Fixed #24921 -- set_autocommit(False) + ORM queries.

This commits lifts the restriction that the outermost atomic block must
be declared with savepoint=False. This restriction was overly cautious.

The logic that makes it safe not to create savepoints for inner blocks
also applies to the outermost block when autocommit is disabled and a
transaction is already active.

This makes it possible to use the ORM after set_autocommit(False).
Previously it didn't work because ORM write operations are protected
with atomic(savepoint=False).

tests/transactions/tests.py

@@ -398,16 +398,18 @@ class AtomicMiscTests(TransactionTestCase):
     available_apps = []
 
     def test_wrap_callable_instance(self):
-        # Regression test for #20028
+        """#20028 -- Atomic must support wrapping callable instances."""
+
         class Callable(object):
             def __call__(self):
                 pass
+
         # Must not raise an exception
         transaction.atomic(Callable())
 
     @skipUnlessDBFeature('can_release_savepoints')
     def test_atomic_does_not_leak_savepoints_on_failure(self):
-        # Regression test for #23074
+        """#23074 -- Savepoints must be released after rollback."""
 
         # Expect an error when rolling back a savepoint that doesn't exist.
         # Done outside of the transaction block to ensure proper recovery.
@@ -426,3 +428,14 @@ class AtomicMiscTests(TransactionTestCase):
 
                 # This is expected to fail because the savepoint no longer exists.
                 connection.savepoint_rollback(sid)
+
+    @skipIf(connection.features.autocommits_when_autocommit_is_off,
+            "This test requires a non-autocommit mode that doesn't autocommit.")
+    def test_orm_query_without_autocommit(self):
+        """#24921 -- ORM queries must be possible after set_autocommit(False)."""
+        transaction.set_autocommit(False)
+        try:
+            Reporter.objects.create(first_name="Tintin")
+        finally:
+            transaction.rollback()
+            transaction.set_autocommit(True)