Make sure multiple calls to _get_session() aren't nested

Fixes bug 924918

async_call_plugin() acquires a xenapi session as does the nested call to
get_xenapi_host(). This can cause a deadlock if multiple greenthreads
all block waiting for the outer sessions to be freed to allocate the
inner session. This change moves the call to get_xenapi_host() to outside
the with statement to ensure calls to _get_session() aren't nested.

Change-Id: I8f5490f40a9ccaf74a276187f66519a5d5f52b2e

1 files changed, 5 insertions, 1 deletions

diff --git a/nova/virt/xenapi_conn.py b/nova/virt/xenapi_conn.py
index 6b67b8b88..6fda205d0 100644
--- a/nova/virt/xenapi_conn.py
+++ b/nova/virt/xenapi_conn.py
@@ -544,10 +544,14 @@ class XenAPISession(object):
 
     def async_call_plugin(self, plugin, fn, args):
         """Call Async.host.call_plugin on a background thread."""
+        # NOTE(johannes): Fetch host before we acquire a session. Since
+        # _get_session() acquires a session too, it can result in a deadlock
+        # if multiple greenthreads race with each other. See bug 924918
+        host = self.get_xenapi_host()
         with self._get_session() as session:
             return tpool.execute(self._unwrap_plugin_exceptions,
                                  session.xenapi.Async.host.call_plugin,
-                                 self.get_xenapi_host(), plugin, fn, args)
+                                 host, plugin, fn, args)
 
     def wait_for_task(self, task, uuid=None):
         """Return the result of the given task. The task is polled