contrib/hgweb.wsgi

transaction: avoid file stat ambiguity only for files in blacklist

Advancing mtime by os.utime() fails for EPERM, if the target file is
owned by another. bff5ccbe5ead and related changes made some code
paths give advancing mtime up in such case, to fix issue5418.

This causes file stat ambiguity (again), if it is owned by another.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

To avoid file stat ambiguity in such case, especially for
.hg/dirstate, ed66ec39933f made vfs.rename() copy the target file, and
advance mtime of renamed one again, if EPERM (see issue5584 for detail).

But straightforward "copy if EPERM" isn't reasonable for truncation of
append-only files at rollbacking, because rollbacking might cost much
for truncation of many filelogs, even though filelogs aren't
filecache-ed.

Therefore, this patch introduces blacklist "checkambigfiles", and
avoids file stat ambiguity only for files specified in this blacklist.

This patch consists of two parts below, which should be applied at
once in order to avoid regression.

  - specify 'checkambig=True' at vfs.open(mode='a') in _playback()
    according to checkambigfiles

  - invoke _playback() with checkambigfiles
    - add transaction.__init__() checkambigfiles argument, for _abort()
      - make localrepo instantiate transaction with _cachedfiles
    - add rollback() checkambigfiles argument, for "hg rollback/recover"
      - make localrepo invoke rollback() with _cachedfiles

After this patch, straightforward "copy if EPERM" will be reasonable
at closing the file opened with checkambig=True, because this policy
is applied only on files, which are listed in blacklist
"checkambigfiles".

# An example WSGI for use with mod_wsgi, edit as necessary
# See https://mercurial-scm.org/wiki/modwsgi for more information

# Path to repo or hgweb config to serve (see 'hg help hgweb')
config = "/path/to/repo/or/config"

# Uncomment and adjust if Mercurial is not installed system-wide
# (consult "installed modules" path from 'hg debuginstall'):
#import sys; sys.path.insert(0, "/path/to/python/lib")

# Uncomment to send python tracebacks to the browser if an error occurs:
#import cgitb; cgitb.enable()

# enable demandloading to reduce startup time
from mercurial import demandimport; demandimport.enable()

from mercurial.hgweb import hgweb
application = hgweb(config)