Separated autocommit and isolation level handling for PostgreSQL.

Autocommit cannot be manipulated independently from an open connection.
This commit introduces a minor change in behavior: entering transaction
management forces opening a databasse connection. This shouldn't be
backwards incompatible in any practical use case.

django/db/backends/postgresql_psycopg2/base.py

@@ -77,21 +77,21 @@ 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)
-        autocommit = self.settings_dict["OPTIONS"].get('autocommit', False)
-        self.features.uses_autocommit = autocommit
-        if autocommit:
-            level = psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT
-        else:
-            level = self.settings_dict["OPTIONS"].get('isolation_level',
-                psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
-        self._set_isolation_level(level)
         self.ops = DatabaseOperations(self)
         self.client = DatabaseClient(self)
         self.creation = DatabaseCreation(self)
         self.introspection = DatabaseIntrospection(self)
         self.validation = BaseDatabaseValidation(self)
 
+        autocommit = opts.get('autocommit', False)
+        self.features.uses_autocommit = autocommit
+        self.features.uses_savepoints = not autocommit
+
     def get_connection_params(self):
         settings_dict = self.settings_dict
         if not settings_dict['NAME']:
@@ -135,11 +135,12 @@ def init_connection_state(self):
 
             if conn_tz != tz:
                 # Set the time zone in autocommit mode (see #17062)
-                self.connection.set_isolation_level(
-                        psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
+                self.set_autocommit(True)
                 self.connection.cursor().execute(
                         self.ops.set_time_zone_sql(), [tz])
         self.connection.set_isolation_level(self.isolation_level)
+        if self.features.uses_autocommit:
+            self.set_autocommit(True)
 
     def create_cursor(self):
         cursor = self.connection.cursor()
@@ -172,42 +173,40 @@ def _enter_transaction_management(self, managed):
         Switch the isolation level when needing transaction support, so that
         the same transaction is visible across all the queries.
         """
-        if self.features.uses_autocommit and managed and not self.isolation_level:
-            level = self.settings_dict["OPTIONS"].get('isolation_level',
-                psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
-            self._set_isolation_level(level)
+        if self.connection is None:             # Force creating a connection.
+            self.cursor().close()
+        if self.features.uses_autocommit and managed and self.autocommit:
+            self.set_autocommit(False)
+            self.features.uses_savepoints = True
 
     def _leave_transaction_management(self, managed):
         """
         If the normal operating mode is "autocommit", switch back to that when
         leaving transaction management.
         """
-        if self.features.uses_autocommit and not managed and self.isolation_level:
-            self._set_isolation_level(psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)
-
-    def _set_isolation_level(self, level):
-        """
-        Do all the related feature configurations for changing isolation
-        levels. This doesn't touch the uses_autocommit feature, since that
-        controls the movement *between* isolation levels.
-        """
-        assert level in range(5)
-        try:
-            if self.connection is not None:
-                self.connection.set_isolation_level(level)
-            if level == psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT:
-                self.set_clean()
-        finally:
-            self.isolation_level = level
-            self.features.uses_savepoints = bool(level)
+        if self.connection is None:             # Force creating a connection.
+            self.cursor().close()
+        if self.features.uses_autocommit and not managed and not self.autocommit:
+            self.rollback()                     # Must terminate transaction first.
+            self.set_autocommit(True)
+            self.features.uses_savepoints = False
+
+    def _set_isolation_level(self, isolation_level):
+        assert isolation_level in range(1, 5)     # Use set_autocommit for level = 0
+        if self.psycopg2_version >= (2, 4, 2):
+            self.connection.set_session(isolation_level=isolation_level)
+        else:
+            self.connection.set_isolation_level(isolation_level)
 
     def _set_autocommit(self, autocommit):
-        if autocommit:
-            level = psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT
+        if self.psycopg2_version >= (2, 4, 2):
+            self.connection.autocommit = autocommit
         else:
-            level = self.settings_dict["OPTIONS"].get('isolation_level',
-                psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED)
-        self._set_isolation_level(level)
+            if autocommit:
+                level = psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT
+            else:
+                level = self.isolation_level
+            self.connection.set_isolation_level(level)
 
     def set_dirty(self):
         if ((self.transaction_state and self.transaction_state[-1]) or
@@ -231,6 +230,11 @@ def is_usable(self):
         else:
             return True
 
+    @cached_property
+    def psycopg2_version(self):
+        version = psycopg2.__version__.split(' ', 1)[0]
+        return tuple(int(v) for v in version.split('.'))
+
     @cached_property
     def pg_version(self):
         with self.temporary_connection():

tests/transactions_regress/tests.py

@@ -274,31 +274,39 @@ def tearDown(self):
             connections[DEFAULT_DB_ALIAS] = self._old_backend
 
     def test_initial_autocommit_state(self):
+        # Autocommit is activated when the connection is created.
+        connection.cursor().close()
+
         self.assertTrue(connection.features.uses_autocommit)
-        self.assertEqual(connection.isolation_level, self._autocommit)
+        self.assertTrue(connection.autocommit)
 
     def test_transaction_management(self):
         transaction.enter_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
 
         transaction.leave_transaction_management()
-        self.assertEqual(connection.isolation_level, self._autocommit)
+        self.assertTrue(connection.autocommit)
 
     def test_transaction_stacking(self):
         transaction.enter_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
 
         transaction.enter_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
 
         transaction.leave_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
 
         transaction.leave_transaction_management()
-        self.assertEqual(connection.isolation_level, self._autocommit)
+        self.assertTrue(connection.autocommit)
 
     def test_enter_autocommit(self):
         transaction.enter_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
         list(Mod.objects.all())
         self.assertTrue(transaction.is_dirty())
@@ -311,9 +319,10 @@ def test_enter_autocommit(self):
         list(Mod.objects.all())
         self.assertFalse(transaction.is_dirty())
         transaction.leave_transaction_management()
+        self.assertFalse(connection.autocommit)
         self.assertEqual(connection.isolation_level, self._serializable)
         transaction.leave_transaction_management()
-        self.assertEqual(connection.isolation_level, self._autocommit)
+        self.assertTrue(connection.autocommit)
 
 
 class TestManyToManyAddTransaction(TransactionTestCase):