Add retries when encountering DatabaseError when updating InstructorTask.

lms/djangoapps/bulk_email/tests/test_err_handling.py

@@ -11,7 +11,7 @@ from django.test.utils import override_settings
 from django.conf import settings
 from django.core.management import call_command
 from django.core.urlresolvers import reverse
-
+from django.db import DatabaseError
 
 from courseware.tests.tests import TEST_DATA_MONGO_MODULESTORE
 from xmodule.modulestore.tests.django_utils import ModuleStoreTestCase
@@ -27,6 +27,7 @@ from instructor_task.subtasks import (
     check_subtask_is_valid,
     update_subtask_status,
     DuplicateTaskException,
+    MAX_DATABASE_LOCK_RETRIES,
 )
 
 
@@ -285,13 +286,29 @@ class TestEmailErrors(ModuleStoreTestCase):
         with self.assertRaisesRegexp(DuplicateTaskException, 'already retried'):
             send_course_email(entry_id, bogus_email_id, to_list, global_email_context, new_subtask_status.to_dict())
 
-    def dont_test_send_email_undefined_email(self):
+    def test_send_email_with_locked_instructor_task(self):
         # test at a lower level, to ensure that the course gets checked down below too.
         entry = InstructorTask.create(self.course.id, "task_type", "task_key", "task_input", self.instructor)
         entry_id = entry.id  # pylint: disable=E1101
+        subtask_id = "subtask-id-locked-model"
+        initialize_subtask_info(entry, "emailed", 100, [subtask_id])
+        subtask_status = SubtaskStatus.create(subtask_id)
+        bogus_email_id = 1001
         to_list = ['test@test.com']
         global_email_context = {'course_title': 'dummy course'}
-        subtask_id = "subtask-id-value"
+        with patch('instructor_task.subtasks.InstructorTask.save') as mock_task_save:
+            mock_task_save.side_effect = DatabaseError
+            with self.assertRaises(DatabaseError):
+                send_course_email(entry_id, bogus_email_id, to_list, global_email_context, subtask_status.to_dict())
+            self.assertEquals(mock_task_save.call_count, MAX_DATABASE_LOCK_RETRIES)
+
+    def test_send_email_undefined_email(self):
+        # test at a lower level, to ensure that the course gets checked down below too.
+        entry = InstructorTask.create(self.course.id, "task_type", "task_key", "task_input", self.instructor)
+        entry_id = entry.id  # pylint: disable=E1101
+        to_list = ['test@test.com']
+        global_email_context = {'course_title': 'dummy course'}
+        subtask_id = "subtask-id-undefined-email"
         initialize_subtask_info(entry, "emailed", 100, [subtask_id])
         subtask_status = SubtaskStatus.create(subtask_id)
         bogus_email_id = 1001

lms/djangoapps/instructor_task/subtasks.py

@@ -10,7 +10,7 @@ from celery.utils.log import get_task_logger
 from celery.states import SUCCESS, READY_STATES, RETRY
 from dogapi import dog_stats_api
 
-from django.db import transaction
+from django.db import transaction, DatabaseError
 from django.core.cache import cache
 
 from instructor_task.models import InstructorTask, PROGRESS, QUEUING
@@ -19,6 +19,9 @@ TASK_LOG = get_task_logger(__name__)
 
 # Lock expiration should be long enough to allow a subtask to complete.
 SUBTASK_LOCK_EXPIRE = 60 * 10  # Lock expires in 10 minutes
+# Number of times to retry if a subtask update encounters a lock on the InstructorTask.
+# (These are recursive retries, so don't make this number too large.)
+MAX_DATABASE_LOCK_RETRIES = 5
 
 
 class DuplicateTaskException(Exception):
@@ -411,8 +414,42 @@ def check_subtask_is_valid(entry_id, current_task_id, new_subtask_status):
         raise DuplicateTaskException(msg)
 
 
+def update_subtask_status(entry_id, current_task_id, new_subtask_status, retry_count=0):
+    """
+    Update the status of the subtask in the parent InstructorTask object tracking its progress.
+
+    Because select_for_update is used to lock the InstructorTask object while it is being updated,
+    multiple subtasks updating at the same time may time out while waiting for the lock.
+    The actual update operation is surrounded by a try/except/else that permits the update to be
+    retried if the transaction times out.
+
+    The subtask lock acquired in the call to check_subtask_is_valid() is released here, only when
+    the attempting of retries has concluded.
+    """
+    try:
+        _update_subtask_status(entry_id, current_task_id, new_subtask_status)
+    except DatabaseError:
+        # If we fail, try again recursively.
+        retry_count += 1
+        if retry_count < MAX_DATABASE_LOCK_RETRIES:
+            TASK_LOG.info("Retrying to update status for subtask %s of instructor task %d with status %s:  retry %d",
+                          current_task_id, entry_id, new_subtask_status, retry_count)
+            dog_stats_api.increment('instructor_task.subtask.retry_after_failed_update')
+            update_subtask_status(entry_id, current_task_id, new_subtask_status, retry_count)
+        else:
+            TASK_LOG.info("Failed to update status after %d retries for subtask %s of instructor task %d with status %s",
+                          retry_count, current_task_id, entry_id, new_subtask_status)
+            dog_stats_api.increment('instructor_task.subtask.failed_after_update_retries')
+            raise
+    finally:
+        # Only release the lock on the subtask when we're done trying to update it.
+        # Note that this will be called each time a recursive call to update_subtask_status()
+        # returns.  Fortunately, it's okay to release a lock that has already been released.
+        _release_subtask_lock(current_task_id)
+
+
 @transaction.commit_manually
-def update_subtask_status(entry_id, current_task_id, new_subtask_status):
+def _update_subtask_status(entry_id, current_task_id, new_subtask_status):
     """
     Update the status of the subtask in the parent InstructorTask object tracking its progress.
 
@@ -493,17 +530,15 @@ def update_subtask_status(entry_id, current_task_id, new_subtask_status):
         entry.subtasks = json.dumps(subtask_dict)
         entry.task_output = InstructorTask.create_output_for_success(task_progress)
 
-        TASK_LOG.info("Task output updated to %s for subtask %s of instructor task %d",
-                      entry.task_output, current_task_id, entry_id)
         TASK_LOG.debug("about to save....")
         entry.save()
+        TASK_LOG.info("Task output updated to %s for subtask %s of instructor task %d",
+                      entry.task_output, current_task_id, entry_id)
     except Exception:
         TASK_LOG.exception("Unexpected error while updating InstructorTask.")
         transaction.rollback()
-        dog_stats_api.increment('instructor_task.subtask.update_exception', tags=[entry.course_id])
+        dog_stats_api.increment('instructor_task.subtask.update_exception')
         raise
     else:
         TASK_LOG.debug("about to commit....")
         transaction.commit()
-    finally:
-        _release_subtask_lock(current_task_id)