hgweb.cgi

lock: avoid unintentional lock acquisition at failure of readlock

Acquiring lock by vfs.makelock() and getting lock holder (aka
"locker") information by vfs.readlock() aren't atomic action.
Therefore, failure of the former doesn't ensure success of the latter.

Before this patch, lock is unintentionally acquired, if ENOENT
causes failure of vfs.readlock() while 5 times retrying, because
lock._trylock() returns to caller silently after retrying, and
lock.lock() assumes that lock._trylock() returns successfully only if
lock is acquired.

In this case, lock symlink (or file) isn't created, even though lock
is treated as acquired in memory.

To avoid this issue, this patch makes lock._trylock() raise
LockHeld(EAGAIN) at the end of it, if lock isn't acquired while
retrying.

An empty "locker" meaning "busy for frequent lock/unlock by many
processes" might appear in an abortion message, if lock acquisition
fails. Therefore, this patch also does:

  - use '%r' to increase visibility of "locker", even if it is empty
  - show hint message to explain what empty "locker" means

#!/usr/bin/env python
#
# An example hgweb CGI script, edit as necessary
# See also https://mercurial-scm.org/wiki/PublishingRepositories

# 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()

from mercurial import demandimport; demandimport.enable()
from mercurial.hgweb import hgweb, wsgicgi
application = hgweb(config)
wsgicgi.launch(application)