On Fri, 12 Mar 2021, James Almer wrote:
I get a packet from a demuxer. It contains two independent portions
(NALUs, OBUs, etc) i want to separate in order to feed them to
something like a multi threaded decoder. And so I create a new
reference to it, resulting in two packets pointing to the same data. I
shrink one to only contain the first portion, and i add the required
byte offset to the data pointer and subtract it to the size field on
the second packet so it contains only the second portion.
The result if i use av_packet_resize() will be two valid, properly
padded packets containing their respective expected data, but if i use
av_shrink_packet(), the result will be the packet with the second
portion featuring padding bytes worth of data zeroed at the start of
its payload.
This looks like an unfortunate example, since you are:
- adding a reference to the packet, so you don't have to duplicate data
- and then want to add zero padding to the partial packet, so you will
duplicate data.
And I think the padding does not have to be zero for the partial packet.
The padding exists AFAIK because something, like an optimized bitstream
reader that tries to process several bytes at the same time, may end up
reading or writing pass the reported end of the buffer.
That means that if reading and it's not all zeroes, it could in theory
have unpredictable results. Hence why everything always zeroes the
padding, even av_shrink_packet().
And yes, what you describe is what some bitstream filters and the
matroska demuxer do. They just create several packet references pointing
to the same data buffer, but using different offsets for the data
pointer. They all have "padding", but in many cases said padding is the
beginning of the payload of another packet, and that's not ideal.
Well, performance reasons come in play and the ability to not copy the
data. Yeah, it does not play nicely with our historical requirement of
zero padding.
I did a little experimenting, and except for subtitles (which have crashed
and burned because of the missing 0 string terminator), most tests handled
non-zero padding well, a few failed tests, mostly for partial source
files, for which I guess it is inevitable?
So I guess for now we will stay in the gray area of "if it does not
crash, then having non-zero padding for some partial packets is
OKish"...
I agree that it is not nice that av_shrink_packet() writes something to
the packet, people may not think about it and misuse it instead of
directly decreaseing pkt->size when they need a partial packet. That is
why I suggested the assert(). One might argue that it kind of breaks
behaviour, but I'd say it does not break it too much, and writing to a
non-writable packet was broken in the first place.
Alternatively we can change av_shrink_packet() to only zero the padding
if the packet is writable, which has the benefit that it will do what
people generally expect, no matter if you throw a writable or a
non-writable packet to it.
A third alternative is to introduce void av_shrink_packet2() in which
you explicitly document that a writable packet is needed and do the
assert there, and deprecate av_shrink_packet().
Not a fan of functions with a 2 suffix. And to be honest, I really don't
care about what we do with av_shrink_packet().
Do you want to keep it around? Ok. Want to deprecate and remove it?
Better. Want to add an assert to it? Not a fan and it may result in
unexpected crashes for library users, but whatever.
I just want to add av_packet_resize() to have a single resize function
that will properly handle AVPackets in their current and any potential
future states.
Ok, then I suggest you add av_resize_packet but keep av_shrink_packet() as
well, and we can add an assert() to it after the release/bump.
Regards,
Marton
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