~bzr-pqm/bzr/bzr.dev

======================
Bazaar Developer Guide
======================

This document describes the Bazaar internals and the development process.
It's meant for people interested in developing Bazaar, and some parts will
also be useful to people developing Bazaar plugins.

If you have any questions or something seems to be incorrect, unclear or
missing, please talk to us in ``irc://irc.freenode.net/#bzr``, or write to
the Bazaar mailing list.  To propose a correction or addition to this
document, send a merge request or new text to the mailing list.

The latest developer documentation can be found online at
http://doc.bazaar.canonical.com/developers/.

Getting Started
###############

Exploring the Bazaar Platform
=============================

Before making changes, it's a good idea to explore the work already
done by others. Perhaps the new feature or improvement you're looking
for is available in another plug-in already? If you find a bug,
perhaps someone else has already fixed it?

To answer these questions and more, take a moment to explore the
overall Bazaar Platform. Here are some links to browse:

* The Plugins page on the Wiki - http://wiki.bazaar.canonical.com/BzrPlugins

* The Bazaar product family on Launchpad - https://launchpad.net/bazaar

* Bug Tracker for the core product - https://bugs.launchpad.net/bzr/

If nothing else, perhaps you'll find inspiration in how other developers
have solved their challenges.

Finding Something To Do
=======================

Ad-hoc performance work can also be done. One useful tool is the 'evil' debug
flag. For instance running ``bzr -Devil commit -m "test"`` will log a backtrace
to the bzr log file for every method call which triggers a slow or non-scalable
part of the bzr library. So checking that a given command with ``-Devil`` has
no backtraces logged to the log file is a good way to find problem function
calls that might be nested deep in the code base.

Planning and Discussing Changes
===============================

There is a very active community around Bazaar. Mostly we meet on IRC
(#bzr on irc.freenode.net) and on the mailing list. To join the Bazaar
community, see http://wiki.bazaar.canonical.com/BzrSupport.

If you are planning to make a change, it's a very good idea to mention it
on the IRC channel and/or on the mailing list. There are many advantages
to involving the community before you spend much time on a change.
These include:

* you get to build on the wisdom of others, saving time

* if others can direct you to similar code, it minimises the work to be done

* it assists everyone in coordinating direction, priorities and effort.

In summary, maximising the input from others typically minimises the
total effort required to get your changes merged. The community is
friendly, helpful and always keen to welcome newcomers.


Bazaar Development in a Nutshell
================================

.. was from http://wiki.bazaar.canonical.com/BzrGivingBack

One of the fun things about working on a version control system like Bazaar is
that the users have a high level of proficiency in contributing back into
the tool.  Consider the following very brief introduction to contributing back
to Bazaar.  More detailed instructions are in the following sections.

Making the change
-----------------

First, get a local copy of the development mainline (See `Why make a local
copy of bzr.dev?`_.)
::

 $ bzr init-repo ~/bzr
 $ cd ~/bzr
 $ bzr branch lp:bzr bzr.dev

Now make your own branch::

 $ bzr branch bzr.dev 123456-my-bugfix

This will give you a branch called "123456-my-bugfix" that you can work on
and commit in. Here, you can study the code, make a fix or a new feature.
Feel free to commit early and often (after all, it's your branch!).

Documentation improvements are an easy place to get started giving back to the
Bazaar project.  The documentation is in the `doc/` subdirectory of the Bazaar
source tree.

When you are done, make sure that you commit your last set of changes as well!
Once you are happy with your changes, ask for them to be merged, as described
below.

Making a Merge Proposal
-----------------------

The Bazaar developers use Launchpad to further enable a truly distributed
style of development.  Anyone can propose a branch for merging into the Bazaar
trunk.  To start this process, you need to push your branch to Launchpad.  To
do this, you will need a Launchpad account and user name, e.g.
`your_lp_username`.  You can push your branch to Launchpad directly from
Bazaar::

  $ bzr push lp:~<your_lp_username>/bzr/meaningful_name_here

After you have pushed your branch, you will need to propose it for merging to
the Bazaar trunk.  Go to
<https://launchpad.net/~<your_lp_username>/bzr/meaningful_name_here> and choose
"Propose for merging into another branch".  Select "lp:bzr" to hand
your changes off to the Bazaar developers for review and merging.

Alternatively, after pushing you can use the ``lp-propose`` command to 
create the merge proposal.

Using a meaningful name for your branch will help you and the reviewer(s)
better track the submission. Use a very succint description of your submission
and prefix it with bug number if needed (lp:~mbp/bzr/484558-merge-directory
for example). Alternatively, you can suffix with the bug number
(lp:~jameinel/bzr/export-file-511987).


Review cover letters
--------------------

Please put a "cover letter" on your merge request explaining:

* the reason **why** you're making this change

* **how** this change achieves this purpose

* anything else you may have fixed in passing

* anything significant that you thought of doing, such as a more
  extensive fix or a different approach, but didn't or couldn't do now

A good cover letter makes reviewers' lives easier because they can decide
from the letter whether they agree with the purpose and approach, and then
assess whether the patch actually does what the cover letter says.
Explaining any "drive-by fixes" or roads not taken may also avoid queries
from the reviewer.  All in all this should give faster and better reviews.
Sometimes writing the cover letter helps the submitter realize something
else they need to do.  The size of the cover letter should be proportional
to the size and complexity of the patch.


Why make a local copy of bzr.dev?
---------------------------------

Making a local mirror of bzr.dev is not strictly necessary, but it means

- You can use that copy of bzr.dev as your main bzr executable, and keep it
  up-to-date using ``bzr pull``.
- Certain operations are faster, and can be done when offline.  For example:

  - ``bzr bundle``
  - ``bzr diff -r ancestor:...``
  - ``bzr merge``

- When it's time to create your next branch, it's more convenient.  When you
  have further contributions to make, you should do them in their own branch::

    $ cd ~/bzr
    $ bzr branch bzr.dev additional_fixes
    $ cd additional_fixes # hack, hack, hack



Understanding the Development Process
=====================================

The development team follows many practices including:

* a public roadmap and planning process in which anyone can participate

* time based milestones everyone can work towards and plan around

* extensive code review and feedback to contributors

* complete and rigorous test coverage on any code contributed

* automated validation that all tests still pass before code is merged
  into the main code branch.

The key tools we use to enable these practices are:

* Launchpad - https://launchpad.net/

* Bazaar - http://bazaar.canonical.com/

* Patch Queue Manager - https://launchpad.net/pqm/

For further information, see <http://wiki.bazaar.canonical.com/BzrDevelopment>.




Preparing a Sandbox for Making Changes to Bazaar
================================================

Bazaar supports many ways of organising your work. See
http://wiki.bazaar.canonical.com/SharedRepositoryLayouts for a summary of the
popular alternatives.

Of course, the best choice for you will depend on numerous factors:
the number of changes you may be making, the complexity of the changes, etc.
As a starting suggestion though:

* create a local copy of the main development branch (bzr.dev) by using
  this command::

    bzr branch lp:bzr bzr.dev

* keep your copy of bzr.dev pristine (by not developing in it) and keep
  it up to date (by using bzr pull)

* create a new branch off your local bzr.dev copy for each issue
  (bug or feature) you are working on.

This approach makes it easy to go back and make any required changes
after a code review. Resubmitting the change is then simple with no
risk of accidentally including edits related to other issues you may
be working on. After the changes for an issue are accepted and merged,
the associated branch can be deleted or archived as you wish.


Navigating the Code Base
========================

.. Was at <http://wiki.bazaar.canonical.com/NewDeveloperIntroduction>

Some of the key files in this directory are:

bzr
    The command you run to start Bazaar itself.  This script is pretty
    short and just does some checks then jumps into bzrlib.

README
    This file covers a brief introduction to Bazaar and lists some of its
    key features.

setup.py
    Installs Bazaar system-wide or to your home directory.  To perform
    development work on Bazaar it is not required to run this file - you
    can simply run the bzr command from the top level directory of your
    development copy. Note: That if you run setup.py this will create a
    'build' directory in your development branch. There's nothing wrong
    with this but don't be confused by it. The build process puts a copy
    of the main code base into this build directory, along with some other
    files. You don't need to go in here for anything discussed in this
    guide.

bzrlib
    Possibly the most exciting folder of all, bzrlib holds the main code
    base. This is where you will go to edit python files and contribute to
    Bazaar.

doc
    Holds documentation on a whole range of things on Bazaar from the
    origination of ideas within the project to information on Bazaar
    features and use cases.  Within this directory there is a subdirectory
    for each translation into a human language.  All the documentation
    is in the ReStructuredText markup language.

doc/developers
    Documentation specifically targeted at Bazaar and plugin developers.
    (Including this document.)

doc/en/release-notes/

    Detailed changes in each Bazaar release (there is one file by series:
    bzr-2.3.txt, bzr-2.4.txt, etc) that can affect users or plugin
    developers.

doc/en/whats-new/

    High-level summaries of changes in each Bazaar release (there is one
    file by series: whats-new-in-2.3.txt, whats-new-in-2.4.txt, etc).


Automatically-generated API reference information is available at
<http://starship.python.net/crew/mwh/bzrlibapi/>.

See also the `Bazaar Architectural Overview
<http://doc.bazaar.canonical.com/developers/overview.html>`_.


Core Topics
###########

Evolving Interfaces
===================

We don't change APIs in stable branches: any supported symbol in a stable
release of bzr must not be altered in any way that would result in
breaking existing code that uses it. That means that method names,
parameter ordering, parameter names, variable and attribute names etc must
not be changed without leaving a 'deprecated forwarder' behind. This even
applies to modules and classes.

If you wish to change the behaviour of a supported API in an incompatible
way, you need to change its name as well. For instance, if I add an optional keyword
parameter to branch.commit - that's fine. On the other hand, if I add a
keyword parameter to branch.commit which is a *required* transaction
object, I should rename the API - i.e. to 'branch.commit_transaction'.

  (Actually, that may break code that provides a new implementation of
  ``commit`` and doesn't expect to receive the parameter.)

When renaming such supported API's, be sure to leave a deprecated_method (or
_function or ...) behind which forwards to the new API. See the
bzrlib.symbol_versioning module for decorators that take care of the
details for you - such as updating the docstring, and issuing a warning
when the old API is used.

For unsupported API's, it does not hurt to follow this discipline, but it's
not required. Minimally though, please try to rename things so that
callers will at least get an AttributeError rather than weird results.


Deprecation decorators
----------------------

``bzrlib.symbol_versioning`` provides decorators that can be attached to
methods, functions, and other interfaces to indicate that they should no
longer be used.  For example::

   @deprecated_method(deprecated_in((0, 1, 4)))
   def foo(self):
        return self._new_foo()

To deprecate a static method you must call ``deprecated_function``
(**not** method), after the staticmethod call::

    @staticmethod
    @deprecated_function(deprecated_in((0, 1, 4)))
    def create_repository(base, shared=False, format=None):

When you deprecate an API, you should not just delete its tests, because
then we might introduce bugs in them.  If the API is still present at all,
it should still work.  The basic approach is to use
``TestCase.applyDeprecated`` which in one step checks that the API gives
the expected deprecation message, and also returns the real result from
the method, so that tests can keep running.

Deprecation warnings will be suppressed for final releases, but not for
development versions or release candidates, or when running ``bzr
selftest``. This gives developers information about whether their code is
using deprecated functions, but avoids confusing users about things they
can't fix.


General Guidelines
==================

Copyright
---------

The copyright policy for bzr was recently made clear in this email (edited
for grammatical correctness)::

    The attached patch cleans up the copyright and license statements in
    the bzr source. It also adds tests to help us remember to add them
    with the correct text.

    We had the problem that lots of our files were "Copyright Canonical
    Development Ltd" which is not a real company, and some other variations
    on this theme. Also, some files were missing the GPL statements.

    I want to be clear about the intent of this patch, since copyright can
    be a little controversial.

    1) The big motivation for this is not to shut out the community, but
    just to clean up all of the invalid copyright statements.

    2) It has been the general policy for bzr that we want a single
    copyright holder for all of the core code. This is following the model
    set by the FSF, which makes it easier to update the code to a new
    license in case problems are encountered. (For example, if we want to
    upgrade the project universally to GPL v3 it is much simpler if there is
    a single copyright holder). It also makes it clearer if copyright is
    ever debated, there is a single holder, which makes it easier to defend
    in court, etc. (I think the FSF position is that if you assign them
    copyright, they can defend it in court rather than you needing to, and
    I'm sure Canonical would do the same).
    As such, Canonical has requested copyright assignments from all of the
    major contributers.

    3) If someone wants to add code and not attribute it to Canonical, there
    is a specific list of files that are excluded from this check. And the
    test failure indicates where that is, and how to update it.

    4) If anyone feels that I changed a copyright statement incorrectly, just
    let me know, and I'll be happy to correct it. Whenever you have large
    mechanical changes like this, it is possible to make some mistakes.

    Just to reiterate, this is a community project, and it is meant to stay
    that way. Core bzr code is copyright Canonical for legal reasons, and
    the tests are just there to help us maintain that.


Miscellaneous Topics
####################

Debugging
=========

Bazaar has a few facilities to help debug problems by going into pdb_, the
Python debugger.

.. _pdb: http://docs.python.org/lib/debugger-commands.html

If the ``BZR_PDB`` environment variable is set
then bzr will go into pdb post-mortem mode when an unhandled exception
occurs.

If you send a SIGQUIT or SIGBREAK signal to bzr then it will drop into the
debugger immediately. SIGQUIT can be generated by pressing Ctrl-\\ on
Unix.  SIGBREAK is generated with Ctrl-Pause on Windows (some laptops have
this as Fn-Pause).  You can continue execution by typing ``c``.  This can
be disabled if necessary by setting the environment variable
``BZR_SIGQUIT_PDB=0``.

All tests inheriting from bzrlib.tests.TestCase can use ``self.debug()``
instead of the longer ``import pdb; pdb.set_trace()``. The former also works
when ``stdin/stdout`` are redirected (by using the original ``stdin/stdout``
file handles at the start of the ``bzr`` script) while the later doesn't.
``bzrlib.debug.set_trace()`` also uses the original ``stdin/stdout`` file
handles.

Debug Flags
===========

Bazaar accepts some global options starting with ``-D`` such as
``-Dhpss``.  These set a value in `bzrlib.debug.debug_flags`, and
typically cause more information to be written to the trace file.  Most
`mutter` calls should be guarded by a check of those flags so that we
don't write out too much information if it's not needed.

Debug flags may have effects other than just emitting trace messages.

Run ``bzr help global-options`` to see them all.

These flags may also be set as a comma-separated list in the
``debug_flags`` option in e.g.  ``~/.bazaar/bazaar.conf``.  (Note that it
must be in this global file, not in the branch or location configuration,
because it's currently only loaded at startup time.)  For instance you may
want to always record hpss traces and to see full error tracebacks::

    debug_flags = hpss, error


Jargon
======

revno
    Integer identifier for a revision on the main line of a branch.
    Revision 0 is always the null revision; others are 1-based
    indexes into the branch's revision history.


Unicode and Encoding Support
============================

This section discusses various techniques that Bazaar uses to handle
characters that are outside the ASCII set.

``Command.outf``
----------------

When a ``Command`` object is created, it is given a member variable
accessible by ``self.outf``.  This is a file-like object, which is bound to
``sys.stdout``, and should be used to write information to the screen,
rather than directly writing to ``sys.stdout`` or calling ``print``.
This file has the ability to translate Unicode objects into the correct
representation, based on the console encoding.  Also, the class attribute
``encoding_type`` will effect how unprintable characters will be
handled.  This parameter can take one of 3 values:

  replace
    Unprintable characters will be represented with a suitable replacement
    marker (typically '?'), and no exception will be raised. This is for
    any command which generates text for the user to review, rather than
    for automated processing.
    For example: ``bzr log`` should not fail if one of the entries has text
    that cannot be displayed.

  strict
    Attempting to print an unprintable character will cause a UnicodeError.
    This is for commands that are intended more as scripting support, rather
    than plain user review.
    For example: ``bzr ls`` is designed to be used with shell scripting. One
    use would be ``bzr ls --null --unknowns | xargs -0 rm``.  If ``bzr``
    printed a filename with a '?', the wrong file could be deleted. (At the
    very least, the correct file would not be deleted). An error is used to
    indicate that the requested action could not be performed.

  exact
    Do not attempt to automatically convert Unicode strings. This is used
    for commands that must handle conversion themselves.
    For example: ``bzr diff`` needs to translate Unicode paths, but should
    not change the exact text of the contents of the files.


``bzrlib.urlutils.unescape_for_display``
----------------------------------------

Because Transports work in URLs (as defined earlier), printing the raw URL
to the user is usually less than optimal. Characters outside the standard
set are printed as escapes, rather than the real character, and local
paths would be printed as ``file://`` URLs. The function
``unescape_for_display`` attempts to unescape a URL, such that anything
that cannot be printed in the current encoding stays an escaped URL, but
valid characters are generated where possible.


C Extension Modules
===================

We write some extensions in C using pyrex. We design these to work in
three scenarios:

 * User with no C compiler
 * User with C compiler
 * Developers

The recommended way to install bzr is to have a C compiler so that the
extensions can be built, but if no C compiler is present, the pure python
versions we supply will work, though more slowly.

For developers we recommend that pyrex be installed, so that the C
extensions can be changed if needed.

For the C extensions, the extension module should always match the
original python one in all respects (modulo speed). This should be
maintained over time.

To create an extension, add rules to setup.py for building it with pyrex,
and with distutils. Now start with an empty .pyx file. At the top add
"include 'yourmodule.py'". This will import the contents of foo.py into this
file at build time - remember that only one module will be loaded at
runtime. Now you can subclass classes, or replace functions, and only your
changes need to be present in the .pyx file.

Note that pyrex does not support all 2.4 programming idioms, so some
syntax changes may be required. I.e.

 - 'from foo import (bar, gam)' needs to change to not use the brackets.
 - 'import foo.bar as bar' needs to be 'import foo.bar; bar = foo.bar'

If the changes are too dramatic, consider
maintaining the python code twice - once in the .pyx, and once in the .py,
and no longer including the .py file.


Making Installers for OS Windows
================================
To build a win32 installer, see the instructions on the wiki page:
http://wiki.bazaar.canonical.com/BzrWin32Installer

Core Developer Tasks
####################

Overview
========

What is a Core Developer?
-------------------------

While everyone in the Bazaar community is welcome and encouraged to
propose and submit changes, a smaller team is reponsible for pulling those
changes together into a cohesive whole. In addition to the general developer
stuff covered above, "core" developers have responsibility for:

* reviewing changes
* planning releases
* managing releases (see `Releasing Bazaar <http://doc.bazaar.canonical.com/developers/releasing.html>`_)

.. note::
  Removing barriers to community participation is a key reason for adopting
  distributed VCS technology. While DVCS removes many technical barriers,
  a small number of social barriers are often necessary instead.
  By documenting how the above things are done, we hope to
  encourage more people to participate in these activities, keeping the
  differences between core and non-core contributors to a minimum.


Communicating and Coordinating
------------------------------

While it has many advantages, one of the challenges of distributed
development is keeping everyone else aware of what you're working on.
There are numerous ways to do this:

#. Assign bugs to yourself in Launchpad
#. Mention it on the mailing list
#. Mention it on IRC

As well as the email notifcations that occur when merge requests are sent
and reviewed, you can keep others informed of where you're spending your
energy by emailing the **bazaar-commits** list implicitly. To do this,
install and configure the Email plugin. One way to do this is add these
configuration settings to your central configuration file (e.g.
``~/.bazaar/bazaar.conf``)::

  [DEFAULT]
  email = Joe Smith <joe.smith@internode.on.net>
  smtp_server = mail.internode.on.net:25

Then add these lines for the relevant branches in ``locations.conf``::

  post_commit_to = bazaar-commits@lists.canonical.com
  post_commit_mailer = smtplib

While attending a sprint, RobertCollins' Dbus plugin is useful for the
same reason. See the documentation within the plugin for information on
how to set it up and configure it.



Planning Releases
=================


Bug Triage
----------

Keeping on top of bugs reported is an important part of ongoing release
planning. Everyone in the community is welcome and encouraged to raise
bugs, confirm bugs raised by others, and nominate a priority. Practically
though, a good percentage of bug triage is often done by the core
developers, partially because of their depth of product knowledge.

With respect to bug triage, core developers are encouraged to play an
active role with particular attention to the following tasks:

* keeping the number of unconfirmed bugs low
* ensuring the priorities are generally right (everything as critical - or
  medium - is meaningless)
* looking out for regressions and turning those around sooner rather than later.

.. note::
  As well as prioritizing bugs and nominating them against a
  target milestone, Launchpad lets core developers offer to mentor others in
  fixing them.


..
   vim: ft=rst tw=74 ai