available, there are several public hosting
      services available.  Some are free to open source projects,
      while others offer paid commercial hosting.  An up-to-date list
      is available at <ulink
	url="http://www.selenic.com/mercurial/wiki/index.cgi/MercurialHosting">http://www.selenic.com/mercurial/wiki/index.cgi/MercurialHosting</ulink>.</para>

    <para id="x_6a0">If you would prefer to host your own repositories, Mercurial
      has built-in support for several popular hosting technologies,
      most notably CGI (Common Gateway Interface), and WSGI (Web
      Services Gateway Interface).  See <xref
	linkend="sec:collab:cgi"/> for details of CGI and WSGI
      configuration.</para>
  </sect1>

  <sect1>
    <title>Collaboration models</title>

    <para id="x_451">With a suitably flexible tool, making decisions about
      workflow is much more of a social engineering challenge than a
      technical one. Mercurial imposes few limitations on how you can
      structure the flow of work in a project, so it's up to you and
      your group to set up and live with a model that matches your own
      particular needs.</para>

    <sect2>
      <title>Factors to keep in mind</title>

      <para id="x_452">The most important aspect of any model that you must keep
	in mind is how well it matches the needs and capabilities of
	the people who will be using it.  This might seem
	self-evident; even so, you still can't afford to forget it for
	a moment.</para>

      <para id="x_453">I once put together a workflow model that seemed to make
	perfect sense to me, but that caused a considerable amount of
	consternation and strife within my development team.  In spite
	of my attempts to explain why we needed a complex set of
	branches, and how changes ought to flow between them, a few
	team members revolted.  Even though they were smart people,
	they didn't want to pay attention to the constraints we were
	operating under, or face the consequences of those constraints
	in the details of the model that I was advocating.</para>

      <para id="x_454">Don't sweep foreseeable social or technical problems under
	the rug. Whatever scheme you put into effect, you should plan
	for mistakes and problem scenarios.  Consider adding automated
	machinery to prevent, or quickly recover from, trouble that
	you can anticipate.  As an example, if you intend to have a
	branch with not-for-release changes in it, you'd do well to
	think early about the possibility that someone might
	accidentally merge those changes into a release branch.  You
	could avoid this particular problem by writing a hook that
	prevents changes from being merged from an inappropriate
	branch.</para>
    </sect2>

    <sect2>
      <title>Informal anarchy</title>

      <para id="x_455">I wouldn't suggest an <quote>anything goes</quote>
	approach as something sustainable, but it's a model that's
	easy to grasp, and it works perfectly well in a few unusual
	situations.</para>

      <para id="x_456">As one example, many projects have a loose-knit group of
	collaborators who rarely physically meet each other.  Some
	groups like to overcome the isolation of working at a distance
	by organizing occasional <quote>sprints</quote>.  In a sprint,
	a number of people get together in a single location (a
	company's conference room, a hotel meeting room, that kind of
	place) and spend several days more or less locked in there,
	hacking intensely on a handful of projects.</para>

      <para id="x_457">A sprint or a hacking session in a coffee shop are the perfect places to use the
	<command role="hg-cmd">hg serve</command> command, since
	<command role="hg-cmd">hg serve</command> does not require any
	fancy server infrastructure.  You can get started with
	<command role="hg-cmd">hg serve</command> in moments, by
	reading <xref linkend="sec:collab:serve"/> below.  Then simply
	tell the person next to you that you're running a server, send
	the URL to them in an instant message, and you immediately
	have a quick-turnaround way to work together.  They can type
	your URL into their web browser and quickly review your
	changes; or they can pull a bugfix from you and verify it; or
	they can clone a branch containing a new feature and try it
	out.</para>

      <para id="x_458">The charm, and the problem, with doing things
	in an ad hoc fashion like this is that only people who know
	about your changes, and where they are, can see them.  Such an
	informal approach simply doesn't scale beyond a handful
	people, because each individual needs to know about
	<emphasis>n</emphasis> different repositories to pull
	from.</para>
    </sect2>

    <sect2>
      <title>A single central repository</title>

      <para id="x_459">For smaller projects migrating from a centralised revision
	control tool, perhaps the easiest way to get started is to
	have changes flow through a single shared central repository.
	This is also the most common <quote>building block</quote> for
	more ambitious workflow schemes.</para>

      <para id="x_45a">Contributors start by cloning a copy of this repository.
	They can pull changes from it whenever they need to, and some
	(perhaps all) developers have permission to push a change back
	when they're ready for other people to see it.</para>

      <para id="x_45b">Under this model, it can still often make sense for people
	to pull changes directly from each other, without going
	through the central repository.  Consider a case in which I
	have a tentative bug fix, but I am worried that if I were to
	publish it to the central repository, it might subsequently
	break everyone else's trees as they pull it.  To reduce the
	potential for damage, I can ask you to clone my repository
	into a temporary repository of your own and test it.  This
	lets us put off publishing the potentially unsafe change until
	it has had a little testing.</para>

      <para id="x_45c">If a team is hosting its own repository in this
	kind of scenario, people will usually use the
	<command>ssh</command> protocol to securely push changes to
	the central repository, as documented in <xref
	  linkend="sec:collab:ssh"/>.  It's also usual to publish a
	read-only copy of the repository over HTTP, as in
	<xref linkend="sec:collab:cgi"/>. Publishing over HTTP
	satisfies the needs of people who don't have push access, and
	those who want to use web browsers to browse the repository's
	history.</para>
    </sect2>

    <sect2>
      <title>A hosted central repository</title>

      <para id="x_6a1">A wonderful thing about public hosting services like
	<ulink url="http://bitbucket.org/">Bitbucket</ulink> is that
	not only do they handle the fiddly server configuration
	details, such as user accounts, authentication, and secure
	wire protocols, they provide additional infrastructure to make
	this model work well.</para>

      <para id="x_6a2">For instance, a well-engineered hosting service will let
	people clone their own copies of a repository with a single
	click.  This lets people work in separate spaces and share
	their changes when they're ready.</para>

      <para id="x_6a3">In addition, a good hosting service will let people
	communicate with each other, for instance to say <quote>there
	  are changes ready for you to review in this
	  tree</quote>.</para>
    </sect2>

    <sect2>
      <title>Working with multiple branches</title>

      <para id="x_45d">Projects of any significant size naturally tend to make
	progress on several fronts simultaneously.  In the case of
	software, it's common for a project to go through periodic
	official releases.  A release might then go into
	<quote>maintenance mode</quote> for a while after its first
	publication; maintenance releases tend to contain only bug
	fixes, not new features.  In parallel with these maintenance
	releases, one or more future releases may be under
	development.  People normally use the word
	<quote>branch</quote> to refer to one of these many slightly
	different directions in which development is
	proceeding.</para>

      <para id="x_45e">Mercurial is particularly well suited to managing a number
	of simultaneous, but not identical, branches.  Each
	<quote>development direction</quote> can live in its own
	central repository, and you can merge changes from one to
	another as the need arises.  Because repositories are
	independent of each other, unstable changes in a development
	branch will never affect a stable branch unless someone
	explicitly merges those changes into the stable branch.</para>

      <para id="x_45f">Here's an example of how this can work in practice.  Let's
	say you have one <quote>main branch</quote> on a central
	server.</para>

      &interaction.branching.init;

      <para id="x_460">People clone it, make changes locally, test them, and push
	them back.</para>

      <para id="x_461">Once the main branch reaches a release milestone, you can
	use the <command role="hg-cmd">hg tag</command> command to
	give a permanent name to the milestone revision.</para>

	&interaction.branching.tag;

      <para id="x_462">Let's say some ongoing
	development occurs on the main branch.</para>

      &interaction.branching.main;

      <para id="x_463">Using the tag that was recorded at the milestone, people
	who clone that repository at any time in the future can use
	<command role="hg-cmd">hg update</command> to get a copy of
	the working directory exactly as it was when that tagged
	revision was committed.</para>

      &interaction.branching.update;

      <para id="x_464">In addition, immediately after the main branch is tagged,
	we can then clone the main branch on the server to a new
	<quote>stable</quote> branch, also on the server.</para>

      &interaction.branching.clone;

      <para id="x_465">If we need to make a change to the stable
	branch, we can then clone <emphasis>that</emphasis>
	repository, make our changes, commit, and push our changes
	back there.</para>

      &interaction.branching.stable;

      <para id="x_466">Because Mercurial repositories are independent, and
	Mercurial doesn't move changes around automatically, the
	stable and main branches are <emphasis>isolated</emphasis>
	from each other.  The changes that we made on the main branch
	don't <quote>leak</quote> to the stable branch, and vice
	versa.</para>

      <para id="x_467">We'll often want all of our bugfixes on the stable
	branch to show up on the main branch, too.  Rather than
	rewrite a bugfix on the main branch, we can simply pull and
	merge changes from the stable to the main branch, and
	Mercurial will bring those bugfixes in for us.</para>

      &interaction.branching.merge;

      <para id="x_468">The main branch will still contain changes that
	are not on the stable branch, but it will also contain all of
	the bugfixes from the stable branch.  The stable branch
	remains unaffected by these changes, since changes are only
	flowing from the stable to the main branch, and not the other
	way.</para>
    </sect2>

    <sect2>
      <title>Feature branches</title>

      <para id="x_469">For larger projects, an effective way to manage change is
	to break up a team into smaller groups.  Each group has a
	shared branch of its own, cloned from a single
	<quote>master</quote> branch used by the entire project.
	People working on an individual branch are typically quite
	isolated from developments on other branches.</para>

      <figure id="fig:collab:feature-branches">
	<title>Feature branches</title>
	<mediaobject>
	  <imageobject><imagedata width="100%" fileref="figs/feature-branches.png"/></imageobject>
	  <textobject><phrase>XXX add text</phrase></textobject>
	</mediaobject>
      </figure>

      <para id="x_46b">When a particular feature is deemed to be in suitable
	shape, someone on that feature team pulls and merges from the
	master branch into the feature branch, then pushes back up to
	the master branch.</para>
    </sect2>

    <sect2>
      <title>The release train</title>

      <para id="x_46c">Some projects are organized on a <quote>train</quote>
	basis: a release is scheduled to happen every few months, and
	whatever features are ready when the <quote>train</quote> is
	ready to leave are allowed in.</para>

      <para id="x_46d">This model resembles working with feature branches.  The
	difference is that when a feature branch misses a train,
	someone on the feature team pulls and merges the changes that
	went out on that train release into the feature branch, and
	the team continues its work on top of that release so that
	their feature can make the next release.</para>
    </sect2>

    <sect2>
      <title>The Linux kernel model</title>

      <para id="x_46e">The development of the Linux kernel has a shallow
	hierarchical structure, surrounded by a cloud of apparent
	chaos.  Because most Linux developers use
	<command>git</command>, a distributed revision control tool
	with capabilities similar to Mercurial, it's useful to
	describe the way work flows in that environment; if you like
	the ideas, the approach translates well across tools.</para>

      <para id="x_46f">At the center of the community sits Linus Torvalds, the
	creator of Linux.  He publishes a single source repository
	that is considered the <quote>authoritative</quote> current
	tree by the entire developer community. Anyone can clone
	Linus's tree, but he is very choosy about whose trees he pulls
	from.</para>

      <para id="x_470">Linus has a number of <quote>trusted lieutenants</quote>.
	As a general rule, he pulls whatever changes they publish, in
	most cases without even reviewing those changes.  Some of
	those lieutenants are generally agreed to be
	<quote>maintainers</quote>, responsible for specific
	subsystems within the kernel.  If a random kernel hacker wants
	to make a change to a subsystem that they want to end up in
	Linus's tree, they must find out who the subsystem's
	maintainer is, and ask that maintainer to take their change.
	If the maintainer reviews their changes and agrees to take
	them, they'll pass them along to Linus in due course.</para>

      <para id="x_471">Individual lieutenants have their own approaches to
	reviewing, accepting, and publishing changes; and for deciding
	when to feed them to Linus.  In addition, there are several
	well known branches that people use for different purposes.
	For example, a few people maintain <quote>stable</quote>
	repositories of older versions of the kernel, to which they
	apply critical fixes as needed.  Some maintainers publish
	multiple trees: one for experimental changes; one for changes
	that they are about to feed upstream; and so on.  Others just
	publish a single tree.</para>

      <para id="x_472">This model has two notable features.  The first is that
	it's <quote>pull only</quote>.  You have to ask, convince, or
	beg another developer to take a change from you, because there
	are almost no trees to which more than one person can push,
	and there's no way to push changes into a tree that someone
	else controls.</para>

      <para id="x_473">The second is that it's based on reputation and acclaim.
	If you're an unknown, Linus will probably ignore changes from
	you without even responding.  But a subsystem maintainer will
	probably review them, and will likely take them if they pass
	their criteria for suitability. The more <quote>good</quote>
	changes you contribute to a maintainer, the more likely they
	are to trust your judgment and accept your changes.  If you're
	well-known and maintain a long-lived branch for something
	Linus hasn't yet accepted, people with similar interests may
	pull your changes regularly to keep up with your work.</para>

      <para id="x_474">Reputation and acclaim don't necessarily cross subsystem
	or <quote>people</quote> boundaries.  If you're a respected
	but specialised storage hacker, and you try to fix a
	networking bug, that change will receive a level of scrutiny
	from a network maintainer comparable to a change from a
	complete stranger.</para>

      <para id="x_475">To people who come from more orderly project backgrounds,
	the comparatively chaotic Linux kernel development process
	often seems completely insane.  It's subject to the whims of
	individuals; people make sweeping changes whenever they deem
	it appropriate; and the pace of development is astounding.
	And yet Linux is a highly successful, well-regarded piece of
	software.</para>
    </sect2>

    <sect2>
      <title>Pull-only versus shared-push collaboration</title>

      <para id="x_476">A perpetual source of heat in the open source community is
	whether a development model in which people only ever pull
	changes from others is <quote>better than</quote> one in which
	multiple people can push changes to a shared
	repository.</para>

      <para id="x_477">Typically, the backers of the shared-push model use tools
	that actively enforce this approach.  If you're using a
	centralised revision control tool such as Subversion, there's
	no way to make a choice over which model you'll use: the tool
	gives you shared-push, and if you want to do anything else,
	you'll have to roll your own approach on top (such as applying
	a patch by hand).</para>

      <para id="x_478">A good distributed revision control tool will
	support both models.  You and your collaborators can then
	structure how you work together based on your own needs and
	preferences, not on what contortions your tools force you
	into.</para>
    </sect2>
    <sect2>
      <title>Where collaboration meets branch management</title>

      <para id="x_479">Once you and your team set up some shared
	repositories and start propagating changes back and forth
	between local and shared repos, you begin to face a related,
	but slightly different challenge: that of managing the
	multiple directions in which your team may be moving at once.
	Even though this subject is intimately related to how your
	team collaborates, it's dense enough to merit treatment of its
	own, in <xref linkend="chap:branch"/>.</para>
    </sect2>
  </sect1>

  <sect1>
    <title>The technical side of sharing</title>

    <para id="x_47a">The remainder of this chapter is devoted to the question of
      sharing changes with your collaborators.</para>
  </sect1>

  <sect1 id="sec:collab:serve">
    <title>Informal sharing with <command role="hg-cmd">hg
	serve</command></title>

    <para id="x_47b">Mercurial's <command role="hg-cmd">hg serve</command>
      command is wonderfully suited to small, tight-knit, and
      fast-paced group environments.  It also provides a great way to
      get a feel for using Mercurial commands over a network.</para>

    <para id="x_47c">Run <command role="hg-cmd">hg serve</command> inside a
      repository, and in under a second it will bring up a specialised
      HTTP server; this will accept connections from any client, and
      serve up data for that repository until you terminate it.
      Anyone who knows the URL of the server you just started, and can
      talk to your computer over the network, can then use a web
      browser or Mercurial to read data from that repository.  A URL
      for a <command role="hg-cmd">hg serve</command> instance running
      on a laptop is likely to look something like
      <literal>http://my-laptop.local:8000/</literal>.</para>

    <para id="x_47d">The <command role="hg-cmd">hg serve</command> command is
      <emphasis>not</emphasis> a general-purpose web server. It can do
      only two things:</para>
    <itemizedlist>
      <listitem><para id="x_47e">Allow people to browse the history of the
	  repository it's serving, from their normal web
	  browsers.</para>
      </listitem>
      <listitem><para id="x_47f">Speak Mercurial's wire protocol, so that people
	  can <command role="hg-cmd">hg clone</command> or <command
	    role="hg-cmd">hg pull</command> changes from that
	  repository.</para>
      </listitem></itemizedlist>
    <para id="x_480">In particular, <command role="hg-cmd">hg serve</command>
      won't allow remote users to <emphasis>modify</emphasis> your
      repository.  It's intended for read-only use.</para>

    <para id="x_481">If you're getting started with Mercurial, there's nothing to
      prevent you from using <command role="hg-cmd">hg serve</command>
      to serve up a repository on your own computer, then use commands
      like <command role="hg-cmd">hg clone</command>, <command
	role="hg-cmd">hg incoming</command>, and so on to talk to that
      server as if the repository was hosted remotely. This can help
      you to quickly get acquainted with using commands on
      network-hosted repositories.</para>

    <sect2>
      <title>A few things to keep in mind</title>

      <para id="x_482">Because it provides unauthenticated read access to all
	clients, you should only use <command role="hg-cmd">hg
	  serve</command> in an environment where you either don't
	care, or have complete control over, who can access your
	network and pull data from your repository.</para>

      <para id="x_483">The <command role="hg-cmd">hg serve</command> command
	knows nothing about any firewall software you might have
	installed on your system or network.  It cannot detect or
	control your firewall software.  If other people are unable to
	talk to a running <command role="hg-cmd">hg serve</command>
	instance, the second thing you should do
	(<emphasis>after</emphasis> you make sure that they're using
	the correct URL) is check your firewall configuration.</para>

      <para id="x_484">By default, <command role="hg-cmd">hg serve</command>
	listens for incoming connections on port 8000.  If another
	process is already listening on the port you want to use, you
	can specify a different port to listen on using the <option
	  role="hg-opt-serve">-p</option> option.</para>

      <para id="x_485">Normally, when <command role="hg-cmd">hg serve</command>
	starts, it prints no output, which can be a bit unnerving.  If
	you'd like to confirm that it is indeed running correctly, and
	find out what URL you should send to your collaborators, start
	it with the <option role="hg-opt-global">-v</option>
	option.</para>
    </sect2>
  </sect1>

  <sect1 id="sec:collab:ssh">
    <title>Using the Secure Shell (ssh) protocol</title>

    <para id="x_486">You can pull and push changes securely over a network
      connection using the Secure Shell (<literal>ssh</literal>)
      protocol.  To use this successfully, you may have to do a little
      bit of configuration on the client or server sides.</para>

    <para id="x_487">If you're not familiar with ssh, it's the name of
      both a command and a network protocol that let you securely
      communicate with another computer.  To use it with Mercurial,
      you'll be setting up one or more user accounts on a server so
      that remote users can log in and execute commands.</para>

    <para id="x_488">(If you <emphasis>are</emphasis> familiar with ssh, you'll
      probably find some of the material that follows to be elementary
      in nature.)</para>

    <sect2>
      <title>How to read and write ssh URLs</title>

      <para id="x_489">An ssh URL tends to look like this:</para>
      <programlisting>ssh://bos@hg.serpentine.com:22/hg/hgbook</programlisting>
      <orderedlist>
	<listitem><para id="x_48a">The <quote><literal>ssh://</literal></quote>
	    part tells Mercurial to use the ssh protocol.</para>
	</listitem>
	<listitem><para id="x_48b">The <quote><literal>bos@</literal></quote>
	    component indicates what username to log into the server
	    as.  You can leave this out if the remote username is the
	    same as your local username.</para>
	</listitem>
	<listitem><para id="x_48c">The
	    <quote><literal>hg.serpentine.com</literal></quote> gives
	    the hostname of the server to log into.</para>
	</listitem>
	<listitem><para id="x_48d">The <quote>:22</quote> identifies the port
	    number to connect to the server on.  The default port is
	    22, so you only need to specify a colon and port number if
	    you're <emphasis>not</emphasis> using port 22.</para>
	</listitem>
	<listitem><para id="x_48e">The remainder of the URL is the local path to
	    the repository on the server.</para>
	</listitem></orderedlist>

      <para id="x_48f">There's plenty of scope for confusion with the path
	component of ssh URLs, as there is no standard way for tools
	to interpret it.  Some programs behave differently than others
	when dealing with these paths. This isn't an ideal situation,
	but it's unlikely to change.  Please read the following
	paragraphs carefully.</para>

      <para id="x_490">Mercurial treats the path to a repository on the server as
	relative to the remote user's home directory.  For example, if
	user <literal>foo</literal> on the server has a home directory
	of <filename class="directory">/home/foo</filename>, then an
	ssh URL that contains a path component of <filename
	  class="directory">bar</filename> <emphasis>really</emphasis>
	refers to the directory <filename
	  class="directory">/home/foo/bar</filename>.</para>

      <para id="x_491">If you want to specify a path relative to another user's
	home directory, you can use a path that starts with a tilde
	character followed by the user's name (let's call them
	<literal>otheruser</literal>), like this.</para>
      <programlisting>ssh://server/~otheruser/hg/repo</programlisting>

      <para id="x_492">And if you really want to specify an
	<emphasis>absolute</emphasis> path on the server, begin the
	path component with two slashes, as in this example.</para>
      <programlisting>ssh://server//absolute/path</programlisting>
    </sect2>

    <sect2>
      <title>Finding an ssh client for your system</title>

      <para id="x_493">Almost every Unix-like system comes with OpenSSH
	preinstalled.  If you're using such a system, run
	<literal>which ssh</literal> to find out if the
	<command>ssh</command> command is installed (it's usually in
	<filename class="directory">/usr/bin</filename>).  In the
	unlikely event that it isn't present, take a look at your
	system documentation to figure out how to install it.</para>

      <para id="x_494">On Windows, the TortoiseHg package is bundled
	with a version of Simon Tatham's excellent
	<command>plink</command> command, and you should not need to
	do any further configuration.</para>
    </sect2>

    <sect2>
      <title>Generating a key pair</title>

      <para id="x_499">To avoid the need to repetitively type a
	password every time you need to use your ssh client, I
	recommend generating a key pair.</para>

      <tip>
	<title>Key pairs are not mandatory</title>

	<para id="x_6a4">Mercurial knows nothing about ssh authentication or key
	  pairs.  You can, if you like, safely ignore this section and
	  the one that follows until you grow tired of repeatedly
	  typing ssh passwords.</para>
      </tip>

      <itemizedlist>
	<listitem>
	  <para id="x_6a5">On a Unix-like system, the
	    <command>ssh-keygen</command> command will do the
	    trick.</para>
	  <para id="x_6a6">On Windows, if you're using TortoiseHg, you may need
	    to download a command named <command>puttygen</command>
	    from <ulink
	      url="http://www.chiark.greenend.org.uk/~sgtatham/putty">the 
	      PuTTY web site</ulink> to generate a key pair.  See
	    <ulink
	      url="http://the.earth.li/~sgtatham/putty/0.60/htmldoc/Chapter8.html#pubkey-puttygen">the 
	      <command>puttygen</command> documentation</ulink> for
	    details of how use the command.</para>
	</listitem>
      </itemizedlist>

      <para id="x_49a">When you generate a key pair, it's usually
	<emphasis>highly</emphasis> advisable to protect it with a
	passphrase.  (The only time that you might not want to do this
	is when you're using the ssh protocol for automated tasks on a
	secure network.)</para>

      <para id="x_49b">Simply generating a key pair isn't enough, however.
	You'll need to add the public key to the set of authorised
	keys for whatever user you're logging in remotely as.  For
	servers using OpenSSH (the vast majority), this will mean
	adding the public key to a list in a file called <filename
	  role="special">authorized_keys</filename> in their <filename
	  role="special" class="directory">.ssh</filename>
	directory.</para>

      <para id="x_49c">On a Unix-like system, your public key will have a
	<filename>.pub</filename> extension.  If you're using
	<command>puttygen</command> on Windows, you can save the
	public key to a file of your choosing, or paste it from the
	window it's displayed in straight into the <filename
	  role="special">authorized_keys</filename> file.</para>
    </sect2>
    <sect2>
      <title>Using an authentication agent</title>

      <para id="x_49d">An authentication agent is a daemon that stores
	passphrases in memory (so it will forget passphrases if you
	log out and log back in again). An ssh client will notice if
	it's running, and query it for a passphrase.  If there's no
	authentication agent running, or the agent doesn't store the
	necessary passphrase, you'll have to type your passphrase
	every time Mercurial tries to communicate with a server on
	your behalf (e.g. whenever you pull or push changes).</para>

      <para id="x_49e">The downside of storing passphrases in an agent is that
	it's possible for a well-prepared attacker to recover the
	plain text of your passphrases, in some cases even if your
	system has been power-cycled. You should make your own
	judgment as to whether this is an acceptable risk.  It
	certainly saves a lot of repeated typing.</para>

      <itemizedlist>
	<listitem>
	  <para id="x_49f">On Unix-like systems, the agent is called
	    <command>ssh-agent</command>, and it's often run
	    automatically for you when you log in.  You'll need to use
	    the <command>ssh-add</command> command to add passphrases
	    to the agent's store.</para>
	</listitem>
	<listitem>
	  <para id="x_6a7">On Windows, if you're using TortoiseHg, the
	    <command>pageant</command> command acts as the agent.  As
	    with <command>puttygen</command>, you'll need to <ulink
	      url="http://www.chiark.greenend.org.uk/%7Esgtatham/putty/download.html">download 
	      <command>pageant</command></ulink> from the PuTTY web
	    site and read <ulink
	      url="http://the.earth.li/~sgtatham/putty/0.60/htmldoc/Chapter9.html#pageant">its 
	      documentation</ulink>.  The <command>pageant</command>
	    command adds an icon to your system tray that will let you
	    manage stored passphrases.</para>
	</listitem>
      </itemizedlist>
    </sect2>

    <sect2>
      <title>Configuring the server side properly</title>

      <para id="x_4a0">Because ssh can be fiddly to set up if you're new to it,
	a variety of things can go wrong.  Add Mercurial
	on top, and there's plenty more scope for head-scratching.
	Most of these potential problems occur on the server side, not
	the client side.  The good news is that once you've gotten a
	configuration working, it will usually continue to work
	indefinitely.</para>

      <para id="x_4a1">Before you try using Mercurial to talk to an ssh server,
	it's best to make sure that you can use the normal
	<command>ssh</command> or <command>putty</command> command to
	talk to the server first.  If you run into problems with using
	these commands directly, Mercurial surely won't work.  Worse,
	it will obscure the underlying problem.  Any time you want to
	debug ssh-related Mercurial problems, you should drop back to
	making sure that plain ssh client commands work first,
	<emphasis>before</emphasis> you worry about whether there's a
	problem with Mercurial.</para>

      <para id="x_4a2">The first thing to be sure of on the server side is that
	you can actually log in from another machine at all.  If you
	can't use <command>ssh</command> or <command>putty</command>
	to log in, the error message you get may give you a few hints
	as to what's wrong.  The most common problems are as
	follows.</para>
      <itemizedlist>
	<listitem><para id="x_4a3">If you get a <quote>connection refused</quote>
	    error, either there isn't an SSH daemon running on the
	    server at all, or it's inaccessible due to firewall
	    configuration.</para>
	</listitem>
	<listitem><para id="x_4a4">If you get a <quote>no route to host</quote>
	    error, you either have an incorrect address for the server
	    or a seriously locked down firewall that won't admit its
	    existence at all.</para>
	</listitem>
	<listitem><para id="x_4a5">If you get a <quote>permission denied</quote>
	    error, you may have mistyped the username on the server,
	    or you could have mistyped your key's passphrase or the
	    remote user's password.</para>
	</listitem></itemizedlist>
      <para id="x_4a6">In summary, if you're having trouble talking to the
	server's ssh daemon, first make sure that one is running at
	all.  On many systems it will be installed, but disabled, by
	default.  Once you're done with this step, you should then
	check that the server's firewall is configured to allow
	incoming connections on the port the ssh daemon is listening
	on (usually 22).  Don't worry about more exotic possibilities
	for misconfiguration until you've checked these two
	first.</para>

      <para id="x_4a7">If you're using an authentication agent on the client side
	to store passphrases for your keys, you ought to be able to
	log into the server without being prompted for a passphrase or
	a password.  If you're prompted for a passphrase, there are a
	few possible culprits.</para>
      <itemizedlist>
	<listitem><para id="x_4a8">You might have forgotten to use
	    <command>ssh-add</command> or <command>pageant</command>
	    to store the passphrase.</para>
	</listitem>
	<listitem><para id="x_4a9">You might have stored the passphrase for the
	    wrong key.</para>
	</listitem></itemizedlist>
      <para id="x_4aa">If you're being prompted for the remote user's password,
	there are another few possible problems to check.</para>
      <itemizedlist>
	<listitem><para id="x_4ab">Either the user's home directory or their
	    <filename role="special" class="directory">.ssh</filename>
	    directory might have excessively liberal permissions.  As
	    a result, the ssh daemon will not trust or read their
	    <filename role="special">authorized_keys</filename> file.
	    For example, a group-writable home or <filename
	      role="special" class="directory">.ssh</filename>
	    directory will often cause this symptom.</para>
	</listitem>
	<listitem><para id="x_4ac">The user's <filename
	      role="special">authorized_keys</filename> file may have
	    a problem. If anyone other than the user owns or can write
	    to that file, the ssh daemon will not trust or read
	    it.</para>
	</listitem></itemizedlist>

      <para id="x_4ad">In the ideal world, you should be able to run the
	following command successfully, and it should print exactly
	one line of output, the current date and time.</para>
      <programlisting>ssh myserver date</programlisting>

      <para id="x_4ae">If, on your server, you have login scripts that print
	banners or other junk even when running non-interactive
	commands like this, you should fix them before you continue,
	so that they only print output if they're run interactively.
	Otherwise these banners will at least clutter up Mercurial's
	output.  Worse, they could potentially cause problems with
	running Mercurial commands remotely.  Mercurial tries to
	detect and ignore banners in non-interactive
	<command>ssh</command> sessions, but it is not foolproof.  (If
	you're editing your login scripts on your server, the usual
	way to see if a login script is running in an interactive
	shell is to check the return code from the command
	<literal>tty -s</literal>.)</para>

      <para id="x_4af">Once you've verified that plain old ssh is working with
	your server, the next step is to ensure that Mercurial runs on
	the server.  The following command should run
	successfully:</para>

      <programlisting>ssh myserver hg version</programlisting>

      <para id="x_4b0">If you see an error message instead of normal <command
	  role="hg-cmd">hg version</command> output, this is usually
	because you haven't installed Mercurial to <filename
	  class="directory">/usr/bin</filename>.  Don't worry if this
	is the case; you don't need to do that.  But you should check
	for a few possible problems.</para>
      <itemizedlist>
	<listitem><para id="x_4b1">Is Mercurial really installed on the server at
	    all?  I know this sounds trivial, but it's worth
	    checking!</para>
	</listitem>
	<listitem><para id="x_4b2">Maybe your shell's search path (usually set
	    via the <envar>PATH</envar> environment variable) is
	    simply misconfigured.</para>
	</listitem>
	<listitem><para id="x_4b3">Perhaps your <envar>PATH</envar> environment
	    variable is only being set to point to the location of the
	    <command>hg</command> executable if the login session is
	    interactive.  This can happen if you're setting the path
	    in the wrong shell login script.  See your shell's
	    documentation for details.</para>
	</listitem>
	<listitem><para id="x_4b4">The <envar>PYTHONPATH</envar> environment
	    variable may need to contain the path to the Mercurial
	    Python modules.  It might not be set at all; it could be
	    incorrect; or it may be set only if the login is
	    interactive.</para>
	</listitem></itemizedlist>

      <para id="x_4b5">If you can run <command role="hg-cmd">hg version</command>
	over an ssh connection, well done! You've got the server and
	client sorted out.  You should now be able to use Mercurial to
	access repositories hosted by that username on that server.
	If you run into problems with Mercurial and ssh at this point,
	try using the <option role="hg-opt-global">--debug</option>
	option to get a clearer picture of what's going on.</para>
    </sect2>
    <sect2>
      <title>Using compression with ssh</title>

      <para id="x_4b6">Mercurial does not compress data when it uses the ssh
	protocol, because the ssh protocol can transparently compress
	data.  However, the default behavior of ssh clients is
	<emphasis>not</emphasis> to request compression.</para>

      <para id="x_4b7">Over any network other than a fast LAN (even a wireless
	network), using compression is likely to significantly speed
	up Mercurial's network operations.  For example, over a WAN,
	someone measured compression as reducing the amount of time
	required to clone a particularly large repository from 51
	minutes to 17 minutes.</para>

      <para id="x_4b8">Both <command>ssh</command> and <command>plink</command>
	accept a <option role="cmd-opt-ssh">-C</option> option which
	turns on compression.  You can easily edit your <filename
	  role="special">~/.hgrc</filename> to enable compression for
	all of Mercurial's uses of the ssh protocol.  Here is how to
	do so for regular <command>ssh</command> on Unix-like systems,
	for example.</para>
      <programlisting>[ui]
ssh = ssh -C</programlisting>

      <para id="x_4b9">If you use <command>ssh</command> on a
	Unix-like system, you can configure it to always use
	compression when talking to your server.  To do this, edit
	your <filename role="special">.ssh/config</filename> file
	(which may not yet exist), as follows.</para>

      <programlisting>Host hg
  Compression yes
  HostName hg.example.com</programlisting>

      <para id="x_4ba">This defines a hostname alias,
	<literal>hg</literal>.  When you use that hostname on the
	<command>ssh</command> command line or in a Mercurial
	<literal>ssh</literal>-protocol URL, it will cause
	<command>ssh</command> to connect to
	<literal>hg.example.com</literal> and use compression.  This
	gives you both a shorter name to type and compression, each of
	which is a good thing in its own right.</para>
    </sect2>
  </sect1>

  <sect1 id="sec:collab:cgi">
    <title>Serving over HTTP using CGI</title>

    <para id="x_6a8">The simplest way to host one or more repositories in a
      permanent way is to use a web server and Mercurial's CGI
      support.</para>

    <para id="x_4bb">Depending on how ambitious you are, configuring Mercurial's
      CGI interface can take anything from a few moments to several
      hours.</para>

    <para id="x_4bc">We'll begin with the simplest of examples, and work our way
      towards a more complex configuration.  Even for the most basic
      case, you're almost certainly going to need to read and modify
      your web server's configuration.</para>

    <note>
      <title>High pain tolerance required</title>

      <para id="x_4bd">Configuring a web server is a complex, fiddly,
	and highly system-dependent activity.  I can't possibly give
	you instructions that will cover anything like all of the
	cases you will encounter. Please use your discretion and
	judgment in following the sections below.  Be prepared to make
	plenty of mistakes, and to spend a lot of time reading your
	server's error logs.</para>

      <para id="x_6a9">If you don't have a strong stomach for tweaking
	configurations over and over, or a compelling need to host
	your own services, you might want to try one of the public
	hosting services that I mentioned earlier.</para>
    </note>

    <sect2>
      <title>Web server configuration checklist</title>

      <para id="x_4be">Before you continue, do take a few moments to check a few
	aspects of your system's setup.</para>

      <orderedlist>
	<listitem><para id="x_4bf">Do you have a web server installed
	    at all? Mac OS X and some Linux distributions ship with
	    Apache, but many other systems may not have a web server
	    installed.</para>
	</listitem>
	<listitem><para id="x_4c0">If you have a web server installed, is it
	    actually running?  On most systems, even if one is
	    present, it will be disabled by default.</para>
	</listitem>
	<listitem><para id="x_4c1">Is your server configured to allow you to run
	    CGI programs in the directory where you plan to do so?
	    Most servers default to explicitly disabling the ability
	    to run CGI programs.</para>
	</listitem></orderedlist>

      <para id="x_4c2">If you don't have a web server installed, and don't have
	substantial experience configuring Apache, you should consider
	using the <literal>lighttpd</literal> web server instead of
	Apache.  Apache has a well-deserved reputation for baroque and
	confusing configuration. While <literal>lighttpd</literal> is
	less capable in some ways than Apache, most of these
	capabilities are not relevant to serving Mercurial
	repositories.  And <literal>lighttpd</literal> is undeniably
	<emphasis>much</emphasis> easier to get started with than
	Apache.</para>
    </sect2>

    <sect2>
      <title>Basic CGI configuration</title>

      <para id="x_4c3">On Unix-like systems, it's common for users to have a
	subdirectory named something like <filename
	  class="directory">public_html</filename> in their home
	directory, from which they can serve up web pages.  A file
	named <filename>foo</filename> in this directory will be
	accessible at a URL of the form
	<literal>http://www.example.com/username/foo</literal>.</para>

      <para id="x_4c4">To get started, find the <filename
	  role="special">hgweb.cgi</filename> script that should be
	present in your Mercurial installation.  If you can't quickly
	find a local copy on your system, simply download one from the
	master Mercurial repository at <ulink
	  url="http://www.selenic.com/repo/hg/raw-file/tip/hgweb.cgi">http://www.selenic.com/repo/hg/raw-file/tip/hgweb.cgi</ulink>.</para>

      <para id="x_4c5">You'll need to copy this script into your <filename
	  class="directory">public_html</filename> directory, and
	ensure that it's executable.</para>
      <programlisting>cp .../hgweb.cgi ~/public_html
chmod 755 ~/public_html/hgweb.cgi</programlisting>
      <para id="x_4c6">The <literal>755</literal> argument to
	<command>chmod</command> is a little more general than just
	making the script executable: it ensures that the script is
	executable by anyone, and that <quote>group</quote> and
	<quote>other</quote> write permissions are
	<emphasis>not</emphasis> set.  If you were to leave those
	write permissions enabled, Apache's <literal>suexec</literal>
	subsystem would likely refuse to execute the script.  In fact,
	<literal>suexec</literal> also insists that the
	<emphasis>directory</emphasis> in which the script resides
	must not be writable by others.</para>
      <programlisting>chmod 755 ~/public_html</programlisting>

      <sect3 id="sec:collab:wtf">
	<title>What could <emphasis>possibly</emphasis> go
	  wrong?</title>

	<para id="x_4c7">Once you've copied the CGI script into place,
	  go into a web browser, and try to open the URL
	  <literal>http://myhostname/~myuser/hgweb.cgi</literal>,
	  <emphasis>but</emphasis> brace yourself for instant failure.
	  There's a high probability that trying to visit this URL
	  will fail, and there are many possible reasons for this.  In
	  fact, you're likely to stumble over almost every one of the
	  possible errors below, so please read carefully.  The
	  following are all of the problems I ran into on a system
	  running Fedora 7, with a fresh installation of Apache, and a
	  user account that I created specially to perform this
	  exercise.</para>

	<para id="x_4c8">Your web server may have per-user directories disabled.
	  If you're using Apache, search your config file for a
	  <literal>UserDir</literal> directive.  If there's none
	  present, per-user directories will be disabled.  If one
	  exists, but its value is <literal>disabled</literal>, then
	  per-user directories will be disabled.  Otherwise, the
	  string after <literal>UserDir</literal> gives the name of
	  the subdirectory that Apache will look in under your home
	  directory, for example <filename
	    class="directory">public_html</filename>.</para>

	<para id="x_4c9">Your file access permissions may be too restrictive.
	  The web server must be able to traverse your home directory
	  and directories under your <filename
	    class="directory">public_html</filename> directory, and
	  read files under the latter too.  Here's a quick recipe to
	  help you to make your permissions more appropriate.</para>
	<programlisting>chmod 755 ~
find ~/public_html -type d -print0 | xargs -0r chmod 755
find ~/public_html -type f -print0 | xargs -0r chmod 644</programlisting>

	<para id="x_4ca">The other possibility with permissions is that you might
	  get a completely empty window when you try to load the
	  script.  In this case, it's likely that your access
	  permissions are <emphasis>too permissive</emphasis>.  Apache's
	  <literal>suexec</literal> subsystem won't execute a script
	  that's group- or world-writable, for example.</para>

	<para id="x_4cb">Your web server may be configured to disallow execution
	  of CGI programs in your per-user web directory.  Here's
	  Apache's default per-user configuration from my Fedora
	  system.</para>

	&ch06-apache-config.lst;

	<para id="x_4cc">If you find a similar-looking
	  <literal>Directory</literal> group in your Apache
	  configuration, the directive to look at inside it is
	  <literal>Options</literal>. Add <literal>ExecCGI</literal>
	  to the end of this list if it's missing, and restart the web
	  server.</para>

	<para id="x_4cd">If you find that Apache serves you the text of the CGI
	  script instead of executing it, you may need to either
	  uncomment (if already present) or add a directive like
	  this.</para>
	<programlisting>AddHandler cgi-script .cgi</programlisting>

	<para id="x_4ce">The next possibility is that you might be served with a
	  colourful Python backtrace claiming that it can't import a
	  <literal>mercurial</literal>-related module.  This is
	  actually progress!  The server is now capable of executing
	  your CGI script.  This error is only likely to occur if
	  you're running a private installation of Mercurial, instead
	  of a system-wide version.  Remember that the web server runs
	  the CGI program without any of the environment variables
	  that you take for granted in an interactive session.  If
	  this error happens to you, edit your copy of <filename
	    role="special">hgweb.cgi</filename> and follow the
	  directions inside it to correctly set your
	  <envar>PYTHONPATH</envar> environment variable.</para>

	<para id="x_4cf">Finally, you are <emphasis>certain</emphasis> to be
	  served with another colourful Python backtrace: this one
	  will complain that it can't find <filename
	    class="directory">/path/to/repository</filename>.  Edit
	  your <filename role="special">hgweb.cgi</filename> script
	  and replace the <filename
	    class="directory">/path/to/repository</filename> string
	  with the complete path to the repository you want to serve
	  up.</para>

	<para id="x_4d0">At this point, when you try to reload the page, you
	  should be presented with a nice HTML view of your
	  repository's history.  Whew!</para>
      </sect3>

      <sect3>
	<title>Configuring lighttpd</title>

	<para id="x_4d1">To be exhaustive in my experiments, I tried configuring
	  the increasingly popular <literal>lighttpd</literal> web
	  server to serve the same repository as I described with
	  Apache above.  I had already overcome all of the problems I
	  outlined with Apache, many of which are not server-specific.
	  As a result, I was fairly sure that my file and directory
	  permissions were good, and that my <filename
	    role="special">hgweb.cgi</filename> script was properly
	  edited.</para>

	<para id="x_4d2">Once I had Apache running, getting
	  <literal>lighttpd</literal> to serve the repository was a
	  snap (in other words, even if you're trying to use
	  <literal>lighttpd</literal>, you should read the Apache
	  section).  I first had to edit the
	  <literal>mod_access</literal> section of its config file to
	  enable <literal>mod_cgi</literal> and
	  <literal>mod_userdir</literal>, both of which were disabled
	  by default on my system.  I then added a few lines to the
	  end of the config file, to configure these modules.</para>
	<programlisting>userdir.path = "public_html"
cgi.assign = (".cgi" =&gt; "" )</programlisting>
	<para id="x_4d3">With this done, <literal>lighttpd</literal> ran
	  immediately for me.  If I had configured
	  <literal>lighttpd</literal> before Apache, I'd almost
	  certainly have run into many of the same system-level
	  configuration problems as I did with Apache.  However, I
	  found <literal>lighttpd</literal> to be noticeably easier to
	  configure than Apache, even though I've used Apache for over
	  a decade, and this was my first exposure to
	  <literal>lighttpd</literal>.</para>
      </sect3>
    </sect2>

    <sect2>
      <title>Sharing multiple repositories with one CGI script</title>

      <para id="x_4d4">The <filename role="special">hgweb.cgi</filename> script
	only lets you publish a single repository, which is an
	annoying restriction.  If you want to publish more than one
	without wracking yourself with multiple copies of the same
	script, each with different names, a better choice is to use
	the <filename role="special">hgwebdir.cgi</filename>
	script.</para>

      <para id="x_4d5">The procedure to configure <filename
	  role="special">hgwebdir.cgi</filename> is only a little more
	involved than for <filename
	  role="special">hgweb.cgi</filename>.  First, you must obtain
	a copy of the script.  If you don't have one handy, you can
	download a copy from the master Mercurial repository at <ulink
	  url="http://www.selenic.com/repo/hg/raw-file/tip/hgwebdir.cgi">http://www.selenic.com/repo/hg/raw-file/tip/hgwebdir.cgi</ulink>.</para>

      <para id="x_4d6">You'll need to copy this script into your <filename
	  class="directory">public_html</filename> directory, and
	ensure that it's executable.</para>

      <programlisting>cp .../hgwebdir.cgi ~/public_html
chmod 755 ~/public_html ~/public_html/hgwebdir.cgi</programlisting>

      <para id="x_4d7">With basic configuration out of the way, try to
	visit <literal>http://myhostname/~myuser/hgwebdir.cgi</literal>
	in your	browser.  It should
	display an empty list of repositories.  If you get a blank
	window or error message, try walking through the list of
	potential problems in <xref
	  linkend="sec:collab:wtf"/>.</para>

      <para id="x_4d8">The <filename role="special">hgwebdir.cgi</filename>
	script relies on an external configuration file.  By default,
	it searches for a file named <filename
	  role="special">hgweb.config</filename> in the same directory
	as itself.  You'll need to create this file, and make it
	world-readable.  The format of the file is similar to a
	Windows <quote>ini</quote> file, as understood by Python's
	<literal>ConfigParser</literal>
	<citation>web:configparser</citation> module.</para>

      <para id="x_4d9">The easiest way to configure <filename
	  role="special">hgwebdir.cgi</filename> is with a section
	named <literal>collections</literal>.  This will automatically
	publish <emphasis>every</emphasis> repository under the
	directories you name.  The section should look like
	this:</para>
      <programlisting>[collections]
/my/root = /my/root</programlisting>
      <para id="x_4da">Mercurial interprets this by looking at the directory name
	on the <emphasis>right</emphasis> hand side of the
	<quote><literal>=</literal></quote> sign; finding repositories
	in that directory hierarchy; and using the text on the
	<emphasis>left</emphasis> to strip off matching text from the
	names it will actually list in the web interface.  The
	remaining component of a path after this stripping has
	occurred is called a <quote>virtual path</quote>.</para>

      <para id="x_4db">Given the example above, if we have a
	repository whose local path is <filename
	  class="directory">/my/root/this/repo</filename>, the CGI
	script will strip the leading <filename
	  class="directory">/my/root</filename> from the name, and
	publish the repository with a virtual path of <filename
	  class="directory">this/repo</filename>.  If the base URL for
	our CGI script is
	<literal>http://myhostname/~myuser/hgwebdir.cgi</literal>, the
	complete URL for that repository will be
	<literal>http://myhostname/~myuser/hgwebdir.cgi/this/repo</literal>.</para>

      <para id="x_4dc">If we replace <filename
	  class="directory">/my/root</filename> on the left hand side
	of this example with <filename
	  class="directory">/my</filename>, then <filename
	  role="special">hgwebdir.cgi</filename> will only strip off
	<filename class="directory">/my</filename> from the repository
	name, and will give us a virtual path of <filename
	  class="directory">root/this/repo</filename> instead of
	<filename class="directory">this/repo</filename>.</para>

      <para id="x_4dd">The <filename role="special">hgwebdir.cgi</filename>
	script will recursively search each directory listed in the
	<literal>collections</literal> section of its configuration
	file, but it will <literal>not</literal> recurse into the
	repositories it finds.</para>

      <para id="x_4de">The <literal>collections</literal> mechanism makes it easy
	to publish many repositories in a <quote>fire and
	  forget</quote> manner.  You only need to set up the CGI
	script and configuration file one time.  Afterwards, you can
	publish or unpublish a repository at any time by simply moving
	it into, or out of, the directory hierarchy in which you've
	configured <filename role="special">hgwebdir.cgi</filename> to
	look.</para>

      <sect3>
	<title>Explicitly specifying which repositories to
	  publish</title>

	<para id="x_4df">In addition to the <literal>collections</literal>
	  mechanism, the <filename
	    role="special">hgwebdir.cgi</filename> script allows you
	  to publish a specific list of repositories.  To do so,
	  create a <literal>paths</literal> section, with contents of
	  the following form.</para>
	<programlisting>[paths]
repo1 = /my/path/to/some/repo
repo2 = /some/path/to/another</programlisting>
	<para id="x_4e0">In this case, the virtual path (the component that will
	  appear in a URL) is on the left hand side of each
	  definition, while the path to the repository is on the
	  right.  Notice that there does not need to be any
	  relationship between the virtual path you choose and the
	  location of a repository in your filesystem.</para>

	<para id="x_4e1">If you wish, you can use both the
	  <literal>collections</literal> and <literal>paths</literal>
	  mechanisms simultaneously in a single configuration
	  file.</para>

	<note>
	  <title>Beware duplicate virtual paths</title>

	  <para id="x_4e2">  If several repositories have the same
	    virtual path, <filename
	      role="special">hgwebdir.cgi</filename> will not report
	    an error.  Instead, it will behave unpredictably.</para>
	</note>
      </sect3>
    </sect2>

    <sect2>
      <title>Downloading source archives</title>

      <para id="x_4e3">Mercurial's web interface lets users download an archive
	of any revision.  This archive will contain a snapshot of the
	working directory as of that revision, but it will not contain
	a copy of the repository data.</para>

      <para id="x_4e4">By default, this feature is not enabled.  To enable it,
	you'll need to add an <envar
	  role="rc-item-web">allow_archive</envar> item to the
	<literal role="rc-web">web</literal> section of your <filename
	  role="special">~/.hgrc</filename>; see below for details.</para>
    </sect2>
    <sect2>
      <title>Web configuration options</title>

      <para id="x_4e5">Mercurial's web interfaces (the <command role="hg-cmd">hg
	  serve</command> command, and the <filename
	  role="special">hgweb.cgi</filename> and <filename
	  role="special">hgwebdir.cgi</filename> scripts) have a
	number of configuration options that you can set.  These
	belong in a section named <literal
	  role="rc-web">web</literal>.</para>
      <itemizedlist>
	<listitem><para id="x_4e6"><envar
	      role="rc-item-web">allow_archive</envar>: Determines
	    which (if any) archive download mechanisms Mercurial
	    supports.  If you enable this feature, users of the web
	    interface will be able to download an archive of whatever
	    revision of a repository they are viewing. To enable the
	    archive feature, this item must take the form of a
	    sequence of words drawn from the list below.</para>
	  <itemizedlist>
	    <listitem><para id="x_4e7"><literal>bz2</literal>: A
		<command>tar</command> archive, compressed using
		<literal>bzip2</literal> compression.  This has the
		best compression ratio, but uses the most CPU time on
		the server.</para>
	    </listitem>
	    <listitem><para id="x_4e8"><literal>gz</literal>: A
		<command>tar</command> archive, compressed using
		<literal>gzip</literal> compression.</para>
	    </listitem>
	    <listitem><para id="x_4e9"><literal>zip</literal>: A
		<command>zip</command> archive, compressed using LZW
		compression.  This format has the worst compression
		ratio, but is widely used in the Windows world.</para>
	    </listitem>
	  </itemizedlist>
	  <para id="x_4ea">  If you provide an empty list, or don't have an
	    <envar role="rc-item-web">allow_archive</envar> entry at
	    all, this feature will be disabled.  Here is an example of
	    how to enable all three supported formats.</para>
	  <programlisting>[web]
allow_archive = bz2 gz zip</programlisting>
	</listitem>
	<listitem><para id="x_4eb"><envar role="rc-item-web">allowpull</envar>:
	    Boolean.  Determines whether the web interface allows
	    remote users to <command role="hg-cmd">hg pull</command>
	    and <command role="hg-cmd">hg clone</command> this
	    repository over HTTP.  If set to <literal>no</literal> or
	    <literal>false</literal>, only the
	    <quote>human-oriented</quote> portion of the web interface
	    is available.</para>
	</listitem>
	<listitem><para id="x_4ec"><envar role="rc-item-web">contact</envar>:
	    String.  A free-form (but preferably brief) string
	    identifying the person or group in charge of the
	    repository.  This often contains the name and email
	    address of a person or mailing list.  It often makes sense
	    to place this entry in a repository's own <filename
	      role="special">.hg/hgrc</filename> file, but it can make
	    sense to use in a global <filename
	      role="special">~/.hgrc</filename> if every repository
	    has a single maintainer.</para>
	</listitem>
	<listitem><para id="x_4ed"><envar role="rc-item-web">maxchanges</envar>:
	    Integer.  The default maximum number of changesets to
	    display in a single page of output.</para>
	</listitem>
	<listitem><para id="x_4ee"><envar role="rc-item-web">maxfiles</envar>:
	    Integer.  The default maximum number of modified files to
	    display in a single page of output.</para>
	</listitem>
	<listitem><para id="x_4ef"><envar role="rc-item-web">stripes</envar>:
	    Integer.  If the web interface displays alternating
	    <quote>stripes</quote> to make it easier to visually align
	    rows when you are looking at a table, this number controls
	    the number of rows in each stripe.</para>
	</listitem>
	<listitem><para id="x_4f0"><envar
	      role="rc-item-web">style</envar>: Controls the template
	    Mercurial uses to display the web interface.  Mercurial
	    ships with several web templates.</para>
	  <itemizedlist>
	    <listitem>
	      <para id="x_6aa"><literal>coal</literal> is monochromatic.</para>
	    </listitem>
	    <listitem>
	      <para id="x_6ab"><literal>gitweb</literal> emulates the visual
		style of git's web interface.</para>
	    </listitem>
	    <listitem>
	      <para id="x_6ac"><literal>monoblue</literal> uses solid blues and
		greys.</para>
	    </listitem>
	    <listitem>
	      <para id="x_6ad"><literal>paper</literal> is the default.</para>
	    </listitem>
	    <listitem>
	      <para id="x_6ae"><literal>spartan</literal> was the default for a
		long time.</para>
	    </listitem>
	  </itemizedlist>
	  <para id="x_6af">You can
	    also specify a custom template of your own; see 
	    <xref linkend="chap:template"/> for details. Here, you can
	    see how to enable the <literal>gitweb</literal>
	    style.</para>
	  <programlisting>[web]
style = gitweb</programlisting>
	</listitem>
	<listitem><para id="x_4f1"><envar role="rc-item-web">templates</envar>:
	    Path.  The directory in which to search for template
	    files.  By default, Mercurial searches in the directory in
	    which it was installed.</para>
	</listitem></itemizedlist>
      <para id="x_4f2">If you are using <filename
	  role="special">hgwebdir.cgi</filename>, you can place a few
	configuration items in a <literal role="rc-web">web</literal>
	section of the <filename
	  role="special">hgweb.config</filename> file instead of a
	<filename role="special">~/.hgrc</filename> file, for
	convenience.  These items are <envar
	  role="rc-item-web">motd</envar> and <envar
	  role="rc-item-web">style</envar>.</para>

      <sect3>
	<title>Options specific to an individual repository</title>

	<para id="x_4f3">A few <literal role="rc-web">web</literal> configuration
	  items ought to be placed in a repository's local <filename
	    role="special">.hg/hgrc</filename>, rather than a user's
	  or global <filename role="special">~/.hgrc</filename>.</para>
	<itemizedlist>
	  <listitem><para id="x_4f4"><envar
		role="rc-item-web">description</envar>: String.  A
	      free-form (but preferably brief) string that describes
	      the contents or purpose of the repository.</para>
	  </listitem>
	  <listitem><para id="x_4f5"><envar role="rc-item-web">name</envar>:
	      String.  The name to use for the repository in the web
	      interface.  This overrides the default name, which is
	      the last component of the repository's path.</para>
	  </listitem></itemizedlist>
      </sect3>

      <sect3>
	<title>Options specific to the <command role="hg-cmd">hg
	    serve</command> command</title>

	<para id="x_4f6">Some of the items in the <literal
	    role="rc-web">web</literal> section of a <filename
	    role="special">~/.hgrc</filename> file are only for use
	  with the <command role="hg-cmd">hg serve</command>
	  command.</para>
	<itemizedlist>
	  <listitem><para id="x_4f7"><envar role="rc-item-web">accesslog</envar>:
	      Path.  The name of a file into which to write an access
	      log.  By default, the <command role="hg-cmd">hg
		serve</command> command writes this information to
	      standard output, not to a file.  Log entries are written
	      in the standard <quote>combined</quote> file format used
	      by almost all web servers.</para>
	  </listitem>
	  <listitem><para id="x_4f8"><envar role="rc-item-web">address</envar>:
	      String.  The local address on which the server should
	      listen for incoming connections.  By default, the server
	      listens on all addresses.</para>
	  </listitem>
	  <listitem><para id="x_4f9"><envar role="rc-item-web">errorlog</envar>:
	      Path.  The name of a file into which to write an error
	      log.  By default, the <command role="hg-cmd">hg
		serve</command> command writes this information to
	      standard error, not to a file.</para>
	  </listitem>
	  <listitem><para id="x_4fa"><envar role="rc-item-web">ipv6</envar>:
	      Boolean.  Whether to use the IPv6 protocol. By default,
	      IPv6 is not used.</para>
	  </listitem>
	  <listitem><para id="x_4fb"><envar role="rc-item-web">port</envar>:
	      Integer.  The TCP port number on which the server should
	      listen.  The default port number used is 8000.</para>
	  </listitem></itemizedlist>
      </sect3>

      <sect3>
	<title>Choosing the right <filename
	    role="special">~/.hgrc</filename> file to add <literal
	    role="rc-web">web</literal> items to</title>

	<para id="x_4fc">It is important to remember that a web server like
	  Apache or <literal>lighttpd</literal> will run under a user
	  ID that is different to yours. CGI scripts run by your
	  server, such as <filename
	    role="special">hgweb.cgi</filename>, will usually also run
	  under that user ID.</para>

	<para id="x_4fd">If you add <literal role="rc-web">web</literal> items to
	  your own personal <filename role="special">~/.hgrc</filename> file, CGI scripts won't read that
	  <filename role="special">~/.hgrc</filename> file.  Those
	  settings will thus only affect the behavior of the <command
	    role="hg-cmd">hg serve</command> command when you run it.
	  To cause CGI scripts to see your settings, either create a
	  <filename role="special">~/.hgrc</filename> file in the
	  home directory of the user ID that runs your web server, or
	  add those settings to a system-wide <filename
	    role="special">hgrc</filename> file.</para>
      </sect3>
    </sect2>
  </sect1>

  <sect1>
    <title>System-wide configuration</title>

    <para id="x_6b0">On Unix-like systems shared by multiple users (such as a
      server to which people publish changes), it often makes sense to
      set up some global default behaviors, such as what theme to use
      in web interfaces.</para>

    <para id="x_6b1">If a file named <filename>/etc/mercurial/hgrc</filename>
      exists, Mercurial will read it at startup time and apply any
      configuration settings it finds in that file.  It will also look
      for files ending in a <literal>.rc</literal> extension in a
      directory named <filename>/etc/mercurial/hgrc.d</filename>, and
      apply any configuration settings it finds in each of those
      files.</para>

    <sect2>
      <title>Making Mercurial more trusting</title>

      <para id="x_6b2">One situation in which a global <filename>hgrc</filename>
	can be useful is if users are pulling changes owned by other
	users.  By default, Mercurial will not trust most of the
	configuration items in a <filename>.hg/hgrc</filename> file
	inside a repository that is owned by a different user. If we
	clone or pull changes from such a repository, Mercurial will
	print a warning stating that it does not trust their
	<filename>.hg/hgrc</filename>.</para>

      <para id="x_6b3">If everyone in a particular Unix group is on the same team
	and <emphasis>should</emphasis> trust each other's
	configuration settings, or we want to trust particular users,
	we can override Mercurial's skeptical defaults by creating a
	system-wide <filename>hgrc</filename> file such as the
	following:</para>

    <programlisting># Save this as e.g. /etc/mercurial/hgrc.d/trust.rc
[trusted]
# Trust all entries in any hgrc file owned by the "editors" or
# "www-data" groups.
groups = editors, www-data

# Trust entries in hgrc files owned by the following users.
users = apache, bobo
</programlisting>
    </sect2>
  </sect1>
</chapter>

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