Implement NullVideoDecoder to avoid crash on unsupported decoders.

There is a use case with external codec factories that only support
encoding but not decoding for a given type. This leads to a crash
due to null being registered as codec (after a DCHECK).

This CL adds a NullVideoDecoder that is used instead of the null to
not crash but log to LS_ERROR.

BUG=webrtc:5249

Review URL: https://codereview.webrtc.org/1657023002

Cr-Commit-Position: refs/heads/master@{#11475}

talk/media/webrtc/webrtcvideoengine2.cc

@@ -2226,10 +2226,9 @@ WebRtcVideoChannel2::WebRtcVideoReceiveStream::CreateOrReuseVideoDecoder(
         webrtc::VideoDecoder::Create(webrtc::VideoDecoder::kH264), type, false);
   }
 
-  // This shouldn't happen, we should not be trying to create something we don't
-  // support.
-  RTC_DCHECK(false);
-  return AllocatedDecoder(NULL, webrtc::kVideoCodecUnknown, false);
+  return AllocatedDecoder(
+      webrtc::VideoDecoder::Create(webrtc::VideoDecoder::kUnsupportedCodec),
+      webrtc::kVideoCodecUnknown, false);
 }
 
 void WebRtcVideoChannel2::WebRtcVideoReceiveStream::ConfigureCodecs(

webrtc/video/video_decoder.cc

@@ -27,8 +27,8 @@ VideoDecoder* VideoDecoder::Create(VideoDecoder::DecoderType codec_type) {
     case kVp9:
       return VP9Decoder::Create();
     case kUnsupportedCodec:
-      RTC_NOTREACHED();
-      return nullptr;
+      LOG(LS_ERROR) << "Creating NullVideoDecoder for unsupported codec.";
+      return new NullVideoDecoder();
   }
   RTC_NOTREACHED();
   return nullptr;
@@ -140,4 +140,40 @@ const char* VideoDecoderSoftwareFallbackWrapper::ImplementationName() const {
   return decoder_->ImplementationName();
 }
 
+NullVideoDecoder::NullVideoDecoder() {}
+
+int32_t NullVideoDecoder::InitDecode(const VideoCodec* codec_settings,
+                                     int32_t number_of_cores) {
+  LOG(LS_ERROR) << "Can't initialize NullVideoDecoder.";
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
+int32_t NullVideoDecoder::Decode(const EncodedImage& input_image,
+    bool missing_frames,
+    const RTPFragmentationHeader* fragmentation,
+    const CodecSpecificInfo* codec_specific_info,
+    int64_t render_time_ms) {
+  LOG(LS_ERROR) << "The NullVideoDecoder doesn't support decoding.";
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
+int32_t NullVideoDecoder::RegisterDecodeCompleteCallback(
+    DecodedImageCallback* callback) {
+  LOG(LS_ERROR)
+      << "Can't register decode complete callback on NullVideoDecoder.";
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
+int32_t NullVideoDecoder::Release() {
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
+int32_t NullVideoDecoder::Reset() {
+  return WEBRTC_VIDEO_CODEC_OK;
+}
+
+const char* NullVideoDecoder::ImplementationName() const {
+  return "NullVideoDecoder";
+}
+
 }  // namespace webrtc

webrtc/video_decoder.h

@@ -123,6 +123,30 @@ class VideoDecoderSoftwareFallbackWrapper : public webrtc::VideoDecoder {
   DecodedImageCallback* callback_;
 };
 
+// Video decoder class to be used for unknown codecs. Doesn't support decoding
+// but logs messages to LS_ERROR.
+class NullVideoDecoder : public VideoDecoder {
+ public:
+  NullVideoDecoder();
+
+  int32_t InitDecode(const VideoCodec* codec_settings,
+                     int32_t number_of_cores) override;
+
+  int32_t Decode(const EncodedImage& input_image,
+                 bool missing_frames,
+                 const RTPFragmentationHeader* fragmentation,
+                 const CodecSpecificInfo* codec_specific_info,
+                 int64_t render_time_ms) override;
+
+  int32_t RegisterDecodeCompleteCallback(
+      DecodedImageCallback* callback) override;
+
+  int32_t Release() override;
+  int32_t Reset() override;
+
+  const char* ImplementationName() const override;
+};
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_VIDEO_DECODER_H_