Add mp3_plugin, support rtsp with mp3 (#4048)

增加mp3_plugin, 解决rtmp推流转rtsp无法播放问题
ref: https://datatracker.ietf.org/doc/html/rfc2250#section-3.5

ext-codec/MP3Rtp.cpp

@@ -0,0 +1,83 @@
+#include "MP3Rtp.h"
+#define MPEG12_HEADER_LEN 4
+
+namespace mediakit {
+
+//MPEG Audio-specific header
+/*
+ 0               1               2               3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|             MBZ               |            Frag_offset        |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+*/
+    
+
+MP3RtpEncoder::MP3RtpEncoder(int sample_rate, int channels, int sample_bit) {
+    _sample_rate = sample_rate;
+    _channels = channels;
+    _sample_bit = sample_bit;
+}
+
+void MP3RtpEncoder::outputRtp(const char *data, size_t len, size_t offset, bool mark, uint64_t stamp) {
+    auto rtp = getRtpInfo().makeRtp(TrackAudio, nullptr, len + 4, mark, stamp);
+    auto payload = rtp->data() + RtpPacket::kRtpTcpHeaderSize + RtpPacket::kRtpHeaderSize;
+    payload[0] = 0;
+    payload[1] = 0;
+    payload[2] = offset >> 8;
+    payload[3] = offset ;
+    memcpy(payload + 4, data, len);
+    RtpCodec::inputRtp(std::move(rtp), false);
+}
+
+bool MP3RtpEncoder::inputFrame(const Frame::Ptr &frame) {
+    auto ptr = frame->data() + frame->prefixSize();
+    auto size = frame->size() - frame->prefixSize();
+    auto remain_size = size;
+    auto max_size = getRtpInfo().getMaxSize() - MPEG12_HEADER_LEN;
+    int  offset = 0;
+    while (remain_size > 0) {
+        if (remain_size <= max_size) {
+            outputRtp(ptr, remain_size, offset, true, frame->dts());
+            break;
+        }
+        outputRtp(ptr, max_size, offset, false, frame->dts());
+        ptr += max_size;
+        remain_size -= max_size;
+        offset += max_size;
+    }
+    return true;
+}
+
+/////////////////////////////////////////////////////////////////////////////////////
+
+MP3RtpDecoder::MP3RtpDecoder() {
+    obtainFrame();
+}
+
+void MP3RtpDecoder::obtainFrame() {
+    _frame = FrameImp::create();
+    _frame->_codec_id = CodecMP3;
+}
+
+bool MP3RtpDecoder::inputRtp(const RtpPacket::Ptr &rtp, bool key_pos) {
+    auto payload_size = rtp->getPayloadSize();
+    if (payload_size <= MPEG12_HEADER_LEN) {
+        return false;
+    }
+    auto stamp = rtp->getStampMS();
+    auto ptr = rtp->getPayload();
+
+    _frame->_buffer.append((char *)ptr + MPEG12_HEADER_LEN, payload_size - MPEG12_HEADER_LEN);
+    flushData();
+
+    _last_dts = stamp;
+    return false;
+}
+
+void MP3RtpDecoder::flushData() {
+    RtpCodec::inputFrame(_frame);
+    obtainFrame();
+}
+
+} // namespace mediakit