[FFmpeg-devel] [PATCH] lavf/mpegts: improve read error handling

[Rodger Combs]

> On Aug 24, 2018, at 03:44, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> 
> On Fri, Aug 24, 2018 at 5:01 AM mypopy at gmail.com <mailto:mypopy at gmail.com> <mypopy at gmail.com <mailto:mypopy at gmail.com>> wrote:
>> 
>> On Fri, Aug 24, 2018 at 5:38 AM Rodger Combs <rodger.combs at gmail.com> wrote:
>>> 
>>> We previously could fail to check errors entirely, or misinterpret read
>> errors
>>> as normal EOFs.
>>> ---
>>> libavformat/mpegts.c | 13 +++++++++++--
>>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/libavformat/mpegts.c b/libavformat/mpegts.c
>>> index 37a6aa8bff..1350213d39 100644
>>> --- a/libavformat/mpegts.c
>>> +++ b/libavformat/mpegts.c
>>> @@ -2475,6 +2475,8 @@ static int mpegts_resync(AVFormatContext *s, int
>> seekback, const uint8_t *curren
>>> 
>>>     for (i = 0; i < ts->resync_size; i++) {
>>>         c = avio_r8(pb);
>>> +        if (pb->error)
>>> +            return pb->error;
>>>         if (avio_feof(pb))
>>>             return AVERROR_EOF;
>>>         if (c == 0x47) {
>>> @@ -2498,8 +2500,13 @@ static int read_packet(AVFormatContext *s, uint8_t
>> *buf, int raw_packet_size,
>>> 
>>>     for (;;) {
>>>         len = ffio_read_indirect(pb, buf, TS_PACKET_SIZE, data);
>>> -        if (len != TS_PACKET_SIZE)
>>> -            return len < 0 ? len : AVERROR_EOF;
>>> +        if (len != TS_PACKET_SIZE) {
>>> +            if (len < 0)
>>> +                return len;
>>> +            if (pb->error)
>>> +                return pb->error;
>>> +            return AVERROR_EOF;
>>> +        }
>>>         /* check packet sync byte */
>>>         if ((*data)[0] != 0x47) {
>>>             /* find a new packet start */
>>> @@ -2670,6 +2677,8 @@ static int mpegts_read_header(AVFormatContext *s)
>>>     /* read the first 8192 bytes to get packet size */
>>>     pos = avio_tell(pb);
>>>     len = avio_read(pb, buf, sizeof(buf));
>>> +    if (len < 0)
>>> +        return len;
>>>     ts->raw_packet_size = get_packet_size(buf, len);
>>>     if (ts->raw_packet_size <= 0) {
>>>         av_log(s, AV_LOG_WARNING, "Could not detect TS packet size,
>> defaulting to non-FEC/DVHS\n");
>>> --
>> As I understand, only after avio_xxx return < 0 to check pb->error, now we
>> coding like:
>>           len = avio_xxx(pb);
>>           if (len < 0)
>>                return len;
>>            if (pb->error)
>>                return pb->error;
>> 
>> It's stranger way for me, consider Unix API read(2), we just check the
>> error after -1 is returned
>> (
>> http://man7.org/linux/man-pages/man2/read.2.html
>> )
>> 
>> we usually catch the error
>> / error
>> number like:
>> 
> 
> The reason for this is to be able to differentiate between EOF and
> read errors. In case of a read error, partial data may still be
> returned, and any short read is otherwise considered EOF before this
> patch.
> The alternative to this would be checking pb->eof_reached, which would
> do the same thing,  just flip the logic on its head.

I'd actually sort of prefer that, since it'd let me call avio_feof() instead of doing a direct member access, but it turns out that eof_reached is set to 1 in error cases as well.

> 
> - Hendrik
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