roachprod,gcloud: return early in ListVolumes if disk mismatch

ListVolumes depends on the described and attached disk commands
returning the same amount of disks in the same order, as there is
not enough info in the responses to join them together. If one
response contains more disks, e.g. if local SSDs are used, then
the function will panic.

Author: DarrylWong

pkg/roachprod/vm/gce/gcloud.go

@@ -874,6 +874,16 @@ func (p *Provider) ListVolumes(l *logger.Logger, v *vm.VM) ([]vm.Volume, error)
 		}
 	}
 
+	// Unfortunately the above responses contain no common identifier to join
+	// on, so we must resort to indexing. This does not work if the number of
+	// attached disks is different from the number of described volumes, i.e.
+	// if the cluster is using local SSDs.
+	if len(attachedDisks) != len(describedVolumes) {
+		err := errors.Newf("expected to find the same number of attached disks (%d) as described volumes (%d)",
+			len(attachedDisks), len(describedVolumes))
+		return nil, errors.WithHint(err, "is the cluster using local SSD?")
+	}
+
 	var volumes []vm.Volume
 	for idx := range attachedDisks {
 		attachedDisk := attachedDisks[idx]