avcodec/ffv1dec: Fix data races emanating from copying whole context

When using frame threading, the FFV1 decoder's update_thread_context()
function copies the whole context and afterwards restores some allocated
fields with backups made earlier. Among these fields are the
ThreadFrames and the source context's ThreadFrames can change
concurrently without any synchronization, leading to data races which
are undefined behaviour even if they don't lead to problems in
practice (as the destination's own ThreadFrames are restored directly
thereafter).

Fix this by only copying the actually needed fields.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/ffv1dec.c

@@ -1072,18 +1072,12 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
     if (dst == src)
         return 0;
 
-    {
-        ThreadFrame picture = fdst->picture, last_picture = fdst->last_picture;
-        uint8_t (*initial_states[MAX_QUANT_TABLES])[32];
-        struct FFV1Context *slice_context[MAX_SLICES];
-        memcpy(initial_states, fdst->initial_states, sizeof(fdst->initial_states));
-        memcpy(slice_context,  fdst->slice_context , sizeof(fdst->slice_context));
-
-        memcpy(fdst, fsrc, sizeof(*fdst));
-        memcpy(fdst->initial_states, initial_states, sizeof(fdst->initial_states));
-        memcpy(fdst->slice_context,  slice_context , sizeof(fdst->slice_context));
-        fdst->picture      = picture;
-        fdst->last_picture = last_picture;
+    copy_fields(fdst, fsrc, fsrc);
+    fdst->use32bit     = fsrc->use32bit;
+    memcpy(fdst->state_transition, fsrc->state_transition,
+           sizeof(fdst->state_transition));
+    memcpy(fdst->quant_table, fsrc->quant_table, sizeof(fsrc->quant_table));
+
         for (i = 0; i<fdst->num_h_slices * fdst->num_v_slices; i++) {
             FFV1Context *fssrc = fsrc->slice_context[i];
             FFV1Context *fsdst = fdst->slice_context[i];
@@ -1091,7 +1085,6 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
         }
         av_assert0(!fdst->plane[0].state);
         av_assert0(!fdst->sample_buffer);
-    }
 
     av_assert1(fdst->max_slice_count == fsrc->max_slice_count);