On 7/4/2020 7:54 PM, Jan Engelhardt wrote:
> Greetings.
>
>
> Between ffmpeg-4.2.3 and ffmpeg-4.3, struct AVCodecContext,
> publicly exposed through /usr/include, has been changed thus:
>
> --- avcodec.h 2020-06-11 11:45:16.000000000 +0200
> +++ avcodec.h 2020-07-01 03:45:24.000000000 +0200
> @@ -3370,6 +2334,24 @@ typedef struct AVCodecContext {
> * - encoding: unused
> */
> int discard_damaged_percentage;
> +
> + /**
> + * The number of samples per frame to maximally accept.
> + *
> + * - decoding: set by user
> + * - encoding: set by user
> + */
> + int64_t max_samples;
> +
> + /**
> + * Bit set of AV_CODEC_EXPORT_DATA_* flags, which affects the kind of
> + * metadata exported in frame, packet, or coded stream side data by
> + * decoders and encoders.
> + *
> + * - decoding: set by user
> + * - encoding: set by user
> + */
> + int export_side_data;
> } AVCodecContext;
>
> #if FF_API_CODEC_GET_SET
>
>
> Second, struct AVDCT, which is publicly exposed through /usr/include, has been
> changed:
>
> --- avdct.h 2020-06-11 11:45:16.000000000 +0200
> +++ avdct.h 2020-07-01 03:45:24.000000000 +0200
> @@ -67,6 +67,10 @@ typedef struct AVDCT {
> ptrdiff_t line_size);
>
> int bits_per_sample;
> +
> + void (*get_pixels_unaligned)(int16_t *block /* align 16 */,
> + const uint8_t *pixels,
> + ptrdiff_t line_size);
> } AVDCT;
>
> /**
>
>
> The abidiff(1) utility confirms this situation such:
>
> $ abidiff
> b42/usr/lib/debug/usr/lib64/libavcodec.so.58.54.100-4.2.3-1.1.x86_64.debug \
>
> b43/usr/lib/debug/usr/lib64/libavcodec.so.58.91.100-4.3-136.2.x86_64.debug
>
> [...] in pointed to type 'struct AVCodecContext' at avcodec.h:526:1:
> type size changed from 8448 to 8576 (in bits)
> [...]
> underlying type 'struct AVDCT' at avdct.h:29:1 changed:
> type size changed from 896 to 960 (in bits)
>
>
> These struct changes constitute an ABI break, and ABI breaks require SO
> version
> bumps, but which was not done for 4.3. This is bad.
>
> A user has summarily reported crashes through {not a proper bug reporting
> medium} already.
> https://build.opensuse.org/package/show/multimedia:libs/ffmpeg-4
Neither of these are breaks as sizeof(AVCodecContext) and sizeof(AVDCT)
are not part of the ABI. Both structs are meant to be allocated using
avcodec_alloc_context3() and avcodec_dct_alloc() respectively, and not
stored on stack or allocated with av_malloc(sizeof()).
Fields can be added at the end of such structs without the need for a
soname bump and remain backwards compatible. A break would be adding a
field in the middle of such structs, moving the offset of other fields,
and that's not what happened here.
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