[1.7.x] Fixed #24318 -- Set the transaction isolation level with psycopg >= 2.4.2.

Backport of 76356d96 from master

django/db/backends/postgresql_psycopg2/base.py

@@ -93,10 +93,6 @@ class DatabaseWrapper(BaseDatabaseWrapper):
     def __init__(self, *args, **kwargs):
         super(DatabaseWrapper, self).__init__(*args, **kwargs)
 
-        opts = self.settings_dict["OPTIONS"]
-        RC = psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED
-        self.isolation_level = opts.get('isolation_level', RC)
-
         self.features = DatabaseFeatures(self)
         self.ops = DatabaseOperations(self)
         self.client = DatabaseClient(self)
@@ -131,7 +127,29 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         return conn_params
 
     def get_new_connection(self, conn_params):
-        return Database.connect(**conn_params)
+        connection = Database.connect(**conn_params)
+
+        # self.isolation_level must be set:
+        # - after connecting to the database in order to obtain the database's
+        #   default when no value is explicitly specified in options.
+        # - before calling _set_autocommit() because if autocommit is on, that
+        #   will set connection.isolation_level to ISOLATION_LEVEL_AUTOCOMMIT;
+        #   and if autocommit is off, on psycopg2 < 2.4.2, _set_autocommit()
+        #   needs self.isolation_level.
+        options = self.settings_dict['OPTIONS']
+        try:
+            self.isolation_level = options['isolation_level']
+        except KeyError:
+            self.isolation_level = connection.isolation_level
+        else:
+            # Set the isolation level to the value from OPTIONS. This isn't
+            # needed on psycopg2 < 2.4.2 because it happens as a side-effect
+            # of _set_autocommit(False).
+            if (self.isolation_level != connection.isolation_level and
+                    self.psycopg2_version >= (2, 4, 2)):
+                connection.set_session(isolation_level=self.isolation_level)
+
+        return connection
 
     def init_connection_state(self):
         settings_dict = self.settings_dict

docs/releases/1.7.5.txt

@@ -19,3 +19,7 @@ Bugfixes
 
 * Fixed crash in ``contrib.sites`` migrations when a default database isn't
   used (:ticket:`24332`).
+
+* Added the ability to set the isolation level on PostgreSQL with psycopg2 ≥
+  2.4.2 (:ticket:`24318`). It was advertised as a new feature in Django 1.6
+  but it didn't work in practice.

tests/backends/tests.py

@@ -233,6 +233,34 @@ class PostgreSQLTests(TestCase):
         finally:
             new_connection.close()
 
+    def test_connect_isolation_level(self):
+        """
+        Regression test for #18130 and #24318.
+        """
+        from psycopg2.extensions import (
+            ISOLATION_LEVEL_READ_COMMITTED as read_committed,
+            ISOLATION_LEVEL_SERIALIZABLE as serializable,
+        )
+
+        # Since this is a django.test.TestCase, a transaction is in progress
+        # and the isolation level isn't reported as 0. This test assumes that
+        # PostgreSQL is configured with the default isolation level.
+
+        # Check the level on the psycopg2 connection, not the Django wrapper.
+        self.assertEqual(connection.connection.isolation_level, read_committed)
+
+        databases = copy.deepcopy(settings.DATABASES)
+        databases[DEFAULT_DB_ALIAS]['OPTIONS']['isolation_level'] = serializable
+        new_connections = ConnectionHandler(databases)
+        new_connection = new_connections[DEFAULT_DB_ALIAS]
+        try:
+            # Start a transaction so the isolation level isn't reported as 0.
+            new_connection.set_autocommit(False)
+            # Check the level on the psycopg2 connection, not the Django wrapper.
+            self.assertEqual(new_connection.connection.isolation_level, serializable)
+        finally:
+            new_connection.close()
+
     def _select(self, val):
         with connection.cursor() as cursor:
             cursor.execute("SELECT %s", (val,))