mercurial/txnutil.py

rust: module policy with importrust

We introduce two rust+c module policies and a new
`policy.importrust()` that makes use of them.

This simple approach provides runtime switching of
implementations, which is crucial for the performance
measurements such as those Octobus does with ASV.

It can also be useful for bug analysis.

It also has the advantage of making conditionals in
Rust callers more uniform, in particular
abstracting over specifics like `demandimport`

At this point, the build stays unchanged, with the rust-cpython based
`rustext` module being built if HGWITHRUSTEXT=cpython.

More transparency for the callers, i.e., just using
`policy.importmod` would be a much longer term and riskier
effort for the following reasons:

1. It would require to define common module boundaries
   for the three or four cases (pure, c, rust+ext, cffi) and that
   is premature with the Rust extension currently under heavy
   development in areas that are outside the scope of the C extensions.
2. It would imply internal API changes that are not currently wished,
   as the case of ancestors demonstrates.
3. The lack of data or property-like attributes (tp_member
   and tp_getset) in current `rust-cpython` makes it impossible to
   achieve direct transparent replacement of pure Python classes by
   Rust extension code, meaning that the caller sometimes has to be able
   to make adjustments or provide additional wrapping.

# 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 __future__ import absolute_import

import errno

from . import (
    encoding,
)

def mayhavepending(root):
    '''return whether 'root' may have pending changes, which are
    visible to this process.
    '''
    return root == encoding.environ.get('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('%s.pending' % filename, **kwargs), True)
        except IOError as inst:
            if inst.errno != errno.ENOENT:
                raise
    return (vfs(filename, **kwargs), False)