index: IndexLock added to provide some kind of critical section for index based git operations in concurrent environments

Author: Byron

lib/git/index.py

@@ -21,7 +21,7 @@
 
 from errors import GitCommandError
 from git.objects import Blob, Tree, Object, Commit
-from git.utils import SHA1Writer, LazyMixin, ConcurrentWriteOperation, join_path_native
+from git.utils import SHA1Writer, LazyMixin, ConcurrentWriteOperation, join_path_native, BlockingLockFile
 
 
 class CheckoutError( Exception ):
@@ -70,6 +70,38 @@ def __del__(self):
 			os.rename(self.tmp_file_path, self.file_path)
 		# END temp file exists
 
+class IndexLock(BlockingLockFile):
+	"""Specialized lock that will wait for the index to be locked and immediately
+	move the lock out of the way. This allows a git command to take the lock 
+	which has to follow right after the lock() call.
+	
+	Right before the lock is released, we will move our original lock back into place.
+	This lock should be used to gain fine-grained control over multiple processes
+	or threads that try to manipulate the index and would otherwise fail.
+	Using this lock, these will wait for each other.
+	
+	NOTE:
+		There is still a small chance that git will fail as file system
+		operations take time as well.
+	"""
+	__slots__ = '_tmp_file_path'
+	
+	def _obtain_lock_or_raise(self):
+		super(IndexLock, self)._obtain_lock_or_raise()
+		self._tmp_file_path = self._lock_file_path() + tempfile.mktemp('','','')
+		# rename lock we aquired to move it out of the way
+		os.rename(self._lock_file_path(), self._tmp_file_path)
+		
+	def _release_lock(self):
+		# move our lock back into place
+		if hasattr(self, '_tmp_file_path') and os.path.isfile(self._tmp_file_path):
+			lock_file = self._lock_file_path()
+			if os.name == 'nt' and os.path.exists(lock_file):
+				os.remove(lock_file)
+			os.rename(self._tmp_file_path, lock_file)
+		# END temp file exists
+		super(IndexLock, self)._release_lock()
+	
 class BlobFilter(object):
 	"""
 	Predicate to be used by iter_blobs allowing to filter only return blobs which