Extending Bazaar with Hooks and Plugins
=======================================

Bazaar offers a powerful extension mechanism for adding capabilities.  In
addition to offering full library API access to all of its structures, which
can be useful for outside programs that would like to interact with Bazaar
branches, Bazaar can also load *plugins* that perform specific tasks.  These
specific tasks are specified by *hooks* that run during certain steps of the
version control process.  

For full documentation on the available hooks, see ``bzr help hooks``.  Among
those, some of the most significant hooks from an administration
standpoint are `pre_commit`, `post_commit` and `post_change_branch_tip`.
A `pre_commit` hook can inspect a commit before it happens and cancel it if
some criteria are not met.  This can be useful for enforcing policies about
the code, such as line-endings or whitespace conventions.  A
`post_commit` hook can take actions based on the commit that just happened,
such as providing various types of notifications.  Finally, a
`post_change_branch_tip` hook is a more general form of a `post_commit`
hook which is used whenever the tip of a branch changes (which can happen in
more ways than just committing).  This too can be used for notification
purposes, as well as for backups and mirroring.

Information on the whole range of Bazaar plugins is available at
http://doc.bazaar.canonical.com/plugins/en/.  For purposes of installation,
plugins are simply python packages.  They can be installed alongside Bazaar in
the ``bzrlib.plugins`` package using each plugin's ``setup.py``.  They can
also be installed in the plugin path which is the user's
``~/.config/breezy/plugins`` directory or can be specified with the
``BZR_PLUGIN_PATH`` environment variable.  See ``bzr help configuration`` for
more on specifying the location of plugins.


Email Notification
------------------

A common need is for every change made on a branch to send an email message to
some address, most often a mailing list.  These plugins provide that capability
in a number of different ways.  

The `email` plugin sends email from each individual developer's computer.  This
can be useful for situations that want to track what each individual developer
is working on.  On the downside, it requires that every developer's branches be
configured individually to use the same plugin.  

The next two plugins `hookless-email` and `email-notifier` address this concern
by running on a central server whenever changes happen on centrally stored
branches.

email
~~~~~

To configure this plugin, simply install the plugin and configure the
``post_commit_to``  option for each branch.  This configuration can be done
in the ``locations.conf`` file or individually in each branch's
``branch.conf`` file.  The sender's email address can be specified as
``post_commit_sender`` if it is different than the email address reported by
``bzr whoami``.  The ``post_commit_mailer`` option specifies how the
mail should be sent.  If it isn't set, email is sent via ``/usr/bin/mail``.
It can also be configured to communicate directly with an SMTP server.
For more details on configuring this plugin, see
http://doc.bazaar.canonical.com/plugins/en/email-plugin.html.  As examples,
consider the following two possible configurations.  A minimal one (uses
``/usr/bin/mail``)

::

  [DEFAULT]
  post_commit_to = projectx-commits@example.com

and a more complicated one (using all of the options)

::

  [DEFAULT]
  post_commit_url = http://www.example.com/code/projectx/trunk
  post_commit_to = projectx-commits@example.com
  post_commit_sender = donotreply@example.com
  post_commit_mailer = smtplib
  smtp_server = mail.example.com:587
  smtp_username = bob
  # smtp_password = 'not specified, will prompt'


hookless-email
~~~~~~~~~~~~~~

This plugin is basically a server-side version of the `email` plugin.  It is
a program that runs either from the command line or as a daemon that monitors
the branches specified on the command line for any changes.  When a change
occurs to any of the monitored branches, it will send an email to the
specified address.  Using our simple example, the following command would send
an email to ``projectx-commits@example.com`` on any of the branches under
``/srv/bzr`` since the last time the command was run.  (This command could be
set up to run at regular intervals, for example from ``cron``.)

::

  $ brz_hookless_email.py --email=projectx-commits@example.com \
  --recurse /srv/bzr

email-notifier
~~~~~~~~~~~~~~

This is a more elaborate version of the `hookless-email` plugin that can send
templated HTML emails, render wiki-style markup in commit messages and update
working copies on the server (similar to `push_and_update`_).  It can also
send emails reporting the creation of new branches or the removal of branches
under a specified directory (here ``/srv/bzr/projectx``).  As it is more
complicated, its configuration is also more complicated and we won't repeat
its documentation here, but a simple configuration that will send emails on
commits and creation/deletion of branches is

::

  [smtp]

  server=smtp.example.com
  # If user is not provided then no authentication will be performed.
  user=bob
  password=pAssW0rd

  [commits]

  # The address to send commit emails to. 
  to=projctx-commits@example.com
  from=$revision.committer

  # A Cheetah template used to construct the subject of the email message.
  subject=$relative_path: $revision_number $summary

  [new-branches]
  to=projectx-commits@example.com
  from=donotreply@example.com
  subject=$relative_path: New branch created

  [removed-branches]
  to=projectx-commits@example.com
  from=donotreply@example.com
  subject=$relative_path: Branch removed

If this file is stored as ``/srv/bzr/email-notifier.conf``, then the command

::
 
  $ brz-email-notifier.py --config=/srv/bzr/email-notifier.conf /srv/bzr/projectx

will watch all branches under the given directory for commits, branch
creations and branch deletions.
  

Feed Generation
---------------

A related concept to sending out emails when branches change is the generation
of news feeds from changes on each branch.  Interested parties can then choose
to follow those news feeds in order to see what is happening on a branch.

branchfeed
~~~~~~~~~~

This plugin creates an ATOM feed for every branch on every branch change
(commit, etc.).  It stores these files as ``.bzr/branch/branch.atom`` inside
each branch.  Currently, it includes the 20 most recent changes in each feed.
To use it, simply install the plugin and set your feed reader to follow the
``branch.atom`` files.

In addition, there are other tools that are not plugins for creating news
feeds from Bazaar branches.  See
http://wiki.bazaar.canonical.com/FeedGenerators for more on those tools.

Mirroring
---------

Sometimes it is useful to ensure that one branch exists as an exact copy of
another.  This can be used to provide simple backup facilities or redundancy
(see `Back-up and restore <backup.html>`_ for more details on backups).  One
way to do this using Bazaar's workflows is to make the branch where changes
happen into a bound branch of the mirror branch.  Then, when commits happen on
the working branch, they will also happen on the mirror branch.  Note that
commits to bound branches do *not* update the mirror branch's working copy, so
if the mirror branch is more than just a backup of the complete history of the
branch, for example if it is being served as a web page, then additional
plugins are necessary.

push_and_update
~~~~~~~~~~~~~~~

This plugin updates Bazaar's ``push`` command to also update the remote
working copy.  It can only work over connections that imply filesystem or SSH
access to the remote working copy (``bzr+ssh://``, ``sftp://`` and
``file://``).  Also, it is only useful when the remote branch is updated with
an explicit ``push`` command.

automirror
~~~~~~~~~~

This plugin is similar to `push_and_update` in that it updates the working
copy of a remote branch.  The difference is that this plugin is designed to
update the remote branch on every change to the working branch.  To configure
this, set the ``post_commit_mirror = URL`` option on a branch.  This option
can include multiple branch URLs separated by commas to create multiple
mirrors.  For example, if we want to mirror our ``/srv/bzr/projectx/trunk``
branch to the URL ``sftp://www.example.com/var/www/projectx`` (for example if
ProjectX were a web project that we wanted to access at
``http://www.example.com/projectx``), then we could include

::
 
  [DEFAULT]
  post_commit_mirror = sftp://www.example.com/var/www/branches/trunk

in the file ``/srv/bzr/projectx/trunk/.bzr/branch/branch.conf``.


Other Useful Plugins
--------------------

pqm (plugin)
~~~~~~~~~~~~

Facilitating interaction with `PQM
<integration.html#patch-queue-manager-pqm>`_, this plugin provides support for 
submitting merge requests to a remote Patch Queue Manager.  PQM provides 
a way to automatically run the test suite before merging changes to the
trunk branch.

testrunner
~~~~~~~~~~

Sometimes referred to as the poor man's PQM, this plugin runs a single command
on the updated revision (in a temporary directory) and if the command returns
0, then the revision can be committed to that branch.  For example, if the
testsuite is run with the command ``nosetests`` in the root of the branch
(which returns 0 if the test suite passes and 1 if it doesn't pass), then one
can set 

::

  [DEFAULT]
  pre_change_branch_tip_test_command = nosetests

in ``.bzr/branch/branch.conf``.

checkeol
~~~~~~~~

This plugin is an example of a `pre_commit` hook that checks the revision
being committed for meeting some policy.  In this case, it checks that all of
the files have the specified line endings.  It uses a configuration file
``.bzreol`` in the root of the working tree (similar to the ``.bzrignore``
file).  This configuration file has sections for line feed endings (LF),
carriage return/line-feed endings (CRLF) and carriage return endings (CR).
For an unusual example that specifies different line endings for different
files, that file might look like

:: 

  [LF]
  *.py
  *.[ch]

  [CRLF]
  *.txt
  *.ini

  [CR]
  foo.mac

or if you simply want to enforce a single line ending convention on the branch
you can use

::
  
  [LF]
  *

This plugin needs to be installed on the server where the branch updates will
happen, and the ``.bzreol`` file must be in each branch where line ending
policies will be enforced.  (Adding it to the branch with ``bzr add .bzreol``
is an easy way to ensure this, although it means that branches on the server
must have working trees.)

text_checker
~~~~~~~~~~~~

This plugin is a more advanced version of `checkeol` that can check such
coding style guidelines such as trailing whitespace, long lines and files that
don't end with a newline.  It is configured using Bazaar's built in rules
specification in ``BZR_HOME/rules`` (see ``bzr help rules`` for more
information.  For different types of undesired changes, you can specify
different types of actions.  For example

::

  [name NEWS README]
  trailing_whitespace=fail
  long_lines=warn
  newline_at_eof=ignore

  [name *.py]
  tabs=fail
  long_line_length=78
  long_lines=fail
  trailing_whitespace=fail

will prevent changes from adding new trailing whitespace to the specified
files and keep all python source files free of tabs and lines over 78
characters.  To commit while violating these rules, one can pass the
``--text-check-warn-only`` option to commit.
