Add locking to the file metadata task

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

HISTORY.rst

@@ -3,6 +3,7 @@
 - Update the gunicorn worker class to synchronous.
 - Update the way the BaseTransformationType metaclass is passed
   to work on Python 3.
+- Add locking to the file metadata document processing task.
 
 3.3.3 (2019-12-05)
 ==================

mayan/apps/file_metadata/literals.py

@@ -1,3 +1,4 @@
 from __future__ import unicode_literals
 
 DEFAULT_EXIF_PATH = '/usr/bin/exiftool'
+LOCK_EXPIRE = 60 * 10  # Adjust to worst case scenario

mayan/apps/file_metadata/tasks.py

@@ -4,8 +4,12 @@ import logging
 
 from django.apps import apps
 
+from mayan.apps.lock_manager.exceptions import LockError
+from mayan.apps.lock_manager.runtime import locking_backend
 from mayan.celery import app
 
+from .literals import LOCK_EXPIRE
+
 from .classes import FileMetadataDriver
 
 logger = logging.getLogger(__name__)
@@ -19,6 +23,17 @@ def task_process_document_version(document_version_id):
 
     document_version = DocumentVersion.objects.get(pk=document_version_id)
 
-    FileMetadataDriver.process_document_version(
-        document_version=document_version
-    )
+    lock_id = 'task_process_document_version-%d' % document_version_id
+    try:
+        logger.debug('trying to acquire lock: %s', lock_id)
+        # Acquire lock to avoid processing the same document version more
+        # than once concurrently
+        lock = locking_backend.acquire_lock(name=lock_id, timeout=LOCK_EXPIRE)
+        logger.debug('acquired lock: %s', lock_id)
+    except LockError:
+        logger.debug('unable to obtain lock: %s' % lock_id)
+    else:
+        FileMetadataDriver.process_document_version(
+            document_version=document_version
+        )
+        lock.release()