[CAPPL-472] Avoid overwriting engines (#15967)

core/services/workflows/syncer/engine_registry.go

@@ -19,10 +19,14 @@ func NewEngineRegistry() *EngineRegistry {
 }
 
 // Add adds an engine to the registry.
-func (r *EngineRegistry) Add(id string, engine services.Service) {
+func (r *EngineRegistry) Add(id string, engine services.Service) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
+	if _, found := r.engines[id]; found {
+		return errors.New("attempting to register duplicate engine")
+	}
 	r.engines[id] = engine
+	return nil
 }
 
 // Get retrieves an engine from the registry.

core/services/workflows/syncer/handler.go

@@ -484,7 +484,11 @@ func (h *eventHandler) workflowRegisteredEvent(
 		return fmt.Errorf("failed to start workflow engine: %w", err)
 	}
 
-	h.engineRegistry.Add(wfID, engine)
+	// This shouldn't happen because we call the handler serially and
+	// check for running engines above, see the call to engineRegistry.IsRunning.
+	if err := h.engineRegistry.Add(wfID, engine); err != nil {
+		return fmt.Errorf("invariant violation: %w", err)
+	}
 
 	return nil
 }