Lock git repository while doing merge

weblate/trans/filelock.py

+import os
+import time
+import errno
+
+class FileLockException(Exception):
+    pass
+
+class FileLock(object):
+    """ A file locking mechanism that has context-manager support so
+        you can use it in a with statement. This should be relatively cross
+        compatible as it doesn't rely on msvcrt or fcntl for the locking.
+    """
+
+    def __init__(self, file_name, timeout=10, delay=.05):
+        """ Prepare the file locker. Specify the file to lock and optionally
+            the maximum timeout and the delay between each attempt to lock.
+        """
+        self.is_locked = False
+        self.lockfile = os.path.join(os.getcwd(), "%s.lock" % file_name)
+        self.file_name = file_name
+        self.timeout = timeout
+        self.delay = delay
+
+
+    def acquire(self):
+        """ Acquire the lock, if possible. If the lock is in use, it check again
+            every `wait` seconds. It does this until it either gets the lock or
+            exceeds `timeout` number of seconds, in which case it throws
+            an exception.
+        """
+        start_time = time.time()
+        while True:
+            try:
+                self.fd = os.open(self.lockfile, os.O_CREAT|os.O_EXCL|os.O_RDWR)
+                break;
+            except OSError as e:
+                if e.errno != errno.EEXIST:
+                    raise
+                if (time.time() - start_time) >= self.timeout:
+                    raise FileLockException("Timeout occured.")
+                time.sleep(self.delay)
+        self.is_locked = True
+
+
+    def release(self):
+        """ Get rid of the lock by deleting the lockfile.
+            When working in a `with` statement, this gets automatically
+            called at the end.
+        """
+        if self.is_locked:
+            os.close(self.fd)
+            os.unlink(self.lockfile)
+            self.is_locked = False
+
+
+    def __enter__(self):
+        """ Activated when used in the with statement.
+            Should automatically acquire a lock to be used in the with block.
+        """
+        if not self.is_locked:
+            self.acquire()
+        return self
+
+
+    def __exit__(self, type, value, traceback):
+        """ Activated at the end of the with statement.
+            It automatically releases the lock if it isn't locked.
+        """
+        if self.is_locked:
+            self.release()
+
+
+    def __del__(self):
+        """ Make sure that the FileLock instance doesn't leave a lockfile
+            lying around.
+        """
+        self.release()

weblate/trans/models.py

@@ -24,6 +24,7 @@ import weblate
 from weblate.lang.models import Language
 from weblate.trans.checks import CHECKS
 from weblate.trans.managers import TranslationManager, UnitManager, DictionaryManager
+from weblate.trans.filelock import FileLock
 from util import is_plural, split_plural, join_plural
 
 logger = logging.getLogger('weblate')
@@ -233,6 +234,20 @@ class SubProject(models.Model):
         '''
         return os.path.join(self.project.get_path(), self.slug)
 
+    def get_lock_path(self):
+        '''
+        Returns full path to subproject git repository.
+        '''
+        return os.path.join(self.project.get_path(), self.slug + '.lock')
+
+    def get_lock(self):
+        '''
+        Returns lock object for current translation instance.
+        '''
+        if not hasattr(self, '__lock__'):
+            self.__lock__ = FileLock(self.get_lock_path())
+        return self.__lock__
+
     def can_push(self):
         '''
         Returns true if push is possible for this subproject.
@@ -387,27 +402,29 @@ class SubProject(models.Model):
         # Update remote repo
         self.pull_repo(False, gitrepo)
 
-        try:
-            # Try to merge it
-            gitrepo.git.merge('origin/%s' % self.branch)
-            logger.info('merged remote into repo %s', self.__unicode__())
-            return True
-        except Exception, e:
-            # In case merge has failer recover and tell admins
-            status = gitrepo.git.status()
-            gitrepo.git.merge('--abort')
-            logger.warning('failed merge on repo %s', self.__unicode__())
-            msg = 'Error:\n%s' % str(e)
-            msg += '\n\nStatus:\n' + status
-            mail_admins(
-                'failed merge on repo %s' % self.__unicode__(),
-                msg
-            )
-            if request is not None:
-                messages.error(request, _('Failed to merge remote branch into %s.') % self.__unicode__())
-            return False
-        finally:
-            del gitrepo
+        # Mege with lock acquired
+        with self.get_lock():
+            try:
+                # Try to merge it
+                gitrepo.git.merge('origin/%s' % self.branch)
+                logger.info('merged remote into repo %s', self.__unicode__())
+                return True
+            except Exception, e:
+                # In case merge has failer recover and tell admins
+                status = gitrepo.git.status()
+                gitrepo.git.merge('--abort')
+                logger.warning('failed merge on repo %s', self.__unicode__())
+                msg = 'Error:\n%s' % str(e)
+                msg += '\n\nStatus:\n' + status
+                mail_admins(
+                    'failed merge on repo %s' % self.__unicode__(),
+                    msg
+                )
+                if request is not None:
+                    messages.error(request, _('Failed to merge remote branch into %s.') % self.__unicode__())
+                return False
+            finally:
+                del gitrepo
 
     def get_mask_matches(self):
         '''