Segfault when input contains some corrupted streams

[LasSino]

Code like this will cause segfault for some inputs with corrupted or invalid streams:

import av
container = av.open("input.m2ts", "r")  # for certain input files
stream = container.streams[1] # select the corrupted stream
print(stream.type) # segfault here

This happens when the stream is corrupted so a general Stream rather than specific stream like VideoStream is created. So the type property invokes:

# av/stream.py
@cython.cclass
class Stream:
    ...

    @property
    def type(self):
        """
        The type of the stream.

        :type: Literal["audio", "video", "subtitle", "data", "attachment"]
        """
        return lib.av_get_media_type_string(self.ptr.codecpar.codec_type)

The underlying av_get_media_type_string function may return NULL for unknown AVMediaType:

// FFmpeg/libavutil/utils.c
const char *av_get_media_type_string(enum AVMediaType media_type)
{
    switch (media_type) {
    case AVMEDIA_TYPE_VIDEO:      return "video";
    case AVMEDIA_TYPE_AUDIO:      return "audio";
    case AVMEDIA_TYPE_DATA:       return "data";
    case AVMEDIA_TYPE_SUBTITLE:   return "subtitle";
    case AVMEDIA_TYPE_ATTACHMENT: return "attachment";
    default:                      return NULL;
    }
}

When NULL is returned, a segfault will be triggered since Cython converts the returned const char * to a Python string without checking for NULL.