[FFmpeg-devel] [PATCH] avio_read data loss with multiple calls to fill_buffer

[Rob Meyers]

avio_read does multiple calls to fill_buffer(). Adjusting things in
fill_buffer was frowned upon; how would you suggest I approach this? The
id3v2 seek doesn't come into play; the return from avio_read is the "full"
read.

The call I added in avio_read is only done when there was accumulated data,
so avoid reallocating the buffer the first time through.

On Thu, May 18, 2017 at 3:53 PM Michael Niedermayer <michael at niedermayer.cc>
wrote:

> On Thu, May 18, 2017 at 01:12:49PM -0700, Rob Meyers wrote:
> > avio_read may make multiple calls to fill_buffer to accumulate bytes
> > to fulfill a request. fill_buffer will overwrite previously read data
> > if it is not prepped. Added a call to 'ffio_ensure_seekback' to adjust
> > the s->buffer to fill_buffer's liking. This isn't necessary for the
> > very first call to fill_buffer, thus the "size1 != size" check.
> > ---
> >  libavformat/aviobuf.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> > index 0a7c39eacd..8e9594cab8 100644
> > --- a/libavformat/aviobuf.c
> > +++ b/libavformat/aviobuf.c
> > @@ -648,6 +648,11 @@ int avio_read(AVIOContext *s, unsigned char *buf,
> int size)
> >                      s->buf_end = s->buffer/* + len*/;
> >                  }
> >              } else {
> > +                if (size1 != size)
> > +                    /* this call will ensure fill_buffer will not
> overwrite previously
> > +                    read data. */
> > +                    ffio_ensure_seekback(s, size1);
>
> ffio_ensure_seekback() should be called when theres a potential
> seek back in the following code.
>
> There is no seekback in avio_read(), avio_read only moves forward.
> Some callers may do a avio_read() and or other function calls and
> then seek back. A caller might do 3 avio_read() and seek back over all
>
> ffio_ensure_seekback() generally should be called by the code that
> later initiates teh seek back
>
> ffio_ensure_seekback() is not free it should only be used when it is
> needed.
>
>
>
> [...]
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> Those who are too smart to engage in politics are punished by being
> governed by those who are dumber. -- Plato
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