avcodec/avcodec, avpacket: Return blank packet on av_packet_ref() failure

Up until now, it was completely unspecified what the content of the
destination packet dst was on error. Depending upon where the error
happened calling av_packet_unref() on dst might be dangerous.

This commit changes this by making sure that dst is blank on error, so
unreferencing it again is safe (and still pointless). This behaviour is
documented.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

doc/APIchanges

@@ -15,6 +15,10 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2020-03-27 - xxxxxxxxxx - lavc 58.77.100 - avcodec.h
+  av_packet_ref() now guarantees to return the destination packet
+  in a blank state on error.
+
 2020-03-10 - xxxxxxxxxx - lavc 58.75.100 - avcodec.h
   Add AV_PKT_DATA_ICC_PROFILE.
 

libavcodec/avcodec.h

@@ -4651,7 +4651,8 @@ void av_packet_free_side_data(AVPacket *pkt);
  * @param dst Destination packet. Will be completely overwritten.
  * @param src Source packet
  *
- * @return 0 on success, a negative AVERROR on error.
+ * @return 0 on success, a negative AVERROR on error. On error, dst
+ *         will be blank (as if returned by av_packet_alloc()).
  */
 int av_packet_ref(AVPacket *dst, const AVPacket *src);
 

libavcodec/avpacket.c

@@ -610,12 +610,13 @@ int av_packet_ref(AVPacket *dst, const AVPacket *src)
 {
     int ret;
 
+    dst->buf = NULL;
+
     ret = av_packet_copy_props(dst, src);
     if (ret < 0)
-        return ret;
+        goto fail;
 
     if (!src->buf) {
-        dst->buf = NULL;
         ret = packet_alloc(&dst->buf, src->size);
         if (ret < 0)
             goto fail;
@@ -637,7 +638,7 @@ int av_packet_ref(AVPacket *dst, const AVPacket *src)
 
     return 0;
 fail:
-    av_packet_free_side_data(dst);
+    av_packet_unref(dst);
     return ret;
 }
 

libavcodec/version.h

@@ -28,7 +28,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR  58
-#define LIBAVCODEC_VERSION_MINOR  76
+#define LIBAVCODEC_VERSION_MINOR  77
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \