pthread_frame: save the FF_DEBUG_THREADS option in PerThreadContext.

Add the debug_threads boolean field to PerThreadContext. For
PerThreadContext *p, p->debug_threads records whether the
FF_DEBUG_THREADS bit is set in p->avctx->debug, and p->debug_threads and
p->avctx->debug are kept in sync. The debug_threads field is defined as
an atomic_int to allow atomic read by another thread in
ff_thread_await_progress().

This fixes the tsan warning that
2e664b9c1e attempted to fix:

WARNING: ThreadSanitizer: data race (pid=452658)
  Write of size 4 at 0x7b640003f4fc by main thread (mutexes: write M248499):
    #0 update_context_from_user [..]/libavcodec/pthread_frame.c:335:19 (5ab42bb1a6f4b068d7863dabe9b2bacc+0xe73859)
[..]
  Previous read of size 4 at 0x7b640003f4fc by thread T130 (mutexes: write M248502, write M248500):
    #0 ff_thread_await_progress [..]/libavcodec/pthread_frame.c:591:26 (5ab42bb1a6f4b068d7863dabe9b2bacc+0xe749a1)

Signed-off-by: Wan-Teh Chang <wtc@google.com>
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavcodec/pthread_frame.c

@@ -107,6 +107,8 @@ typedef struct PerThreadContext {
 
     int hwaccel_serializing;
     int async_serializing;
+
+    atomic_int debug_threads;       ///< Set if the FF_DEBUG_THREADS option is set.
 } PerThreadContext;
 
 /**
@@ -398,6 +400,9 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
         pthread_mutex_unlock(&p->mutex);
         return ret;
     }
+    atomic_store_explicit(&p->debug_threads,
+                          (p->avctx->debug & FF_DEBUG_THREADS) != 0,
+                          memory_order_relaxed);
 
     release_delayed_buffers(p);
 
@@ -566,7 +571,7 @@ void ff_thread_report_progress(ThreadFrame *f, int n, int field)
     p = f->owner[field]->internal->thread_ctx;
 
     pthread_mutex_lock(&p->progress_mutex);
-    if (f->owner[field]->debug&FF_DEBUG_THREADS)
+    if (atomic_load_explicit(&p->debug_threads, memory_order_relaxed))
         av_log(f->owner[field], AV_LOG_DEBUG,
                "%p finished %d field %d\n", progress, n, field);
 
@@ -588,7 +593,7 @@ void ff_thread_await_progress(ThreadFrame *f, int n, int field)
     p = f->owner[field]->internal->thread_ctx;
 
     pthread_mutex_lock(&p->progress_mutex);
-    if (f->owner[field]->debug&FF_DEBUG_THREADS)
+    if (atomic_load_explicit(&p->debug_threads, memory_order_relaxed))
         av_log(f->owner[field], AV_LOG_DEBUG,
                "thread awaiting %d field %d from %p\n", n, field, progress);
     while (atomic_load_explicit(&progress[field], memory_order_relaxed) < n)
@@ -823,6 +828,8 @@ int ff_frame_thread_init(AVCodecContext *avctx)
 
         if (err) goto error;
 
+        atomic_init(&p->debug_threads, (copy->debug & FF_DEBUG_THREADS) != 0);
+
         err = AVERROR(pthread_create(&p->thread, NULL, frame_worker_thread, p));
         p->thread_init= !err;
         if(!p->thread_init)