Segmentation fault when calling FlushAllDownStream

[l-law]

Issue

It is found that when calling ChunkingHandler::OnFlushRequest with 2 different stream indexes, BufferWriter::WriteToFile will crash due to dereferencing a null pointer.

Setup

The setup was MediaHandler(derived class)->ChunkingHandler->MP4Muxer
MediaHandler takes 2 streams(video+audio), then feed into the one ChunkingHandler and one MP4Muxer, since a muxed MP4 stream is required.
When calling MediaHandler::FlushAllDownStream, it is found that SingleSegmentSegmenter::DoFinalize was called before SingleSegmentSegmenter::DoFinalizeSegment. In DoFinalize, temp_file_ was released already, therefore in DoFinalizeSegment, temp_file_.get() is a null pointer, and leads to the crash in BufferWriter::WriteToFile.

Is this a wrong usage of ChunkingHandler/MP4Muxer, or a bug in MP4Muxer?

More Information

Call stacks when calling DoFinalize and DoFinalizeSegment:

#0 0x00007fffeffb0dc7 in shaka::media::mp4::SingleSegmentSegmenter::DoFinalize() ()
#1 0x00007fffeffae0a9 in shaka::media::mp4::Segmenter::Finalize() ()
#2 0x00007fffeffa48ca in shaka::media::mp4::MP4Muxer::Finalize() ()
#3 0x00007fffeffa41ea in shaka::media::Muxer::OnFlushRequest(unsigned long) ()
#4 0x00007fffeffe706a in shaka::media::MediaHandler::FlushDownstream(unsigned long) ()
#5 0x00007fffeff9b62b in shaka::media::ChunkingHandler::OnFlushRequest(unsigned long) ()
#6 0x00007fffeffe716c in shaka::media::MediaHandler::FlushAllDownstreams() ()

#0 0x00007fffeffe5710 in shaka::media::BufferWriter::WriteToFile(shaka::File*) ()
#1 0x00007fffeffb1be6 in shaka::media::mp4::SingleSegmentSegmenter::DoFinalizeSegment() ()
#2 0x00007fffeffb0400 in shaka::media::mp4::Segmenter::FinalizeSegment(unsigned long, shaka::media::SegmentInfo const&) ()
#3 0x00007fffeffa42d5 in shaka::media::mp4::MP4Muxer::FinalizeSegment(unsigned long, shaka::media::SegmentInfo const&) ()
#4 0x00007fffeffe8b61 in shaka::media::Muxer::Process(std::unique_ptr<shaka::media::StreamData, std::default_deleteshaka::media::StreamData >) ()
#5 0x00007fffeffe6846 in shaka::media::MediaHandler::Dispatch(std::unique_ptr<shaka::media::StreamData, std::default_deleteshaka::media::StreamData >) ()
#6 0x00007fffeff9b229 in shaka::media::ChunkingHandler::OnFlushRequest(unsigned long) ()
#7 0x00007fffeffe6eac in shaka::media::MediaHandler::FlushAllDownstreams() ()

shaka-packager version: 858fcd8

[kqyang]

MP4Muxer is called a muxer, but it actually only supports one stream. We'd like to add support for muxing a/v streams in muxers, but it is low priority.