Re: [FFmpeg-devel] [PATCH 1/2] avformat/demux: use io_close2 when closing avfromat

[Kacper Michajlow <kasper93@gmail.com>]

On Sat, 17 May 2025 at 15:03, Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
>
> Andreas Rheinhardt:
> > Kacper Michajłow:
> >> It's not valid to call avio_close() on context that has not been open
> >> with avio_open().
> >>
> >> This fixes use of custom IO. (io_open / io_close2 callbacks)
> >>
> >> Note that by default io_close2 is set to io_close2_default() which calls
> >> avio_close(), so default case will work the same as before.
> >>
> >> Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
> >> ---
> >>  libavformat/demux.c | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/libavformat/demux.c b/libavformat/demux.c
> >> index 2795863567..ecd4f40da9 100644
> >> --- a/libavformat/demux.c
> >> +++ b/libavformat/demux.c
> >> @@ -383,11 +383,10 @@ void avformat_close_input(AVFormatContext **ps)
> >>          if (ffifmt(s->iformat)->read_close)
> >>              ffifmt(s->iformat)->read_close(s);
> >>
> >> +    ff_format_io_close(s, &pb);
> >>      avformat_free_context(s);
> >>
> >>      *ps = NULL;
> >> -
> >> -    avio_close(pb);
> >>  }
> >>
> >>  static void force_codec_ids(AVFormatContext *s, AVStream *st)
> >
> > avformat_open_input() sets AVFMT_FLAG_CUSTOM_IO when using custom IO and
> > avformat_close_input() checks for this and does not free the
> > AVFormatContext's AVIOContext in this case (but does avio_close(NULL) in
> > this case).
> > Your patch furthermore presumes that the custom IO context is compatible
> > with io_close2, which need not be true at all (one can use custom IO
> > without ever touching io_open/io_close2 pointers; I expect users that
> > use custom IO, but not formats that open new AVIOContexts to do so).
>
> On reading a bit further, init_input() uses io_open to open the
> AVIOContext in case no AVIOContext was supplied (and one is needed).
> This means that the AVFMT_FLAG_CUSTOM_IO flag is misleading, as it is
> possible to use custom IO even with this flag unset. It also means that
> your patch is likely correct.

Exactly. AVFMT_FLAG_CUSTOM_IO is used when an "external" AVIOContext
is provided. This is different when "custom" IO is used by changing
io_open/io_close2, where the lifetime of AVIOContext is still managed
internally, but uses custom implementation. I was initially confused
in exactly the same way you were, but those two "custom/external" IO
are not actually related.

Note that avformat_close_input() already zeroes the `pb` pointer if
the AVIOContext shouldn't be released, so ff_format_io_close() is noop
as expected for the AVFMT_FLAG_CUSTOM_IO case. (same as avio_close()
was).

- Kacper
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