[FFmpeg-devel] [PATCH 1/3] lavu/error: add AVERROR_REDO.

[Marton Balint]

On Fri, 27 Nov 2015, Nicolas George wrote:

> Le sextidi 6 frimaire, an CCXXIV, Marton Balint a écrit :
>> Maybe I am missing something, but the existing error AVERROR(EINTR) cannot
>> be used for this?
>
> It would be less broken than EAGAIN, since it is almost always treated like
> that by Unix code.
>
> But without the loop in utils.c (patch 2/3 in this series), the result is as
> bad or worse than EAGAIN, since the application sees it as an error code.
> And this is this loop that wm4 and Ronald are bikeshedding.

I agree that a loop is necessary if a demuxer can return an error with 
such semantics to maintain compatibility.

> But in principle, I am quite against reusing codes like that. The error
> message for EINTR is "Interrupted function" or more frequently "Interrupted
> system call", and nothing was interrupted, especially not a system call.

The only reaseon for considering this if we return AVERROR(errno) directly 
from some syscalls to factorize code... probably not likely to happen, so 
ok.

> The
> bug in the FLV demuxer at the origin of this threads was precisely caused
> because EAGAIN was reused for a slightly different semantic.
>
> (As a side note, it seems that EINTR is already abused in a completely
> different way by the MPEG-TS demuxer; so much for the fabled review
> process.)

Yes, it should be fixed...

> On the other hand, I can see a point for making this error code MORE
> specific, for example AVERROR_DATA_DISCARDED "Data was discarded".

Okay, there are two different kind of semantics here:

1) Packets were discarded becuase the user did not want them, or they were 
empty packets (stuffing). This "error" does not indicate any problem with 
the stream.

2) Data was discarded becuase it was garbage. (sync byte lost case)
This should simply be an AVERROR_INVALIDDATA? Or we should add a new error 
code and pass it simply to the user?

Anyway, IMHO the important thing is to define/document that the error code 
you introduce is only used for the first case.

Because EAGAIN is used for both at the moment I guess and neither of these 
is EAGAIN...

Regards,
Marton