[torchcodec] Return subsequent frame if FFMPEG seeks past end of the frame (#178)

Summary:
Pull Request resolved: #178

The way get_frame_displayed_at works is as follows:

We call avformat_seek_file with:

`min_ts=-inf ts=timestamp max_ts=timestamp`

https://ffmpeg.org/doxygen/7.0/group__lavf__decoding.html#ga3b40fc8d2fda6992ae6ea2567d71ba30

This should in theory never seek past our timestamp.

However for some videos ffmpeg does seek past our timestamp.

And that causes us to never return a valid frame in that case.

The fix is to warn the user and return the first frame after the timestamp if this ever happens.

Differential Revision: D61139386

Author: ahmadsharif1

src/torchcodec/decoders/_core/VideoDecoder.cpp

@@ -934,14 +934,26 @@ VideoDecoder::DecodedOutput VideoDecoder::getFrameDisplayedAtTimestampNoDemux(
     }
   }
   setCursorPtsInSeconds(seconds);
-  return getDecodedOutputWithFilter(
-      [seconds, this](int frameStreamIndex, AVFrame* frame) {
-        StreamInfo& stream = streams_[frameStreamIndex];
-        double frameStartTime = ptsToSeconds(frame->pts, stream.timeBase);
-        double frameEndTime =
-            ptsToSeconds(frame->pts + getDuration(frame), stream.timeBase);
-        return seconds >= frameStartTime && seconds < frameEndTime;
-      });
+  return getDecodedOutputWithFilter([seconds, this](
+                                        int frameStreamIndex, AVFrame* frame) {
+    StreamInfo& stream = streams_[frameStreamIndex];
+    double frameStartTime = ptsToSeconds(frame->pts, stream.timeBase);
+    double frameEndTime =
+        ptsToSeconds(frame->pts + getDuration(frame), stream.timeBase);
+    if (frameStartTime > seconds) {
+      // FFMPEG seeked past the frame we are looking for even though we
+      // set max_ts to be our needed timestamp in avformat_seek_file()
+      // in maybeSeekToBeforeDesiredPts().
+      // In this case we return the very next frame instead of throwing an
+      // exception.
+      std::cerr
+          << "TorchCodec: WARNING: ffmpeg seeked past requested timestamp="
+          << seconds << ". Returning frame with startTime=" << frameStartTime
+          << " endTime=" << frameEndTime << std::endl;
+      return true;
+    }
+    return seconds >= frameStartTime && seconds < frameEndTime;
+  });
 }
 
 void VideoDecoder::validateUserProvidedStreamIndex(uint64_t streamIndex) {