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

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
 }