Add _core._get_backend_details() utility for testing and debugging (#987)

Author: NicolasHug

src/torchcodec/_core/BetaCudaDeviceInterface.cpp

@@ -699,4 +699,9 @@ void BetaCudaDeviceInterface::convertAVFrameToFrameOutput(
       avFrame, device_, nppCtx_, nvdecStream, preAllocatedOutputTensor);
 }
 
+std::string BetaCudaDeviceInterface::getDetails() {
+  return std::string("Beta CUDA Device Interface. Using ") +
+      (cpuFallback_ ? "CPU fallback." : "NVDEC.");
+}
+
 } // namespace facebook::torchcodec

src/torchcodec/_core/BetaCudaDeviceInterface.h

@@ -59,6 +59,8 @@ class BetaCudaDeviceInterface : public DeviceInterface {
   int frameReadyForDecoding(CUVIDPICPARAMS* picParams);
   int frameReadyInDisplayOrder(CUVIDPARSERDISPINFO* dispInfo);
 
+  std::string getDetails() override;
+
  private:
   int sendCuvidPacket(CUVIDSOURCEDATAPACKET& cuvidPacket);
 

src/torchcodec/_core/CpuDeviceInterface.cpp

@@ -346,4 +346,8 @@ torch::Tensor CpuDeviceInterface::convertAVFrameToTensorUsingFilterGraph(
   return rgbAVFrameToTensor(filterGraph_->convert(avFrame));
 }
 
+std::string CpuDeviceInterface::getDetails() {
+  return std::string("CPU Device Interface.");
+}
+
 } // namespace facebook::torchcodec

src/torchcodec/_core/CpuDeviceInterface.h

@@ -39,6 +39,8 @@ class CpuDeviceInterface : public DeviceInterface {
       std::optional<torch::Tensor> preAllocatedOutputTensor =
           std::nullopt) override;
 
+  std::string getDetails() override;
+
  private:
   int convertAVFrameToTensorUsingSwScale(
       const UniqueAVFrame& avFrame,

src/torchcodec/_core/CudaDeviceInterface.cpp

@@ -284,9 +284,12 @@ void CudaDeviceInterface::convertAVFrameToFrameOutput(
       frameOutput.data = cpuFrameOutput.data.to(device_);
     }
 
+    usingCPUFallback_ = true;
     return;
   }
 
+  usingCPUFallback_ = false;
+
   // Above we checked that the AVFrame was on GPU, but that's not enough, we
   // also need to check that the AVFrame is in AV_PIX_FMT_NV12 format (8 bits),
   // because this is what the NPP color conversion routines expect. This SHOULD
@@ -351,4 +354,12 @@ std::optional<const AVCodec*> CudaDeviceInterface::findCodec(
   return std::nullopt;
 }
 
+std::string CudaDeviceInterface::getDetails() {
+  // Note: for this interface specifically the fallback is only known after a
+  // frame has been decoded, not before: that's when FFmpeg decides to fallback,
+  // so we can't know earlier.
+  return std::string("FFmpeg CUDA Device Interface. Using ") +
+      (usingCPUFallback_ ? "CPU fallback." : "NVDEC.");
+}
+
 } // namespace facebook::torchcodec

src/torchcodec/_core/CudaDeviceInterface.h

@@ -40,6 +40,8 @@ class CudaDeviceInterface : public DeviceInterface {
       std::optional<torch::Tensor> preAllocatedOutputTensor =
           std::nullopt) override;
 
+  std::string getDetails() override;
+
  private:
   // Our CUDA decoding code assumes NV12 format. In order to handle other
   // kinds of input, we need to convert them to NV12. Our current implementation
@@ -60,6 +62,8 @@ class CudaDeviceInterface : public DeviceInterface {
   // maybeConvertAVFrameToNV12().
   std::unique_ptr<FiltersContext> nv12ConversionContext_;
   std::unique_ptr<FilterGraph> nv12Conversion_;
+
+  bool usingCPUFallback_ = false;
 };
 
 } // namespace facebook::torchcodec

src/torchcodec/_core/DeviceInterface.h

@@ -119,6 +119,10 @@ class DeviceInterface {
     avcodec_flush_buffers(codecContext_.get());
   }
 
+  virtual std::string getDetails() {
+    return "";
+  }
+
  protected:
   torch::Device device_;
   SharedAVCodecContext codecContext_;

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -1702,4 +1702,9 @@ double SingleStreamDecoder::getPtsSecondsForFrame(int64_t frameIndex) {
       streamInfo.allFrames[frameIndex].pts, streamInfo.timeBase);
 }
 
+std::string SingleStreamDecoder::getDeviceInterfaceDetails() const {
+  TORCH_CHECK(deviceInterface_ != nullptr, "Device interface doesn't exist.");
+  return deviceInterface_->getDetails();
+}
+
 } // namespace facebook::torchcodec

src/torchcodec/_core/SingleStreamDecoder.h

@@ -186,6 +186,8 @@ class SingleStreamDecoder {
   DecodeStats getDecodeStats() const;
   void resetDecodeStats();
 
+  std::string getDeviceInterfaceDetails() const;
+
  private:
   // --------------------------------------------------------------------------
   // STREAMINFO AND ASSOCIATED STRUCTS

src/torchcodec/_core/__init__.py

@@ -14,6 +14,7 @@
 )
 from .ops import (
     _add_video_stream,
+    _get_backend_details,
     _get_key_frame_indices,
     _test_frame_pts_equality,
     add_audio_stream,