view mercurial/txnutil.py @ 50801:c51b178b0b7e

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.

# txnutil.py - transaction related utilities
#
#  Copyright FUJIWARA Katsunori <[e-mail]> and others
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.


from . import encoding


def mayhavepending(root):
    """return whether 'root' may have pending changes, which are
    visible to this process.
    """
    return root == encoding.environ.get(b'HG_PENDING')


def trypending(root, vfs, filename, **kwargs):
    """Open  file to be read according to HG_PENDING environment variable

    This opens '.pending' of specified 'filename' only when HG_PENDING
    is equal to 'root'.

    This returns '(fp, is_pending_opened)' tuple.
    """
    if mayhavepending(root):
        try:
            return (vfs(b'%s.pending' % filename, **kwargs), True)
        except FileNotFoundError:
            pass
    return (vfs(filename, **kwargs), False)