[FFmpeg-devel] [PATCH] avcodec/pthread_frame: Don't update the first thread ctx before freeing

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Currently, ff_frame_thread_free() uses the last worker thread
to updates the first worker thread via update_context_from_thread()
immediately before freeing all these worker threads. This is
a remnant of the time in which the first worker was special.
(E.g. the first worker shared its AVCodecInternal with the public
AVCodecContext.)

But these times are over (none of the uses of is_copy matter
for ff_frame_thread_free()); nowadays the only thing that
update_context_from_thread() does is referencing a few
buffers/frames and replacing them with other references instead.
These new references will then be freed immediately thereafter
when the first worker thread is freed. Ensuring that the code is
free of double-frees is achieved by using reference-counted structures
(or in case of AVChannelLayouts: by giving each worker its own copy).

Some archaeology:
a) Updating the first worker thread from the last one used
has been done since frame-threading was added in
37b00b47cbeecd66bb34c5c7c534d016d6e8da24.
b) The precursor to ff_mpv_common_end() checked for is_copy
before freeing pictures (i.e. it only freed them for the first
worker thread).
c) Commits c2dfb1e37cc72bf144545c4410a4621cbff5c4b1 and
e33811bd2686411233cb0eb4a4ee45eb99d7e736 modified the
update_thread_context function of the H.264 decoder
so that it could fail before calling ff_mpeg_update_thread_context().
d) This led to a double free/an assert violation with an H.264
sample for which ff_mpeg_update_thread_context() is not reached
for the final update_context_from_thread(). Commit
a6e4796fbf0aa9b13451a8ef917ecc4e80d1d272 added code to fix this
sample.
e) This issue was fixed (even with the last mentioned commit reverted)
when the H.264 decoder was deMpegEncContextized in commit
b7fe35c9e50e1701274364adf7280bf4a02b092b (merging commit
2c541554076cc8a72e7145d4da30389ca763f32f).
f) mpegvideo.c stopped using is_copy when it was switched to refcounted
frames in 759001c534287a96dc96d1e274665feb7059145d.
g) 1f4cf92cfbd3accbae582ac63126ed5570ddfd37 removed the init_thread_copy
callbacks; now no FFCodec.close callback checks for is_copy at all
any more.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
---
 libavcodec/pthread_frame.c | 7 -------
 1 file changed, 7 deletions(-)

diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
index c667706206..8faea75a49 100644
--- a/libavcodec/pthread_frame.c
+++ b/libavcodec/pthread_frame.c
@@ -714,13 +714,6 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
         }
     }
 
-    if (fctx->prev_thread && fctx->prev_thread != fctx->threads)
-        if (update_context_from_thread(fctx->threads->avctx, fctx->prev_thread->avctx, 0) < 0) {
-            av_log(avctx, AV_LOG_ERROR, "Final thread update failed\n");
-            fctx->prev_thread->avctx->internal->is_copy = fctx->threads->avctx->internal->is_copy;
-            fctx->threads->avctx->internal->is_copy = 1;
-        }
-
     for (i = 0; i < thread_count; i++) {
         PerThreadContext *p = &fctx->threads[i];
         AVCodecContext *ctx = p->avctx;
-- 
2.32.0

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