avcodec: deprecate thread_safe_callbacks

They add considerable complexity to frame-threading implementation,
which includes an unavoidably leaking error path, while the advantages
of this option to the users are highly dubious.

It should be always possible and desirable for the callers to make their
get_buffer2() implementation thread-safe, so deprecate this option.

doc/APIchanges

@@ -15,6 +15,11 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2020-xx-xx - xxxxxxxxxx - lavc 58.114.100 - avcodec.h
+  Deprecate AVCodecContext.thread_safe_callbacks. Starting with
+  LIBAVCODEC_VERSION_MAJOR=60, user callbacks must always be
+  thread-safe when frame threading is used.
+
 2020-11-25 - xxxxxxxxxx - lavc 58.113.100 - avcodec.h
   Adds a new flag AV_CODEC_EXPORT_DATA_FILM_GRAIN for export_side_data.
 

doc/multithreading.txt

@@ -20,8 +20,7 @@ Slice threading -
 
 Frame threading -
 * Restrictions with slice threading also apply.
-* For best performance, the client should set thread_safe_callbacks if it
-  provides a thread-safe get_buffer() callback.
+* Custom get_buffer2() and get_format() callbacks must be thread-safe.
 * There is one frame of delay added for every thread beyond the first one.
   Clients must be able to handle this; the pkt_dts and pkt_pts fields in
   AVFrame will work as usual.

fftools/ffmpeg.c

@@ -2927,7 +2927,9 @@ static int init_input_stream(int ist_index, char *error, int error_len)
         ist->dec_ctx->opaque                = ist;
         ist->dec_ctx->get_format            = get_format;
         ist->dec_ctx->get_buffer2           = get_buffer;
+#if LIBAVCODEC_VERSION_MAJOR < 60
         ist->dec_ctx->thread_safe_callbacks = 1;
+#endif
 
         av_opt_set_int(ist->dec_ctx, "refcounted_frames", 1, 0);
         if (ist->dec_ctx->codec_id == AV_CODEC_ID_DVB_SUBTITLE &&

libavcodec/avcodec.h

@@ -1319,9 +1319,9 @@ typedef struct AVCodecContext {
      *
      * Some decoders do not support linesizes changing between frames.
      *
-     * If frame multithreading is used and thread_safe_callbacks is set,
-     * this callback may be called from a different thread, but not from more
-     * than one at once. Does not need to be reentrant.
+     * If frame multithreading is used, this callback may be called from a
+     * different thread, but not from more than one at once. Does not need to be
+     * reentrant.
      *
      * @see avcodec_align_dimensions2()
      *
@@ -1808,6 +1808,7 @@ typedef struct AVCodecContext {
      */
     int active_thread_type;
 
+#if FF_API_THREAD_SAFE_CALLBACKS
     /**
      * Set by the client if its custom get_buffer() callback can be called
      * synchronously from another thread, which allows faster multithreaded decoding.
@@ -1815,8 +1816,14 @@ typedef struct AVCodecContext {
      * Ignored if the default get_buffer() is used.
      * - encoding: Set by user.
      * - decoding: Set by user.
+     *
+     * @deprecated the custom get_buffer2() callback should always be
+     *   thread-safe. Thread-unsafe get_buffer2() implementations will be
+     *   invalid once this field is removed.
      */
+    attribute_deprecated
     int thread_safe_callbacks;
+#endif
 
     /**
      * The codec may call this to execute several independent things.

libavcodec/pthread_frame.c

@@ -89,6 +89,7 @@ typedef struct PerThreadContext {
 
     atomic_int state;
 
+#if FF_API_THREAD_SAFE_CALLBACKS
     /**
      * Array of frames passed to ff_thread_release_buffer().
      * Frames are released after all threads referencing them are finished.
@@ -102,6 +103,7 @@ typedef struct PerThreadContext {
 
     const enum AVPixelFormat *available_formats; ///< Format array for get_format()
     enum AVPixelFormat result_format;            ///< get_format() result
+#endif
 
     int die;                        ///< Set when the thread should exit.
 
@@ -137,8 +139,10 @@ typedef struct FrameThreadContext {
                                     */
 } FrameThreadContext;
 
+#if FF_API_THREAD_SAFE_CALLBACKS
 #define THREAD_SAFE_CALLBACKS(avctx) \
 ((avctx)->thread_safe_callbacks || (avctx)->get_buffer2 == avcodec_default_get_buffer2)
+#endif
 
 static void async_lock(FrameThreadContext *fctx)
 {
@@ -178,8 +182,14 @@ static attribute_align_arg void *frame_worker_thread(void *arg)
 
         if (p->die) break;
 
-        if (!codec->update_thread_context && THREAD_SAFE_CALLBACKS(avctx))
+FF_DISABLE_DEPRECATION_WARNINGS
+        if (!codec->update_thread_context
+#if FF_API_THREAD_SAFE_CALLBACKS
+            && THREAD_SAFE_CALLBACKS(avctx)
+#endif
+            )
             ff_thread_finish_setup(avctx);
+FF_ENABLE_DEPRECATION_WARNINGS
 
         /* If a decoder supports hwaccel, then it must call ff_get_format().
          * Since that call must happen before ff_thread_finish_setup(), the
@@ -344,7 +354,11 @@ static int update_context_from_user(AVCodecContext *dst, AVCodecContext *src)
 
     dst->frame_number     = src->frame_number;
     dst->reordered_opaque = src->reordered_opaque;
+#if FF_API_THREAD_SAFE_CALLBACKS
+FF_DISABLE_DEPRECATION_WARNINGS
     dst->thread_safe_callbacks = src->thread_safe_callbacks;
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
 
     if (src->slice_count && src->slice_offset) {
         if (dst->slice_count < src->slice_count) {
@@ -360,6 +374,7 @@ static int update_context_from_user(AVCodecContext *dst, AVCodecContext *src)
     return 0;
 }
 
+#if FF_API_THREAD_SAFE_CALLBACKS
 /// Releases the buffers that this decoding thread was the last user of.
 static void release_delayed_buffers(PerThreadContext *p)
 {
@@ -380,6 +395,7 @@ static void release_delayed_buffers(PerThreadContext *p)
         pthread_mutex_unlock(&fctx->buffer_mutex);
     }
 }
+#endif
 
 static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
                          AVPacket *avpkt)
@@ -403,7 +419,9 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
                           (p->avctx->debug & FF_DEBUG_THREADS) != 0,
                           memory_order_relaxed);
 
+#if FF_API_THREAD_SAFE_CALLBACKS
     release_delayed_buffers(p);
+#endif
 
     if (prev_thread) {
         int err;
@@ -433,6 +451,8 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
     pthread_cond_signal(&p->input_cond);
     pthread_mutex_unlock(&p->mutex);
 
+#if FF_API_THREAD_SAFE_CALLBACKS
+FF_DISABLE_DEPRECATION_WARNINGS
     /*
      * If the client doesn't have a thread-safe get_buffer(),
      * then decoding threads call back to the main thread,
@@ -466,6 +486,8 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
             pthread_mutex_unlock(&p->progress_mutex);
         }
     }
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
 
     fctx->prev_thread = p;
     fctx->next_decoding++;
@@ -657,7 +679,7 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
 {
     FrameThreadContext *fctx = avctx->internal->thread_ctx;
     const AVCodec *codec = avctx->codec;
-    int i, j;
+    int i;
 
     park_frame_worker_threads(fctx, thread_count);
 
@@ -690,7 +712,9 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
         if (codec->close && p->avctx)
             codec->close(p->avctx);
 
+#if FF_API_THREAD_SAFE_CALLBACKS
         release_delayed_buffers(p);
+#endif
         av_frame_free(&p->frame);
     }
 
@@ -704,9 +728,11 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
         pthread_cond_destroy(&p->output_cond);
         av_packet_unref(&p->avpkt);
 
-        for (j = 0; j < p->released_buffers_allocated; j++)
+#if FF_API_THREAD_SAFE_CALLBACKS
+        for (int j = 0; j < p->released_buffers_allocated; j++)
             av_frame_free(&p->released_buffers[j]);
         av_freep(&p->released_buffers);
+#endif
 
         if (p->avctx) {
             if (codec->priv_class)
@@ -889,7 +915,9 @@ void ff_thread_flush(AVCodecContext *avctx)
         av_frame_unref(p->frame);
         p->result = 0;
 
+#if FF_API_THREAD_SAFE_CALLBACKS
         release_delayed_buffers(p);
+#endif
 
         if (avctx->codec->flush)
             avctx->codec->flush(p->avctx);
@@ -899,10 +927,16 @@ void ff_thread_flush(AVCodecContext *avctx)
 int ff_thread_can_start_frame(AVCodecContext *avctx)
 {
     PerThreadContext *p = avctx->internal->thread_ctx;
+FF_DISABLE_DEPRECATION_WARNINGS
     if ((avctx->active_thread_type&FF_THREAD_FRAME) && atomic_load(&p->state) != STATE_SETTING_UP &&
-        (avctx->codec->update_thread_context || !THREAD_SAFE_CALLBACKS(avctx))) {
+        (avctx->codec->update_thread_context
+#if FF_API_THREAD_SAFE_CALLBACKS
+         || !THREAD_SAFE_CALLBACKS(avctx)
+#endif
+         )) {
         return 0;
     }
+FF_ENABLE_DEPRECATION_WARNINGS
     return 1;
 }
 
@@ -916,8 +950,14 @@ static int thread_get_buffer_internal(AVCodecContext *avctx, ThreadFrame *f, int
     if (!(avctx->active_thread_type & FF_THREAD_FRAME))
         return ff_get_buffer(avctx, f->f, flags);
 
+FF_DISABLE_DEPRECATION_WARNINGS
     if (atomic_load(&p->state) != STATE_SETTING_UP &&
-        (avctx->codec->update_thread_context || !THREAD_SAFE_CALLBACKS(avctx))) {
+        (avctx->codec->update_thread_context
+#if FF_API_THREAD_SAFE_CALLBACKS
+         || !THREAD_SAFE_CALLBACKS(avctx)
+#endif
+         )) {
+FF_ENABLE_DEPRECATION_WARNINGS
         av_log(avctx, AV_LOG_ERROR, "get_buffer() cannot be called after ff_thread_finish_setup()\n");
         return -1;
     }
@@ -935,6 +975,10 @@ static int thread_get_buffer_internal(AVCodecContext *avctx, ThreadFrame *f, int
     }
 
     pthread_mutex_lock(&p->parent->buffer_mutex);
+#if !FF_API_THREAD_SAFE_CALLBACKS
+    err = ff_get_buffer(avctx, f->f, flags);
+#else
+FF_DISABLE_DEPRECATION_WARNINGS
     if (THREAD_SAFE_CALLBACKS(avctx)) {
         err = ff_get_buffer(avctx, f->f, flags);
     } else {
@@ -954,6 +998,8 @@ static int thread_get_buffer_internal(AVCodecContext *avctx, ThreadFrame *f, int
     }
     if (!THREAD_SAFE_CALLBACKS(avctx) && !avctx->codec->update_thread_context)
         ff_thread_finish_setup(avctx);
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
     if (err)
         av_buffer_unref(&f->progress);
 
@@ -962,6 +1008,8 @@ static int thread_get_buffer_internal(AVCodecContext *avctx, ThreadFrame *f, int
     return err;
 }
 
+#if FF_API_THREAD_SAFE_CALLBACKS
+FF_DISABLE_DEPRECATION_WARNINGS
 enum AVPixelFormat ff_thread_get_format(AVCodecContext *avctx, const enum AVPixelFormat *fmt)
 {
     enum AVPixelFormat res;
@@ -987,6 +1035,8 @@ enum AVPixelFormat ff_thread_get_format(AVCodecContext *avctx, const enum AVPixe
 
     return res;
 }
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
 
 int ff_thread_get_buffer(AVCodecContext *avctx, ThreadFrame *f, int flags)
 {
@@ -998,12 +1048,16 @@ int ff_thread_get_buffer(AVCodecContext *avctx, ThreadFrame *f, int flags)
 
 void ff_thread_release_buffer(AVCodecContext *avctx, ThreadFrame *f)
 {
+#if FF_API_THREAD_SAFE_CALLBACKS
+FF_DISABLE_DEPRECATION_WARNINGS
     PerThreadContext *p = avctx->internal->thread_ctx;
     FrameThreadContext *fctx;
     AVFrame *dst;
     int ret = 0;
     int can_direct_free = !(avctx->active_thread_type & FF_THREAD_FRAME) ||
                           THREAD_SAFE_CALLBACKS(avctx);
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
 
     if (!f->f)
         return;
@@ -1014,6 +1068,9 @@ void ff_thread_release_buffer(AVCodecContext *avctx, ThreadFrame *f)
     av_buffer_unref(&f->progress);
     f->owner[0] = f->owner[1] = NULL;
 
+#if !FF_API_THREAD_SAFE_CALLBACKS
+    av_frame_unref(f->f);
+#else
     // when the frame buffers are not allocated, just reset it to clean state
     if (can_direct_free || !f->f->buf[0]) {
         av_frame_unref(f->f);
@@ -1055,4 +1112,5 @@ fail:
             memset(f->f->extended_buf, 0, f->f->nb_extended_buf * sizeof(*f->f->extended_buf));
         av_frame_unref(f->f);
     }
+#endif
 }

libavcodec/thread.h

@@ -96,6 +96,7 @@ void ff_thread_report_progress(ThreadFrame *f, int progress, int field);
  */
 void ff_thread_await_progress(ThreadFrame *f, int progress, int field);
 
+#if FF_API_THREAD_SAFE_CALLBACKS
 /**
  * Wrapper around get_format() for frame-multithreaded codecs.
  * Call this function instead of avctx->get_format().
@@ -105,6 +106,9 @@ void ff_thread_await_progress(ThreadFrame *f, int progress, int field);
  * @param fmt The list of available formats.
  */
 enum AVPixelFormat ff_thread_get_format(AVCodecContext *avctx, const enum AVPixelFormat *fmt);
+#else
+#define ff_thread_get_format ff_get_format
+#endif
 
 /**
  * Wrapper around get_buffer() for frame-multithreaded codecs.

libavcodec/utils.c

@@ -678,6 +678,19 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, const AVCodec *code
         goto free_and_end;
     }
 
+#if FF_API_THREAD_SAFE_CALLBACKS
+FF_DISABLE_DEPRECATION_WARNINGS
+    if ((avctx->thread_type & FF_THREAD_FRAME) &&
+        avctx->get_buffer2 != avcodec_default_get_buffer2 &&
+        !avctx->thread_safe_callbacks) {
+        av_log(avctx, AV_LOG_WARNING, "Requested frame threading with a "
+               "custom get_buffer2() implementation which is not marked as "
+               "thread safe. This is not supported anymore, make your "
+               "callback thread-safe.\n");
+    }
+FF_ENABLE_DEPRECATION_WARNINGS
+#endif
+
     avctx->codec = codec;
     if ((avctx->codec_type == AVMEDIA_TYPE_UNKNOWN || avctx->codec_type == codec->type) &&
         avctx->codec_id == AV_CODEC_ID_NONE) {

libavcodec/version.h

@@ -28,7 +28,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR  58
-#define LIBAVCODEC_VERSION_MINOR 113
+#define LIBAVCODEC_VERSION_MINOR 114
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \
@@ -147,5 +147,8 @@
 #ifndef FF_API_AVPRIV_PUT_BITS
 #define FF_API_AVPRIV_PUT_BITS     (LIBAVCODEC_VERSION_MAJOR < 59)
 #endif
+#ifndef FF_API_THREAD_SAFE_CALLBACKS
+#define FF_API_THREAD_SAFE_CALLBACKS (LIBAVCODEC_VERSION_MAJOR < 60)
+#endif
 
 #endif /* AVCODEC_VERSION_H */