lavf: add an AVClass to AVStream on next major bump

Also add a function to retrieve that class, analogously to
avformat_get_class().

This will be useful for adding an AVOption for dispositions.

doc/APIchanges

@@ -14,6 +14,10 @@ libavutil:     2021-04-27
 
 API changes, most recent first:
 
+2021-11-xx - xxxxxxxxxx - lavf 59.9.100 - avformat.h
+  Add av_stream_get_class(). Schedule adding AVStream.av_class at libavformat
+  major version 60.
+
 2021-11-12 - xxxxxxxxxx - lavu 57.8.100 - hwcontext_vulkan.h
   Added AVFrame.sem_value, AVVulkanDeviceContext.queue_family_encode_index,
   nb_encode_queues, queue_family_decode_index, and nb_decode_queues.

libavformat/avformat.h

@@ -909,6 +909,13 @@ typedef struct AVIndexEntry {
  * sizeof(AVStream) must not be used outside libav*.
  */
 typedef struct AVStream {
+#if FF_API_AVSTREAM_CLASS
+    /**
+     * A class for @ref avoptions. Set on stream creation.
+     */
+    const AVClass *av_class;
+#endif
+
     int index;    /**< stream index in AVFormatContext */
     /**
      * Format-specific stream ID.
@@ -1864,6 +1871,14 @@ void avformat_free_context(AVFormatContext *s);
  */
 const AVClass *avformat_get_class(void);
 
+/**
+ * Get the AVClass for AVStream. It can be used in combination with
+ * AV_OPT_SEARCH_FAKE_OBJ for examining options.
+ *
+ * @see av_opt_find().
+ */
+const AVClass *av_stream_get_class(void);
+
 /**
  * Add a new stream to a media file.
  *

libavformat/utils.c

@@ -721,6 +721,17 @@ void avformat_free_context(AVFormatContext *s)
     av_free(s);
 }
 
+static const AVClass stream_class = {
+    .class_name     = "AVStream",
+    .item_name      = av_default_item_name,
+    .version        = LIBAVUTIL_VERSION_INT,
+};
+
+const AVClass *av_stream_get_class(void)
+{
+    return &stream_class;
+}
+
 AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
 {
     FFFormatContext *const si = ffformatcontext(s);
@@ -745,6 +756,10 @@ AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
         return NULL;
     st = &sti->pub;
 
+#if FF_API_AVSTREAM_CLASS
+    st->av_class = &stream_class;
+#endif
+
     st->codecpar = avcodec_parameters_alloc();
     if (!st->codecpar)
         goto fail;

libavformat/version.h

@@ -32,7 +32,7 @@
 // Major bumping may affect Ticket5467, 5421, 5451(compatibility with Chromium)
 // Also please add any ticket numbers that you believe might be affected here
 #define LIBAVFORMAT_VERSION_MAJOR  59
-#define LIBAVFORMAT_VERSION_MINOR   8
+#define LIBAVFORMAT_VERSION_MINOR   9
 #define LIBAVFORMAT_VERSION_MICRO 100
 
 #define LIBAVFORMAT_VERSION_INT AV_VERSION_INT(LIBAVFORMAT_VERSION_MAJOR, \
@@ -58,6 +58,7 @@
 #define FF_API_LAVF_PRIV_OPT            (LIBAVFORMAT_VERSION_MAJOR < 60)
 #define FF_API_COMPUTE_PKT_FIELDS2      (LIBAVFORMAT_VERSION_MAJOR < 60)
 #define FF_API_AVIOCONTEXT_WRITTEN      (LIBAVFORMAT_VERSION_MAJOR < 60)
+#define FF_API_AVSTREAM_CLASS           (LIBAVFORMAT_VERSION_MAJOR > 59)
 
 
 #define FF_API_R_FRAME_RATE            1