Avoid unnecessary calls to avcodec_receive_frame in decoding loop (#502)

Author: NicolasHug

src/torchcodec/decoders/_core/VideoDecoder.cpp

@@ -971,43 +971,44 @@ VideoDecoder::AVFrameStream VideoDecoder::decodeAVFrame(
     }
 
     if (reachedEOF) {
-      // We don't have any more packets to send to the decoder. So keep on
-      // pulling frames from its internal buffers.
+      // We don't have any more packets to receive. So keep on pulling frames
+      // from its internal buffers.
       continue;
     }
 
     // We still haven't found the frame we're looking for. So let's read more
     // packets and send them to the decoder.
     ReferenceAVPacket packet(autoAVPacket);
-    ffmpegStatus = av_read_frame(formatContext_.get(), packet.get());
-    decodeStats_.numPacketsRead++;
+    do {
+      ffmpegStatus = av_read_frame(formatContext_.get(), packet.get());
+      decodeStats_.numPacketsRead++;
+
+      if (ffmpegStatus == AVERROR_EOF) {
+        // End of file reached. We must drain the codec by sending a nullptr
+        // packet.
+        ffmpegStatus = avcodec_send_packet(
+            streamInfo.codecContext.get(),
+            /*avpkt=*/nullptr);
+        if (ffmpegStatus < AVSUCCESS) {
+          throw std::runtime_error(
+              "Could not flush decoder: " +
+              getFFMPEGErrorStringFromErrorCode(ffmpegStatus));
+        }
+
+        reachedEOF = true;
+        break;
+      }
 
-    if (ffmpegStatus == AVERROR_EOF) {
-      // End of file reached. We must drain the codec by sending a nullptr
-      // packet.
-      ffmpegStatus = avcodec_send_packet(
-          streamInfo.codecContext.get(),
-          /*avpkt=*/nullptr);
       if (ffmpegStatus < AVSUCCESS) {
         throw std::runtime_error(
-            "Could not flush decoder: " +
+            "Could not read frame from input file: " +
             getFFMPEGErrorStringFromErrorCode(ffmpegStatus));
       }
+    } while (packet->stream_index != activeStreamIndex_);
 
-      // We've reached the end of file so we can't read any more packets from
-      // it, but the decoder may still have frames to read in its buffer.
-      // Continue iterating to try reading frames.
-      reachedEOF = true;
-      continue;
-    }
-
-    if (ffmpegStatus < AVSUCCESS) {
-      throw std::runtime_error(
-          "Could not read frame from input file: " +
-          getFFMPEGErrorStringFromErrorCode(ffmpegStatus));
-    }
-
-    if (packet->stream_index != activeStreamIndex_) {
+    if (reachedEOF) {
+      // We don't have any more packets to send to the decoder. So keep on
+      // pulling frames from its internal buffers.
       continue;
     }