lavc: convert frame threading to the receive_frame() pattern

Reorganize the code such that the frame threading code does not call the
decoders directly, but instead calls back into the generic decoding
code. This avoids duplicating the logic that wraps the decoder
invocation and will be useful in the following commits.

Further work by Timo Rothenpieler <timo@rothenpieler.org>.

5 files changed, 235 insertions, 123 deletions

diff --git a/libavcodec/avcodec.c b/libavcodec/avcodec.c
index 525fe516bd..0401f5be99 100644
--- a/libavcodec/avcodec.c
+++ b/libavcodec/avcodec.c
@@ -381,10 +381,13 @@ void avcodec_flush_buffers(AVCodecContext *avctx)
 
     avci->draining      = 0;
     avci->draining_done = 0;
-    av_frame_unref(avci->buffer_frame);
-    av_packet_unref(avci->buffer_pkt);
+    if (avci->buffer_frame)
+        av_frame_unref(avci->buffer_frame);
+    if (avci->buffer_pkt)
+        av_packet_unref(avci->buffer_pkt);
 
-    if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME)
+    if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME &&
+        !avci->is_frame_mt)
         ff_thread_flush(avctx);
     else if (ffcodec(avctx->codec)->flush)
         ffcodec(avctx->codec)->flush(avctx);
diff --git a/libavcodec/avcodec_internal.h b/libavcodec/avcodec_internal.h
index 816f39ae76..2f0aaab93b 100644
--- a/libavcodec/avcodec_internal.h
+++ b/libavcodec/avcodec_internal.h
@@ -84,16 +84,23 @@ void ff_thread_free(struct AVCodecContext *s);
 void ff_thread_flush(struct AVCodecContext *avctx);
 
 /**
- * Submit a new frame to a decoding thread.
- * Returns the next available frame in picture. *got_picture_ptr
- * will be 0 if none is available.
- * The return value on success is the size of the consumed packet for
- * compatibility with FFCodec.decode. This means the decoder
- * has to consume the full packet.
+ * Submit available packets for decoding to worker threads, return a
+ * decoded frame if available. Returns AVERROR(EAGAIN) if none is available.
  *
- * Parameters are the same as FFCodec.decode.
+ * Parameters are the same as FFCodec.receive_frame.
  */
-int ff_thread_decode_frame(struct AVCodecContext *avctx, struct AVFrame *frame,
-                           int *got_picture_ptr, struct AVPacket *avpkt);
+int ff_thread_receive_frame(AVCodecContext *avctx, AVFrame *frame);
+
+/**
+ * Do the actual decoding and obtain a decoded frame from the decoder, if
+ * available. When frame threading is used, this is invoked by the worker
+ * threads, otherwise by the top layer directly.
+ */
+int ff_decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame);
+
+/**
+ * Get a packet for decoding. This gets invoked by the worker threads.
+ */
+int ff_thread_get_packet(AVCodecContext *avctx, AVPacket *pkt);
 
 #endif // AVCODEC_AVCODEC_INTERNAL_H
diff --git a/libavcodec/decode.c b/libavcodec/decode.c
index 9a0e7f3ca5..2a552146fe 100644
--- a/libavcodec/decode.c
+++ b/libavcodec/decode.c
@@ -191,6 +191,11 @@ fail:
     return ret;
 }
 
+#if !HAVE_THREADS
+#define ff_thread_get_packet(avctx, pkt) (AVERROR_BUG)
+#define ff_thread_receive_frame(avctx, frame) (AVERROR_BUG)
+#endif
+
 static int decode_get_packet(AVCodecContext *avctx, AVPacket *pkt)
 {
     AVCodecInternal *avci = avctx->internal;
@@ -224,6 +229,13 @@ int ff_decode_get_packet(AVCodecContext *avctx, AVPacket *pkt)
     if (avci->draining)
         return AVERROR_EOF;
 
+    /* If we are a worker thread, get the next packet from the threading
+     * context. Otherwise we are the main (user-facing) context, so we get the
+     * next packet from the input filterchain.
+     */
+    if (avctx->internal->is_frame_mt)
+        return ff_thread_get_packet(avctx, pkt);
+
     while (1) {
         int ret = decode_get_packet(avctx, pkt);
         if (ret == AVERROR(EAGAIN) &&
@@ -396,15 +408,11 @@ static inline int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame,
         return AVERROR_EOF;
 
     if (!pkt->data &&
-        !(avctx->codec->capabilities & AV_CODEC_CAP_DELAY ||
-          avctx->active_thread_type & FF_THREAD_FRAME))
+        !(avctx->codec->capabilities & AV_CODEC_CAP_DELAY))
         return AVERROR_EOF;
 
     got_frame = 0;
 
-    if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME) {
-        consumed = ff_thread_decode_frame(avctx, frame, &got_frame, pkt);
-    } else {
         consumed = codec->cb.decode(avctx, frame, &got_frame, pkt);
 
         if (!(codec->caps_internal & FF_CODEC_CAP_SETS_PKT_DTS))
@@ -417,7 +425,6 @@ FF_DISABLE_DEPRECATION_WARNINGS
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
         }
-    }
     emms_c();
 
     if (avctx->codec->type == AVMEDIA_TYPE_VIDEO) {
@@ -583,12 +590,11 @@ static int decode_simple_receive_frame(AVCodecContext *avctx, AVFrame *frame)
     return 0;
 }
 
-static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
+int ff_decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
 {
     AVCodecInternal *avci = avctx->internal;
-    DecodeContext     *dc = decode_ctx(avci);
     const FFCodec *const codec = ffcodec(avctx->codec);
-    int ret, ok;
+    int ret;
 
     av_assert0(!frame->buf[0]);
 
@@ -609,6 +615,20 @@ static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
     if (ret == AVERROR_EOF)
         avci->draining_done = 1;
 
+    return ret;
+}
+
+static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
+{
+    AVCodecInternal *avci = avctx->internal;
+    DecodeContext     *dc = decode_ctx(avci);
+    int ret, ok;
+
+    if (avctx->active_thread_type & FF_THREAD_FRAME)
+        ret = ff_thread_receive_frame(avctx, frame);
+    else
+        ret = ff_decode_receive_frame_internal(avctx, frame);
+
     /* preserve ret */
     ok = detect_colorspace(avctx, frame);
     if (ok < 0) {
@@ -1921,7 +1941,8 @@ void ff_decode_flush_buffers(AVCodecContext *avctx)
     dc->pts_correction_last_pts =
     dc->pts_correction_last_dts = INT64_MIN;
 
-    av_bsf_flush(avci->bsf);
+    if (avci->bsf)
+        av_bsf_flush(avci->bsf);
 
     dc->nb_draining_errors = 0;
     dc->draining_started   = 0;
diff --git a/libavcodec/internal.h b/libavcodec/internal.h
index 3519525ab0..b67c57afca 100644
--- a/libavcodec/internal.h
+++ b/libavcodec/internal.h
@@ -54,6 +54,13 @@ typedef struct AVCodecInternal {
     int is_copy;
 
     /**
+     * This field is set to 1 when frame threading is being used and the parent
+     * AVCodecContext of this AVCodecInternal is a worker-thread context (i.e.
+     * one of those actually doing the decoding), 0 otherwise.
+     */
+    int is_frame_mt;
+
+    /**
      * Audio encoders can set this flag during init to indicate that they
      * want the small last frame to be padded to a multiple of pad_samples.
      */
diff --git a/libavcodec/pthread_frame.c b/libavcodec/pthread_frame.c
index fd356bd190..6e6d9d67df 100644
--- a/libavcodec/pthread_frame.c
+++ b/libavcodec/pthread_frame.c
@@ -34,6 +34,7 @@
 #include "hwaccel_internal.h"
 #include "hwconfig.h"
 #include "internal.h"
+#include "packet_internal.h"
 #include "pthread_internal.h"
 #include "refstruct.h"
 #include "thread.h"
@@ -66,6 +67,12 @@ enum {
     INITIALIZED,    ///< Thread has been properly set up
 };
 
+typedef struct DecodedFrames {
+    AVFrame  **f;
+    size_t  nb_f;
+    size_t  nb_f_allocated;
+} DecodedFrames;
+
 typedef struct ThreadFrameProgress {
     atomic_int progress[2];
 } ThreadFrameProgress;
@@ -90,8 +97,10 @@ typedef struct PerThreadContext {
 
     AVPacket       *avpkt;          ///< Input packet (for decoding) or output (for encoding).
 
-    AVFrame *frame;                 ///< Output frame (for decoding) or input (for encoding).
-    int     got_frame;              ///< The output of got_picture_ptr from the last avcodec_decode_video() call.
+    /**
+     * Decoded frames from a single decode iteration.
+     */
+    DecodedFrames df;
     int     result;                 ///< The result of the last codec decode/encode() call.
 
     atomic_int state;
@@ -128,14 +137,17 @@ typedef struct FrameThreadContext {
     pthread_cond_t async_cond;
     int async_lock;
 
+    DecodedFrames df;
+    int result;
+
+    /**
+     * Packet to be submitted to the next thread for decoding.
+     */
+    AVPacket *next_pkt;
+
     int next_decoding;             ///< The next context to submit a packet to.
     int next_finished;             ///< The next context to return output from.
 
-    int delaying;                  /**<
-                                    * Set for the first N packets, where N is the number of threads.
-                                    * While it is set, ff_thread_en/decode_frame won't return any results.
-                                    */
-
     /* hwaccel state for thread-unsafe hwaccels is temporarily stored here in
      * order to transfer its ownership to the next decoding thread without the
      * need for extra synchronization */
@@ -178,6 +190,52 @@ static void thread_set_name(PerThreadContext *p)
     ff_thread_setname(name);
 }
 
+// get a free frame to decode into
+static AVFrame *decoded_frames_get_free(DecodedFrames *df)
+{
+    if (df->nb_f == df->nb_f_allocated) {
+        AVFrame **tmp = av_realloc_array(df->f, df->nb_f + 1,
+                                         sizeof(*df->f));
+        if (!tmp)
+            return NULL;
+        df->f = tmp;
+
+        df->f[df->nb_f] = av_frame_alloc();
+        if (!df->f[df->nb_f])
+            return NULL;
+
+        df->nb_f_allocated++;
+    }
+
+    av_assert0(!df->f[df->nb_f]->buf[0]);
+
+    return df->f[df->nb_f];
+}
+
+static void decoded_frames_pop(DecodedFrames *df, AVFrame *dst)
+{
+    AVFrame *tmp_frame = df->f[0];
+    av_frame_move_ref(dst, tmp_frame);
+    memmove(df->f, df->f + 1, (df->nb_f - 1) * sizeof(*df->f));
+    df->f[--df->nb_f] = tmp_frame;
+}
+
+static void decoded_frames_flush(DecodedFrames *df)
+{
+    for (size_t i = 0; i < df->nb_f; i++)
+        av_frame_unref(df->f[i]);
+    df->nb_f = 0;
+}
+
+static void decoded_frames_free(DecodedFrames *df)
+{
+    for (size_t i = 0; i < df->nb_f_allocated; i++)
+        av_frame_free(&df->f[i]);
+    av_freep(&df->f);
+    df->nb_f           = 0;
+    df->nb_f_allocated = 0;
+}
+
 /**
  * Codec worker thread.
  *
@@ -195,6 +253,8 @@ static attribute_align_arg void *frame_worker_thread(void *arg)
 
     pthread_mutex_lock(&p->mutex);
     while (1) {
+        int ret;
+
         while (atomic_load(&p->state) == STATE_INPUT_READY && !p->die)
             pthread_cond_wait(&p->input_cond, &p->mutex);
 
@@ -218,16 +278,31 @@ static attribute_align_arg void *frame_worker_thread(void *arg)
             p->hwaccel_serializing = 1;
         }
 
-        av_frame_unref(p->frame);
-        p->got_frame = 0;
-        p->result = codec->cb.decode(avctx, p->frame, &p->got_frame, p->avpkt);
+        ret = 0;
+        while (ret >= 0) {
+            AVFrame *frame;
 
-        if ((p->result < 0 || !p->got_frame) && p->frame->buf[0])
-            av_frame_unref(p->frame);
+            /* get the frame which will store the output */
+            frame = decoded_frames_get_free(&p->df);
+            if (!frame) {
+                p->result = AVERROR(ENOMEM);
+                goto alloc_fail;
+            }
+
+            /* do the actual decoding */
+            ret = ff_decode_receive_frame_internal(avctx, frame);
+            if (ret == 0)
+                p->df.nb_f++;
+            else if (ret < 0 && frame->buf[0])
+                av_frame_unref(frame);
+
+            p->result = (ret == AVERROR(EAGAIN)) ? 0 : ret;
+        }
 
         if (atomic_load(&p->state) == STATE_SETTING_UP)
             ff_thread_finish_setup(avctx);
 
+alloc_fail:
         if (p->hwaccel_serializing) {
             /* wipe hwaccel state for thread-unsafe hwaccels to avoid stale
              * pointers lying around;
@@ -422,18 +497,21 @@ static int update_context_from_user(AVCodecContext *dst, const AVCodecContext *s
 }
 
 static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
-                         AVPacket *avpkt)
+                         AVPacket *in_pkt)
 {
     FrameThreadContext *fctx = p->parent;
     PerThreadContext *prev_thread = fctx->prev_thread;
     const AVCodec *codec = p->avctx->codec;
     int ret;
 
-    if (!avpkt->size && !(codec->capabilities & AV_CODEC_CAP_DELAY))
-        return 0;
-
     pthread_mutex_lock(&p->mutex);
 
+    av_packet_unref(p->avpkt);
+    av_packet_move_ref(p->avpkt, in_pkt);
+
+    if (AVPACKET_IS_EMPTY(p->avpkt))
+        p->avctx->internal->draining = 1;
+
     ret = update_context_from_user(p->avctx, user_avctx);
     if (ret) {
         pthread_mutex_unlock(&p->mutex);
@@ -444,7 +522,6 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
                           memory_order_relaxed);
 
     if (prev_thread) {
-        int err;
         if (atomic_load(&prev_thread->state) == STATE_SETTING_UP) {
             pthread_mutex_lock(&prev_thread->progress_mutex);
             while (atomic_load(&prev_thread->state) == STATE_SETTING_UP)
@@ -452,10 +529,16 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
             pthread_mutex_unlock(&prev_thread->progress_mutex);
         }
 
-        err = update_context_from_thread(p->avctx, prev_thread->avctx, 0);
-        if (err) {
-            pthread_mutex_unlock(&p->mutex);
-            return err;
+        /* codecs without delay might not be prepared to be called repeatedly here during
+         * flushing (vp3/theora), and also don't need to be, since from this point on, they
+         * will always return EOF anyway */
+        if (!p->avctx->internal->draining ||
+            (codec->capabilities & AV_CODEC_CAP_DELAY)) {
+            ret = update_context_from_thread(p->avctx, prev_thread->avctx, 0);
+            if (ret) {
+                pthread_mutex_unlock(&p->mutex);
+                return ret;
+            }
         }
     }
 
@@ -467,70 +550,47 @@ static int submit_packet(PerThreadContext *p, AVCodecContext *user_avctx,
         FFSWAP(void*,            p->avctx->internal->hwaccel_priv_data, fctx->stash_hwaccel_priv);
     }
 
-    av_packet_unref(p->avpkt);
-    ret = av_packet_ref(p->avpkt, avpkt);
-    if (ret < 0) {
-        pthread_mutex_unlock(&p->mutex);
-        av_log(p->avctx, AV_LOG_ERROR, "av_packet_ref() failed in submit_packet()\n");
-        return ret;
-    }
-
     atomic_store(&p->state, STATE_SETTING_UP);
     pthread_cond_signal(&p->input_cond);
     pthread_mutex_unlock(&p->mutex);
 
     fctx->prev_thread = p;
-    fctx->next_decoding++;
+    fctx->next_decoding = (fctx->next_decoding + 1) % p->avctx->thread_count;
 
     return 0;
 }
 
-int ff_thread_decode_frame(AVCodecContext *avctx,
-                           AVFrame *picture, int *got_picture_ptr,
-                           AVPacket *avpkt)
+int ff_thread_receive_frame(AVCodecContext *avctx, AVFrame *frame)
 {
     FrameThreadContext *fctx = avctx->internal->thread_ctx;
-    int finished = fctx->next_finished;
-    PerThreadContext *p;
-    int err;
+    int ret = 0;
 
     /* release the async lock, permitting blocked hwaccel threads to
      * go forward while we are in this function */
     async_unlock(fctx);
 
-    /*
-     * Submit a packet to the next decoding thread.
-     */
-
-    p = &fctx->threads[fctx->next_decoding];
-    err = submit_packet(p, avctx, avpkt);
-    if (err)
-        goto finish;
-
-    /*
-     * If we're still receiving the initial packets, don't return a frame.
-     */
-
-    if (fctx->next_decoding > (avctx->thread_count-1-(avctx->codec_id == AV_CODEC_ID_FFV1)))
-        fctx->delaying = 0;
+    /* submit packets to threads while there are no buffered results to return */
+    while (!fctx->df.nb_f && !fctx->result) {
+        PerThreadContext *p;
 
-    if (fctx->delaying) {
-        *got_picture_ptr=0;
-        if (avpkt->size) {
-            err = avpkt->size;
+        /* get a packet to be submitted to the next thread */
+        av_packet_unref(fctx->next_pkt);
+        ret = ff_decode_get_packet(avctx, fctx->next_pkt);
+        if (ret < 0 && ret != AVERROR_EOF)
             goto finish;
-        }
-    }
 
-    /*
-     * Return the next available frame from the oldest thread.
-     * If we're at the end of the stream, then we have to skip threads that
-     * didn't output a frame/error, because we don't want to accidentally signal
-     * EOF (avpkt->size == 0 && *got_picture_ptr == 0 && err >= 0).
-     */
+        ret = submit_packet(&fctx->threads[fctx->next_decoding], avctx,
+                            fctx->next_pkt);
+        if (ret < 0)
+             goto finish;
 
-    do {
-        p = &fctx->threads[finished++];
+        /* do not return any frames until all threads have something to do */
+        if (fctx->next_decoding != fctx->next_finished &&
+            !avctx->internal->draining)
+            continue;
+
+        p                   = &fctx->threads[fctx->next_finished];
+        fctx->next_finished = (fctx->next_finished + 1) % avctx->thread_count;
 
         if (atomic_load(&p->state) != STATE_INPUT_READY) {
             pthread_mutex_lock(&p->progress_mutex);
@@ -539,35 +599,26 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
             pthread_mutex_unlock(&p->progress_mutex);
         }
 
-        av_frame_move_ref(picture, p->frame);
-        *got_picture_ptr = p->got_frame;
-        picture->pkt_dts = p->avpkt->dts;
-        err = p->result;
-
-        /*
-         * A later call with avkpt->size == 0 may loop over all threads,
-         * including this one, searching for a frame/error to return before being
-         * stopped by the "finished != fctx->next_finished" condition.
-         * Make sure we don't mistakenly return the same frame/error again.
-         */
-        p->got_frame = 0;
-        p->result = 0;
-
-        if (finished >= avctx->thread_count) finished = 0;
-    } while (!avpkt->size && !*got_picture_ptr && err >= 0 && finished != fctx->next_finished);
-
-    update_context_from_thread(avctx, p->avctx, 1);
-
-    if (fctx->next_decoding >= avctx->thread_count) fctx->next_decoding = 0;
+        update_context_from_thread(avctx, p->avctx, 1);
+        fctx->result = p->result;
+        p->result    = 0;
+        if (p->df.nb_f)
+            FFSWAP(DecodedFrames, fctx->df, p->df);
+    }
 
-    fctx->next_finished = finished;
+    /* a thread may return multiple frames AND an error
+     * we first return all the frames, then the error */
+    if (fctx->df.nb_f) {
+        decoded_frames_pop(&fctx->df, frame);
+        ret = 0;
+    } else {
+        ret = fctx->result;
+        fctx->result = 0;
+    }
 
-    /* return the size of the consumed packet if no error occurred */
-    if (err >= 0)
-        err = avpkt->size;
 finish:
     async_lock(fctx);
-    return err;
+    return ret;
 }
 
 void ff_thread_report_progress(ThreadFrame *f, int n, int field)
@@ -675,7 +726,6 @@ static void park_frame_worker_threads(FrameThreadContext *fctx, int thread_count
                 pthread_cond_wait(&p->output_cond, &p->progress_mutex);
             pthread_mutex_unlock(&p->progress_mutex);
         }
-        p->got_frame = 0;
     }
 
     async_lock(fctx);
@@ -728,12 +778,13 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
             }
 
             ff_refstruct_unref(&ctx->internal->pool);
+            av_packet_free(&ctx->internal->in_pkt);
             av_packet_free(&ctx->internal->last_pkt_props);
             av_freep(&ctx->internal);
             av_buffer_unref(&ctx->hw_frames_ctx);
         }
 
-        av_frame_free(&p->frame);
+        decoded_frames_free(&p->df);
 
         ff_pthread_free(p, per_thread_offsets);
         av_packet_free(&p->avpkt);
@@ -741,6 +792,9 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
         av_freep(&p->avctx);
     }
 
+    decoded_frames_free(&fctx->df);
+    av_packet_free(&fctx->next_pkt);
+
     av_freep(&fctx->threads);
     ff_pthread_free(fctx, thread_ctx_offsets);
 
@@ -799,13 +853,17 @@ static av_cold int init_thread(PerThreadContext *p, int *threads_to_free,
     if (err < 0)
         return err;
 
-    if (!(p->frame = av_frame_alloc()) ||
-        !(p->avpkt = av_packet_alloc()))
+    if (!(p->avpkt = av_packet_alloc()))
         return AVERROR(ENOMEM);
 
+    copy->internal->is_frame_mt = 1;
     if (!first)
         copy->internal->is_copy = 1;
 
+    copy->internal->in_pkt = av_packet_alloc();
+    if (!copy->internal->in_pkt)
+        return AVERROR(ENOMEM);
+
     copy->internal->last_pkt_props = av_packet_alloc();
     if (!copy->internal->last_pkt_props)
         return AVERROR(ENOMEM);
@@ -865,8 +923,11 @@ int ff_frame_thread_init(AVCodecContext *avctx)
         return err;
     }
 
+    fctx->next_pkt = av_packet_alloc();
+    if (!fctx->next_pkt)
+        return AVERROR(ENOMEM);
+
     fctx->async_lock = 1;
-    fctx->delaying = 1;
 
     if (codec->p.type == AVMEDIA_TYPE_VIDEO)
         avctx->delay = avctx->thread_count - 1;
@@ -907,17 +968,18 @@ void ff_thread_flush(AVCodecContext *avctx)
     }
 
     fctx->next_decoding = fctx->next_finished = 0;
-    fctx->delaying = 1;
     fctx->prev_thread = NULL;
+
+    decoded_frames_flush(&fctx->df);
+    fctx->result = 0;
+
     for (i = 0; i < avctx->thread_count; i++) {
         PerThreadContext *p = &fctx->threads[i];
-        // Make sure decode flush calls with size=0 won't return old frames
-        p->got_frame = 0;
-        av_frame_unref(p->frame);
+
+        decoded_frames_flush(&p->df);
         p->result = 0;
 
-        if (ffcodec(avctx->codec)->flush)
-            ffcodec(avctx->codec)->flush(p->avctx);
+        avcodec_flush_buffers(p->avctx);
     }
 }
 
@@ -1000,3 +1062,15 @@ void ff_thread_release_ext_buffer(ThreadFrame *f)
     if (f->f)
         av_frame_unref(f->f);
 }
+
+int ff_thread_get_packet(AVCodecContext *avctx, AVPacket *pkt)
+{
+    PerThreadContext *p = avctx->internal->thread_ctx;
+
+    if (!AVPACKET_IS_EMPTY(p->avpkt)) {
+        av_packet_move_ref(pkt, p->avpkt);
+        return 0;
+    }
+
+    return avctx->internal->draining ? AVERROR_EOF : AVERROR(EAGAIN);
+}