fix: Dispatch all emsg boxes, even if they are ID3 (shaka-project#5428)

The 'metadata' event was introduced in v4.3, and ID3 metadata from
'emsg' boxes dispatched through that event was no longer dispatched
through the more general 'emsg' events.

Now we will no longer exclude ID3 emsg boxes from the more general
'emsg' event. This restores compatibility with Shaka v4.2.

lib/media/streaming_engine.js

@@ -1982,7 +1982,6 @@ shaka.media.StreamingEngine = class {
     const messageData = box.reader.readBytes(
         box.reader.getLength() - box.reader.getPosition());
 
-
     // See DASH sec. 5.10.3.3.1
     // If a DASH client detects an event message box with a scheme that is not
     // defined in MPD, the client is expected to ignore it.
@@ -1992,23 +1991,8 @@ shaka.media.StreamingEngine = class {
       // A special scheme in DASH used to signal manifest updates.
       if (schemeId == 'urn:mpeg:dash:event:2012') {
         this.playerInterface_.onManifestUpdate();
-      } else if (schemeId == 'https://aomedia.org/emsg/ID3' ||
-        schemeId == 'https://developer.apple.com/streaming/emsg-id3') {
-        // See https://aomediacodec.github.io/id3-emsg/
-        const frames = shaka.util.Id3Utils.getID3Frames(messageData);
-        if (frames.length && reference) {
-          /** @private {shaka.extern.ID3Metadata} */
-          const metadata = {
-            cueTime: reference.startTime,
-            data: messageData,
-            frames: frames,
-            dts: reference.startTime,
-            pts: reference.startTime,
-          };
-          this.playerInterface_.onMetadata(
-              [metadata], /* offset= */ 0, reference.endTime);
-        }
       } else {
+        // All other schemes are dispatched as a general 'emsg' event.
         /** @type {shaka.extern.EmsgInfo} */
         const emsg = {
           startTime: startTime,
@@ -2026,7 +2010,38 @@ shaka.media.StreamingEngine = class {
         const eventName = shaka.util.FakeEvent.EventName.Emsg;
         const data = (new Map()).set('detail', emsg);
         const event = new shaka.util.FakeEvent(eventName, data);
+        // A user can call preventDefault() on a cancelable event.
+        event.cancelable = true;
+
         this.playerInterface_.onEvent(event);
+
+        if (event.defaultPrevented) {
+          // If the caller uses preventDefault() on the 'emsg' event, don't
+          // process any further, and don't generate an ID3 'metadata' event
+          // for the same data.
+          return;
+        }
+
+        // Additionally, ID3 events generate a 'metadata' event.  This is a
+        // pre-parsed version of the metadata blob already dispatched in the
+        // 'emsg' event.
+        if (schemeId == 'https://aomedia.org/emsg/ID3' ||
+            schemeId == 'https://developer.apple.com/streaming/emsg-id3') {
+          // See https://aomediacodec.github.io/id3-emsg/
+          const frames = shaka.util.Id3Utils.getID3Frames(messageData);
+          if (frames.length && reference) {
+            /** @private {shaka.extern.ID3Metadata} */
+            const metadata = {
+              cueTime: reference.startTime,
+              data: messageData,
+              frames: frames,
+              dts: reference.startTime,
+              pts: reference.startTime,
+            };
+            this.playerInterface_.onMetadata(
+                [metadata], /* offset= */ 0, reference.endTime);
+          }
+        }
       }
     }
   }

lib/player.js

@@ -99,7 +99,9 @@ goog.requireType('shaka.routing.Payload');
 
 /**
  * @event shaka.Player.EmsgEvent
- * @description Fired when a non-typical emsg is found in a segment.
+ * @description Fired when an emsg box is found in a segment.
+ *   If the application calls preventDefault() on this event, further parsing
+ *   will not happen, and no 'metadata' event will be raised for ID3 payloads.
  * @property {string} type
  *   'emsg'
  * @property {shaka.extern.EmsgInfo} detail

test/media/streaming_engine_unit.js

@@ -3130,21 +3130,45 @@ describe('StreamingEngine', () => {
       expect(onManifestUpdate).toHaveBeenCalled();
     });
 
-    it('triggers metadata event', async () => {
+    it('triggers both emsg event and metadata event for ID3', async () => {
       setSegment0(emsgSegmentV0ID3);
       videoStream.emsgSchemeIdUris = [id3SchemeUri];
 
+      onEvent.and.callFake((emsgEvent) => {
+        expect(emsgEvent.type).toBe('emsg');
+      });
+
       // Here we go!
       streamingEngine.switchVariant(variant);
       streamingEngine.switchTextStream(textStream);
       await streamingEngine.start();
       playing = true;
       await runTest();
 
-      expect(onEvent).not.toHaveBeenCalled();
+      expect(onEvent).toHaveBeenCalled();
       expect(onMetadata).toHaveBeenCalled();
     });
 
+    it('only triggers emsg event for ID3 if event canceled', async () => {
+      setSegment0(emsgSegmentV0ID3);
+      videoStream.emsgSchemeIdUris = [id3SchemeUri];
+
+      onEvent.and.callFake((emsgEvent) => {
+        expect(emsgEvent.type).toBe('emsg');
+        emsgEvent.preventDefault();
+      });
+
+      // Here we go!
+      streamingEngine.switchVariant(variant);
+      streamingEngine.switchTextStream(textStream);
+      await streamingEngine.start();
+      playing = true;
+      await runTest();
+
+      expect(onEvent).toHaveBeenCalled();
+      expect(onMetadata).not.toHaveBeenCalled();
+    });
+
     it('event start matches presentation time', async () => {
       // This box has a non-zero event time, which doesn't matter.
       setSegment0(emsgSegmentV1NonZeroStart);