Allow for bad revision on empty list.

Looks like k8s might return that in some cases (customK8sResourceClient
checks for it).

libcalico-go/lib/backend/watchersyncer/watchercache.go

@@ -220,7 +220,17 @@ func (wc *watcherCache) resyncAndCreateWatcher(ctx context.Context) {
 			// Store the current watch revision.  This gets updated on any new add/modified event.
 			wc.logger.Logger.WithField("revision", l.Revision).Debug("List completed.")
 			if l.Revision == "" || l.Revision == "0" {
-				wc.logger.Panic("BUG: List returned empty/zero revision.  Watch would be inconsistent.")
+				if len(l.KVPairs) == 0 {
+					// Got a bad revision but there are no items.  This may mean that the datastore
+					// returns an unhelpful "not found" error instead of an empty list.  Revert to a
+					// poll until some items show up.
+					wc.logger.Info("List returned no items and an empty/zero revision, reverting to poll.")
+					wc.currentWatchRevision = "0"
+					performFullResync = true
+					wc.resyncBlockedUntil = time.Now().Add(WatchPollInterval)
+					continue
+				}
+				wc.logger.Panic("BUG: List returned items with empty/zero revision.  Watch would be inconsistent.")
 			}
 			wc.currentWatchRevision = l.Revision