fixed transcribe cannot fallback for specific endpoint

lib/tasks/stt-task.js

@@ -260,8 +260,19 @@ class SttTask extends Task {
     ep.addCustomEventListener(event, handler);
   }
 
-  removeCustomEventListeners() {
-    this.eventHandlers.forEach((h) => h.ep.removeCustomEventListener(h.event, h.handler));
+  removeCustomEventListeners(ep) {
+    if (ep) {
+      // for specific endpoint
+      this.eventHandlers.filter((h) => h.ep === ep).forEach((h) => {
+        h.ep.removeCustomEventListener(h.event, h.handler);
+      });
+      this.eventHandlers = this.eventHandlers.filter((h) => h.ep !== ep);
+      return;
+    } else {
+      // for all endpoints
+      this.eventHandlers.forEach((h) => h.ep.removeCustomEventListener(h.event, h.handler));
+      this.eventHandlers = [];
+    }
   }
 
   async _initSpeechCredentials(cs, vendor, label) {
@@ -333,7 +344,8 @@ class SttTask extends Task {
     return this.fallbackVendor && this.isHandledByPrimaryProvider && !this.cs.hasFallbackAsr;
   }
 
-  async _initFallback() {
+  // gather does not need to provide ep, but transcribe does as it might has 2 legs
+  async _initFallback(ep) {
     assert(this.fallbackVendor, 'fallback failed without fallbackVendor configuration');
     this.logger.info(`Failed to use primary STT provider, fallback to ${this.fallbackVendor}`);
     this.isHandledByPrimaryProvider = false;
@@ -346,7 +358,7 @@ class SttTask extends Task {
     this.data.recognizer.label = this.label;
     this.sttCredentials = await this._initSpeechCredentials(this.cs, this.vendor, this.label);
     // cleanup previous listener from previous vendor
-    this.removeCustomEventListeners();
+    this.removeCustomEventListeners(ep);
   }
 
   async compileHintsForCobalt(ep, hostport, model, token, hints) {

lib/tasks/transcribe.js

@@ -786,7 +786,7 @@ class TaskTranscribe extends SttTask {
       try {
         this.notifyError({ msg: 'ASR error',
           details:`STT Vendor ${this.vendor} error: ${evt.error || evt.reason}`, failover: 'in progress'});
-        await this._initFallback();
+        await this._initFallback(_ep);
         let channel = 1;
         if (this.ep !== _ep) {
           channel = 2;