lock: add a method to prepare the lock for inheritance

This is part of a series that will allow locks to be inherited by subprocesses
in limited circumstances.

mercurial/lock.py

@@ -1,187 +1,209 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 errno
 import os
 import socket
 import time
 import warnings
 
 from . import (
     error,
     util,
 )
 
 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, parentlock=None):
         self.vfs = vfs
         self.f = file
         self.held = 0
         self.timeout = timeout
         self.releasefn = releasefn
         self.acquirefn = acquirefn
         self.desc = desc
         self.parentlock = parentlock
         self._parentheld = False
         self._inherited = False
         self.postrelease  = []
         self.pid = os.getpid()
         self.delay = self.lock()
         if self.acquirefn:
             self.acquirefn()
 
     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 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 = socket.gethostname()
         lockname = '%s:%s' % (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.testlock()
                     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)
 
+    def prepinherit(self):
+        """prepare for the lock to be inherited by a Mercurial subprocess
+
+        Returns 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(
+                'prepinherit can only be called while lock is held')
+        if self._inherited:
+            raise error.LockInheritanceContractViolation(
+                'prepinherit cannot be called while lock is already inherited')
+        if self.releasefn:
+            self.releasefn()
+        if self._parentheld:
+            lockname = self.parentlock
+        else:
+            lockname = '%s:%s' % (lock._host, self.pid)
+        self._inherited = True
+        return lockname
+
     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 os.getpid() != self.pid:
                 # we forked, and are not the parent
                 return
             try:
                 if self.releasefn:
                     self.releasefn()
             finally:
                 try:
                     self.vfs.unlink(self.f)
                 except OSError:
                     pass
             for callback in self.postrelease:
                 callback()
 
 def release(*locks):
     for lock in locks:
         if lock is not None:
             lock.release()