mercurial/txnutil.py

branchmap: add a cache validation cache, avoid expensive re-hash on every use

In a pathological `hg log` case, we end up executing the branchmap validity
checking twice per commit displayed. Or maybe we always do, and I just noticed
because it's really slow in this repo for some reason.

Before:
```
  Time (mean ± σ):      9.816 s ±  0.071 s    [User: 9.435 s, System: 0.392 s]

  Range (min … max):    9.709 s …  9.920 s
```

After:
```
  Time (mean ± σ):      8.671 s ±  0.078 s    [User: 8.309 s, System: 0.392 s]

  Range (min … max):    8.594 s …  8.816 s
```

Differential Revision: https://phab.mercurial-scm.org/D9023

# 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(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 IOError as inst:
            if inst.errno != errno.ENOENT:
                raise
    return (vfs(filename, **kwargs), False)