xdsclient: make sending requests more deterministic

xds/internal/xdsclient/transport/ads/ads_stream.go

@@ -102,6 +102,7 @@
 	nonce               string                         // Last received nonce. Should be reset when the stream breaks.
 	bufferedRequests    chan struct{}                  // Channel to buffer requests when writing is blocked.
 	subscribedResources map[string]*ResourceWatchState // Map of subscribed resource names to their state.
+	pendingWrite        bool                           // True if there is a pending write for this resource type.
 }
 
 // StreamImpl provides the functionality associated with an ADS (Aggregated
@@ -203,6 +204,7 @@
 	// Create state for the newly subscribed resource. The watch timer will
 	// be started when a request for this resource is actually sent out.
 	state.subscribedResources[name] = &ResourceWatchState{State: ResourceWatchStateStarted}
+	state.pendingWrite = true
 
 	// Send a request for the resource type with updated subscriptions.
 	s.requestCh.Put(typ)
@@ -233,6 +235,7 @@
 		rs.ExpiryTimer.Stop()
 	}
 	delete(state.subscribedResources, name)
+	state.pendingWrite = true
 
 	// Send a request for the resource type with updated subscriptions.
 	s.requestCh.Put(typ)
@@ -346,17 +349,7 @@
 		return nil
 	}
 
-	names := resourceNames(state.subscribedResources)
-	if err := s.sendMessageLocked(stream, names, typ.TypeURL(), state.version, state.nonce, nil); err != nil {
-		return err
-
-	}
-	select {
-	case <-state.bufferedRequests:
-	default:
-	}
-	s.startWatchTimersLocked(typ, names)
-	return nil
+	return s.sendMessageIfWritePendingLocked(stream, typ, state)
 }
 
 // sendExisting sends out discovery requests for existing resources when
@@ -385,18 +378,10 @@
 			continue
 		}
 
-		names := resourceNames(state.subscribedResources)
-		if s.logger.V(2) {
-			s.logger.Infof("Re-requesting resources %v of type %q, as the stream has been recreated", names, typ.TypeURL())
-		}
-		if err := s.sendMessageLocked(stream, names, typ.TypeURL(), state.version, state.nonce, nil); err != nil {
+		state.pendingWrite = true
+		if err := s.sendMessageIfWritePendingLocked(stream, typ, state); err != nil {
 			return err
 		}
-		select {
-		case <-state.bufferedRequests:
-		default:
-		}
-		s.startWatchTimersLocked(typ, names)
 	}
 	return nil
 }
@@ -413,11 +398,9 @@
 	for typ, state := range s.resourceTypeState {
 		select {
 		case <-state.bufferedRequests:
-			names := resourceNames(state.subscribedResources)
-			if err := s.sendMessageLocked(stream, names, typ.TypeURL(), state.version, state.nonce, nil); err != nil {
+			if err := s.sendMessageIfWritePendingLocked(stream, typ, state); err != nil {
 				return err
 			}
-			s.startWatchTimersLocked(typ, names)
 		default:
 			// No buffered request.
 			continue
@@ -426,6 +409,38 @@
 	return nil
 }
 
+// sendMessageIfWritePendingLocked attempts to sends a discovery request to the
+// server, if there is a pending write for the given resource type.
+//
+// If the request is successfully sent, the pending write field is cleared and
+// watch timers are started for the resources in the request.
+//
+// Caller needs to hold c.mu.
+func (s *StreamImpl) sendMessageIfWritePendingLocked(stream transport.StreamingCall, typ xdsresource.Type, state *resourceTypeState) error {
+	if !state.pendingWrite {
+		if s.logger.V(2) {
+			s.logger.Infof("Skipping sending request for type %q, because all subscribed resources were already sent", typ.TypeURL())
+		}
+		return nil
+	}
+
+	names := resourceNames(state.subscribedResources)
+	if err := s.sendMessageLocked(stream, names, typ.TypeURL(), state.version, state.nonce, nil); err != nil {
+		return err
+	}
+	state.pendingWrite = false
+
+	// Drain the buffered requests channel because we just sent a request for this
+	// resource type.
+	select {
+	case <-state.bufferedRequests:
+	default:
+	}
+
+	s.startWatchTimersLocked(typ, names)
+	return nil
+}
+
 // sendMessageLocked sends a discovery request to the server, populating the
 // different fields of the message with the given parameters. Returns a non-nil
 // error if the request could not be sent.