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
self.parentlock is None (this is default value), and lock._readlock()
returns None for ENOENT at vfs.readlock(), because these None are
recognized as equal to each other.

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 retries lock acquisition immediately,
if lock._readlock() returns None "locker".

This issue will be covered by a test added in subsequent patch,
because simple emulation of ENOENT at vfs.readlock() easily causes
another issue.  "raising ENOENT only at the first vfs.readlock()
invocation" is too complicated for unit test, IMHO.

mercurial/lock.py

@@ -1,264 +1,267 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 socket
 import time
 import warnings
 
 from . import (
     encoding,
     error,
     pycompat,
     util,
 )
 
 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 = socket.gethostname()
     if pycompat.ispy3:
         result = result.encode(pycompat.sysstr(encoding.encoding), 'replace')
     if pycompat.sysplatform.startswith('linux'):
         try:
             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
 
 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, file, timeout=-1, releasefn=None, acquirefn=None,
                  desc=None, inheritchecker=None, parentlock=None):
         self.vfs = vfs
         self.f = file
         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
         self.postrelease  = []
         self.pid = self._getpid()
         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 util.getpid() to make testing easier
         return util.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 = '%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
+
                     # 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)
 
     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 _testlock(self, locker):
         if locker is None:
             return None
         try:
             host, pid = locker.split(":", 1)
         except ValueError:
             return locker
         if host != lock._host:
             return locker
         try:
             pid = int(pid)
         except ValueError:
             return locker
         if util.testpid(pid):
             return locker
         # if locker dead, break lock.  must do this with another lock
         # held, or can race and break valid lock.
         try:
             l = lock(self.vfs, self.f + '.break', timeout=0)
             self.vfs.unlink(self.f)
             l.release()
         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(
                 'inherit can only be called while lock is held')
         if self._inherited:
             raise error.LockInheritanceContractViolation(
                 '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 = '%s:%s' % (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()