set autocommit for psycopg2 pre-ping

Fixed issue where the pool "pre ping" feature would implicitly start a
transaction, which would then interfere with custom transactional flags
such as PostgreSQL's "read only" mode when used with the psycopg2 driver.

Fixes: #6621
Change-Id: I29117c393e50c090cc2587efcccfe1e986738928

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/psycopg2.py b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
index 3ef01e9e9..19ca14265 100644
--- a/lib/sqlalchemy/dialects/postgresql/psycopg2.py
+++ b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
@@ -844,6 +844,25 @@ class PGDialect_psycopg2(PGDialect):
     def get_deferrable(self, connection):
         return connection.deferrable
 
+    def do_ping(self, dbapi_connection):
+        cursor = None
+        try:
+            dbapi_connection.autocommit = True
+            cursor = dbapi_connection.cursor()
+            try:
+                cursor.execute(self._dialect_specific_select_one)
+            finally:
+                cursor.close()
+                if not dbapi_connection.closed:
+                    dbapi_connection.autocommit = False
+        except self.dbapi.Error as err:
+            if self.is_disconnect(err, dbapi_connection, cursor):
+                return False
+            else:
+                raise
+        else:
+            return True
+
     def on_connect(self):
         extras = self._psycopg2_extras()
         extensions = self._psycopg2_extensions()