All config options used within Mercurial should be registered.

Config Option in Core
=====================

Config options used by Mercurial core are registered in the
``mercurial.configitems`` module.

Simple entry
------------

A registration entry typically looks like::

    coreconfigitem('section', 'option',
        default=MyDefaultValue,
    )

Once registered, Mercurial will know that ``section.option`` is a legitimate
config option and that ``MyDefaultValue`` should be used if no other values are
defined in configuration files.

Complex default value
---------------------

If the default provided is a callable, it is called to retrieve the default
value when accessing the config option. This is useful for default value that
are mutable like the empty list::

    coreconfigitem('pager', 'ignore',
        default=list,
    )

In addition, there are cases where the default is not fixed, but computed from
other properties. In this case, use the ``dynamicdefault`` object as value for
the ``default`` parameters. A default value is then explicitly required when
reading the option::

    # registration
    coreconfigitem('web', 'name',
        default=dynamicdefault,
    )

    # usage
    ui.config('web', 'name', dirnam)

Free form options
-----------------

Some config sections use free form options (e.g. ``paths``). You can register
them using the ``generic`` parameters::

    coreconfigitem('paths', '.*',
        default=None,
        generic=True,
    )

When ``generic=True`` is set, the option name is matched as a regular expression
(rooted to string start). It can be used to select specific sub parameters::

    coreconfigitem('merge-tools', br'.*\.args$',
        default="$local $base $other",
        generic=True,
        priority=-1,
    )

The ``priority`` parameter control the order used to match the generic pattern
(lower first).

Config Option in Extensions
===========================

General case
------------

Extensions should register config items through the ``registrar`` API (also used
for commands and others)::

    configtable = {}
    configitem = registrar.configitem(configtable)

    configitem('blackbox', 'dirty',
        default=False,
    )

The ``dynamicdefault`` object is then available as
``configitem.dynamicdefault``.

Supporting older version
------------------------

The register was introduced in Mercurial 4.3, the ``generic`` parameter was
introduced in 4.4. Starting with Mercurial 4.4, all core options were registered
and developer warnings are emitted when accessing unregistered option.

Extensions supporting version older than Mercurial-4.3 cannot rely on the
default value registered. The simplest way to register option while still
supporting older version is to use ``dynamicdefault`` for option requiring a
default value. The existing code passing an explicit default can then stay in
use until compatibility to Mercurial 4.2 is dropped.

As reminder here are the default value for each config types:
- config:      None
- configbool:  False
- configbytes: 0
- configdate:  None
- configint:   None
- configlist:  []
- configpath: None