feat(client): implement server-initiated mode with WebSocket support

cmd/client/main.go

@@ -5,21 +5,24 @@ import (
 	"flag"
 	"fmt"
 	"log"
+	"net/http"
 	"os"
 	"os/signal"
 	"path/filepath"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
 	"github.com/Sendspin/sendspin-go/pkg/discovery"
 	"github.com/Sendspin/sendspin-go/pkg/sendspin"
+	"github.com/gorilla/websocket"
 
 	"rpi-sendspin/internal/pipewire"
 )
 
 func main() {
-	server := flag.String("server", "localhost:8927", "server address")
+	server := flag.String("server", "", "server address for client-initiated mode (dial out). Leave empty for server-initiated mode: advertise via mDNS and wait for the server to connect.")
 	name := flag.String("name", hostname(), "player name")
 	device := flag.String("device", "", "PipeWire sink node name (default: system default)")
 	listDevices := flag.Bool("list-devices", false, "list available PipeWire sinks and exit")
@@ -119,23 +122,109 @@ func main() {
 	} else {
 		log.Printf("output: PipeWire default sink")
 	}
-	log.Printf("connecting to %s as %q...", *server, *name)
-
-	if err := player.Connect(); err != nil {
-		log.Fatalf("connect: %v", err)
-	}
-	if err := player.Play(); err != nil {
-		log.Fatalf("play: %v", err)
-	}
 
 	sig := make(chan os.Signal, 1)
 	signal.Notify(sig, os.Interrupt, syscall.SIGTERM)
-	<-sig
+
+	if *server == "" {
+		// Server-initiated mode: the server discovers us via mDNS and dials
+		// our /sendspin endpoint. We just listen and let player.Accept drive
+		// each connection.
+		if *noMDNS {
+			log.Printf("warning: --no-mdns with no --server — the server cannot discover this player")
+		}
+		runServerInitiated(player, *mdnsPort, sig)
+	} else {
+		// Client-initiated mode: dial the configured server.
+		log.Printf("connecting to %s as %q...", *server, *name)
+		if err := player.Connect(); err != nil {
+			log.Fatalf("connect: %v", err)
+		}
+		if err := player.Play(); err != nil {
+			log.Fatalf("play: %v", err)
+		}
+		<-sig
+	}
 
 	log.Println("stopping")
 	player.Stop()
 }
 
+// runServerInitiated runs an HTTP/WebSocket server on the mDNS-advertised
+// port and hands each accepted /sendspin connection to player.Accept. Only
+// one session runs at a time; a newer connection evicts the current one
+// (newest-wins) so a server that reconnects isn't blocked behind a stale
+// socket. Blocks until a signal arrives on sig.
+func runServerInitiated(player *sendspin.Player, port int, sig <-chan os.Signal) {
+	upgrader := websocket.Upgrader{
+		CheckOrigin: func(*http.Request) bool { return true },
+	}
+
+	var (
+		mu      sync.Mutex // serializes sessions; held for a connection's lifetime
+		connMu  sync.Mutex // guards curConn
+		curConn *websocket.Conn
+	)
+
+	handler := func(w http.ResponseWriter, r *http.Request) {
+		conn, err := upgrader.Upgrade(w, r, nil)
+		if err != nil {
+			log.Printf("websocket upgrade failed: %v", err)
+			return
+		}
+
+		// Evict any in-flight session so its Accept returns and releases mu.
+		connMu.Lock()
+		if curConn != nil {
+			curConn.Close()
+		}
+		curConn = conn
+		connMu.Unlock()
+
+		mu.Lock()
+		defer mu.Unlock()
+
+		// We may have been superseded while waiting for the lock.
+		connMu.Lock()
+		superseded := curConn != conn
+		connMu.Unlock()
+		if superseded {
+			conn.Close()
+			return
+		}
+
+		if err := player.Accept(conn); err != nil {
+			log.Printf("session ended: %v", err)
+		}
+
+		connMu.Lock()
+		if curConn == conn {
+			curConn = nil
+		}
+		connMu.Unlock()
+	}
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/sendspin", handler)
+	srv := &http.Server{Addr: fmt.Sprintf(":%d", port), Handler: mux}
+
+	go func() {
+		log.Printf("server-initiated mode: listening on :%d/sendspin, waiting for a server to connect", port)
+		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("listen: %v", err)
+		}
+	}()
+
+	<-sig
+
+	// Drop the active session first so the blocked handler returns, then let
+	// Shutdown drain the (now-unblocked) server.
+	player.CloseConnection()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	srv.Shutdown(ctx)
+}
+
 func hostname() string {
 	h, _ := os.Hostname()
 	if h == "" {
@@ -154,11 +243,20 @@ func hostname() string {
 // during reconnect could in theory move the player to a different server,
 // but for the same-machine echo-cancel use case that's acceptable.
 func buildMuter(server, muteServer, muteLoopback string) *pipewire.Muter {
-	if muteLoopback == "" && muteServer == "" {
+	if muteLoopback == "" {
+		if muteServer != "" {
+			log.Printf("muter: --mute-server set without --mute-loopback; loopback mute disabled")
+		}
 		return nil
 	}
-	if muteLoopback == "" || muteServer == "" {
-		log.Printf("muter: --mute-loopback and --mute-server must be set together; loopback mute disabled")
+	// Server-initiated (listen) mode has no fixed --server to compare
+	// against, so arm the muter purely on --mute-loopback.
+	if server == "" {
+		log.Printf("muter: will mute PipeWire node %q while streaming (server-initiated mode)", muteLoopback)
+		return pipewire.NewMuter(muteLoopback)
+	}
+	if muteServer == "" {
+		log.Printf("muter: --mute-loopback and --mute-server must be set together in client-initiated mode; loopback mute disabled")
 		return nil
 	}
 	if !sameServerAddr(server, muteServer) {

contrib/systemd/sendspin-client.service

@@ -4,8 +4,10 @@ After=pipewire.service network-online.target
 Wants=pipewire.service
 
 [Service]
+# No --server: server-initiated mode. The player advertises via mDNS and
+# waits for Music Assistant to discover and connect to it. Pass --server
+# host:port only to force the old client-initiated (dial-out) behavior.
 ExecStart=%h/.local/bin/sendspin-client \
-    --server 192.168.1.100:8927 \
     --name %H \
     --device alsa_output.usb-Device-00.analog-stereo \
     --volume 80

third_party/sendspin-go/pkg/sendspin/player.go

@@ -14,6 +14,7 @@ import (
 	"github.com/Sendspin/sendspin-go/pkg/audio/output"
 	"github.com/Sendspin/sendspin-go/pkg/protocol"
 	"github.com/Sendspin/sendspin-go/pkg/sync"
+	"github.com/gorilla/websocket"
 )
 
 // ReconnectConfig controls automatic reconnect behavior after the protocol
@@ -216,6 +217,55 @@ func (p *Player) Connect() error {
 	return nil
 }
 
+// Accept drives the player over a server-initiated WebSocket connection
+// (the server discovered this player via mDNS and dialed it). It builds a
+// receiver over the accepted conn, starts audio consumption, and BLOCKS
+// until the connection drops or the player is closed — so callers running a
+// listener can serialize one session at a time and re-accept on return.
+//
+// Unlike Connect, Accept does not start the dial-based reconnect loop: in
+// server-initiated mode the server redials, so reconnection is the listener's
+// responsibility, not the player's. The caller transfers ownership of conn.
+func (p *Player) Accept(conn *websocket.Conn) error {
+	recv, err := p.buildReceiver("server-initiated")
+	if err != nil {
+		return err
+	}
+	if err := recv.AcceptConn(conn); err != nil {
+		return err
+	}
+
+	p.receiver = recv
+	p.state.Connected = true
+	p.notifyStateChange()
+
+	go p.consumeAudio(recv)
+
+	// Block for the lifetime of this connection so the caller's accept loop
+	// serializes sessions (one server connection at a time).
+	select {
+	case <-recv.Done():
+	case <-p.ctx.Done():
+	}
+
+	recv.Close()
+	p.state.Connected = false
+	p.state.State = "idle"
+	p.notifyStateChange()
+
+	return nil
+}
+
+// CloseConnection drops the currently active receiver, if any, without
+// tearing down the Player or its audio output. In server-initiated mode a
+// listener calls this to evict a stale/superseded session before accepting
+// a newer connection; the blocked Accept for the old session then returns.
+func (p *Player) CloseConnection() {
+	if p.receiver != nil {
+		p.receiver.Close()
+	}
+}
+
 func (p *Player) buildReceiver(addr string) (*Receiver, error) {
 	p.ensureCapsResolved()
 	return NewReceiver(ReceiverConfig{

third_party/sendspin-go/pkg/sendspin/receiver.go

@@ -17,6 +17,7 @@ import (
 	"github.com/Sendspin/sendspin-go/pkg/audio/decode"
 	"github.com/Sendspin/sendspin-go/pkg/protocol"
 	"github.com/Sendspin/sendspin-go/pkg/sync"
+	"github.com/gorilla/websocket"
 )
 
 // Time-sync burst parameters. Mirrors sendspin-cpp's TimeBurst defaults and
@@ -109,6 +110,12 @@ type Receiver struct {
 	// from r.clockSync.ServerMicrosNow so tests can drive the
 	// timestamp-deferral path with a fake clock.
 	clockNow func() int64
+
+	// closeOnce guards Close so it is idempotent: in server-initiated mode
+	// a listener may evict a session via CloseConnection (which closes the
+	// receiver) while Accept's own deferred Close also fires. Without this
+	// the second close(r.output) panics.
+	closeOnce stdsync.Once
 }
 
 // NewReceiver creates a new Receiver with the given configuration.
@@ -189,17 +196,59 @@ func (r *Receiver) Stats() ReceiverStats {
 	return stats
 }
 
-// Connect establishes a connection to the server, performs initial clock sync,
-// and starts background goroutines for connection watching and clock sync.
+// Connect establishes a client-initiated connection to the server (dials
+// ServerAddr), performs initial clock sync, and starts background goroutines
+// for connection watching and clock sync.
 func (r *Receiver) Connect() error {
+	clientConfig, err := r.buildClientConfig()
+	if err != nil {
+		return err
+	}
+
+	r.client = protocol.NewClient(clientConfig)
+
+	if err := r.client.Connect(); err != nil {
+		return fmt.Errorf("connection failed: %w", err)
+	}
+
+	log.Printf("Connected to server: %s", r.serverAddr)
+	return r.startSession()
+}
+
+// AcceptConn drives the receiver protocol over an already-established
+// WebSocket connection (server-initiated mode: the server discovered this
+// player via mDNS and dialed it). The client still sends client/hello first
+// per spec, so the handshake and message loop are identical to Connect once
+// the socket exists. The caller transfers ownership of conn; Close() (or a
+// dropped connection) closes it.
+func (r *Receiver) AcceptConn(conn *websocket.Conn) error {
+	clientConfig, err := r.buildClientConfig()
+	if err != nil {
+		return err
+	}
+
+	r.client = protocol.NewClientFromConn(clientConfig, conn)
+
+	if err := r.client.Start(); err != nil {
+		return fmt.Errorf("handshake failed: %w", err)
+	}
+
+	log.Printf("Accepted server-initiated connection from %s", conn.RemoteAddr())
+	return r.startSession()
+}
+
+// buildClientConfig validates required fields and assembles the protocol
+// client config (advertised formats, device info, role support) shared by
+// both the client-initiated (Connect) and server-initiated (AcceptConn) paths.
+func (r *Receiver) buildClientConfig() (protocol.Config, error) {
 	if r.config.ClientID == "" {
-		return fmt.Errorf("ReceiverConfig.ClientID is required (resolve via sendspin.ResolveClientID)")
+		return protocol.Config{}, fmt.Errorf("ReceiverConfig.ClientID is required (resolve via sendspin.ResolveClientID)")
 	}
 
 	supportedFormats := buildSupportedFormats(r.config.PreferredCodec, r.config.MaxSampleRate, r.config.MaxBitDepth)
 	logAdvertisedFormats(supportedFormats, r.config.MaxSampleRate, r.config.MaxBitDepth)
 
-	clientConfig := protocol.Config{
+	return protocol.Config{
 		ServerAddr: r.serverAddr,
 		ClientID:   r.config.ClientID,
 		Name:       r.config.PlayerName,
@@ -222,15 +271,13 @@ func (r *Receiver) Connect() error {
 		VisualizerV1Support: &protocol.VisualizerV1Support{
 			BufferCapacity: r.config.BufferCapacity,
 		},
-	}
+	}, nil
+}
 
-	r.client = protocol.NewClient(clientConfig)
-
-	if err := r.client.Connect(); err != nil {
-		return fmt.Errorf("connection failed: %w", err)
-	}
-
-	log.Printf("Connected to server: %s", r.serverAddr)
+// startSession marks the receiver connected, runs the initial clock-sync
+// burst, and launches the background goroutines. Shared by Connect and
+// AcceptConn; r.client must already be started.
+func (r *Receiver) startSession() error {
 	r.connected = true
 
 	if err := r.performInitialSync(); err != nil {
@@ -795,29 +842,31 @@ func (r *Receiver) notifyError(err error) {
 }
 
 func (r *Receiver) Close() error {
-	// Send goodbye BEFORE cancelling the context so the message
-	// reaches the server while the connection is still alive.
-	if r.client != nil {
-		r.client.SendGoodbye("shutdown")
-	}
-
-	r.cancel()
-
-	if r.client != nil {
-		r.client.Close()
-	}
-
-	if r.scheduler != nil {
-		r.scheduler.Stop()
-	}
-
-	if r.decoder != nil {
-		if err := r.decoder.Close(); err != nil {
-			log.Printf("Receiver: decoder close error: %v", err)
+	r.closeOnce.Do(func() {
+		// Send goodbye BEFORE cancelling the context so the message
+		// reaches the server while the connection is still alive.
+		if r.client != nil {
+			r.client.SendGoodbye("shutdown")
 		}
-	}
 
-	close(r.output)
+		r.cancel()
+
+		if r.client != nil {
+			r.client.Close()
+		}
+
+		if r.scheduler != nil {
+			r.scheduler.Stop()
+		}
+
+		if r.decoder != nil {
+			if err := r.decoder.Close(); err != nil {
+				log.Printf("Receiver: decoder close error: %v", err)
+			}
+		}
+
+		close(r.output)
+	})
 
 	return nil
 }