CohortMembership Transaction Fixes

An issue arose recently due to ATOMIC_REQUESTS being turned on by default. It
turns out that CohortMemberships had been somewhat relying on the old default
transaction handling in order to keep CohortMemberships and the underlying
CourseUserGroup.users values in-sync.

To fix this, I've made all updates to Cohortmemberships go through an
outer_atomic(read_committed=True) block. This, is conjunction with the already
present select_for_update(), will no longer allow 2 simultaneous requests to
modify objects in memory without sharing them. Only one process will be
touching a given CohortMembership at any given time, and all changes will be
immediately comitted to the database, where the other process will see them.

I've also included some changes to get_cohort(), add_user_to_cohort(), and
remove_user_from_cohort() in order to properly make use of the new
CohortMembership system.

lms/djangoapps/instructor_task/tasks_helper.py

@@ -1505,7 +1505,6 @@ def cohort_students_and_upload(_xmodule_instance_args, _entry_id, course_id, tas
                 continue
 
             try:
-                with outer_atomic():
                 add_user_to_cohort(cohorts_status[cohort_name]['cohort'], username_or_email)
                 cohorts_status[cohort_name]['Students Added'] += 1
                 task_progress.succeeded += 1

openedx/core/djangoapps/course_groups/cohorts.py

@@ -179,22 +179,30 @@ def get_cohort(user, course_key, assign=True, use_cached=False):
     if not course_cohort_settings.is_cohorted:
         return request_cache.data.setdefault(cache_key, None)
 
-    # If course is cohorted, check if the user already has a cohort.
     try:
-        cohort = CourseUserGroup.objects.get(
+        membership = CohortMembership.objects.get(
             course_id=course_key,
-            group_type=CourseUserGroup.COHORT,
-            users__id=user.id,
+            user_id=user.id,
         )
-        return request_cache.data.setdefault(cache_key, cohort)
-    except CourseUserGroup.DoesNotExist:
+        return request_cache.data.setdefault(cache_key, membership.course_user_group)
+    except CohortMembership.DoesNotExist:
         # Didn't find the group. If we do not want to assign, return here.
         if not assign:
             # Do not cache the cohort here, because in the next call assign
             # may be True, and we will have to assign the user a cohort.
             return None
 
-    # Otherwise assign the user a cohort.
+    membership = CohortMembership.objects.create(
+        user=user,
+        course_user_group=_get_default_cohort(course_key)
+    )
+    return request_cache.data.setdefault(cache_key, membership.course_user_group)
+
+
+def _get_default_cohort(course_key):
+    """
+    Helper method to get a default cohort for assignment in get_cohort
+    """
     course = courses.get_course(course_key)
     cohorts = get_course_cohorts(course, assignment_type=CourseCohort.RANDOM)
     if cohorts:
@@ -205,11 +213,7 @@ def get_cohort(user, course_key, assign=True, use_cached=False):
             course_id=course_key,
             assignment_type=CourseCohort.RANDOM
         ).course_user_group
-
-    membership = CohortMembership(course_user_group=cohort, user=user)
-    membership.save()
-
-    return request_cache.data.setdefault(cache_key, cohort)
+    return cohort
 
 
 def migrate_cohort_settings(course):
@@ -350,7 +354,7 @@ def add_user_to_cohort(cohort, username_or_email):
     user = get_user_by_username_or_email(username_or_email)
 
     membership = CohortMembership(course_user_group=cohort, user=user)
-    membership.save()
+    membership.save()  # This will handle both cases, creation and updating, of a CohortMembership for this user.
 
     tracker.emit(
         "edx.cohort.user_add_requested",

openedx/core/djangoapps/course_groups/models.py

@@ -7,7 +7,10 @@ import logging
 
 from django.contrib.auth.models import User
 from django.db import models, transaction, IntegrityError
+from util.db import outer_atomic
 from django.core.exceptions import ValidationError
+from django.db.models.signals import pre_delete
+from django.dispatch import receiver
 from xmodule_django.models import CourseKeyField
 
 log = logging.getLogger(__name__)
@@ -85,23 +88,22 @@ class CohortMembership(models.Model):
             raise ValidationError("Non-matching course_ids provided")
 
     def save(self, *args, **kwargs):
+        self.full_clean(validate_unique=False)
+
         # Avoid infinite recursion if creating from get_or_create() call below.
+        # This block also allows middleware to use CohortMembership.get_or_create without worrying about outer_atomic
         if 'force_insert' in kwargs and kwargs['force_insert'] is True:
+            with transaction.atomic():
+                self.course_user_group.users.add(self.user)
+                self.course_user_group.save()
                 super(CohortMembership, self).save(*args, **kwargs)
             return
 
-        self.full_clean(validate_unique=False)
-
-        # This loop has been created to allow for optimistic locking, and retrial in case of losing a race condition.
-        # The limit is 2, since select_for_update ensures atomic updates. Creation is the only possible race condition.
-        max_retries = 2
-        success = False
-        for __ in range(max_retries):
+        # This block will transactionally commit updates to CohortMembership and underlying course_user_groups.
+        # Note the use of outer_atomic, which guarantees that operations are committed to the database on block exit.
+        # If called from a view method, that method must be marked with @transaction.non_atomic_requests.
+        with outer_atomic(read_committed=True):
 
-            with transaction.atomic():
-
-                try:
-                    with transaction.atomic():
             saved_membership, created = CohortMembership.objects.select_for_update().get_or_create(
                 user__id=self.user.id,
                 course_id=self.course_id,
@@ -110,10 +112,12 @@ class CohortMembership(models.Model):
                     'user': self.user
                 }
             )
-                except IntegrityError:  # This can happen if simultaneous requests try to create a membership
-                    continue
 
-                if not created:
+            # If the membership was newly created, all the validation and course_user_group logic was settled
+            # with a call to self.save(force_insert=True), which gets handled above.
+            if created:
+                return
+
             if saved_membership.course_user_group == self.course_user_group:
                 raise ValueError("User {user_name} already present in cohort {cohort_name}".format(
                     user_name=self.user.username,
@@ -123,17 +127,24 @@ class CohortMembership(models.Model):
             self.previous_cohort_name = saved_membership.course_user_group.name
             self.previous_cohort_id = saved_membership.course_user_group.id
             self.previous_cohort.users.remove(self.user)
+            self.previous_cohort.save()
 
             saved_membership.course_user_group = self.course_user_group
             self.course_user_group.users.add(self.user)
+            self.course_user_group.save()
 
             super(CohortMembership, saved_membership).save(update_fields=['course_user_group'])
 
-            success = True
-            break
 
-        if not success:
-            raise IntegrityError("Unable to save membership after {} tries, aborting.".format(max_retries))
+# Needs to exist outside class definition in order to use 'sender=CohortMembership'
+@receiver(pre_delete, sender=CohortMembership)
+def remove_user_from_cohort(sender, instance, **kwargs):  # pylint: disable=unused-argument
+    """
+    Ensures that when a CohortMemebrship is deleted, the underlying CourseUserGroup
+    has its users list updated to reflect the change as well.
+    """
+    instance.course_user_group.users.remove(instance.user)
+    instance.course_user_group.save()
 
 
 class CourseUserGroupPartitionGroup(models.Model):

openedx/core/djangoapps/course_groups/views.py

@@ -10,6 +10,8 @@ from django.core.urlresolvers import reverse
 from django.http import Http404, HttpResponseBadRequest
 from django.views.decorators.http import require_http_methods
 from util.json_request import expect_json, JsonResponse
+from util.db import outer_atomic
+from django.db import transaction
 from django.contrib.auth.decorators import login_required
 from django.utils.translation import ugettext
 
@@ -23,7 +25,7 @@ from edxmako.shortcuts import render_to_response
 
 from . import cohorts
 from lms.djangoapps.django_comment_client.utils import get_discussion_category_map, get_discussion_categories_ids
-from .models import CourseUserGroup, CourseUserGroupPartitionGroup
+from .models import CourseUserGroup, CourseUserGroupPartitionGroup, CohortMembership
 
 log = logging.getLogger(__name__)
 
@@ -299,6 +301,7 @@ def users_in_cohort(request, course_key_string, cohort_id):
                                'users': user_info})
 
 
+@transaction.non_atomic_requests
 @ensure_csrf_cookie
 @require_POST
 def add_users_to_cohort(request, course_key_string, cohort_id):
@@ -384,16 +387,22 @@ def remove_user_from_cohort(request, course_key_string, cohort_id):
         return json_http_response({'success': False,
                                    'msg': 'No username specified'})
 
-    cohort = cohorts.get_cohort_by_id(course_key, cohort_id)
     try:
         user = User.objects.get(username=username)
-        cohort.users.remove(user)
-        return json_http_response({'success': True})
     except User.DoesNotExist:
         log.debug('no user')
         return json_http_response({'success': False,
                                    'msg': "No user '{0}'".format(username)})
 
+    try:
+        membership = CohortMembership.objects.get(user=user, course_id=course_key)
+        membership.delete()
+
+    except CohortMembership.DoesNotExist:
+        pass
+
+    return json_http_response({'success': True})
+
 
 def debug_cohort_mgmt(request, course_key_string):
     """