ffmpeg: exporting RTP header extension via packet side data

Exports RTP header extension as base64 encoded side data of type AV_PKT_DATA_STRINGS_METADATA.

--- ffmpeg-4.2.7/libavformat/rtpdec.c   2022-05-13 23:07:14.000000000 +0000
+++ ffmpeg-4.2.7-patched/libavformat/rtpdec.c   2024-12-28 12:40:26.348880247 +0000
@@ -23,6 +23,7 @@
 #include "libavutil/avstring.h"
 #include "libavutil/intreadwrite.h"
 #include "libavutil/time.h"
+#include "libavutil/base64.h"
 
 #include "avformat.h"
 #include "network.h"
@@ -630,6 +631,7 @@
     unsigned int ssrc;
     int payload_type, seq, flags = 0;
     int ext, csrc;
+    uint8_t *rtp_ext;
     AVStream *st;
     uint32_t timestamp;
     int rv = 0;
@@ -704,6 +706,16 @@
     // now perform timestamp things....
     finalize_packet(s, pkt, timestamp);
 
+    if (ext) {
+        rtp_ext = av_packet_new_side_data(pkt, AV_PKT_DATA_STRINGS_METADATA, 8 + AV_BASE64_SIZE(ext));
+
+        if (rtp_ext) {
+            memset(rtp_ext, 0, 8 + AV_BASE64_SIZE(ext));
+            memcpy(rtp_ext, "rtp_ext", 8);
+            av_base64_encode(rtp_ext + 8, AV_BASE64_SIZE(ext), buf - ext, ext);
+        }
+    }
+
     return rv;
 }
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