comparison mercurial/lock.py @ 36701:d77c3b023393

lock: block signal interrupt while making a lock file

On Windows where symlink isn't supported, util.makelock() could leave an empty
file if interrupted immediately after os.open(). This empty lock never dies
as it has no process id recorded.

    ld = os.open(pathname, os.O_CREAT | os.O_WRONLY | os.O_EXCL)
    # an interrupt may occur here
    os.write(ld, info)
    os.close(ld)

This was a long-standing bug of TortoiseHg which runs a command-server and
kills it by CTRL_C_EVENT, reported by random Windows users.

https://bitbucket.org/tortoisehg/thg/issues/4873/#comment-43591129

At first, I tried to fix makelock() to clean up a stale lock file, which
turned out to be hard because any instructions may be interrupted by a
signal.

    ld = None
    try:
        # CALL_FUNCTION  # os.open(...)
        # an interrupt may occur here
        # STORE_FAST     # ld = ...
        ld = os.open(pathname, os.O_CREAT | os.O_WRONLY | os.O_EXCL)
        os.write(ld, info)
        ...
        return True
    except:
        if ld:
            ...
            os.unlink(pathname)
        return False

So I decided to block signals by temporarily replacing the signal handlers
so makelcok() and held = 1 will never be interrupted.

Many thanks to Fernando Najera for investigating the issue.

36700:e437de3881c1

from __future__ import absolute_import

import contextlib
import errno
import os
import socket
import time
import warnings

from .i18n import _

            result += '/%x' % os.stat('/proc/self/ns/pid').st_ino
        except OSError as ex:
            if ex.errno not in (errno.ENOENT, errno.EACCES, errno.ENOTDIR):
                raise
    return result

def trylock(ui, vfs, lockname, timeout, warntimeout, *args, **kwargs):
    """return an acquired lock or raise an a LockHeld exception

    This function is responsible to issue warnings and or debug messages about

        lockname = '%s:%d' % (lock._host, self.pid)
        retry = 5
        while not self.held and retry:
            retry -= 1
            try:
                self.vfs.makelock(lockname, self.f)
                self.held = 1
            except (OSError, IOError) as why:
                if why.errno == errno.EEXIST:
                    locker = self._readlock()
                    if locker is None:
                        continue

36701:d77c3b023393

from __future__ import absolute_import

import contextlib
import errno
import os
import signal
import socket
import time
import warnings

from .i18n import _

            result += '/%x' % os.stat('/proc/self/ns/pid').st_ino
        except OSError as ex:
            if ex.errno not in (errno.ENOENT, errno.EACCES, errno.ENOTDIR):
                raise
    return result

@contextlib.contextmanager
def _delayedinterrupt():
    """Block signal interrupt while doing something critical

    This makes sure that the code block wrapped by this context manager won't
    be interrupted.

    For Windows developers: It appears not possible to guard time.sleep()
    from CTRL_C_EVENT, so please don't use time.sleep() to test if this is
    working.
    """
    assertedsigs = []
    blocked = False
    orighandlers = {}

    def raiseinterrupt(num):
        if (num == getattr(signal, 'SIGINT', None) or
            num == getattr(signal, 'CTRL_C_EVENT', None)):
            raise KeyboardInterrupt
        else:
            raise error.SignalInterrupt
    def catchterm(num, frame):
        if blocked:
            assertedsigs.append(num)
        else:
            raiseinterrupt(num)

    try:
        # save handlers first so they can be restored even if a setup is
        # interrupted between signal.signal() and orighandlers[] =.
        for name in ['CTRL_C_EVENT', 'SIGINT', 'SIGBREAK', 'SIGHUP', 'SIGTERM']:
            num = getattr(signal, name, None)
            if num and num not in orighandlers:
                orighandlers[num] = signal.getsignal(num)
        try:
            for num in orighandlers:
                signal.signal(num, catchterm)
        except ValueError:
            pass # in a thread? no luck

        blocked = True
        yield
    finally:
        # no simple way to reliably restore all signal handlers because
        # any loops, recursive function calls, except blocks, etc. can be
        # interrupted. so instead, make catchterm() raise interrupt.
        blocked = False
        try:
            for num, handler in orighandlers.items():
                signal.signal(num, handler)
        except ValueError:
            pass # in a thread?

    # re-raise interrupt exception if any, which may be shadowed by a new
    # interrupt occurred while re-raising the first one
    if assertedsigs:
        raiseinterrupt(assertedsigs[0])

def trylock(ui, vfs, lockname, timeout, warntimeout, *args, **kwargs):
    """return an acquired lock or raise an a LockHeld exception

    This function is responsible to issue warnings and or debug messages about

        lockname = '%s:%d' % (lock._host, self.pid)
        retry = 5
        while not self.held and retry:
            retry -= 1
            try:
                with _delayedinterrupt():
                    self.vfs.makelock(lockname, self.f)
                    self.held = 1
            except (OSError, IOError) as why:
                if why.errno == errno.EEXIST:
                    locker = self._readlock()
                    if locker is None:
                        continue