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−# Support code for event tracing in Mercurial. Lives in demandimport
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−# so it can also be used in demandimport.
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−#
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−# Copyright 2018 Google LLC.
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−#
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−# This software may be used and distributed according to the terms of the
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−# GNU General Public License version 2 or any later version.
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−from __future__ import absolute_import
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−
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−import contextlib
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−import os
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−
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−_pipe = None
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−_checked = False
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−
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−@contextlib.contextmanager
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−def log(whencefmt, *whenceargs):
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−    global _pipe, _session, _checked
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−    if _pipe is None:
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−        if _checked:
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−            yield
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−            return
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−        _checked = True
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−        if 'HGCATAPULTSERVERPIPE' not in os.environ:
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−            yield
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−            return
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−        _pipe = open(os.environ['HGCATAPULTSERVERPIPE'], 'w', 1)
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−        _session = os.environ.get('HGCATAPULTSESSION', 'none')
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−    whence = whencefmt % whenceargs
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−    try:
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−        # Both writes to the pipe are wrapped in try/except to ignore
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−        # errors, as we can see mysterious errors in here if the pager
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−        # is active. Presumably other conditions could trigger
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−        # problems too.
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−        try:
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−            _pipe.write('START %s %s\n' % (_session, whence))
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−        except IOError:
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−            pass
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−        yield
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−    finally:
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−        try:
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−            _pipe.write('END %s %s\n' % (_session, whence))
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−        except IOError:
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−            pass