[FFmpeg-devel] [PATCH 1/3] avcodec/avpacket: add av_packet_resize()
James Almer
jamrial at gmail.com
Thu Mar 11 20:40:17 EET 2021
On 3/11/2021 3:26 PM, Andreas Rheinhardt wrote:
> James Almer:
>> On 3/11/2021 2:48 PM, Andreas Rheinhardt wrote:
>>> James Almer:
>>>> On 3/11/2021 2:27 PM, Andreas Rheinhardt wrote:
>>>>> James Almer:
>>>>>> On 3/11/2021 1:56 PM, Andreas Rheinhardt wrote:
>>>>>>> James Almer:
>>>>>>>> On 3/11/2021 1:11 PM, Marton Balint wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On Thu, 11 Mar 2021, James Almer wrote:
>>>>>>>>>
>>>>>>>>>> This function acts as a replacement for both av_grow_packet() and
>>>>>>>>>> av_shrink_packet(), the latter which is now deprecated and will be
>>>>>>>>>> removed as
>>>>>>>>>> it does not correctly handle non-writable packets.
>>>>>>>>>
>>>>>>>>> I don't think this is a good idea, av_shrink_packet cannot fail,
>>>>>>>>> av_grow_packet can. By using the same function you are losing the
>>>>>>>>> information if the end result should be checked or not.
>>>>>>>>
>>>>>>>> I'm not sure i follow. av_shrink_packet() is not being changed at
>>>>>>>> all,
>>>>>>>> just deprecated, scheduled for removal, and its use discouraged.
>>>>>>>
>>>>>>> I'd argue that a deprecation is actually a change.
>>>>>>>
>>>>>>>> Maybe i should have split this in two, one to add
>>>>>>>> av_packet_resize() and
>>>>>>>> one to deprecate av_shrink_packet(), to avoid confusion.
>>>>>>>>
>>>>>>>> In any case, the fact av_shrink_packet() cannot fail is the
>>>>>>>> reason I'm
>>>>>>>> getting rid of it. It's zeroing the padding without bothering to
>>>>>>>> check
>>>>>>>> if the packet is writable at all. And we can't have it attempt to
>>>>>>>> make
>>>>>>>> it writable because it can't then report if it failed to reallocate
>>>>>>>> the
>>>>>>>> buffer. So this patch here deprecates it for being a function that
>>>>>>>> predates reference counted buffers and is not able to properly
>>>>>>>> handle
>>>>>>>> them, and adds a replacement for it that also supersedes
>>>>>>>> av_grow_packet() while at it.
>>>>>>>>
>>>>>>>
>>>>>>> Yet you are not documenting that av_packet_resize can't fail if it is
>>>>>>> shrinking a packet known to be writable; ergo all unchecked uses of
>>>>>>> this
>>>>>>> function in the second and third patch are API abuse.
>>>>>>
>>>>>> I can add checks for all of them if you prefer. I didn't because they
>>>>>> are all internal uses known to (in theory) not fail.
>>>>>>
>>>>>
>>>>> No, I don't prefer checks for stuff that can't fail. I'd rather prefer
>>>>> if it were documented that it can't fail in these cases.
>>>>
>>>> I'm in general against adding that kind of constrain on a function's
>>>> documentation because you never know how it or what it processes could
>>>> be extended in the future.
>>>> Right now it can't fail in that scenario, true, but in the future we
>>>> could add some feature to AVPacket that would need to be handled by this
>>>> function that could start making it fail in that same scenario, and
>>>> suddenly, the doxy is no longer correct, and the function needs to be
>>>> replaced because it became unable to handle the new functionality.
>>>>
>>>> If a function can fail at all, then the library user should always make
>>>> sure to check the return value, and not be told there's one specific
>>>> scenario where they don't need to.
>>>>
>>>
>>> In this case I am against this patch.
>>
>> To add to what explained said above, it's the entire reason making
>> av_shrink_packet() return void was a mistake. It worked great before
>> reference counted buffers, but it was short sighted and the function is
>> now technically unusable after they were introduced, and why we need to
>> replace it now.
>> This patchset wouldn't exist had it been designed to return an int.
>>
>
> The function is very well useable, but only for its use-case of already
> writable packets. (Not documenting this when the refcounted API was
> introduced was an error; or not deprecating it and replacing it with a
> function that explicitly says so.)
It should have been deprecated and replaced back then, but wasn't. And
so I'm righting that wrong now.
>
>> What you're asking for is to make the exact same mistake again, and it
>> could very well come to bite us in the future, again. It's definitely
>> not a good reason to try and block this patch at all.
>> av_shrink_packet has four lines, two of which do the actual work. It is
> extremely simple and could nearly be inlined. If any future extension
> requires adding something that can fail to this, then said future
> extension sounds like a huge step backwards.
Well, then you're calling reference counted buffers a step backwards,
because their addition required making a compliant shrink + zero padding
function be able to fail, as already explained.
Unnecessary constrains like the one you suggest are proven to be short
sighted and not future proof, and all just to let the caller not check a
return value in one very specific scenario.
>
>>>
>>>>>
>>>>>>>
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>> Marton
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>>>>>>>> ---
>>>>>>>>>> libavcodec/avpacket.c | 19 +++++++++++++++----
>>>>>>>>>> libavcodec/packet.h | 16 ++++++++++++++++
>>>>>>>>>> libavcodec/version.h | 3 +++
>>>>>>>>>> 3 files changed, 34 insertions(+), 4 deletions(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/libavcodec/avpacket.c b/libavcodec/avpacket.c
>>>>>>>>>> index 32cb71fcf0..7d0dbadbed 100644
>>>>>>>>>> --- a/libavcodec/avpacket.c
>>>>>>>>>> +++ b/libavcodec/avpacket.c
>>>>>>>>>> @@ -100,6 +100,7 @@ int av_new_packet(AVPacket *pkt, int size)
>>>>>>>>>> return 0;
>>>>>>>>>> }
>>>>>>>>>>
>>>>>>>>>> +#if FF_API_SHRINK_PACKET
>>>>>>>>>> void av_shrink_packet(AVPacket *pkt, int size)
>>>>>>>>>> {
>>>>>>>>>> if (pkt->size <= size)
>>>>>>>>>> @@ -107,16 +108,26 @@ void av_shrink_packet(AVPacket *pkt, int
>>>>>>>>>> size)
>>>>>>>>>> pkt->size = size;
>>>>>>>>>> memset(pkt->data + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
>>>>>>>>>> }
>>>>>>>>>> +#endif
>>>>>>>>>>
>>>>>>>>>> int av_grow_packet(AVPacket *pkt, int grow_by)
>>>>>>>>>> {
>>>>>>>>>> - int new_size;
>>>>>>>>>> av_assert0((unsigned)pkt->size <= INT_MAX -
>>>>>>>>>> AV_INPUT_BUFFER_PADDING_SIZE);
>>>>>>>>>> if ((unsigned)grow_by >
>>>>>>>>>> INT_MAX - (pkt->size + AV_INPUT_BUFFER_PADDING_SIZE))
>>>>>>>>>> return AVERROR(ENOMEM);
>>>>>>>>>>
>>>>>>>>>> - new_size = pkt->size + grow_by +
>>>>>>>>>> AV_INPUT_BUFFER_PADDING_SIZE;
>>>>>>>>>> + return av_packet_resize(pkt, pkt->size + grow_by);
>>>>>>>>>> +}
>>>>>>>>>> +
>>>>>>>>>> +int av_packet_resize(AVPacket *pkt, int size)
>>>>>>>>>> +{
>>>>>>>>>> + int new_size;
>>>>>>>>>> +
>>>>>>>>>> + if (size < 0 || size > INT_MAX -
>>>>>>>>>> AV_INPUT_BUFFER_PADDING_SIZE)
>>>>>>>>>> + return AVERROR(EINVAL);
>>>>>>>>>> +
>>>>>>>>>> + new_size = size + AV_INPUT_BUFFER_PADDING_SIZE;
>>>>>>>>>> if (pkt->buf) {
>>>>>>>>>> size_t data_offset;
>>>>>>>>>> uint8_t *old_data = pkt->data;
>>>>>>>>>> @@ -143,10 +154,10 @@ int av_grow_packet(AVPacket *pkt, int
>>>>>>>>>> grow_by)
>>>>>>>>>> if (!pkt->buf)
>>>>>>>>>> return AVERROR(ENOMEM);
>>>>>>>>>> if (pkt->size > 0)
>>>>>>>>>> - memcpy(pkt->buf->data, pkt->data, pkt->size);
>>>>>>>>>> + memcpy(pkt->buf->data, pkt->data, FFMIN(pkt->size,
>>>>>>>>>> size));
>>>>>>>>>> pkt->data = pkt->buf->data;
>>>>>>>>>> }
>>>>>>>>>> - pkt->size += grow_by;
>>>>>>>>>> + pkt->size = size;
>>>>>>>>>> memset(pkt->data + pkt->size, 0,
>>>>>>>>>> AV_INPUT_BUFFER_PADDING_SIZE);
>>>>>>>>>>
>>>>>>>>>> return 0;
>>>>>>>>>> diff --git a/libavcodec/packet.h b/libavcodec/packet.h
>>>>>>>>>> index 3d9013d783..1720d973f5 100644
>>>>>>>>>> --- a/libavcodec/packet.h
>>>>>>>>>> +++ b/libavcodec/packet.h
>>>>>>>>>> @@ -484,13 +484,29 @@ void av_init_packet(AVPacket *pkt);
>>>>>>>>>> */
>>>>>>>>>> int av_new_packet(AVPacket *pkt, int size);
>>>>>>>>>>
>>>>>>>>>> +#if FF_API_SHRINK_PACKET
>>>>>>>>>> /**
>>>>>>>>>> * Reduce packet size, correctly zeroing padding
>>>>>>>>>> *
>>>>>>>>>> * @param pkt packet
>>>>>>>>>> * @param size new size
>>>>>>>>>> + *
>>>>>>>>>> + * @deprecated Use av_packet_resize
>>>>>>>>>> */
>>>>>>>>>> +attribute_deprecated
>>>>>>>>>> void av_shrink_packet(AVPacket *pkt, int size);
>>>>>>>>>> +#endif
>>>>>>>>>> +
>>>>>>>>>> +/**
>>>>>>>>>> + * Resize the payload of a packet, correctly zeroing padding and
>>>>>>>>>> avoiding data
>>>>>>>>>> + * copy if possible.
>>>>>>>>>> + *
>>>>>>>>>> + * @param pkt packet
>>>>>>>>>> + * @param size new size
>>>>>>>>>> + *
>>>>>>>>>> + * @return 0 on success, a negative AVERROR on error
>>>>>>>>>> + */
>>>>>>>>>> +int av_packet_resize(AVPacket *pkt, int size);
>>>>>>>>>>
>>>>>>>>>> /**
>>>>>>>>>> * Increase packet size, correctly zeroing padding
>>>>>>>>>> diff --git a/libavcodec/version.h b/libavcodec/version.h
>>>>>>>>>> index 3124ec8061..6c362b43e2 100644
>>>>>>>>>> --- a/libavcodec/version.h
>>>>>>>>>> +++ b/libavcodec/version.h
>>>>>>>>>> @@ -162,5 +162,8 @@
>>>>>>>>>> #ifndef FF_API_GET_FRAME_CLASS
>>>>>>>>>> #define FF_API_GET_FRAME_CLASS (LIBAVCODEC_VERSION_MAJOR < 60)
>>>>>>>>>> #endif
>>>>>>>>>> +#ifndef FF_API_SHRINK_PACKET
>>>>>>>>>> +#define FF_API_SHRINK_PACKET (LIBAVCODEC_VERSION_MAJOR <
>>>>>>>>>> 60)
>>>>>>>>>> +#endif
>>>>>>>>>>
>>>>>>>>>> #endif /* AVCODEC_VERSION_H */
>>>>>>>>>> --
>>>>>>>>>> 2.30.2
>>>>>>>>>>
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