avcodec/avpacket: add av_packet_make_writable()

Useful as well to quickly make a packet reference counted when it
isn't already so.
Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2018-03-21 - xxxxxxx - lavc 58.15.100 - avcodec.h
+  Add av_packet_make_writable().
+
 2018-03-18 - xxxxxxx - lavu 56.11.100 - frame.h
   Add AV_FRAME_DATA_QP_TABLE_PROPERTIES and AV_FRAME_DATA_QP_TABLE_DATA.
 

libavcodec/avcodec.h

@@ -4518,6 +4518,17 @@ void av_packet_move_ref(AVPacket *dst, AVPacket *src);
  */
 int av_packet_copy_props(AVPacket *dst, const AVPacket *src);
 
+/**
+ * Create a writable reference for the data described by a given packet,
+ * avoiding data copy if possible.
+ *
+ * @param pkt Packet whose data should be made writable.
+ *
+ * @return 0 on success, a negative AVERROR on failure. On failure, the
+ *         packet is unchanged.
+ */
+int av_packet_make_writable(AVPacket *pkt);
+
 /**
  * Convert valid timing fields (timestamps / durations) in a packet from one
  * timebase to another. Timestamps with unknown values (AV_NOPTS_VALUE) will be

libavcodec/avpacket.c

@@ -652,6 +652,30 @@ void av_packet_move_ref(AVPacket *dst, AVPacket *src)
     src->size = 0;
 }
 
+int av_packet_make_writable(AVPacket *pkt)
+{
+    AVBufferRef *buf = NULL;
+    int ret;
+
+    if (pkt->buf && av_buffer_is_writable(pkt->buf))
+        return 0;
+
+    if (!pkt->data)
+        return AVERROR(EINVAL);
+
+    ret = packet_alloc(&buf, pkt->size);
+    if (ret < 0)
+        return ret;
+    if (pkt->size)
+        memcpy(buf->data, pkt->data, pkt->size);
+
+    av_buffer_unref(&pkt->buf);
+    pkt->buf  = buf;
+    pkt->data = buf->data;
+
+    return 0;
+}
+
 void av_packet_rescale_ts(AVPacket *pkt, AVRational src_tb, AVRational dst_tb)
 {
     if (pkt->pts != AV_NOPTS_VALUE)

libavcodec/version.h

@@ -28,7 +28,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR  58
-#define LIBAVCODEC_VERSION_MINOR  14
+#define LIBAVCODEC_VERSION_MINOR  15
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \