avformat/matroskadec: use refcounted buffers in EbmlBin

Data in EbmlBin objects is never changed after being read from the
input file (save for two specific cases with encoded CodePrivate), so
using AVBufferRef we can prevent unnecessary copy of data by instead
creating new references to said constant data.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -104,6 +104,7 @@ typedef struct EbmlList {
 
 typedef struct EbmlBin {
     int      size;
+    AVBufferRef *buf;
     uint8_t *data;
     int64_t  pos;
 } EbmlBin;
@@ -962,14 +963,19 @@ static int ebml_read_ascii(AVIOContext *pb, int size, char **str)
  */
 static int ebml_read_binary(AVIOContext *pb, int length, EbmlBin *bin)
 {
-    av_fast_padded_malloc(&bin->data, &bin->size, length);
-    if (!bin->data)
-        return AVERROR(ENOMEM);
+    int ret;
 
+    ret = av_buffer_realloc(&bin->buf, length + AV_INPUT_BUFFER_PADDING_SIZE);
+    if (ret < 0)
+        return ret;
+    memset(bin->buf->data + length, 0, AV_INPUT_BUFFER_PADDING_SIZE);
+
+    bin->data = bin->buf->data;
     bin->size = length;
     bin->pos  = avio_tell(pb);
     if (avio_read(pb, bin->data, length) != length) {
-        av_freep(&bin->data);
+        av_buffer_unref(&bin->buf);
+        bin->data = NULL;
         bin->size = 0;
         return AVERROR(EIO);
     }
@@ -1252,7 +1258,7 @@ static void ebml_free(EbmlSyntax *syntax, void *data)
             av_freep(data_off);
             break;
         case EBML_BIN:
-            av_freep(&((EbmlBin *) data_off)->data);
+            av_buffer_unref(&((EbmlBin *) data_off)->buf);
             break;
         case EBML_LEVEL1:
         case EBML_NEST:
@@ -2036,12 +2042,13 @@ static int get_qt_codec(MatroskaTrack *track, uint32_t *fourcc, enum AVCodecID *
      * by expanding/shifting the data by 4 bytes and storing the data
      * size at the start. */
     if (ff_codec_get_id(codec_tags, AV_RL32(track->codec_priv.data))) {
-        uint8_t *p = av_realloc(track->codec_priv.data,
-                                track->codec_priv.size + 4);
-        if (!p)
-            return AVERROR(ENOMEM);
-        memmove(p + 4, p, track->codec_priv.size);
-        track->codec_priv.data = p;
+        int ret = av_buffer_realloc(&track->codec_priv.buf,
+                                    track->codec_priv.size + 4 + AV_INPUT_BUFFER_PADDING_SIZE);
+        if (ret < 0)
+            return ret;
+
+        track->codec_priv.data = track->codec_priv.buf->data;
+        memmove(track->codec_priv.data + 4, track->codec_priv.data, track->codec_priv.size);
         track->codec_priv.size += 4;
         AV_WB32(track->codec_priv.data, track->codec_priv.size);
     }
@@ -2162,8 +2169,19 @@ static int matroska_parse_tracks(AVFormatContext *s)
                            "Failed to decode codec private data\n");
                 }
 
-                if (codec_priv != track->codec_priv.data)
-                    av_free(codec_priv);
+                if (codec_priv != track->codec_priv.data) {
+                    av_buffer_unref(&track->codec_priv.buf);
+                    if (track->codec_priv.data) {
+                        track->codec_priv.buf = av_buffer_create(track->codec_priv.data,
+                                                                 track->codec_priv.size + AV_INPUT_BUFFER_PADDING_SIZE,
+                                                                 NULL, NULL, 0);
+                        if (!track->codec_priv.buf) {
+                            av_freep(&track->codec_priv.data);
+                            track->codec_priv.size = 0;
+                            return AVERROR(ENOMEM);
+                        }
+                    }
+                }
             }
         }