hgdemandimport/tracing.py

errors: catch urllib errors specifically instead of using safehasattr()

Before this patch, we would catch `IOError` and `OSError` and check if
the instance had a `.code` member (indicates `HTTPError`) or a
`.reason` member (indicates the more generic `URLError`). It seems to
me that can simply catch those exception specifically instead, so
that's what this code does. The existing code is from fbe8834923c5
(commands: report http exceptions nicely, 2005-06-17), so I suspect
it's just that there was no `urllib2` (where `URLError` lives) back
then.

The old code mentioned `SSLError` in a comment. The new code does
*not* try to catch that. The documentation for `ssl.SSLError` says
that it has a `.reason` property, but `python -c 'import ssl;
print(dir(ssl.SSLError("foo", Exception("bar"))))` doesn't mention
that property on either Python 2 or Python 3 on my system. It also
seems that `sslutil` is pretty careful about converting `ssl.SSLError`
to `error.Abort`. It also is carefult to not assume that instances of
the exception have a `.reason`. So I at least don't want to catch
`ssl.SSLError` and handle it the same way as `URLError` because that
would likely result in a crash. I also wonder if we don't need to
handle it at all (because `sslutil` might handle all the cases). It's
now early in the release cycle, so perhaps we can just see how it
goes?

Differential Revision: https://phab.mercurial-scm.org/D9318

# 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