mercurial/configitems.py

configitems: declare items in a TOML file

Mercurial ships with Rust code that also needs to read from the config.
Having a way of presenting `configitems` to both Python and Rust is needed
to prevent duplication, drift, and have the appropriate devel warnings.

Abstracting away from Python means choosing a config format. No single format
is perfect, and I have yet to come across a developer that doesn't hate all of
them in some way.
Since we have a strict no-dependencies policy for Mercurial, we either need
to use whatever comes with Python, vendor a library, or implement a custom
format ourselves.

Python stdlib means using JSON, which doesn't support comments and isn't great
for humans, or `configparser` which is an obscure, untyped format that nobody
uses and doesn't have a commonplace Rust parser.
Implementing a custom format is error-prone, tedious and subject to the
same issues as picking an existing format.

Vendoring opens us to the vast array of common config formats. The ones
being picked for most modern software are YAML and TOML. YAML is older and
common in the Python community, but TOML is much simpler and less error-prone.
I would much rather be responsible for the <1000 lines of `tomli`, on top of
TOML being the choice of the Rust community, with robust crates for reading it.

The structure of `configitems.toml` is explained inline.

# configitems.py - centralized declaration of configuration option
#
#  Copyright 2017 Pierre-Yves David <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.


import functools
import re

from .utils import resourceutil

from . import (
    encoding,
    error,
)

try:
    import tomllib  # pytype: disable=import-error

    tomllib.load  # trigger lazy import
except ModuleNotFoundError:
    # Python <3.11 compat
    from .thirdparty import tomli as tomllib


def loadconfigtable(ui, extname, configtable):
    """update config item known to the ui with the extension ones"""
    for section, items in sorted(configtable.items()):
        knownitems = ui._knownconfig.setdefault(section, itemregister())
        knownkeys = set(knownitems)
        newkeys = set(items)
        for key in sorted(knownkeys & newkeys):
            msg = b"extension '%s' overwrites config item '%s.%s'"
            msg %= (extname, section, key)
            ui.develwarn(msg, config=b'warn-config')

        knownitems.update(items)


class configitem:
    """represent a known config item

    :section: the official config section where to find this item,
       :name: the official name within the section,
    :default: default value for this item,
    :alias: optional list of tuples as alternatives,
    :generic: this is a generic definition, match name using regular expression.
    """

    def __init__(
        self,
        section,
        name,
        default=None,
        alias=(),
        generic=False,
        priority=0,
        experimental=False,
        documentation="",
    ):
        self.section = section
        self.name = name
        self.default = default
        self.documentation = documentation
        self.alias = list(alias)
        self.generic = generic
        self.priority = priority
        self.experimental = experimental
        self._re = None
        if generic:
            self._re = re.compile(self.name)


class itemregister(dict):
    """A specialized dictionary that can handle wild-card selection"""

    def __init__(self):
        super(itemregister, self).__init__()
        self._generics = set()

    def update(self, other):
        super(itemregister, self).update(other)
        self._generics.update(other._generics)

    def __setitem__(self, key, item):
        super(itemregister, self).__setitem__(key, item)
        if item.generic:
            self._generics.add(item)

    def get(self, key):
        baseitem = super(itemregister, self).get(key)
        if baseitem is not None and not baseitem.generic:
            return baseitem

        # search for a matching generic item
        generics = sorted(self._generics, key=(lambda x: (x.priority, x.name)))
        for item in generics:
            # we use 'match' instead of 'search' to make the matching simpler
            # for people unfamiliar with regular expression. Having the match
            # rooted to the start of the string will produce less surprising
            # result for user writing simple regex for sub-attribute.
            #
            # For example using "color\..*" match produces an unsurprising
            # result, while using search could suddenly match apparently
            # unrelated configuration that happens to contains "color."
            # anywhere. This is a tradeoff where we favor requiring ".*" on
            # some match to avoid the need to prefix most pattern with "^".
            # The "^" seems more error prone.
            if item._re.match(key):
                return item

        return None


def sanitize_item(item):
    """Apply the transformations that are encoded on top of the pure data"""

    # Set the special defaults
    default_type_key = "default-type"
    default_type = item.pop(default_type_key, None)
    if default_type == "dynamic":
        item["default"] = dynamicdefault
    elif default_type == "list_type":
        item["default"] = list
    elif default_type == "lambda":
        assert isinstance(item["default"], list)
        default = [e.encode() for e in item["default"]]
        item["default"] = lambda: default
    elif default_type == "lazy_module":
        item["default"] = lambda: encoding.encoding
    else:
        if default_type is not None:
            msg = "invalid default config type %r for '%s.%s'"
            msg %= (default_type, item["section"], item["name"])
            raise error.ProgrammingError(msg)

    # config expects bytes
    alias = item.get("alias")
    if alias:
        item["alias"] = [(k.encode(), v.encode()) for (k, v) in alias]
    if isinstance(item.get("default"), str):
        item["default"] = item["default"].encode()
    item["section"] = item["section"].encode()
    item["name"] = item["name"].encode()


def read_configitems_file():
    """Returns the deserialized TOML structure from the configitems file"""
    with resourceutil.open_resource(b"mercurial", b"configitems.toml") as fp:
        return tomllib.load(fp)


def configitems_from_toml(items):
    """Register the configitems from the *deserialized* toml file"""
    for item in items["items"]:
        sanitize_item(item)
        coreconfigitem(**item)

    templates = items["templates"]

    for application in items["template-applications"]:
        template_items = templates[application["template"]]

        for template_item in template_items:
            item = template_item.copy()
            prefix = application.get("prefix", "")
            item["section"] = application["section"]
            if prefix:
                item["name"] = f'{prefix}.{item["suffix"]}'
            else:
                item["name"] = item["suffix"]

            sanitize_item(item)
            item.pop("suffix", None)
            coreconfigitem(**item)


def import_configitems_from_file():
    as_toml = read_configitems_file()
    configitems_from_toml(as_toml)


coreitems = {}


def _register(configtable, *args, **kwargs):
    item = configitem(*args, **kwargs)
    section = configtable.setdefault(item.section, itemregister())
    if item.name in section:
        msg = b"duplicated config item registration for '%s.%s'"
        raise error.ProgrammingError(msg % (item.section, item.name))
    section[item.name] = item


# special value for case where the default is derived from other values
dynamicdefault = object()

# Registering actual config items


def getitemregister(configtable):
    f = functools.partial(_register, configtable)
    # export pseudo enum as configitem.*
    f.dynamicdefault = dynamicdefault
    return f


coreconfigitem = getitemregister(coreitems)

import_configitems_from_file()