fix: allow the playback on platforms when low latency APIs are not su…

…pported (shaka-project#4485)

Close shaka-project#3590

lib/media/streaming_engine.js

@@ -1240,7 +1240,16 @@ shaka.media.StreamingEngine = class {
       if (this.config_.lowLatencyMode && isReadableStreamSupported && isMP4 &&
           !reference.hlsAes128Key) {
         let remaining = new Uint8Array(0);
+        let processingResult = false;
+        let callbackCalled = false;
         const streamDataCallback = async (data) => {
+          if (processingResult) {
+            // If the fallback result processing was triggered, don't also
+            // append the buffer here.  In theory this should never happen,
+            // but it does on some older TVs.
+            return;
+          }
+          callbackCalled = true;
           this.destroyer_.ensureNotDestroyed();
           if (this.fatalError_) {
             return;
@@ -1271,7 +1280,33 @@ shaka.media.StreamingEngine = class {
           }
         };
 
-        await this.fetch_(mediaState, reference, streamDataCallback);
+        const result =
+            await this.fetch_(mediaState, reference, streamDataCallback);
+        if (!callbackCalled) {
+          // In some environments, we might be forced to use network plugins
+          // that don't support streamDataCallback. In those cases, as a
+          // fallback, append the buffer here.
+          processingResult = true;
+          this.destroyer_.ensureNotDestroyed();
+          if (this.fatalError_) {
+            return;
+          }
+
+          // If the text stream gets switched between fetch_() and append_(),
+          // the new text parser is initialized, but the new init segment is
+          // not fetched yet.  That would cause an error in
+          // TextParser.parseMedia().
+          // See http://b/168253400
+          if (mediaState.waitingToClearBuffer) {
+            shaka.log.info(logPrefix, 'waitingToClearBuffer, skip append');
+            mediaState.performingUpdate = false;
+            this.scheduleUpdate_(mediaState, 0);
+            return;
+          }
+
+          await this.append_(
+              mediaState, presentationTime, stream, reference, result);
+        }
       } else {
         if (this.config_.lowLatencyMode && !isReadableStreamSupported) {
           shaka.log.warning('Low latency streaming mode is enabled, but ' +