Fix screen mirroring audio: native FDK-AAC decoder, buffer overflow fix, CI packaging

[Copilot]

Screen mirroring had no audio output. Root cause was multi-layered: AAC-ELD frames were wrapped in ADTS/LOAS containers that FFmpeg couldn't parse correctly, the RaopBuffer dequeue had a short overflow causing playback stalls, and the CI workflow was missing required native dependencies.

Native FDK-AAC decoder

• Added NativeFdkAacEldDecoder using P/Invoke to libfdk-aac-2.dll with TT_MP4_RAW transport, matching the approach in itskenny0/airplayreceiver and UxPlay
• Feeds raw decrypted AAC-ELD frames directly — no ADTS/LOAS framing needed
• AudioSpecificConfig f8e85000 (AOT=39, 44100Hz, stereo) matches UxPlay exactly
• Falls back to FFmpeg subprocess → SharpJaad if native lib unavailable

Buffer overflow fix (audio stuttering)

• RaopBufferDequeue cast buflen to short, overflowing negative when seq gap > 32767 → dequeue stops entirely, queue grows unbounded
• Changed to ushort arithmetic to handle full 16-bit sequence number range
• Set no_resend=true during mirroring — real-time audio should skip missing packets, not wait

Thread safety

• Separate IBufferedCipher instances per socket handler (CBC mode is stateful)
• _bufferLock synchronizes RaopBuffer access between concurrent control/data handlers
• FlushAsync now acquires lock before clearing buffer

Protocol fixes

• Added SETPEERS RTSP request handler (was silently dropped, no regex match)
• Fixed plist integer casting: Convert.ToInt32() instead of direct (int) unbox on Int16

CI & documentation

• Build FDK-AAC from source via MSYS2+CMake in CI, bundle libfdk-aac-2.dll + FFmpeg in artifacts
• Rewrote README.md, added README_zh.md

Debug logging

Comprehensive [DEBUG-*] prefixed logging throughout the audio pipeline for future diagnostics:
[DEBUG-FDK], [DEBUG-DECRYPT], [DEBUG-DECODE], [DEBUG-QUEUE], [DEBUG-RTSP], [DEBUG-SETUP], [DEBUG-TEARDOWN]

Original prompt

此项目在使用屏幕镜像的时候没有声音，但日志却正常：“Initializing audio output...
Audio device created: 44100Hz, 16-bit, 2 channels
Will call Init() after prebuffering 10 packets
Audio output initialized successfully
Video pipe created: \.\pipe\AirPlayVideo
Connect with: ffplay -f h264 -probesize 32 -analyzeduration 0 -fflags nobuffer -flags low_delay \.\pipe\AirPlayVideo
Video output initialized successfully
Waiting for video player to connect...
IP address 240e:390:3c1:7460::a6d
IP address 240e:390:3c1:7460:fac8:bb16:3565:b33a
IP address 240e:390:3c1:7460:8163:2eae:8d83:9dfb
IP address fe80::126d:dcfa:6b21:6f27%22
IP address 192.168.0.99
IP address 2001:0:14c9:d502:83f:2303:25b7:b7c8
IP address fe80::83f:2303:25b7:b7c8%20
NIC 'WLAN 6'
NIC 'Teredo Tunneling Pseudo-Interface'
Video player connected!
Client connected: 192.168.0.215:52614
PairVerified: True
ET: 32
Client connected: 192.168.0.215:52615
Client connected: 192.168.0.215:52616
Volume set to 1.00 (AirPlay dB: 0.0)
Audio type: 96 mpeg4-generic/44100/2, 96 mode=AAC-eld; constantDuration=480
Initializing recevie audio data from socket..
Initializing recevie audio control from socket..
FFmpeg AAC-ELD decoder started: 44100Hz, 2ch, 16bit, frameLength=480
FFmpeg AAC-ELD decoder started: 44100Hz, 2ch, 16bit, frameLength=480
Video: 300 frames written | Type: P | 1920x1080”，请修复此问题（关于屏幕镜像的音频部分你可以参照 https://github.com/itskenny0/airplayreceiver.git ）

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[Copilot]

Pull request overview

This PR addresses missing audio during AirPlay screen mirroring by reworking the AAC-ELD decoding path, improving RAOP buffering behavior during mirroring, and updating CI packaging/docs to include required native dependencies.

Changes:

• Added a native FDK-AAC AAC-ELD decoder (P/Invoke) with fallback to FFmpeg-based decoding.
• Fixed RAOP buffer dequeue overflow behavior and adjusted mirroring mode to skip resend waits.
• Updated CI to bundle FFmpeg + libfdk-aac, and refreshed English/Chinese READMEs.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 25 comments.

Show a summary per file

File	Description
README.md	Rewritten quick start, dependencies, and architecture notes (EN).
README_zh.md	Added Chinese documentation with build/run instructions.
AirPlay/Services/AudioOutputService.cs	Added more detailed audio output diagnostics.
AirPlay/Models/TcpListeners/Request.cs	Added SETPEERS request type detection.
AirPlay/Models/Enums/RequestType.cs	Added SETPEERS enum entry.
AirPlay/Models/Enums/RequestConst.cs	Added SETPEERS magic-number constant.
AirPlay/Listeners/Bases/BaseUdpListener.cs	Added UDP listener start/stop diagnostics and stored ports.
AirPlay/Listeners/Bases/BaseTcpListener.cs	Added SETPEERS to regex splitter + extra RTSP diagnostics.
AirPlay/Listeners/AudioListener.cs	Thread-safety improvements, mirroring resend behavior, buffer overflow fix, extensive debug logging.
AirPlay/Listeners/AirTunesListener.cs	Pass mirroring flag into AudioListener; RTSP logging; OPTIONS advertises SETPEERS.
AirPlay/Decoders/Implementations/NativeFdkAacEldDecoder.cs	New native FDK-AAC AAC-ELD decoder via P/Invoke.
AirPlay/Decoders/Implementations/FFmpegAacEldDecoder.cs	Switched FFmpeg decoder to LOAS/LATM framing and improved diagnostics.
AirPlay/AirPlayService.cs	Added PCM receive diagnostics before pushing to audio output.
.github/workflows/build.yml	Downloads FFmpeg, builds libfdk-aac in CI, and bundles into artifacts.

Comments suppressed due to low confidence (30)

AirPlay/Listeners/AudioListener.cs:646

• This comparison is always false.

                for (seqnum = raop_buffer.FirstSeqNum; Utilities.SeqNumCmp(seqnum, raop_buffer.LastSeqNum) < 0; seqnum++)

AirPlay/AirPlayService.cs:22

• The contents of this container are never accessed.

        private List<byte> _audiobuf;

AirPlay/Listeners/Bases/BaseTcpListener.cs:126

• Poor error handling: empty catch block.

                    catch (System.IO.IOException) { }

AirPlay/Services/AudioOutputService.cs:99

• Poor error handling: empty catch block.

                        catch { }

AirPlay/Services/AudioOutputService.cs:242

• Poor error handling: empty catch block.

                catch { }

AirPlay/Services/AudioOutputService.cs:276

• Poor error handling: empty catch block.

                catch { }

AirPlay/Listeners/AudioListener.cs:642

• Condition always evaluates to 'false'.

            if (Utilities.SeqNumCmp(raop_buffer.FirstSeqNum, raop_buffer.LastSeqNum) < 0)

AirPlay/Listeners/AirTunesListener.cs:344

• Inefficient use of 'ContainsKey' and indexer.

                                if (stream.ContainsKey("audioFormat"))

AirPlay/Listeners/AirTunesListener.cs:352

• Inefficient use of 'ContainsKey' and indexer.

                                if (stream.ContainsKey("ct"))

AirPlay/Listeners/AirTunesListener.cs:356

• Inefficient use of 'ContainsKey' and indexer.

                                if (stream.ContainsKey("spf"))

AirPlay/Listeners/AirTunesListener.cs:360

• Inefficient use of 'ContainsKey' and indexer.

                                if (stream.ContainsKey("controlPort"))

AirPlay/Listeners/AirTunesListener.cs:504

• Inefficient use of 'ContainsKey' and indexer.

                if (request.Headers.ContainsKey("Content-Type"))

AirPlay/Listeners/AirTunesListener.cs:564

• Inefficient use of 'ContainsKey' and indexer.

                if (request.Headers.ContainsKey("RTP-Info"))

AirPlay/Models/TcpListeners/Request.cs:210

• Inefficient use of 'ContainsKey' and indexer.

            if(_headers.ContainsKey("CSeq"))

AirPlay/Listeners/Bases/BaseTcpListener.cs:121

• String concatenation in loop: use 'StringBuilder'.

                        raw += string.Join(string.Empty, buffer.Take(readCount).Select(b => b.ToString("X2")));

AirPlay/Listeners/Bases/BaseTcpListener.cs:206

• String concatenation in loop: use 'StringBuilder'.

                format += $"{header.Name}: {string.Join(",", header.Values)}\r\n";

AirPlay/Listeners/Bases/BaseTcpListener.cs:230

• This assignment to e is useless, since its value is never read.

            catch (System.IO.IOException e)

AirPlay/Listeners/AirTunesListener.cs:201

• This assignment to signatureBuffer is useless, since its value is never read.

                        signatureBuffer = aesCtr128Encrypt.ProcessBytes(signatureBuffer);

AirPlay/Listeners/AirTunesListener.cs:227

• This assignment to mode is useless, since its value is never read.

                        var mode = body[14];

AirPlay/Listeners/AirTunesListener.cs:363

• This assignment to controlPort is useless, since its value is never read.

                                    var controlPort = (ushort)((short)stream["controlPort"]);

AirPlay/Listeners/AirTunesListener.cs:423

• This assignment to timingPort is useless, since its value is never read.

                                var timingPort = (ushort)((short)plist["timingPort"]);

AirPlay/Listeners/AirTunesListener.cs:526

• This assignment to start is useless, since its value is never read.

                                var start = long.Parse(pVals[0]);

AirPlay/Listeners/AirTunesListener.cs:527

• This assignment to current is useless, since its value is never read.

                                var current = long.Parse(pVals[1]);

AirPlay/Listeners/AirTunesListener.cs:528

• This assignment to end is useless, since its value is never read.

                                var end = long.Parse(pVals[2]);

AirPlay/Listeners/AirTunesListener.cs:536

• This assignment to image is useless, since its value is never read.

                        var image = request.Body;

AirPlay/Listeners/AirTunesListener.cs:543

• This assignment to output is useless, since its value is never read.

                        var output = dmap.Decode(request.Body);

AirPlay/Decoders/Implementations/FFmpegAacEldDecoder.cs:381

• These 'if' statements can be combined.

                    if (_ffmpegProcess != null && !_ffmpegProcess.HasExited)
                    {
                        if (!_ffmpegProcess.WaitForExit(PROCESS_EXIT_TIMEOUT_MS))
                        {
                            _ffmpegProcess.Kill();
                            _ffmpegProcess.WaitForExit(PROCESS_EXIT_TIMEOUT_MS);
                        }
                    }

AirPlay/Listeners/AudioListener.cs:34

• Field '_raopBuffer' can be 'readonly'.

        private RaopBuffer _raopBuffer;

AirPlay/Models/TcpListeners/Request.cs:23

• Field '_headers' can be 'readonly'.

        private HeadersCollection _headers;

AirPlay/Listeners/Bases/BaseTcpListener.cs:223

• Both branches of this 'if' statement write to the same variable - consider using '?' to express intent better.

            if (bodyBuffer?.Any() == true)
            {
                payload = formatBuffer.Concat(bodyBuffer).ToArray();
            }
            else
            {
                payload = formatBuffer;
            }

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💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Fill allocates and frees unmanaged memory (AllocHGlobal/FreeHGlobal) on every frame. In real-time audio this can become a hotspot and cause jitter/glitches. Consider pinning the managed input buffer (fixed) or reusing a single unmanaged buffer across calls to avoid per-frame allocations and copies.

Suggested change

	IntPtr ptr = Marshal.AllocHGlobal(pBuffer.Length);
	try
	{
	Marshal.Copy(pBuffer, 0, ptr, pBuffer.Length);
	IntPtr* pBufferPtr = stackalloc IntPtr[1];
	pBufferPtr[0] = ptr;
	int ret = (int)aacDecoder_Fill(_decoder, pBufferPtr, &bufferSize, &bytesValid);
	if (ret == 0 && _decodeCallCount <= 5)
	Console.WriteLine($"[DEBUG-FDK] Fill OK: bufferSize={pBuffer.Length}, bytesValid after={bytesValid}");
	return ret;
	}
	finally
	{
	Marshal.FreeHGlobal(ptr);
	}
	fixed (byte* bufferPtr = pBuffer)
	{
	IntPtr* pBufferPtr = stackalloc IntPtr[1];
	pBufferPtr[0] = (IntPtr)bufferPtr;
	int ret = (int)aacDecoder_Fill(_decoder, pBufferPtr, &bufferSize, &bytesValid);
	if (ret == 0 && _decodeCallCount <= 5)
	Console.WriteLine($"[DEBUG-FDK] Fill OK: bufferSize={pBuffer.Length}, bytesValid after={bytesValid}");
	return ret;
	}

[Copilot]

The comments state Linux uses libfdk-aac.so.2, but FDK_AAC_LIB is hardcoded to libfdk-aac-2. On non-Windows platforms this name is unlikely to resolve without a matching symlink, so native decoding will fail even if the library is installed. Consider adding a platform-specific resolver (NativeLibrary.SetDllImportResolver) or updating the library name strategy/documentation to match actual SONAMEs.

Suggested change

	private const string FDK_AAC_LIB = "libfdk-aac-2";
	#if WINDOWS
	private const string FDK_AAC_LIB = "libfdk-aac-2";
	#elif LINUX
	private const string FDK_AAC_LIB = "libfdk-aac.so.2";
	#else
	// Default to the common UNIX-like SONAME; adjust if targeting other platforms.
	private const string FDK_AAC_LIB = "libfdk-aac.so.2";
	#endif

[Copilot]

The workflow downloads a moving latest FFmpeg ZIP without any integrity verification (checksum/signature) and then executes the binaries. For supply-chain safety and reproducibility, consider pinning to a specific release/build (or at least validating a published SHA256) before copying into artifacts.

[Copilot]

This step assumes MSYS2 is already present at C:\msys64 and that pacman is available from the selected bash shell. To make CI more robust against runner image changes, consider installing MSYS2 explicitly via msys2/setup-msys2@v2 (and running the build in an MSYS2 shell), rather than relying on a preinstalled location.

[Copilot]

if (packet.Length >= 12) is always true because packet is a fixed 50k buffer. This should be checking the received byte count (dret >= 12) before reading header fields / decrypting / queueing, otherwise short/empty reads can still run through this path.

Suggested change

	if (packet.Length >= 12)
	if (dret >= 12)

[Copilot]

Inefficient use of 'ContainsKey' and indexer.

Suggested change

	var cseq = request.Headers.ContainsKey("CSeq") ? request.Headers["CSeq"] : "?";
	request.Headers.TryGetValue("CSeq", out var cseq);
	cseq ??= "?";

[Copilot]

This assignment to next_timestamp is useless, since its value is never read.

Suggested change

uint next_timestamp = (uint)((packet[16] << 24) | (packet[17] << 16) | (packet[18] << 8) | packet[19]);

[Copilot]

These 'if' statements can be combined.

[Copilot]

Field '_controlSequenceNumber' can be 'readonly'.

Suggested change

	private ushort _controlSequenceNumber = 0;
	private readonly ushort _controlSequenceNumber = 0;

[Copilot]

Field '_isMirroring' can be 'readonly'.

Suggested change

	private bool _isMirroring = false;
	private readonly bool _isMirroring = false;