lavc: add avcodec_free_context().

Right now, the caller has to manually manage some allocated
AVCodecContext fields, like extradata or subtitle_header. This is
fragile and prone to leaks, especially if we want to add more such
fields in the future.

The only reason for this behaviour is so that the AVStream codec context
can be reused for decoding. Such reuse is discouraged anyway, so this
commit is the first step to deprecating it.

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2013-12-xx
 
 API changes, most recent first:
 
+2014-04-xx - xxxxxxx - lavc 55.52.0 - avcodec.h
+  Add avcodec_free_context(). From now on it should be used for freeing
+  AVCodecContext.
+
 2014-05-xx - xxxxxxx - lavf 55.17.0 - avformat.h
   Add AVMFT_FLAG_BITEXACT flag. Muxers now use it instead of checking
   CODEC_FLAG_BITEXACT on the first stream.

libavcodec/avcodec.h

@@ -3147,8 +3147,7 @@ void avcodec_register_all(void);
 
 /**
  * Allocate an AVCodecContext and set its fields to default values. The
- * resulting struct can be deallocated by calling avcodec_close() on it followed
+ * resulting struct should be freed with avcodec_free_context().
- * by av_free().
  *
  * @param codec if non-NULL, allocate private data and initialize defaults
  *              for the given codec. It is illegal to then call avcodec_open2()
@@ -3162,6 +3161,12 @@ void avcodec_register_all(void);
  */
 AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);
 
+/**
+ * Free the codec context and everything associated with it and write NULL to
+ * the provided pointer.
+ */
+void avcodec_free_context(AVCodecContext **avctx);
+
 /**
  * Set the fields of the given AVCodecContext to default values corresponding
  * to the given codec (defaults may be codec-dependent).

libavcodec/options.c

@@ -135,6 +135,21 @@ AVCodecContext *avcodec_alloc_context3(const AVCodec *codec)
     return avctx;
 }
 
+void avcodec_free_context(AVCodecContext **pavctx)
+{
+    AVCodecContext *avctx = *pavctx;
+
+    if (!avctx)
+        return;
+
+    avcodec_close(avctx);
+
+    av_freep(&avctx->extradata);
+    av_freep(&avctx->subtitle_header);
+
+    av_freep(pavctx);
+}
+
 int avcodec_copy_context(AVCodecContext *dest, const AVCodecContext *src)
 {
     const AVCodec *orig_codec = dest->codec;

libavcodec/version.h

@@ -29,7 +29,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR 55
-#define LIBAVCODEC_VERSION_MINOR 51
+#define LIBAVCODEC_VERSION_MINOR 52
 #define LIBAVCODEC_VERSION_MICRO  0
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \