lavc/internal: document the precise meaning of AVCodecInternal.draining

Also, set draining=1 in case a bitstream filter returns an
internally-triggered EOF. While no bitstream filters currently inserted
by decoders will do that, that may change in the future and it is better
to cover this case.

libavcodec/decode.c

@@ -213,8 +213,6 @@ static int decode_get_packet(AVCodecContext *avctx, AVPacket *pkt)
     int ret;
 
     ret = av_bsf_receive_packet(avci->bsf, pkt);
-    if (ret == AVERROR_EOF)
-        avci->draining = 1;
     if (ret < 0)
         return ret;
 
@@ -247,14 +245,14 @@ int ff_decode_get_packet(AVCodecContext *avctx, AVPacket *pkt)
         if (ret == AVERROR(EAGAIN) &&
             (!AVPACKET_IS_EMPTY(avci->buffer_pkt) || dc->draining_started)) {
             ret = av_bsf_send_packet(avci->bsf, avci->buffer_pkt);
-            if (ret < 0) {
-                av_packet_unref(avci->buffer_pkt);
-                return ret;
-            }
-
+            if (ret >= 0)
                 continue;
+
+            av_packet_unref(avci->buffer_pkt);
         }
 
+        if (ret == AVERROR_EOF)
+            avci->draining = 1;
         return ret;
     }
 }

libavcodec/internal.h

@@ -123,7 +123,11 @@ typedef struct AVCodecInternal {
     void *hwaccel_priv_data;
 
     /**
-     * checks API usage: after codec draining, flush is required to resume operation
+     * decoding: AVERROR_EOF has been returned from ff_decode_get_packet(); must
+     *           not be used by decoders that use the decode() callback, as they
+     *           do not call ff_decode_get_packet() directly.
+     *
+     * encoding: a flush frame has been submitted to avcodec_send_frame().
      */
     int draining;