~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://people.canonical.com/~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