run-tests: mechanism to report exceptions during test execution Sometimes when running tests you introduce a ton of exceptions. The most extreme example of this is running Mercurial with Python 3, which currently spews thousands of exceptions when running the test harness. This commit adds an opt-in feature to run-tests.py to aggregate exceptions encountered by `hg` when running tests. When --exceptions is used, the test harness enables the "logexceptions" extension in the test environment. This extension wraps the Mercurial function to handle exceptions and writes information about the exception to a random filename in a directory defined by the test harness via an environment variable. At the end of the test harness, these files are parsed, aggregated, and a list of all unique Mercurial frames triggering exceptions is printed in order of frequency. This feature is intended to aid Python 3 development. I've only really tested it on Python 3. There is no shortage of improvements that could be made. e.g. we could write a separate file containing the exception report - maybe even an HTML report. We also don't capture which tests demonstrate the exceptions, so there's no turnkey way to test whether a code change made an exception disappear. Perfect is the enemy of good. I think the current patch is useful enough to land. Whoever uses it can send patches to imprve its usefulness. Differential Revision: https://phab.mercurial-scm.org/D1477

# txnutil.py - transaction related utilities
#
#  Copyright FUJIWARA Katsunori <foozy@lares.dti.ne.jp> 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('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('%s.pending' % filename, **kwargs), True)
        except IOError as inst:
            if inst.errno != errno.ENOENT:
                raise
    return (vfs(filename, **kwargs), False)