lavc/utils: Introduce ff_bprint_to_codecpar_extradata for avformat

It will be used by text subtitle demuxers to construct format instructions
straight into extradata. They all currently a similar function that accepts
an AVCodecContext instead.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/internal.h

@@ -22,6 +22,8 @@
 #define AVFORMAT_INTERNAL_H
 
 #include <stdint.h>
+
+#include "libavutil/bprint.h"
 #include "avformat.h"
 #include "os_support.h"
 
@@ -596,4 +598,9 @@ int ff_reshuffle_raw_rgb(AVFormatContext *s, AVPacket **ppkt, AVCodecContext *en
  */
 int ff_get_packet_palette(AVFormatContext *s, AVPacket *pkt, int ret, uint32_t *palette);
 
+/**
+ * Finalize buf into extradata and set its size appropriately.
+ */
+int ff_bprint_to_codecpar_extradata(AVCodecParameters *par, struct AVBPrint *buf);
+
 #endif /* AVFORMAT_INTERNAL_H */

libavformat/utils.c

@@ -4866,3 +4866,26 @@ int ff_get_packet_palette(AVFormatContext *s, AVPacket *pkt, int ret, uint32_t *
 
     return 0;
 }
+
+int ff_bprint_to_codecpar_extradata(AVCodecParameters *par, struct AVBPrint *buf)
+{
+    int ret;
+    char *str;
+
+    ret = av_bprint_finalize(buf, &str);
+    if (ret < 0)
+        return ret;
+    if (!av_bprint_is_complete(buf)) {
+        av_free(str);
+        return AVERROR(ENOMEM);
+    }
+
+    par->extradata = str;
+    /* Note: the string is NUL terminated (so extradata can be read as a
+     * string), but the ending character is not accounted in the size (in
+     * binary formats you are likely not supposed to mux that character). When
+     * extradata is copied, it is also padded with AV_INPUT_BUFFER_PADDING_SIZE
+     * zeros. */
+    par->extradata_size = buf->len;
+    return 0;
+}