view mercurial/help/internals/config.txt @ 35935:00b9e26d727b

sshpeer: establish SSH connection before class instantiation

We want to move the handshake to before peers are created so
we can instantiate a different peer class depending on the
results of the handshake. This necessitates moving the SSH
process invocation to outside the peer class.

As part of the code move, some variables were renamed for
clarity. util.popen4() returns stdin, stdout, and stderr in
their typical file descriptor order. However, stdin and stdout
were being mapped to "pipeo" and "pipei" respectively. "o"
for "stdin" and "i" for "stdout" is a bit confusing. Although
it does make sense for "output" and "input" from the perspective
of the client. But in the context of the new function, it makes
sense to refer to these as their file descriptor names.

In addition, the last use of self._path disappeared, so we stop
setting that attribute and we can delete the redundant URL
parsing necessary to set it.

Differential Revision: https://phab.mercurial-scm.org/D2031

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 values 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 the value
for the ``default`` parameter. A default value is then explicitly required when
reading the option::

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

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

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 controls 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 versions
-------------------------

The registrar was introduced in Mercurial 4.3, and 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 versions older than Mercurial 4.3 cannot rely on the
default value being registered. The simplest way to register an option while
still supporting an older version is to use ``dynamicdefault`` for options
requiring a default value. The existing code passing an explicit default can
then stay in use until compatibility with Mercurial 4.2 is dropped.

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