CA-422282 Fix race condition about xenops cache

There is race condition about vm cache between pool_migrate_complete
and VM event.
In the cross-pool migration case, it is designed to create vm with
power_state Halted in XAPI db. In pool_migrate_complete, add_caches
create an empty xenops_chae for the VM, then refresh_vm compares the
cache powerstate None with its real state Running to update the
right powerstate to XAPI db.
In the fail case, it is found that:
-> VM event 1 update_vm
-> pool_migrate_complete add_caches (cache power_state None)
-> pool_migrate_complete refresh_vm
-> VM event 1 update cache (cache power_state Running)
-> VM event 2 update_vm (Running <-> Running, XAPI DB not update)
When pool_migrate_complete add_caches, the cache update of previous
VM event 1 breaks the design intention.

This commit add a wait in pool_migrate_complete to ensure all
in-flight events complete before add_caches. Then there will be no
race condition.

Signed-off-by: Changlei Li <changlei.li@cloud.com>

Author: changlei-li

ocaml/xapi/xapi_vm_migrate.ml

@@ -492,6 +492,7 @@ let pool_migrate_complete ~__context ~vm ~host:_ =
   if Xapi_xenops.vm_exists_in_xenopsd queue_name dbg id then (
     remove_stale_pcis ~__context ~vm ;
     Xapi_xenops.set_resident_on ~__context ~self:vm ;
+    Xapi_xenops.Events_from_xenopsd.wait queue_name dbg id () ;
     Xapi_xenops.add_caches id ;
     Xapi_xenops.refresh_vm ~__context ~self:vm ;
     Monitor_dbcalls_cache.clear_cache_for_vm ~vm_uuid:id