Add additional logs for remote dispatch and change to use a non-empty placeholder

internal/dispatch/remote/cluster.go

@@ -423,6 +423,7 @@
 const (
 	secondaryCursorPrefix = "$$secondary:"
 	primaryDispatcher     = "$primary"
+	noDispatcherResults   = "$$no_dispatcher_results"
 )
 
 func publishClient[R any](ctx context.Context, client receiver[R], reqKey string, stream dispatch.Stream[R], secondaryDispatchName string) error {
@@ -565,7 +566,7 @@
 
 	wg.Add(len(validSecondaryDispatchers))
 
-	returnedResultsDispatcherName := atomic.NewString("")
+	returnedResultsDispatcherName := atomic.NewString(noDispatcherResults)
 
 	primarySleeper := cr.newPrimarySleeper(reqKey,
 		tuple.FromCoreRelationReference(req.GetResourceRelation()),
@@ -574,6 +575,12 @@
 	)
 
 	runHandler := func(name string, clusterClient ClusterClient) {
+		if name == "" {
+			log.Warn().Msg("attempting to run a dispatch handler with an empty name, skipping")
+			wg.Done()
+			return
+		}
+
 		ctx := contexts[name].ctx
 		log.Debug().Str("dispatcher", name).Msg("running secondary dispatcher")
 		defer wg.Done()
@@ -644,20 +651,22 @@
 
 					// If a valid result, and we have not yet returned any results, try take a "lock" on
 					// returning results to ensure only a single dispatcher returns results.
-					swapped := returnedResultsDispatcherName.CompareAndSwap("", name)
+					swapped := returnedResultsDispatcherName.CompareAndSwap(noDispatcherResults, name)
 					if !swapped {
 						// Another dispatcher has started returning results, so terminate.
 						log.Trace().Str("dispatcher", name).Msg("another dispatcher has already returned results")
 						return
 					}
 
+					log.Trace().Str("dispatcher", name).Msg("this dispatcher is the first to return results, will publish them and cancel the others")
 					dispatchCounter.WithLabelValues(reqKey, name).Add(1)
 
 					// Cancel all other contexts to prevent them from running, or stop them
 					// from running if started.
 					log.Trace().Str("dispatcher", name).Msg("canceling other dispatchers")
 					for key, ctxAndCancel := range contexts {
 						if key != name {
+							log.Trace().Str("canceling-dispatcher", key).Msg("canceling dispatcher context")
 							ctxAndCancel.cancel()
 						}
 					}
@@ -706,8 +715,14 @@
 
 	// Check for the first dispatcher that returned results and return its error, if any.
 	resultHandlerName := returnedResultsDispatcherName.Load()
-	if resultHandlerName != "" {
+	if resultHandlerName == "" {
+		log.Error().Msg("got empty result handler name; this should never happen")
+		return spiceerrors.MustBugf("got empty result handler name")
+	}
+
+	if resultHandlerName != noDispatcherResults {
 		if err, ok := errorsByDispatcherName.Load(resultHandlerName); ok {
+			log.Warn().Err(err).Str("dispatcher", resultHandlerName).Msg("dispatcher that returned results encountered an error during streaming")
 			return err
 		}
 		return nil