lock: fix race in lock-breaking code

With low frequency, I see hg pulls fail with output like:
abort: no such file or directory: .hg/store/lock

I think what happens is, in lock.py, in:

    def _testlock(self, locker):
        if not self._lockshouldbebroken(locker):
            return locker

        # if locker dead, break lock.  must do this with another lock
        # held, or can race and break valid lock.
        try:
            with lock(self.vfs, self.f + b'.break', timeout=0):
                self.vfs.unlink(self.f)
        except error.LockError:
            return locker

if a lock is breakable on disk, and two hg processes concurrently get
to the "if locker dead" comment, a possible interleaving is: process1
finishes executing the function and then process2 finishes executing
the function. If that happens, process2 will either get ENOENT in
self.vfs.unlink (resulting in the spurious failure above), or break a
valid lock and potentially cause repository corruption.

The fix is simple enough: make sure the lock is breakable _inside_ the
critical section, because only then can we know that no other process
can invalidate our knowledge on the lock on disk.

I don't think there are tests for this. I've tested this manually
with:

diff --git a/mercurial/lock.py b/mercurial/lock.py
--- a/mercurial/lock.py
+++ b/mercurial/lock.py
@@ -351,6 +351,8 @@ class lock(object):
         if not self._lockshouldbebroken(locker):
             return locker

+        import random
+        time.sleep(1. + random.random())
         # if locker dead, break lock.  must do this with another lock
         # held, or can race and break valid lock.
         try:
@@ -358,6 +360,7 @@ class lock(object):
                 self.vfs.unlink(self.f)
         except error.LockError:
             return locker
+        time.sleep(1)

     def testlock(self):
         """return id of locker if lock is valid, else None.

and I see this change of behavior before/after this commit:

 $ $hg init repo
 $ cd repo
 $ ln -s $HOSTNAME/effffffc:987654321 .hg/wlock
 $ touch a
 $ $hg commit -Am_ & $hg commit -Am _; wait
-abort: No such file or directory: '/tmp/repo/.hg/wlock'
 adding a
+warning: ignoring unknown working parent 679a8959a8ca!
+nothing changed

Differential Revision: https://phab.mercurial-scm.org/D7199

mercurial/lock.py

@@ -1,441 +1,444 b''
 # lock.py - simple advisory locking scheme for mercurial
 #
 # Copyright 2005, 2006 Matt Mackall <[e-mail]>
 #
 # 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 errno
 import os
 import signal
 import socket
 import time
 import warnings
 
 from .i18n import _
 from .pycompat import getattr
 
 from . import (
     encoding,
     error,
     pycompat,
     util,
 )
 
 from .utils import procutil
 
 
 def _getlockprefix():
     """Return a string which is used to differentiate pid namespaces
 
     It's useful to detect "dead" processes and remove stale locks with
     confidence. Typically it's just hostname. On modern linux, we include an
     extra Linux-specific pid namespace identifier.
     """
     result = encoding.strtolocal(socket.gethostname())
     if pycompat.sysplatform.startswith(b'linux'):
         try:
             result += b'/%x' % os.stat(b'/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 [
             b'CTRL_C_EVENT',
             b'SIGINT',
             b'SIGBREAK',
             b'SIGHUP',
             b'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
     the held lock while trying to acquires it."""
 
     def printwarning(printer, locker):
         """issue the usual "waiting on lock" message through any channel"""
         # show more details for new-style locks
         if b':' in locker:
             host, pid = locker.split(b":", 1)
             msg = _(
                 b"waiting for lock on %s held by process %r on host %r\n"
             ) % (
                 pycompat.bytestr(l.desc),
                 pycompat.bytestr(pid),
                 pycompat.bytestr(host),
             )
         else:
             msg = _(b"waiting for lock on %s held by %r\n") % (
                 l.desc,
                 pycompat.bytestr(locker),
             )
         printer(msg)
 
     l = lock(vfs, lockname, 0, *args, dolock=False, **kwargs)
 
     debugidx = 0 if (warntimeout and timeout) else -1
     warningidx = 0
     if not timeout:
         warningidx = -1
     elif warntimeout:
         warningidx = warntimeout
 
     delay = 0
     while True:
         try:
             l._trylock()
             break
         except error.LockHeld as inst:
             if delay == debugidx:
                 printwarning(ui.debug, inst.locker)
             if delay == warningidx:
                 printwarning(ui.warn, inst.locker)
             if timeout <= delay:
                 raise error.LockHeld(
                     errno.ETIMEDOUT, inst.filename, l.desc, inst.locker
                 )
             time.sleep(1)
             delay += 1
 
     l.delay = delay
     if l.delay:
         if 0 <= warningidx <= l.delay:
             ui.warn(_(b"got lock after %d seconds\n") % l.delay)
         else:
             ui.debug(b"got lock after %d seconds\n" % l.delay)
     if l.acquirefn:
         l.acquirefn()
     return l
 
 
 class lock(object):
     '''An advisory lock held by one process to control access to a set
     of files.  Non-cooperating processes or incorrectly written scripts
     can ignore Mercurial's locking scheme and stomp all over the
     repository, so don't do that.
 
     Typically used via localrepository.lock() to lock the repository
     store (.hg/store/) or localrepository.wlock() to lock everything
     else under .hg/.'''
 
     # lock is symlink on platforms that support it, file on others.
 
     # symlink is used because create of directory entry and contents
     # are atomic even over nfs.
 
     # old-style lock: symlink to pid
     # new-style lock: symlink to hostname:pid
 
     _host = None
 
     def __init__(
         self,
         vfs,
         fname,
         timeout=-1,
         releasefn=None,
         acquirefn=None,
         desc=None,
         inheritchecker=None,
         parentlock=None,
         signalsafe=True,
         dolock=True,
     ):
         self.vfs = vfs
         self.f = fname
         self.held = 0
         self.timeout = timeout
         self.releasefn = releasefn
         self.acquirefn = acquirefn
         self.desc = desc
         self._inheritchecker = inheritchecker
         self.parentlock = parentlock
         self._parentheld = False
         self._inherited = False
         if signalsafe:
             self._maybedelayedinterrupt = _delayedinterrupt
         else:
             self._maybedelayedinterrupt = util.nullcontextmanager
         self.postrelease = []
         self.pid = self._getpid()
         if dolock:
             self.delay = self.lock()
             if self.acquirefn:
                 self.acquirefn()
 
     def __enter__(self):
         return self
 
     def __exit__(self, exc_type, exc_value, exc_tb):
         self.release()
 
     def __del__(self):
         if self.held:
             warnings.warn(
                 "use lock.release instead of del lock",
                 category=DeprecationWarning,
                 stacklevel=2,
             )
 
             # ensure the lock will be removed
             # even if recursive locking did occur
             self.held = 1
 
         self.release()
 
     def _getpid(self):
         # wrapper around procutil.getpid() to make testing easier
         return procutil.getpid()
 
     def lock(self):
         timeout = self.timeout
         while True:
             try:
                 self._trylock()
                 return self.timeout - timeout
             except error.LockHeld as inst:
                 if timeout != 0:
                     time.sleep(1)
                     if timeout > 0:
                         timeout -= 1
                     continue
                 raise error.LockHeld(
                     errno.ETIMEDOUT, inst.filename, self.desc, inst.locker
                 )
 
     def _trylock(self):
         if self.held:
             self.held += 1
             return
         if lock._host is None:
             lock._host = _getlockprefix()
         lockname = b'%s:%d' % (lock._host, self.pid)
         retry = 5
         while not self.held and retry:
             retry -= 1
             try:
                 with self._maybedelayedinterrupt():
                     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
 
                     # special case where a parent process holds the lock -- this
                     # is different from the pid being different because we do
                     # want the unlock and postrelease functions to be called,
                     # but the lockfile to not be removed.
                     if locker == self.parentlock:
                         self._parentheld = True
                         self.held = 1
                         return
                     locker = self._testlock(locker)
                     if locker is not None:
                         raise error.LockHeld(
                             errno.EAGAIN,
                             self.vfs.join(self.f),
                             self.desc,
                             locker,
                         )
                 else:
                     raise error.LockUnavailable(
                         why.errno, why.strerror, why.filename, self.desc
                     )
 
         if not self.held:
             # use empty locker to mean "busy for frequent lock/unlock
             # by many processes"
             raise error.LockHeld(
                 errno.EAGAIN, self.vfs.join(self.f), self.desc, b""
             )
 
     def _readlock(self):
         """read lock and return its value
 
         Returns None if no lock exists, pid for old-style locks, and host:pid
         for new-style locks.
         """
         try:
             return self.vfs.readlock(self.f)
         except (OSError, IOError) as why:
             if why.errno == errno.ENOENT:
                 return None
             raise
 
     def _lockshouldbebroken(self, locker):
         if locker is None:
             return False
         try:
             host, pid = locker.split(b":", 1)
         except ValueError:
             return False
         if host != lock._host:
             return False
         try:
             pid = int(pid)
         except ValueError:
             return False
         if procutil.testpid(pid):
             return False
         return True
 
     def _testlock(self, locker):
         if not self._lockshouldbebroken(locker):
             return locker
 
         # if locker dead, break lock.  must do this with another lock
         # held, or can race and break valid lock.
         try:
             with lock(self.vfs, self.f + b'.break', timeout=0):
+                locker = self._readlock()
+                if not self._lockshouldbebroken(locker):
+                    return locker
                 self.vfs.unlink(self.f)
         except error.LockError:
             return locker
 
     def testlock(self):
         """return id of locker if lock is valid, else None.
 
         If old-style lock, we cannot tell what machine locker is on.
         with new-style lock, if locker is on this machine, we can
         see if locker is alive.  If locker is on this machine but
         not alive, we can safely break lock.
 
         The lock file is only deleted when None is returned.
 
         """
         locker = self._readlock()
         return self._testlock(locker)
 
     @contextlib.contextmanager
     def inherit(self):
         """context for the lock to be inherited by a Mercurial subprocess.
 
         Yields a string that will be recognized by the lock in the subprocess.
         Communicating this string to the subprocess needs to be done separately
         -- typically by an environment variable.
         """
         if not self.held:
             raise error.LockInheritanceContractViolation(
                 b'inherit can only be called while lock is held'
             )
         if self._inherited:
             raise error.LockInheritanceContractViolation(
                 b'inherit cannot be called while lock is already inherited'
             )
         if self._inheritchecker is not None:
             self._inheritchecker()
         if self.releasefn:
             self.releasefn()
         if self._parentheld:
             lockname = self.parentlock
         else:
             lockname = b'%s:%d' % (lock._host, self.pid)
         self._inherited = True
         try:
             yield lockname
         finally:
             if self.acquirefn:
                 self.acquirefn()
             self._inherited = False
 
     def release(self):
         """release the lock and execute callback function if any
 
         If the lock has been acquired multiple times, the actual release is
         delayed to the last release call."""
         if self.held > 1:
             self.held -= 1
         elif self.held == 1:
             self.held = 0
             if self._getpid() != self.pid:
                 # we forked, and are not the parent
                 return
             try:
                 if self.releasefn:
                     self.releasefn()
             finally:
                 if not self._parentheld:
                     try:
                         self.vfs.unlink(self.f)
                     except OSError:
                         pass
             # The postrelease functions typically assume the lock is not held
             # at all.
             if not self._parentheld:
                 for callback in self.postrelease:
                     callback()
                 # Prevent double usage and help clear cycles.
                 self.postrelease = None
 
 
 def release(*locks):
     for lock in locks:
         if lock is not None:
             lock.release()