hgdemandimport/tracing.py

remotefilelog: add a developer option to wait for background processes

In order to block the main command on the subprocess exiting, we ensure the
repo's ui object will call the subprocess.wait() method to ensure the top-level
hg process doesn't exit until all background processes have also done so.

Currently, in the tests, most operation spawning background process as followed
by commands waiting for these operations to complete. However this waiting is
racy. First because it seems like we can start waiting before the background
operation actually start, in which case it is prematurely detected as "done".
Second, because some commands may spawn multiple background operation for the
same operation (eg: rebase can apparently trigger multiple prefetch). The
current approach could be updated to maybe handle the first issue, but the
second one will never be properly handled.

In most case, we do not care that the bg process keep running after the command
end. (Since we explicitly wait for them to end before doing anything else).  So
we add an option to wait on the background process before exiting the command.
We'll put it in use in the next changeset.

Differential Revision: https://phab.mercurial-scm.org/D7585

# Support code for event tracing in Mercurial. Lives in demandimport
# so it can also be used in demandimport.
#
# Copyright 2018 Google LLC.
#
# 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 contextlib
import os

_pipe = None
_checked = False
_session = 'none'


def _isactive():
    global _pipe, _session, _checked
    if _pipe is None:
        if _checked:
            return False
        _checked = True
        if 'HGCATAPULTSERVERPIPE' not in os.environ:
            return False
        _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
        _session = os.environ.get('HGCATAPULTSESSION', 'none')
    return True


@contextlib.contextmanager
def log(whencefmt, *whenceargs):
    if not _isactive():
        yield
        return
    whence = whencefmt % whenceargs
    try:
        # Both writes to the pipe are wrapped in try/except to ignore
        # errors, as we can see mysterious errors in here if the pager
        # is active. Presumably other conditions could trigger
        # problems too.
        try:
            _pipe.write('START %s %s\n' % (_session, whence))
        except IOError:
            pass
        yield
    finally:
        try:
            _pipe.write('END %s %s\n' % (_session, whence))
        except IOError:
            pass


def counter(label, amount, *labelargs):
    if not _isactive():
        return
    l = label % labelargs
    # See above in log() for why this is in a try/except.
    try:
        _pipe.write('COUNTER %s %d %s\n' % (_session, amount, l))
    except IOError:
        pass