Fixed #16047 -- Restore autocommit state correctly on psycopg2

When the postgresql_psycopg2 backend was used with DB-level autocommit
mode enabled, after entering transaction management and then leaving
it, the isolation level was never set back to autocommit mode.

Thanks brodie for report and working on this issue.

tests/regressiontests/transactions_regress/tests.py

@@ -1,11 +1,11 @@
 from __future__ import absolute_import
 
 from django.core.exceptions import ImproperlyConfigured
-from django.db import connection, transaction
+from django.db import connection, connections, transaction, DEFAULT_DB_ALIAS
 from django.db.transaction import commit_on_success, commit_manually, TransactionManagementError
 from django.test import TransactionTestCase, skipUnlessDBFeature
 from django.test.utils import override_settings
-from django.utils.unittest import skipIf
+from django.utils.unittest import skipIf, skipUnless
 
 from .models import Mod, M2mA, M2mB
 
@@ -175,6 +175,62 @@ class TestTransactionClosing(TransactionTestCase):
         self.test_failing_query_transaction_closed()
 
 
+class TestPostgresAutocommit(TransactionTestCase):
+    """
+    Tests to make sure psycopg2's autocommit mode is restored after entering
+    and leaving transaction management. Refs #16047.
+    """
+    def setUp(self):
+        from psycopg2.extensions import (ISOLATION_LEVEL_AUTOCOMMIT,
+                                         ISOLATION_LEVEL_READ_COMMITTED)
+        self._autocommit = ISOLATION_LEVEL_AUTOCOMMIT
+        self._read_committed = ISOLATION_LEVEL_READ_COMMITTED
+
+        # We want a clean backend with autocommit = True, so
+        # first we need to do a bit of work to have that.
+        self._old_backend = connections[DEFAULT_DB_ALIAS]
+        settings = self._old_backend.settings_dict.copy()
+        opts = settings['OPTIONS'].copy()
+        opts['autocommit'] = True
+        settings['OPTIONS'] = opts
+        new_backend = self._old_backend.__class__(settings, DEFAULT_DB_ALIAS)
+        connections[DEFAULT_DB_ALIAS] = new_backend
+
+    def test_initial_autocommit_state(self):
+        self.assertTrue(connection.features.uses_autocommit)
+        self.assertEqual(connection.isolation_level, self._autocommit)
+
+    def test_transaction_management(self):
+        transaction.enter_transaction_management()
+        transaction.managed(True)
+        self.assertEqual(connection.isolation_level, self._read_committed)
+
+        transaction.leave_transaction_management()
+        self.assertEqual(connection.isolation_level, self._autocommit)
+
+    def test_transaction_stacking(self):
+        transaction.enter_transaction_management()
+        transaction.managed(True)
+        self.assertEqual(connection.isolation_level, self._read_committed)
+
+        transaction.enter_transaction_management()
+        self.assertEqual(connection.isolation_level, self._read_committed)
+
+        transaction.leave_transaction_management()
+        self.assertEqual(connection.isolation_level, self._read_committed)
+
+        transaction.leave_transaction_management()
+        self.assertEqual(connection.isolation_level, self._autocommit)
+
+    def tearDown(self):
+        connections[DEFAULT_DB_ALIAS] = self._old_backend
+
+TestPostgresAutocommit = skipUnless(connection.vendor == 'postgresql',
+    "This test only valid for PostgreSQL")(TestPostgresAutocommit)
+TestPostgresAutoCommit = skipUnlessDBFeature('supports_transactions')(
+    TestPostgresAutocommit)
+
+
 class TestManyToManyAddTransaction(TransactionTestCase):
     def test_manyrelated_add_commit(self):
         "Test for https://code.djangoproject.com/ticket/16818"