Fixed #22291 -- Avoided shadowing deadlock exceptions on MySQL.

Thanks err for the report.

django/db/transaction.py

@@ -231,7 +231,13 @@ class Atomic(object):
                     if sid is None:
                         connection.needs_rollback = True
                     else:
-                        connection.savepoint_rollback(sid)
+                        try:
+                            connection.savepoint_rollback(sid)
+                        except DatabaseError:
+                            # If rolling back to a savepoint fails, mark for
+                            # rollback at a higher level and avoid shadowing
+                            # the original exception.
+                            connection.needs_rollback = True
                 else:
                     # Roll back transaction
                     connection.rollback()

tests/transactions/tests.py

@@ -1,9 +1,15 @@
 from __future__ import unicode_literals
 
 import sys
+try:
+    import threading
+except ImportError:
+    threading = None
+import time
 from unittest import skipIf, skipUnless
 
-from django.db import connection, transaction, DatabaseError, IntegrityError
+from django.db import (connection, transaction,
+    DatabaseError, IntegrityError, OperationalError)
 from django.test import TransactionTestCase, skipIfDBFeature
 from django.utils import six
 
@@ -333,6 +339,45 @@ class AtomicErrorsTests(TransactionTestCase):
         self.assertEqual(Reporter.objects.get(pk=r1.pk).last_name, "Calculus")
 
 
+@skipUnless(connection.vendor == 'mysql', "MySQL-specific behaviors")
+class AtomicMySQLTests(TransactionTestCase):
+
+    available_apps = ['transactions']
+
+    @skipIf(threading is None, "Test requires threading")
+    def test_implicit_savepoint_rollback(self):
+        """MySQL implicitly rolls back savepoints when it deadlocks (#22291)."""
+
+        other_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.exclude(id=1).update(id=2)
+            finally:
+                # This is the thread-local connection, not the main connection.
+                connection.close()
+
+        other_thread = threading.Thread(target=other_thread)
+        other_thread.start()
+        other_thread_ready.wait()
+
+        with six.assertRaisesRegex(self, 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")
+
+        other_thread.join()
+
+
 class AtomicMiscTests(TransactionTestCase):
 
     available_apps = []