Distinguish resize from offline migrate in scheduler

The scheduler reacts to the scheduler-hint `_nova_check_type`. To be
able to have specific filters/weighers for resize, we set this
scheduler-hint to "resize" already.

Since resizes and offline migrations take the same code path and only
differ in having a flavor change or not, we used to set "resize" also
for offline migrations.

In the future, we want to be able to distinguish between offline
migrations and resizes, because one is triggered by customers and the
other can only be triggered by admins. Therefore, we introduce another
`_nova_check_type` "migrate". There's also already another
`_nova_check_type` "live_migrate", so "migrate" does not clash there.

To help in finding an offline migration scheduling request in the
scheduler, we add the helper function `request_is_migrate()` in the same
spirit as the already existing functions for "resize" and
"live_migrate".

Change-Id: Id8915a2eb52025e31604af8dde53343ab980ca3e

nova/compute/api.py

@@ -4143,7 +4143,10 @@ def resize(self, context, instance, flavor_id=None, clean_shutdown=True,
                     validate_pci=True)
 
         # Not to be confused with scheduler_hint (singular)
-        scheduler_hints = {'_nova_check_type': ['resize']}
+        if flavor_id:
+            scheduler_hints = {'_nova_check_type': ['resize']}
+        else:
+            scheduler_hints = {'_nova_check_type': ['migrate']}
         filter_properties = {'ignore_hosts': [],
                              'scheduler_hints': scheduler_hints}
 

nova/scheduler/utils.py

@@ -1239,6 +1239,19 @@ def request_is_live_migrate(spec_obj):
     return check_type == 'live_migrate'
 
 
+def request_is_migrate(spec_obj):
+    """Returns True if request is for an offline migration
+
+    :param spec_obj: An objects.RequestSpec to examine (or None).
+    """
+    if not spec_obj:
+        return False
+    if 'scheduler_hints' not in spec_obj:
+        return False
+    check_type = spec_obj.get_scheduler_hint('_nova_check_type')
+    return check_type == 'migrate'
+
+
 def claim_resources(ctx, client, spec_obj, instance_uuid, alloc_req,
         allocation_request_version=None):
     """Given an instance UUID (representing the consumer of resources) and the