tests/fakemergerecord.py

perf: add threading capability to perfbdiff

Since we are releasing the GIL during diffing, it is interesting to see how a
thread pool would perform on diffing. We add a new `--threads` argument to
commands. Synchronizing the thread pool is a bit complex because we want to be
able to reuse it from one run to another.

On my computer (i7 with 4 cores + hyperthreading), I get the following data for
about 12000 revisions:
threads wall        comb    wall gain   comb overhead
none    31.596715   31.59    0.00%       0.00%
1       31.621228   31.62   -0.08%       0.09%
2       16.406202   32.8    48.08%       3.83%
3       11.598334   34.76   63.29%      10.03%
4        9.205421   36.77   70.87%      16.40%
5        8.517604   42.51   73.04%      34.57%
6        7.94645    47.58   74.85%      50.62%
7        7.434972   51.92   76.47%      64.36%
8        7.070638   55.34   77.62%      75.18%

Compared to the feature disabled (threads=0), the overhead is negligible with
the threading code (threads=1), and the gain is already 48% with two threads.

# Extension to write out fake unsupported records into the merge state
#
#

from __future__ import absolute_import

from mercurial import (
    merge,
    registrar,
)

cmdtable = {}
command = registrar.command(cmdtable)

@command('fakemergerecord',
         [('X', 'mandatory', None, 'add a fake mandatory record'),
          ('x', 'advisory', None, 'add a fake advisory record')], '')
def fakemergerecord(ui, repo, *pats, **opts):
    with repo.wlock():
        ms = merge.mergestate.read(repo)
        records = ms._makerecords()
        if opts.get('mandatory'):
            records.append(('X', 'mandatory record'))
        if opts.get('advisory'):
            records.append(('x', 'advisory record'))
        ms._writerecords(records)