volume list: Don't call nova if no volume is attached

Currently 'openstack volume list' calls nova to resolve server UUIDs
to server names. This is not required if:
1. no volume is attached to an instance
2. no volume exists in deployment

This patch fixes this by checking volume statuses and, if any volume has
status 'in-use', we will call nova to resolve server names.
Note that we don't check for 'reserved', 'attaching', 'detaching'
states since those are transition states and doesn't guarantee that
the volume is actually attached to the instance.

Change-Id: Ic4d89db69244d3fba44d4b69c79b3e7632ee3d53

openstackclient/volume/v2/volume.py

@@ -490,19 +490,6 @@ class ListVolume(command.Lister):
             column_headers = copy.deepcopy(columns)
             column_headers[4] = 'Attached to'
 
-        # Cache the server list
-        server_cache = {}
-        try:
-            compute_client = self.app.client_manager.compute
-            for s in compute_client.servers.list():
-                server_cache[s.id] = s
-        except Exception:
-            # Just forget it if there's any trouble
-            pass
-        AttachmentsColumnWithCache = functools.partial(
-            AttachmentsColumn, server_cache=server_cache
-        )
-
         project_id = None
         if parsed_args.project:
             project_id = identity_common.find_project(
@@ -533,6 +520,28 @@ class ListVolume(command.Lister):
             marker=parsed_args.marker,
             limit=parsed_args.limit,
         )
+
+        do_server_list = False
+
+        for vol in data:
+            if vol.status == 'in-use':
+                do_server_list = True
+                break
+
+        # Cache the server list
+        server_cache = {}
+        if do_server_list:
+            try:
+                compute_client = self.app.client_manager.compute
+                for s in compute_client.servers.list():
+                    server_cache[s.id] = s
+            except Exception:
+                # Just forget it if there's any trouble
+                pass
+        AttachmentsColumnWithCache = functools.partial(
+            AttachmentsColumn, server_cache=server_cache
+        )
+
         column_headers = utils.backward_compat_col_lister(
             column_headers, parsed_args.columns, {'Display Name': 'Name'}
         )