Fixed #23727 -- Inhibited the post_migrate signal when using serialized_rollback.

When using a TransactionTestCase with serialized_rollback=True,
after creating the database and running its migrations (along with
emitting the post_migrate signal), the contents of the database
are serialized to _test_serialized_contents.

After the first test case, _fixture_teardown() would flush the
tables but then the post_migrate signal would be emitted and new
rows (with new PKs) would be created in the django_content_type
table. Then in any subsequent test cases in a suite,
_fixture_setup() attempts to deserialize the content of
 _test_serialized_contents, but these rows are identical to the
rows already in the database except for their PKs.  This causes an
IntegrityError due to the unique constraint in the
django_content_type table.

This change made it so that in the above scenario the post_migrate
signal is not emitted after flushing the tables, since it will be
repopulated during fixture_setup().

4 files changed, 46 insertions, 1 deletions

diff --git a/AUTHORS b/AUTHORS
index b553edd095..3fa10dc0a3 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -698,6 +698,7 @@ answer newbie questions, and generally made Django that much better:
     Tome Cvitan <tome@cvitan.com>
     Tomek Paczkowski <tomek@hauru.eu>
     Tom Insam
+    Tommy Beadle <tbeadle@gmail.com>
     Tom Tobin
     torne-django@wolfpuppy.org.uk
     Travis Cline <travis.cline@gmail.com>
diff --git a/django/test/testcases.py b/django/test/testcases.py
index 93ad8b9354..c4ee8984a9 100644
--- a/django/test/testcases.py
+++ b/django/test/testcases.py
@@ -940,10 +940,19 @@ class TransactionTestCase(SimpleTestCase):
         # when flushing only a subset of the apps
         for db_name in self._databases_names(include_mirrors=False):
             # Flush the database
+            inhibit_post_migrate = (
+                self.available_apps is not None
+                or (
+                    # Inhibit the post_migrate signal when using serialized
+                    # rollback to avoid trying to recreate the serialized data.
+                    self.serialized_rollback and
+                    hasattr(connections[db_name], '_test_serialized_contents')
+                )
+            )
             call_command('flush', verbosity=0, interactive=False,
                          database=db_name, reset_sequences=False,
                          allow_cascade=self.available_apps is not None,
-                         inhibit_post_migrate=self.available_apps is not None)
+                         inhibit_post_migrate=inhibit_post_migrate)
 
     def assertQuerysetEqual(self, qs, values, transform=repr, ordered=True, msg=None):
         items = six.moves.map(transform, qs)
diff --git a/docs/topics/testing/overview.txt b/docs/topics/testing/overview.txt
index d345e9c36d..fc18eb0e4a 100644
--- a/docs/topics/testing/overview.txt
+++ b/docs/topics/testing/overview.txt
@@ -250,6 +250,13 @@ The initial serialization is usually very quick, but if you wish to exclude
 some apps from this process (and speed up test runs slightly), you may add
 those apps to :setting:`TEST_NON_SERIALIZED_APPS`.
 
+.. versionchanged:: 1.9
+
+To prevent serialized data from being loaded twice, setting
+``serialized_rollback=True`` disables the
+:data:`~django.db.models.signals.post_migrate` signal when flushing the test
+database.
+
 Other test conditions
 ---------------------
 
diff --git a/tests/test_utils/test_transactiontestcase.py b/tests/test_utils/test_transactiontestcase.py
new file mode 100644
index 0000000000..34588ddefd
--- /dev/null
+++ b/tests/test_utils/test_transactiontestcase.py
@@ -0,0 +1,28 @@
+from django.test import TransactionTestCase, mock
+
+
+class TestSerializedRollbackInhibitsPostMigrate(TransactionTestCase):
+    """
+    TransactionTestCase._fixture_teardown() inhibits the post_migrate signal
+    for test classes with serialized_rollback=True.
+    """
+    available_apps = ['test_utils']
+    serialized_rollback = True
+
+    def setUp(self):
+        # self.available_apps must be None to test the serialized_rollback
+        # condition.
+        self.available_apps = None
+
+    def tearDown(self):
+        self.available_apps = ['test_utils']
+
+    @mock.patch('django.test.testcases.call_command')
+    def test(self, call_command):
+        # with a mocked call_command(), this doesn't have any effect.
+        self._fixture_teardown()
+        call_command.assert_called_with(
+            'flush', interactive=False, allow_cascade=False,
+            reset_sequences=False, inhibit_post_migrate=True,
+            database='default', verbosity=0,
+        )