[FFmpeg-devel] [PATCH] Use AVERROR(ENOSYS) when it makes sense

[Howard Chu]

Michael Niedermayer wrote:
> On Tue, Apr 13, 2010 at 04:41:15PM -0700, Howard Chu wrote:
>> Michael Niedermayer wrote:
>>> On Mon, Apr 12, 2010 at 12:42:05AM +0200, Stefano Sabatini wrote:
>>>> On date Sunday 2010-04-11 23:33:15 +0200, Michael Niedermayer encoded:
>>>>> On Sun, Apr 11, 2010 at 10:13:30PM +0200, Stefano Sabatini wrote:
>>>>>> On date Saturday 2010-04-03 13:46:24 +0200, Stefano Sabatini encoded:
>>>>>>> Hi, as in subject.
>>>>>>> --
>>>>>>> FFmpeg = Fucking and Fabulous Moronic Peaceful Elitarian Genius
>>>>>>
>>>>>>>    From e66cec2193ee289f742120f92db17f33e7651fba Mon Sep 17 00:00:00
>>>>>>> 2001
>>>>>>> From: Stefano Sabatini<stefano.sabatini-lala at poste.it>
>>>>>>> Date: Tue, 16 Mar 2010 22:48:37 +0100
>>>>>>> Subject: [PATCH 2/5] Make url_seek() return AVERROR(ENOSYS) rather
>>>>>>> than AVERROR(EPIPE) in
>>>>>>>    the case where the seek operation is not defined in the protocol
>>>>>>>    handler.
>>>>>>>
>>>>>>> ---
>>>>>>>    libavformat/avio.c |    2 +-
>>>>>>>    1 files changed, 1 insertions(+), 1 deletions(-)
>>>>>>>
>>>>>>> diff --git a/libavformat/avio.c b/libavformat/avio.c
>>>>>>> index af9e049..3fb64d1 100644
>>>>>>> --- a/libavformat/avio.c
>>>>>>> +++ b/libavformat/avio.c
>>>>>>> @@ -201,7 +201,7 @@ int64_t url_seek(URLContext *h, int64_t pos, int
>>>>>>> whence)
>>>>>>>        int64_t ret;
>>>>>>>
>>>>>>>        if (!h->prot->url_seek)
>>>>>>> -        return AVERROR(EPIPE);
>>>>>>> +        return AVERROR(ENOSYS);
>>>>>>>        ret = h->prot->url_seek(h, pos, whence&   ~AVSEEK_FORCE);
>>>>>>>        return ret;
>>>>>>>    }
>>>>>>> --
>>>>>>> 1.7.0
>>>>>>>

>> Leaving it EPIPE makes for some other headaches. Most of the code uses
>> ENOSYS for unimplemented functions, so using EPIPE here makes an awkward
>> inconsistency. Callers expect ENOSYS.
>
> what is "here", the code above quotes 3 functions i dont see any immedeate
> relation between any of them and your code

aviobuf.c:
int64_t av_url_read_fseek(ByteIOContext *s, int stream_index,
                           int64_t timestamp, int flags)
{
     URLContext *h = s->opaque;
     int64_t ret;
     if (!s->read_seek)
         return AVERROR(ENOSYS);
     ret = s->read_seek(h, stream_index, timestamp, flags);
     if(ret >= 0) {
         int64_t pos;
         s->buf_ptr = s->buf_end; // Flush buffer
         pos = s->seek(h, 0, SEEK_CUR);
         if (pos >= 0)
             s->pos = pos;
         else if (pos != AVERROR(ENOSYS))
             ret = pos;
     }
     return ret;
}

s->seek() will point to url_seek(). For the RTMP stream h->prot->url_seek is 
not implemented, so s->seek() will return EPIPE. av_url_read_fseek() was 
expecting ENOSYS, not EPIPE, so it will pass EPIPE thru to the caller.
flv_read_seek() will return that to its caller. It will get passed on upward 
and the read_seek() will be treated as failing instead of succeeding.

Breakpoint 1, av_url_read_fseek (s=0x0, stream_index=0, timestamp=496, flags=0)
     at aviobuf.c:739
739	{
(gdb) bt
#0  av_url_read_fseek (s=0x0, stream_index=0, timestamp=496, flags=0)
     at aviobuf.c:739
#1  0x0000000000449380 in flv_read_seek (s=0x11ccea0, stream_index=0,
     ts=10905, flags=0) at flvdec.c:455
#2  0x00000000004a4897 in av_seek_frame (s=0x11ccea0, stream_index=0,
     timestamp=10905, flags=0) at utils.c:1674
#3  0x00000000004a49f2 in avformat_seek_file (s=0x11ccea0, stream_index=-1,
     min_ts=905416, ts=10905414, max_ts=9223372036854775807, flags=0)
     at utils.c:1704
#4  0x00000000004293e8 in decode_thread (arg=<value optimized out>)
     at ffplay.c:2506
#5  0x00007ffff67c57f5 in ?? () from /usr/lib/libSDL-1.2.so.0
#6  0x00007ffff680b3d9 in ?? () from /usr/lib/libSDL-1.2.so.0
#7  0x00007ffff621c9ca in start_thread () from /lib/libpthread.so.0
#8  0x00007ffff65196dd in clone () from /lib/libc.so.6
#9  0x0000000000000000 in ?? ()
(gdb)

>
> [...]
>
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-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/