Always yield to other greenthreads after database calls

Since common database backends (eg MySQLdb and sqlite) are implemented
in C extensions, eventlet cannot monkey patch the socket calls to ensure
greenthread context switches occur during network I/O.

Previously, this was done by sprinkling explicit greenthread.sleep(0)
calls in areas known to cause problems. Switch that to always do a
yield after execute.

Change-Id: If91e49fbe212a02710b6ba4abd456222552e2575

1 files changed, 14 insertions, 1 deletions

diff --git a/nova/db/sqlalchemy/session.py b/nova/db/sqlalchemy/session.py
index 6e754be7e..6c349e0c2 100644
--- a/nova/db/sqlalchemy/session.py
+++ b/nova/db/sqlalchemy/session.py
@@ -21,6 +21,7 @@
 import re
 import time
 
+from eventlet import greenthread
 from sqlalchemy.exc import DisconnectionError, OperationalError
 import sqlalchemy.interfaces
 import sqlalchemy.orm
@@ -66,6 +67,16 @@ def add_regexp_listener(dbapi_con, con_record):
     dbapi_con.create_function('regexp', 2, regexp)
 
 
+def greenthread_yield(dbapi_con, con_record):
+    """
+    Ensure other greenthreads get a chance to execute by forcing a context
+    switch. With common database backends (eg MySQLdb and sqlite), there is
+    no implicit yield caused by network I/O since they are implemented by
+    C libraries that eventlet cannot monkey patch.
+    """
+    greenthread.sleep(0)
+
+
 def ping_listener(dbapi_conn, connection_rec, connection_proxy):
     """
     Ensures that MySQL connections checked out of the
@@ -122,9 +133,11 @@ def get_engine():
 
         _ENGINE = sqlalchemy.create_engine(FLAGS.sql_connection, **engine_args)
 
+        sqlalchemy.event.listen(_ENGINE, 'checkin', greenthread_yield)
+
         if 'mysql' in connection_dict.drivername:
             sqlalchemy.event.listen(_ENGINE, 'checkout', ping_listener)
-        elif "sqlite" in connection_dict.drivername:
+        elif 'sqlite' in connection_dict.drivername:
             if not FLAGS.sqlite_synchronous:
                 sqlalchemy.event.listen(_ENGINE, 'connect',
                                         synchronous_switch_listener)