Opus: Let the decoder interpret 2-byte payloads as DTX/CNG packets

webrtc/modules/audio_coding/codecs/opus/opus_interface.c

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 265 matching lines @@
   opus_decoder_ctl(inst->decoder, OPUS_RESET_STATE);
   inst->in_dtx_mode = 0;
 }
 
 /* For decoder to determine if it is to output speech or comfort noise. */
 static int16_t DetermineAudioType(OpusDecInst* inst, size_t encoded_bytes) {
   // Audio type becomes comfort noise if |encoded_byte| is 1 and keeps
   // to be so if the following |encoded_byte| are 0 or 1.
   if (encoded_bytes == 0 && inst->in_dtx_mode) {
     return 2;  // Comfort noise.
-  } else if (encoded_bytes == 1) {
+  } else if (encoded_bytes == 1 || encoded_bytes == 2) {
+    // TODO(henrik.lundin): There is a slight risk that a 2-byte payload is in
+    // fact a 1-byte TOC with a 1-byte payload. That will be erroneously
+    // interpreted as comfort noise output, but such a payload is probably
+    // faulty anyway.
     inst->in_dtx_mode = 1;
     return 2;  // Comfort noise.
   } else {
     inst->in_dtx_mode = 0;
     return 0;  // Speech.
   }
 }
 
 /* |frame_size| is set to maximum Opus frame size in the normal case, and
  * is set to the number of samples needed for PLC in case of losses.
@@ skipping 177 matching lines @@
     return 0;
   }
 
   for (n = 0; n < channels; n++) {
     if (frame_data[0][0] & (0x80 >> ((n + 1) * (frames + 1) - 1)))
       return 1;
   }
 
   return 0;
 }