lavc/decode: track whether the caller started draining with a separate flag

Decoding pipeline has multiple stages, some of which may have their own
delay (e.g. bitstream filters). The code currently uses
AVCodecInternal.draining to track all of them, but they do not have to
all be in sync.

libavcodec/decode.c

@@ -55,6 +55,11 @@ typedef struct DecodeContext {
 
     /* to prevent infinite loop on errors when draining */
     int nb_draining_errors;
+
+    /**
+     * The caller has submitted a NULL packet on input.
+     */
+    int draining_started;
 } DecodeContext;
 
 static DecodeContext *decode_ctx(AVCodecInternal *avci)
@@ -626,12 +631,13 @@ FF_ENABLE_DEPRECATION_WARNINGS
 int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
 {
     AVCodecInternal *avci = avctx->internal;
+    DecodeContext     *dc = decode_ctx(avci);
     int ret;
 
     if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))
         return AVERROR(EINVAL);
 
-    if (avctx->internal->draining)
+    if (dc->draining_started)
         return AVERROR_EOF;
 
     if (avpkt && !avpkt->size && avpkt->data)
@@ -642,7 +648,8 @@ int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacke
         ret = av_packet_ref(avci->buffer_pkt, avpkt);
         if (ret < 0)
             return ret;
-    }
+    } else
+        dc->draining_started = 1;
 
     ret = av_bsf_send_packet(avci->bsf, avci->buffer_pkt);
     if (ret < 0) {
@@ -1756,6 +1763,7 @@ AVBufferRef *ff_hwaccel_frame_priv_alloc(AVCodecContext *avctx,
 void ff_decode_flush_buffers(AVCodecContext *avctx)
 {
     AVCodecInternal *avci = avctx->internal;
+    DecodeContext     *dc = decode_ctx(avci);
 
     av_packet_unref(avci->last_pkt_props);
     av_packet_unref(avci->in_pkt);
@@ -1765,7 +1773,8 @@ void ff_decode_flush_buffers(AVCodecContext *avctx)
 
     av_bsf_flush(avci->bsf);
 
-    decode_ctx(avci)->nb_draining_errors = 0;
+    dc->nb_draining_errors = 0;
+    dc->draining_started   = 0;
 }
 
 AVCodecInternal *ff_decode_internal_alloc(void)