lavf: replace AVStream.codec with AVStream.codecpar

Currently, AVStream contains an embedded AVCodecContext instance, which
is used by demuxers to export stream parameters to the caller and by
muxers to receive stream parameters from the caller. It is also used
internally as the codec context that is passed to parsers.

In addition, it is also widely used by the callers as the decoding (when
demuxer) or encoding (when muxing) context, though this has been
officially discouraged since Libav 11.

There are multiple important problems with this approach:
    - the fields in AVCodecContext are in general one of
        * stream parameters
        * codec options
        * codec state
      However, it's not clear which ones are which. It is consequently
      unclear which fields are a demuxer allowed to set or a muxer allowed to
      read. This leads to erratic behaviour depending on whether decoding or
      encoding is being performed or not (and whether it uses the AVStream
      embedded codec context).
    - various synchronization issues arising from the fact that the same
      context is used by several different APIs (muxers/demuxers,
      parsers, bitstream filters and encoders/decoders) simultaneously, with
      there being no clear rules for who can modify what and the different
      processes being typically delayed with respect to each other.
    - avformat_find_stream_info() making it necessary to support opening
      and closing a single codec context multiple times, thus
      complicating the semantics of freeing various allocated objects in the
      codec context.

Those problems are resolved by replacing the AVStream embedded codec
context with a newly added AVCodecParameters instance, which stores only
the stream parameters exported by the demuxers or read by the muxers.

1 files changed, 15 insertions, 9 deletions

diff --git a/libavformat/avformat.h b/libavformat/avformat.h
index b63541d411..6db17f70cf 100644
--- a/libavformat/avformat.h
+++ b/libavformat/avformat.h
@@ -704,18 +704,13 @@ typedef struct AVStream {
      * encoding: set by the user, replaced by libavformat if left unset
      */
     int id;
+#if FF_API_LAVF_AVCTX
     /**
-     * Codec context associated with this stream. Allocated and freed by
-     * libavformat.
-     *
-     * - decoding: The demuxer exports codec information stored in the headers
-     *             here.
-     * - encoding: The user sets codec information, the muxer writes it to the
-     *             output. Mandatory fields as specified in AVCodecContext
-     *             documentation must be set even if this AVCodecContext is
-     *             not actually used for encoding.
+     * @deprecated use the codecpar struct instead
      */
+    attribute_deprecated
     AVCodecContext *codec;
+#endif
     void *priv_data;
 
 #if FF_API_LAVF_FRAC
@@ -818,6 +813,17 @@ typedef struct AVStream {
     int event_flags;
 #define AVSTREAM_EVENT_FLAG_METADATA_UPDATED 0x0001 ///< The call resulted in updated metadata.
 
+    /*
+     * Codec parameters associated with this stream. Allocated and freed by
+     * libavformat in avformat_new_stream() and avformat_free_context()
+     * respectively.
+     *
+     * - demuxing: filled by libavformat on stream creation or in
+     *             avformat_find_stream_info()
+     * - muxing: filled by the caller before avformat_write_header()
+     */
+    AVCodecParameters *codecpar;
+
     /*****************************************************************
      * All fields below this line are not part of the public API. They
      * may not be used outside of libavformat and can be changed and