FFmpeg docs say that you must return AVERROR_EOF from the read callback, not zero. Still, Telegram just propagates the return value from IODevice::read() call, which returns zero in case of EOF.
I don't know whether this commit has any effect on the upstream build, but it fixes a bug in Debian build of Telegram, which is using FFmpeg 5.1 instead of 4.4. Still, it's also useful in the upstream, as it makes work with FFmpeg more correct.
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