Custom `AVIOContext` fails to open input file

[adamhewitt627]

I'm submitting a bug report.

• Do you want to request a feature or report a bug?
  This could be a bug with ffmpeg, but seems to be a failure with the managed/native callbacks. Our code worked under ffmpeg 4.3, and broke in 5.0. I checked again and it's still broken in ffmpeg6. I will work on a repro example, but wanted to get this filed.

• What is the current behavior?

1. Custom AVIOContext fails to open for reading a file.
2. The Read callback executes once, but then an AccessViolationException is thrown.

• *If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem:

• What is the expected behavior?
  Reading a file with a custom IO context should work. (Interestingly, it does work for writing a file)

• Please tell us about your environment:

• version: ffmpeg6, Windows 11, .NET6.0 WPF application.

[adamhewitt627]

Here is a sample project, just a small console app that tries to open the file. FfmegCustomIO.zip

1. Fill in the ffmpeg.RootPath with your environment.
2. Fill in the file path to some mp4, avi, etc.
3. Run the app - should blow up on avformat_open_input. Breakpoints will show that the Read callback is executed once.

[adamhewitt627]

Interesting, removing this line appears to resolve it but also calls Seek (which I wouldn't expect)

Reference->seekable = stream.CanSeek ? 1 : 0;

[zgabi]

I haven't tried your code, but it looks very suspicious that
write_flag: stream.CanWrite ? 1 : 0,
and
write_packet: null,

I think you should set the write flag to 0 if the write delegate is null.

[adamhewitt627]

True, I didn't get that line entirely pulled out of the minimum repro. But that will also always be 0 because the file is being opened readonly.

[Ruslan-B]

I think it could be related issue #217

[Ruslan-B]

I'll try to check integrity of AVIOContext structure.
As indeed when commenting out Reference->seekable = stream.CanSeek ? 1 : 0; it seems to be working fine.

[dubiousconst282]

The crash in avformat_open_input() seems to be happening somewhere around here: https://github.com/FFmpeg/FFmpeg/blob/5bad4856035ca5ed571e9d7d9b1d503a5c9ef0a5/libavformat/demux.c#L258-L272

The C# struct layout for AVIOContext seem to mismatch the one in the Win32 build. In particular, the field pb->seekable (at offset 0x90 in the C# struct) is swapped with pb->xxx_whitelist.

I didn't want to build ffmpeg myself, so I confirmed this by setting a dummy value like pb->seekable = 0x1234567, which ends up being passed to av_strdup() via rcx:

Also I don't think that the C# layout is wrong as it matches the clang output, so the maybe the problem is specific to Windows builds?

Edit: note that it's not necessary to set seekable manually because avio_alloc_context() already does so when you pass a non-null seek callback (ref). There seems to be no other way to check for the AVIO_SEEKABLE_NORMAL flag, so I'm just checking for pb->seek.Pointer != IntPtr.Zero; instead.

[Ruslan-B]

I updated state of research and passible actions in #217

[AnthonyParkssonos]

btw i encountered this with ffmpeg 5 feeding it a buffer of FLAC and consequently getting free() invalid pointer every time. i have been hitting my head against GDB for the past day trying to do something like this:

/// buffer comes from network, 
uint8_t buffer[4096] = // assume mock read 
AVFormatContext* pFormat = avformat_alloc_context()
AVIOContext* pAvioCtx = avio_alloc_context(buffer, 4096, 0, nullptr, nullptr, nullptr, nullptr);
pFormat->pb = pAvioCtx;
// SIGABRT free() invalid pointer occurs every time inside this ->
if (avformat_open_input(&pFormat, nullptr, nullptr, nullptr)) {
     log("couldn't open input");
}