[FFmpeg-devel] ABI break in 4.3

[Tomas Härdin]

sön 2020-07-05 klockan 10:43 +0200 skrev Jan Engelhardt:
> On Sunday 2020-07-05 01:04, James Almer wrote:
> > [[the header file says:
> > * You can use AVOptions (av_opt* / av_set/get*()) to access these fields from user
> > * applications.]]
> 
> A "can" can be read as "need not". Perhaps that is what we are seeing with
> blender which seems to access av fields directly, and has no noticable 
> av_set_ calls.
> 
> writeffmpeg.c:  if ((of->oformat->flags & AVFMT_GLOBALHEADER)) {
> writeffmpeg.c:  if (of->oformat->flags & AVFMT_GLOBALHEADER) {
> writeffmpeg.c:  of->oformat = fmt;
> writeffmpeg.c:    of->packet_size = rd->ffcodecdata.mux_packet_size;
> writeffmpeg.c:  of->max_delay = (int)(0.7 * AV_TIME_BASE);
> writeffmpeg.c:  BLI_strncpy(of->filename, name, sizeof(of->filename));
> writeffmpeg.c:    if (avio_open(&of->pb, name, AVIO_FLAG_WRITE) < 0) {
> writeffmpeg.c:        &of->metadata, context->stamp_data, ffmpeg_add_metadata_callback, false);
> writeffmpeg.c:  if (of->pb) {
> writeffmpeg.c:    avio_close(of->pb);

This looks like perfectly legitimate code to me. But, if Blender is
using libav*'s APIs incorrectly somewhere then Blender should be fixed.

> Or, summarized: A program may have been built with 4.3 but is combined
> with 4.2.3 at runtime, then this can happen:
> 
> 	a = avcodec_dct_alloc(); // allocates 896
> #ifdef HAVE_STRUCT_AVDCT_GET_PIXELS_UNALIGNED
> 	a->get_pixels_unaligned = ffunc; // boom accessing byte ~952
> #endif

"Doctor it hurts when I do this!"

Downgrading to a .so file with a lower minor version number than the
program is built against can never be expected to work. Else we
couldn't add new functions without a major bump.

/Tomas