~bzr-pqm/bzr/bzr.dev

================================

Simplifying Bazaar Configuration

================================

Goal

====

Not all needs can be addressed by the default values used inside bzr and

bzrlib, no matter how well they are chosen (and they are ;).

Options that are rarely used don't deserve a corresponding command line

switch in one or several commands.

Many parts of ``bzrlib`` depends on some constants though and the user

should be able to customize the behavior to suit his needs so these

constants need to become configuration options or more generally, be easier

to set.

These options can be set from the command-line, acquired from an environment

variable or recorded in a configuration file.

To simplify writing (and reading), this document refers to the old and new

config designs:

* the old design is using ``Config`` as a base class for all config files,

* the new design use ``ConfigStacks`` of ``Section`` from config ``Stores``.

Current issues

==============

* Many parts of ``bzrlib`` declare constants and there is no way for the

  user to look at or modify them (see http://pad.lv/832061).

* The old design requires a configuration object to create, modify or delete

  a configuration option in a given configuration file.  ``bzr config``

  makes it almost transparent for the user. Internally though, not all cases

  are handled: only BranchConfig implements chained configs, nothing is

  provided at the repository level and too many plugins define their own

  section or even their own config file. (config.Stack now provides a way to

  chain config files, BranchStack properly implements the desired behavior,

  ``bzr config`` uses the new design).

* ``locations.conf`` defines the options that need to override any setting

  in ``branch.conf`` for both local and remotes branches (but some remote

  branch options just ignore ``locations.conf``). Many users want a way to

  define default values for options that are not defined in ``branch.conf``

  (and even more users think that ``locations.conf`` provide default values,

  see also http://pad.lv/843211 and http://pad.lv/832046). This could be

  approximated today by *not* defining these options in ``branch.conf`` but

  in ``locations.conf`` instead. This workaround doesn't allow a user to

  define defaults in ``locations.conf`` and override them in

  ``branch.conf``. (Allowing sections in 'bazaar.conf' (or introducing a new

  defaults.conf' allowing sections) can now address that. Defining and using

  a new file is easier as it avoids handling a migration path for

  bazaar.conf and doesn't require banning the use of sections for special

  purpose needs (ALIASES and BOOKMARKS for example)).

* Defining a new option requires adding a new method in the ``Config``

  object to get access to features like:

  * should the option be inherited by more specific sections, (this was more

    or less the default in the old design, it is addressed by section

    matchers in the new one).

  * should the inherited value append the relative path between the

    section one and the location it applies to (see http://pad.lv/832013),

  * the default value (including calling any python code that may be

    required to calculate this value)(see http://pad.lv/832064),

  * priority between sections in various config files (this is defined by

    the section matcher associated with a given config store for stacks,

    http://pad.lv/832046 is about adding a section matcher with clearer

    semantics than the one used for locations.conf).

  A related problem is that, in the actual implementation, some

  configuration options have defined methods, others don't and this is

  inconsistent. (Using only Stacks addresses that).

* Access to the 'active' configuration option value from the command line

  doesn't give access to the specific section. (This is only a concern if

  the user has no other way to address a specific configuration option

  including Store and Section when using ``bzr config``) (see http://pad.lv/725234).

* Rules for configuration options are not clearly defined for remote

  branches (they may differ between dumb and smart servers the former will

  use the local ``bazaar.conf`` and ``locations.conf`` files while the later

  will use (or ignore ?) the remote ones).

* The features offered by the Bazaar configuration files should be easily

  accessible to plugin authors either by supporting plugin configuration

  options in the configuration files or allowing the plugins to define their

  own configuration files. (Separating Section, Store and Stack starts

  addressing that, a stack registry should complete the needed means).

* While the actual configuration files support sections, they are used in

  mutually exclusive ways that make it impossible to offer the same set of

  features to all configuration files:

  * ``bazaar.conf`` use arbitrary names for sections. ``DEFAULT`` is used

    for global options, ``ALIASES`` are used to define command aliases,

    plugins can define their own sections, some plugins do that

    (``bzr-bookmarks`` use ``BOOKMARKS`` for example), some other define

    their own sections (this is addressed with the new design by using only

    the ``DEFAULT`` section and ignore the others. When needed, one can

    create a specific stack to get access to a specific section).

  * ``locations.conf`` use globs as section names. This provides an easy

    way to associate a set of options to a matching working tree or

    branch, including remote ones.

  * ``branch.conf`` doesn't use any section.

* There is no easy way to get configuration options for a given repository

  or an arbitrary path. Working trees and branches are generally organized

  in hierarchies and being able to share the option definitions is an often

  required feature. This can also address some needs exhibited by various

  branch schemes like looms, pipeline, colocated branches and nested

  trees. Being able to specify options *in* a working tree could also help

  support conflict resolution options for a given file, directory or

  subtree (see http://pad.lv/359320).

* Since sections allow different definitions for the same option, a total

  order should be defined between sections to select the right definition

  for a given context (paths or globs for ``locations.conf`` but other

  schemes can be used, window names for qbzr for example). Allowing globs

  for section names is harmful in this respect since the order is currently

  defined as being the lexicographical one. The caveat here is that if the

  order is always defined for a given set of sections it can change when one

  or several globs are modified and the user may get surprising and unwanted

  results in these cases. The lexicographical order is otherwise fine to

  define what section is more specific than another. (This may not be a

  problem in real life since longer globs are generally more specific than

  shorter ones and explicit paths should also be longer than matching

  globs. That may leave a glob and a path of equal length in a gray area but

  in practice using ``bzr config`` should give enough feedback to address

  them. See also http://pad.lv/832046 asking for a less magical section matcher).

* Internally, configuration files (and their fallbacks, ``bazaar.conf`` and

  ``locations.conf`` for ``branch.conf``) are read every time *one* option is

  queried. Likewise, setting or deleting a configuration option implies

  writing the configuration file *immediately* after re-reading the file to

  avoid racing updates (see http://pad.lv/832042).

* The current implementation use a mix of transport-based and direct file

  systems operations (Addressed by Store implementation relying on

  transports only).

* While the underlying ``ConfigObj`` implementation provides an

  interpolation feature, the ``bzrlib`` implementation doesn't provide an

  easy handling of templates where other configuration options can be

  interpolated. Instead, ``locations.conf`` (and only it) allows for

  ``appendpath`` and ``norecurse``. (Partially implemented, cross-section

  and cross-file interpolation still to be implemented, see

  http://pad.lv/832013 for the remaining parts).

* Inherited list values can't be modified, a more specific configuration can

  only redefine the whole list.

* There is no easy way to define dicts (the most obvious one being to use a

  dedicated section which is already overloaded). Using embedded sections

  for this would not be practical either if we keep using a no-name section

  for default values. In a few known cases, a bencoded dict is stored in a

  config value, so while this isn't user-friendly, not providing a better

  alternative shouldn't be a concern. A possible, limited, implementation

  can be envisioned: limiting the dict to a single level only, with simple

  names as keys and unicode strings as values. The keys can then be mapped

  to options prefixed with the dict name.

Proposed implementation

=======================

Configuration files definition

------------------------------

While of course configurations files can be versioned they are not intended

to be accessed in sync with the files they refer to (one can imagine

handling versioned properties this way but this is *not* what the bazaar

configuration files are targeted at). ``bzr`` will always refer to

configuration files as they exist on disk when an option is queried or set.

The configuration files are generally local to the file system but some of

them can be accessed remotely (``branch.conf``, ``repo.conf``).

Naming

------

Option names are organized into a name space for a given configuration file

(or a set of related configuration files). One such set includes

``bazaar.conf``, ``locations.conf``, ``branch.conf``, etc. Plugins can

define their own sets for their own needs.

Using a name space is meant to help:

* avoid collisions between bzr and plugins and between plugins,

* discover the available options and making them easier to remember,

* organise the documentation for the option set.

Using valid python identifiers is recommended but not enforced (but we may

do so in the future).

The option name space is organized by topic:

* bzrlib options are grouped by topic (``branch``, ``tree``, ``repo``)

* plugins are encouraged (but not required) to prefix their specific options

  with their name (``qbzr.`` for qbzr)

* collisions are detected at registration time so users are protected from

  incompatibilities between plugins,

* options that need to be used by several plugins (or shared between ``bzr``

  core and plugins) should be discussed but these discussions are already

  happening so the risk of misuse is low enough.

Value

-----

All option values are text. They are provided as Unicode strings to API

users with some refinements:

* boolean values can be obtained for a set of acceptable strings (yes/no,

  y/n, on/off, etc), (implemented with the ``from_unicode`` parameter)

* a list of strings from a value containing a comma separated list of

  strings.

Since the configuration files can be edited by the user, ``bzr`` doesn't

expect their content to be valid at all times. Instead, the code using

options should be ready to handle *invalid* values by warning the user and

falling back to a default value.

Likely, if an option is not defined in any configuration file, the code

should fallback to a default value (helpers should be provided by the API to

handle common cases: warning the user, getting a particular type of value,

returning a default value)(most of that is now handled at Option definition).

This also ensures compatibility with values provided via environment

variables or from the command line (where no validation can be expected

either)(done in the new design, some cases missing, see http://pad.lv/832064).

Option expansion

----------------

Some option values can be templates and contain references to other

options. This is especially useful to define URLs in sections shared for

multiple branches for example. It can also be used to describe commands

where some parameters are set by ``bzrlib`` at runtime.

Since option values are text-only, and to avoid clashing with other option

expansion (also known as interpolation) syntaxes, references are enclosed

with curly brackets::

  push_location = lp:~{launchpad_username}/bzr/{nick}

In the example above, ``launchpad_username`` is an already defined

configuration option while ``nick`` is the branch nickname and is set when a

configuration applies to a given branch.

The interpolation implementation should accept an additional dict so that

``bzrlib`` or plugins can define references that can be expanded without

being existing configuration options::

  diff_command={cmd} {cmd_opts} {file_a} {file_b}

There are two common errors that should be handled when handling interpolation:

* loops: when a configuration value refers to itself, directly or indirectly,

* undefined references: when a configuration value refers to an unknown option.

The loop handling can be modified to allow cross-sections and cross-files

interpolation: if an option refers to itself (directly or indirectly) during

an expansion, the fallback sections or files can be queried for its value.

This allows list values to refer to the definition in the less specific

configurations::

  bazaar.conf:

    debug_flags = hpss

  branch.conf for mybranch:

    debug_flags = {debug_flags}, hpssdetail

  $ bzr -d mybranch config debug_flags

  hpss, hpssdetail

Undefined references are detected if they are not defined in any

configuration. This will trigger errors while displaying the value. Diagnosing

typos should be doable in this case.

Configuration file syntax

-------------------------

The configuration file is mostly an ``ini-file``. It contains ``name = value``

lines grouped in sections. A section starts with a string enclosed in squared

brackets ('[section_name]`), this string uniquely identifies the section in

the file. Comments are allowed by prefixing them with the '#' character.

A section is named by the path (or some other unuique identifier) it should

apply to (more examples below).

When sections are used, they provide a finer grain of configuration by

defining option sets that apply to some working trees, branches,

repositories (or any kind of context) or part of them. The relationship

between a given context and the sections it applies to is defined by the

config file.

So far, Bazaar uses a glob in ``locations.conf`` and select the sections

that apply to a given url (or a local path).

The subset is defined by the common leading path or a glob.

Different kinds of section names can be defined:

* a full url: used to described options for remote branches and

  repositories (LocationMatcher supports this).

* local absolute path: used for working trees, branches or repositories

  on the local disks (LocationMatcher supports this).

* relative path: the path is relative to the configuration file and can be

  used for colocated branches or threads in a loom, i.e any working tree,

  branch or repository that is located in a place related to the

  configuration file path. Some configuration files may define this path

  relationship in specific ways to make them easier to use (i.e. if a config

  file is somewhere below ``.bzr`` and refers to threads in a loom for

  example, the relative path would be the thread name, it doesn't have to be

  an *exact* relative path, as long as its interpretation is unambiguous and

  clear for the user) (No section matchers support this so far, needs to

  file a bug)

Section matching

----------------

Section names define another name space (than the option names one) with an

entirely different purpose: in a given configuration file, for a given

context, only some sections will be relevant and will be queried in a

specific order.

This matching is specific to each config file and implemented by the

SectionMatcher objects.

Whatever this matching does, the user can observe the results with the ``bzr

config`` command which displays the sections in the order they are queried.

LocationMatcher

~~~~~~~~~~~~~~~

The context here is either:

* an URL,

* a local path.

Note that for both the provided context and the section names, if an URL uses

a ``file:///`` form, it is converted to a local path.

The sections names can use globs for each path component

(i.e. ``/dir/*/subdir`` is allowed but ``/dir/\*\*/subdir`` will match

``/dir/a/subdir`` but not ``/dir/a/b/subdir``. 

The reason is that the ordering is defined by sorting the section names

matching the context on the number of path components followed by the path

itself in lexicographical order. This results in most specific sections being

queried before the more generic ones.

PathMatcher

~~~~~~~~~~~

``LocationMatcher`` has some obscure (for unaware users) edge cases and

limitations that can be surprising. ``PathMatcher`` aims at addressing these

issues by providing simpler rules while still giving full control to the

user (http://pad.lv/832046).

The context here is a local path, absolute or relative. If the path is

relative it is interpreted from the file base directory.

Note that 'base directory' for configuration files in Bazaar directories is

really:

* the home directory for files under ``~/.bazaar``,

* the ``.bzr`` base directory for files under ``.bzr``.

The order is the one observed in the file so most generic values are specified

first and most specific ones last. As such, the order in the file is the

opposite of the one displayed by ``bzr config`` which displays most specific

values first. This seems to be the most natural order in both cases.

A section matches if the section name is a prefix of the context path

(relative paths being converted to absolute on the fly).

The Option object

-----------------

(copied from a recent version of bzr.dev for easier reading, refer to the

original for an up to date version)

The Option object is used to define its properties:

* name: a name: a valid python identifier (even if it's not used as an

  identifier in python itself). This is also used to register the option.

* default: the default value that Stack.get() should return if no

  value can be found for the option.

* default_from_env: a list of environment variables. The first variable set

  will provide a default value overriding 'default' which remains the

  default value if *no* environment variable is set.

* help: a doc string describing the option, the first line should be a

  summary and can be followed by a blank line and a more detailed

  explanation.

* from_unicode: a callable accepting a unicode string and returning a

  suitable value for the option. If the string cannot be coerced it should

  return None.

* invalid: the action to be taken when an invalid value is encountered in a

  store (during a Stack.get()).

The Section object

------------------

Options are grouped into sections which share some properties with the well

known dict objects:

* the key is the name,

* you can get, set and remove an option,

* the value is a unicode string.

MutableSection is needed to set or remove an option, ReadOnlySection should

be used otherwise.

The Store object

----------------

This is an implementation-level object that should rarely be used directly.

* it can be local or remote

* locking

  All lock operations should be implemented via transport objects. (True for

  Store).

* option life cycle

  Working trees, branches and repositories should define a config attribute

  following the same life cycle as their lock: the associated config file is

  read once and written once if needed. This should minimize the file system

  accesses or the network requests. There is no known racing scenarios for

  configuration options, changing the existing implementation to this less

  constrained one shouldn't introduce any. Yet, in order to detect such

  racing scenarios, we can add a check that the current content of the

  configuration file is the expected one before writing the new content and

  emit warnings if differences occur. The checks should be performed for the

  modified values only. As of today (and in the foreseeable future), the

  size of the configuration files are small enough to be kept in memory (see

  http://pad.lv/832042).

The Stack object

-----------------

This the object that provides access to the needed features:

* getting an option value,

* setting an option value,

* deleting an option value,

* handling a list of configuration files and for each of them a section

  matcher providing the sections that should be tried in the given order to

  find an option.

* handling a Store and a section where option creation, modification and

  deletion will occur.

Depending on the files involved, a working tree, branch or repository object

(or more generally a context) should be provided to access the corresponding

configuration files. Note that providing a working tree object also

implicitly provides the associated branch and repository object so only one

of them is required (or none for configuration files specific to the user

like ``bazaar.conf``).

Getting an option value

~~~~~~~~~~~~~~~~~~~~~~~

Depending on the option, there are various places where it can be defined

and several ways to override these settings when needed.

The following lists all possible places where a configuration option can

be defined, but some options will make sense in only some of them. The

first to define a value for an option wins (None is therefore used to

express that an option is not set).

* command-line

  ``-Ooption=value`` see http://pad.lv/491196.

* ``~/.bazaar/locations.conf``

  When an option is set in ``locations.conf`` it overrides any other

  configuration file. This should be used with care as it allows setting a

  different value than what is recommended by the project

* ``tree`` (Not Implemented Yet)

  The options related to the working tree.

  This includes all options related to commits, ignored files, junk files,

  etc.

  Note that the sections defined there can use relative paths if some

  options should apply to a subtree or some specific files only.

  See http://pad.lv/430538 and http://pad.lv/654998.

* ``branch`` located in ``.bzr/branch/branch.conf``

  The options related to the branch.

  Sections can be defined for colocated branches or loom threads.

* ``repository`` (Not Implemented Yet)

  The options related to the repository.

  Using an option to describe whether or not a repository is shared could

  help address http://pad.lv/342119 but this will probably requires a format

  bump).

* ``project`` (Not Implemented Yet)

  The options common to all branches and working trees for a project.

* ``organization`` (Not Implemented Yet)

  The options common to all branches and working trees for an organization.

  See http://pad.lv/419854.

* ``system`` (Not Implemented Yet but see http://pad.lv/419854 and

  https://code.launchpad.net/~thomir/bzr/add-global-config/+merge/69592)

  The options common to all users of a system (may be /etc/bzr/defaults

  or /usr/local/etc/bzr/defaults or

  /Library/Preferences/com.canonical.defaults  or c:\windows\bazaar.conf

  (someone fix this one please ;) depending on the OS).

* ``bazaar.conf``

  The options the user has selected for the host he is using.

  Sections can be defined for both remote and local branches to define

  default values (i.e. the most common use of ``locations.conf`` today).

* default (implemented by the OptionRegistry)

  The options defined in the ``bzr`` source code.

  This will be implemented via the Option objects.

Plugins can define additional configuration files as they see fit and

insert them in this list, see their documentation for details.

Compatibility

=============

There are ways to keep the same files while ensuring compatibility via various

tricks but there are cases where using new files to replace the old ones is

definitely easier:

* no need to ensure that the new files are correctly handled by old bzr

  versions,

* making it clear for users that there is a switch and let them migrate at

  their own pace.

The known cases so far are described below.

Obvious at this point:

* Branch provides ``get_config`` for the old design and ``get_config_stack``

  for the new design so that both designs are supported. Once the store

  sharing is implemented, we may want to use an attribute for the stack and

  deprecate both ``get_config`` and ``get_config_stack``.

* Sections names in ``bazaar.conf`` are arbitrary (except ``DEFAULT``) so

  it's easier to leave the file untouched and let plugin authors and users

  migrate away (or not) from them. For ``bzr`` itself, that means

  ``DEFAULT`` is the only section used for most of the options and provides

  user defaults. ``ALIASES`` requires a specific stack but only the ``bzr

  alias`` command cares about that.

* Option policies should be deprecated:

  * The ``norecurse`` policy is useless, all options are recursive by

    default. If specific values are needed for specific paths, they can just

    be defined as such (in the appropriate sections or files).

  * The ``appendpath`` policy should be implemented via interpolation and a

    ``relpath`` option provided by the configuration framework

    (http://pad.lv/832013).

* Section order in ``locations.conf`` has issues which make a migration to a

  different way to organize the sections (hence the file content) far easier

  with a new file.

* ``locations.conf`` is really for overrides but many users have been using it

  to provide defaults. There is no way to know if the whole content has been

  used for defaults or overrides or a mix of both. So there is no way to

  migrate this automatically.

Unclear at this point:

* [BOOKMARKS] section can be replaced by ``bookmarks.xxx`` options (the

  bookmarks plugins already uses ``bookmarks_xxx`` in branch.conf since no

  sections were supported there). The easiest here is probably to just merge

  the plugin into core and use the appropriate option names consistently. A

  ``config:`` directory service may even be better as any option can be used

  as a bookmark. This allows things like::

    [/whatever/path]

    my_push = lp:<launchpad.login>/xxx/{nick}

    web_site=ftp://example.com/

    bzr push config:web_site

  Which means we completely replace the plugin and don't need to care about

  migrating the section.

* [ALIASES] section can be replaced by corresponding bzr.alias.xxx

  options. This could be automated by creating the corresponding options ?

* I don't know about other sections, feedback welcome. Plugin authors are

  encouraged to migrate to the new name space scheme by prefixing their

  options with their plugin name.

Notes

=====

These are random notes about concepts, ideas or issues not implemented yet.

Developer facing concepts

-------------------------

Option

~~~~~~

* list of allowed Config IDs (this allows a list of possible config files in

  bazaar.conf only option and use it while bootstrapping the config

  creations). 

* blacklist of config IDs (some options *can't* be stored (modified) by the

  user)

An alternative is to just let the devs decide which stack they use for a

given option, ``stacked_on_location`` for example is said to relate to the

branch only and changing it or setting it in a different config file may not

be appropriate. This may not be a good example as there is also the

``default_stack_on`` option which can be set only in ``control.conf``

though...

Stack

~~~~~

* a lazy cache for the option values (should be reset on modifications as

  interpolations will make it tricky to update incrementally) (see FIXME in

  config.py Stack.get()))

* ensures that the Stores involved generate as less IOs as possible (see

  http://pad.lv/832042)

* ensures that the transaction is the object life time (i.e. modifications

  will be taken into account *iff* they are committed explicitly).

StackRegistry

~~~~~~~~~~~~~

* ensures that a config ID is a unique identifier

* register Stacks

Store

~~~~~

* ensures that the transaction is the object life time (i.e. modifications

  will be taken into account *iff* they are committed explicitly).

Examples

--------

store examples:

~~~~~~~~~~~~~~~

* ConfigObj (bazaar.conf)

* DB (<scheme>://bazaar.launchpad.net/bazaar.conf)

Why and when locking config files matter

----------------------------------------

This is relevant for http://pad.lv/832042.

``bzr`` behavior, as well as the objects it acts upon, is configured via a

set of so-called configuration files.

These files allow to define working trees, branches and repositories, their

relationships and how ``bzr`` should handle them.

The default behavior of ``bzr`` is aimed at making this configuration as

transparent as possible by keeping track of how these objects are created

and modified when they are used. In short, they are useless until you want

to change the default behavior in some specific context.

We mostly **read** config options. Therefore all we care about is to

guarantee that:

* we get a valid config file at all times when reading,

* we always leave a valid config file when writing (via the rename dance)

From there, conceptually, all operations can clearly define whether or not

they need to modify a config file and do so only when they succeed. All

modifications occurring during such an operation are delayed until the very

end of the operation.

Now, we want to minimize the overlapping times where one bzr operation has

changed a value and another concurrent operation is unaware of this

modification.

These overlapping periods are *as of today* rare.

The only known case, http://pad.lv/525571 has been fixed in bzr-2.1.3. The

bug there was triggered when two processes tried to write the same config

file at the same time leaving an invalid file in the end.

Such a period can be recognized and detected though: when changing an option

value, if the preserved original value is different in the config file,

someone else modified it and the operation can be invalid because it relied

on the original value.

For the sake of the example, if an option value represent a global unique ID

via a simple counter (very bad idea), if two operations try to increment it,

both will use the same value that won't be unique anymore. Checking the

value present in the file when trying to save the updated value with

identify such a collision.

An assumption is floating around: it should be enough to report when an

operation is modifying an already modified option and observe that no-one

reports such occurrences.

Note that this assumption is made in a context where *no* known scenarios

exist in the bzr code base not in any plugin (for a best effort value of

'any', feedback highly welcome, bug reports even ;)

With this in mind, we can change the definition of config options, stores

and stacks to ensure that:

* a config file is read only once when in read access,

* a config file is read only once and written only once when in write

  access, adding the check mentioned above will require *one* additional

  read.

A reader can then safely assume that reading a config file gives it a valid

(and coherent) definition of the configuration when the operation

starts. All the operation has to do is to declare which config files may be

modified by an operation (whether or not we can be liberal on this 'may be'

is yet to be defined).