Add MySQLPingListener() back

The use of wrap_db() to catch low-level problems in the connection
pool appears to be incompatible with new versions of sqlalchemy
(0.7.4/precise).  This re-instates the use of a event listener to
catch the error of 'mysql has gone away'.  Keeps the wrapping
logic in place for use before the connection pool has been constructed,
specifically to allow glance to retry its initial database connection on
service startup.

Fixes bug 967887

Change-Id: Ia732377d3404104b8acf7fe4ada164ec7ecf705f

glance/registry/db/api.py

@@ -27,7 +27,8 @@
 
 import sqlalchemy
 from sqlalchemy import asc, create_engine, desc
-from sqlalchemy.exc import IntegrityError, OperationalError, DBAPIError
+from sqlalchemy.exc import IntegrityError, OperationalError, DBAPIError,\
+    DisconnectionError
 from sqlalchemy.orm import exc
 from sqlalchemy.orm import joinedload
 from sqlalchemy.orm import sessionmaker
@@ -70,6 +71,27 @@
     ]
 
 
+class MySQLPingListener(object):
+
+    """
+    Ensures that MySQL connections checked out of the
+    pool are alive.
+
+    Borrowed from:
+    http://groups.google.com/group/sqlalchemy/msg/a4ce563d802c929f
+    """
+
+    def checkout(self, dbapi_con, con_record, con_proxy):
+        try:
+            dbapi_con.cursor().execute('select 1')
+        except dbapi_con.OperationalError, ex:
+            if ex.args[0] in (2006, 2013, 2014, 2045, 2055):
+                logger.warn('Got mysql server has gone away: %s', ex)
+                raise DisconnectionError("Database server went away")
+            else:
+                raise
+
+
 def configure_db(conf):
     """
     Establish the database, create an engine if needed, and
@@ -88,9 +110,11 @@ def configure_db(conf):
                        'echo': False,
                        'convert_unicode': True
                        }
+        if 'mysql' in connection_dict.drivername:
+            engine_args['listeners'] = [MySQLPingListener()]
+
         try:
             _ENGINE = create_engine(sql_connection, **engine_args)
-            _ENGINE.create = wrap_db_error(_ENGINE.create)
             _ENGINE.connect = wrap_db_error(_ENGINE.connect)
             _ENGINE.connect()
         except Exception, err:
@@ -129,12 +153,7 @@ def get_session(autocommit=True, expire_on_commit=False):
         _MAKER = sessionmaker(bind=_ENGINE,
                               autocommit=autocommit,
                               expire_on_commit=expire_on_commit)
-    session = _MAKER()
-    session.query = wrap_db_error(session.query)
-    session.flush = wrap_db_error(session.flush)
-    session.execute = wrap_db_error(session.execute)
-    session.begin = wrap_db_error(session.begin)
-    return session
+    return _MAKER()
 
 
 def is_db_connection_error(args):