[FFmpeg-devel] [PATCH 1/5] avformat/aviobuf: set AVIOContext->error on bprint buffer ENOMEM

[Marton Balint]

On Fri, 31 Dec 2021, Andreas Rheinhardt wrote:

> Marton Balint:
>> This makes sure the error condition is kept in AVIOContext even if the user
>> does not check the return value of avio_read_to_bprint or
>> ff_read_line_to_bprint.
>>
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>>  libavformat/aviobuf.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
>> index 29d4bd7510..6f8a822ee3 100644
>> --- a/libavformat/aviobuf.c
>> +++ b/libavformat/aviobuf.c
>> @@ -875,8 +875,10 @@ static int64_t read_string_to_bprint_overwrite(AVIOContext *s, AVBPrint *bp,
>>      if (ret < 0)
>>          return ret;
>>
>> -    if (!av_bprint_is_complete(bp))
>> +    if (!av_bprint_is_complete(bp)) {
>> +        s->error = AVERROR(ENOMEM);
>>          return AVERROR(ENOMEM);
>> +    }
>>
>>      return bp->len;
>>  }
>> @@ -1351,8 +1353,10 @@ int avio_read_to_bprint(AVIOContext *h, AVBPrint *pb, size_t max_size)
>>          if (ret <= 0)
>>              return ret;
>>          av_bprint_append_data(pb, buf, ret);
>> -        if (!av_bprint_is_complete(pb))
>> +        if (!av_bprint_is_complete(pb)) {
>> +            h->error = AVERROR(ENOMEM);
>>              return AVERROR(ENOMEM);
>> +        }
>>          max_size -= ret;
>>      }
>>      return 0;
>>
>
> I don't really see the point of this: It is not a real read error that
> should stick to the AVIOContext (which can still be used afterwards
> without any issue).
> If the user does not check the errors, then the user
> has no one to blame but himself for missing errors.

AVIO read/write behaviour is to store IO errors in the context so the user 
does not have to check for them in every call. It is not well documented 
which calls should be checked always, so the user might be under the 
impression that errors during read/write may be checked sometime 
later.

Admittedly, ENOMEM is not an IO error, but I think it is better to store 
that as well in the context to keep the behaviour consistent, because 
in case of ENOMEM avio_read_to_bprint reads and drops undefined amount of 
data, so the context will also be in an undefined state.

Other possibilities:
  - make avio_read_to_bprint read all the data regardless of AVBPrint 
fullness
  - mark avio_read_to_bprint av_warn_unused_result.
  - both :)

But these also forces the user to check return values... So I kind of like 
my original approach better, because it maintains avio_read/write call 
behaviour that it is safe to check errors sometime later.

Regards,
Marton


>
> - Andreas
>
> PS: If the AVBPrint API had a documented way of marking data as used,
> one could avoid those stack buffers and use the AVBPrint buffer directly
> with av_bprint_get_buffer(). (Marking data as used would be equivalent
> to incrementing len and ensuring that the buffer stays zero-terminated.)
> If this were done, no already read data would be lost in case of a later
> allocation failure.
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