avformat/matroskadec: Reuse AVFormatInternal.parse_pkt

Before 8d78e90a6b the Matroska demuxer
used stack packets to hold temporary packets; now it uses a temporary
packet allocated by the Matroska demuxer. Yet because it used stack
packets the code has always properly reset the packet on error, while
on success these temporary packets were put into a packet list via
avpriv_packet_list_put(), which already resets the source packet.
This means that this code is compatible with just reusing
AVFormatInternal.parse_pkt (which is unused while one is in the
demuxer's read_packet() function). Compared to before 8d78e90a6
this no longer wastes one initialization per AVPacket read
(the resetting of the stack packet performed by av_packet_move_ref()
in avpriv_packet_list_put() was for naught).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/internal.h

@@ -98,6 +98,9 @@ struct AVFormatInternal {
      * for short periods that are guaranteed not to overlap
      * with calls to av_read_frame() (or ff_read_packet())
      * or with each other.
+     * It may be used by demuxers as a replacement for
+     * stack packets (unless they call one of the aforementioned
+     * functions with their own AVFormatContext).
      * Every user has to ensure that this packet is blank
      * after using it.
      */

libavformat/matroskadec.c

@@ -381,6 +381,8 @@ typedef struct MatroskaDemuxContext {
     /* byte position of the segment inside the stream */
     int64_t segment_start;
 
+    /* This packet coincides with AVFormatInternal.parse_pkt
+     * and is not owned by us. */
     AVPacket *pkt;
 
     /* the packet queue */
@@ -2945,9 +2947,7 @@ static int matroska_read_header(AVFormatContext *s)
     }
     ebml_free(ebml_syntax, &ebml);
 
-    matroska->pkt = av_packet_alloc();
-    if (!matroska->pkt)
-        return AVERROR(ENOMEM);
+    matroska->pkt = s->internal->parse_pkt;
 
     /* The next thing is a segment. */
     pos = avio_tell(matroska->ctx->pb);
@@ -3528,7 +3528,6 @@ static int matroska_parse_frame(MatroskaDemuxContext *matroska,
     if (!pkt_size && !additional_size)
         goto no_output;
 
-    av_packet_unref(pkt);
     if (!buf)
         pkt->buf = av_buffer_create(pkt_data, pkt_size + AV_INPUT_BUFFER_PADDING_SIZE,
                                     NULL, NULL, 0);
@@ -3902,7 +3901,6 @@ static int matroska_read_close(AVFormatContext *s)
     int n;
 
     matroska_clear_queue(matroska);
-    av_packet_free(&matroska->pkt);
 
     for (n = 0; n < matroska->tracks.nb_elem; n++)
         if (tracks[n].type == MATROSKA_TRACK_TYPE_AUDIO)