Fixed #27683 -- Made MySQL default to the read committed isolation level.

Thanks Shai Berger for test help and Adam Johnson for review.

2 files changed, 22 insertions, 10 deletions

diff --git a/tests/backends/test_mysql.py b/tests/backends/test_mysql.py
index 637c3e377c..298ca9265f 100644
--- a/tests/backends/test_mysql.py
+++ b/tests/backends/test_mysql.py
@@ -70,6 +70,15 @@ class MySQLTests(TestCase):
                 self.isolation_values[self.other_isolation_level]
             )
 
+    def test_default_isolation_level(self):
+        # If not specified in settings, the default is read committed.
+        with get_connection() as new_connection:
+            new_connection.settings_dict['OPTIONS'].pop('isolation_level', None)
+            self.assertEqual(
+                self.get_isolation_level(new_connection),
+                self.isolation_values[self.read_committed]
+            )
+
     def test_isolation_level_validation(self):
         new_connection = connection.copy()
         new_connection.settings_dict['OPTIONS']['isolation_level'] = 'xxx'
diff --git a/tests/transactions/tests.py b/tests/transactions/tests.py
index 033619c0c8..8290bce1e5 100644
--- a/tests/transactions/tests.py
+++ b/tests/transactions/tests.py
@@ -375,18 +375,17 @@ class AtomicMySQLTests(TransactionTestCase):
     @skipIf(threading is None, "Test requires threading")
     def test_implicit_savepoint_rollback(self):
         """MySQL implicitly rolls back savepoints when it deadlocks (#22291)."""
+        Reporter.objects.create(id=1)
+        Reporter.objects.create(id=2)
 
-        other_thread_ready = threading.Event()
+        main_thread_ready = threading.Event()
 
         def other_thread():
             try:
                 with transaction.atomic():
-                    Reporter.objects.create(id=1, first_name="Tintin")
-                    other_thread_ready.set()
-                    # We cannot synchronize the two threads with an event here
-                    # because the main thread locks. Sleep for a little while.
-                    time.sleep(1)
-                    # 2) ... and this line deadlocks. (see below for 1)
+                    Reporter.objects.select_for_update().get(id=1)
+                    main_thread_ready.wait()
+                    # 1) This line locks... (see below for 2)
                     Reporter.objects.exclude(id=1).update(id=2)
             finally:
                 # This is the thread-local connection, not the main connection.
@@ -394,14 +393,18 @@ class AtomicMySQLTests(TransactionTestCase):
 
         other_thread = threading.Thread(target=other_thread)
         other_thread.start()
-        other_thread_ready.wait()
 
         with self.assertRaisesMessage(OperationalError, 'Deadlock found'):
             # Double atomic to enter a transaction and create a savepoint.
             with transaction.atomic():
                 with transaction.atomic():
-                    # 1) This line locks... (see above for 2)
-                    Reporter.objects.create(id=1, first_name="Tintin")
+                    Reporter.objects.select_for_update().get(id=2)
+                    main_thread_ready.set()
+                    # The two threads can't be synchronized with an event here
+                    # because the other thread locks. Sleep for a little while.
+                    time.sleep(1)
+                    # 2) ... and this line deadlocks. (see above for 1)
+                    Reporter.objects.exclude(id=2).update(id=1)
 
         other_thread.join()