Fixing IntegrityError happened on concurrent requests for course enrollment

ECOM-1730

common/djangoapps/student/models.py

@@ -1030,16 +1030,32 @@ class CourseEnrollment(models.Model):
         if user.id is None:
             user.save()
 
-        enrollment, created = cls.objects.get_or_create(
-            user=user,
-            course_id=course_key,
-        )
+        try:
+            enrollment, created = cls.objects.get_or_create(
+                user=user,
+                course_id=course_key,
+            )
 
-        # If we *did* just create a new enrollment, set some defaults
-        if created:
-            enrollment.mode = CourseMode.DEFAULT_MODE_SLUG
-            enrollment.is_active = False
-            enrollment.save()
+            # If we *did* just create a new enrollment, set some defaults
+            if created:
+                enrollment.mode = CourseMode.DEFAULT_MODE_SLUG
+                enrollment.is_active = False
+                enrollment.save()
+
+        except IntegrityError:
+            log.info(
+                (
+                    "An integrity error occurred while getting-or-creating the enrollment"
+                    "for course key %s and student %s. This can occur if two processes try to get-or-create "
+                    "the enrollment at the same time and the database is set to REPEATABLE READ. We will try "
+                    "committing the transaction and retrying."
+                ),
+                course_key, user
+            )
+            enrollment = cls.objects.get(
+                user=user,
+                course_id=course_key,
+            )
 
         return enrollment
 

common/djangoapps/student/tests/test_enrollment.py

@@ -8,12 +8,13 @@ from nose.plugins.attrib import attr
 
 from django.conf import settings
 from django.core.urlresolvers import reverse
+from django.db import IntegrityError
 from course_modes.models import CourseMode
 from xmodule.modulestore.tests.django_utils import SharedModuleStoreTestCase
 from xmodule.modulestore.tests.factories import CourseFactory
 from util.testing import UrlResetMixin
 from embargo.test_utils import restrict_course
-from student.tests.factories import UserFactory, CourseModeFactory
+from student.tests.factories import UserFactory, CourseModeFactory, CourseEnrollmentFactory
 from student.models import CourseEnrollment, CourseFullError
 from student.roles import (
     CourseInstructorRole,
@@ -281,3 +282,18 @@ class EnrollmentTest(UrlResetMixin, SharedModuleStoreTestCase):
             params['email_opt_in'] = email_opt_in
 
         return self.client.post(reverse('change_enrollment'), params)
+
+    def test_get_or_create_integrity_error(self):
+        """Verify that get_or_create_enrollment handles IntegrityError."""
+
+        CourseEnrollmentFactory.create(user=self.user, course_id=self.course.id)
+
+        with patch.object(CourseEnrollment.objects, "get_or_create") as mock_get_or_create:
+            mock_get_or_create.side_effect = IntegrityError
+            enrollment = CourseEnrollment.get_or_create_enrollment(
+                self.user,
+                self.course.id
+            )
+
+        self.assertEqual(enrollment.user, self.user)
+        self.assertEqual(enrollment.course.id, self.course.id)