first

internal/discovery/client.go

@@ -0,0 +1,54 @@
+package discovery
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"strings"
+	"time"
+)
+
+const requestMagic = "SPEKTR_DISCOVER"
+const responsePrefix = "SPEKTR|"
+
+func Discover(ctx context.Context, port int) (string, error) {
+	conn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		return "", err
+	}
+	defer conn.Close()
+
+	target := &net.UDPAddr{IP: net.IPv4bcast, Port: port}
+	if _, err := conn.WriteToUDP([]byte(requestMagic), target); err != nil {
+		return "", err
+	}
+
+	if err := conn.SetReadDeadline(time.Now().Add(2 * time.Second)); err != nil {
+		return "", err
+	}
+
+	buf := make([]byte, 256)
+	for {
+		select {
+		case <-ctx.Done():
+			return "", ctx.Err()
+		default:
+		}
+
+		n, _, err := conn.ReadFromUDP(buf)
+		if err != nil {
+			if ne, ok := err.(net.Error); ok && ne.Timeout() {
+				return "", fmt.Errorf("discovery timeout")
+			}
+			return "", err
+		}
+
+		resp := strings.TrimSpace(string(buf[:n]))
+		if strings.HasPrefix(resp, responsePrefix) {
+			addr := strings.TrimSpace(strings.TrimPrefix(resp, responsePrefix))
+			if addr != "" {
+				return addr, nil
+			}
+		}
+	}
+}

internal/discovery/server.go

@@ -0,0 +1,42 @@
+package discovery
+
+import (
+	"context"
+	"net"
+	"strings"
+	"time"
+)
+
+func Serve(ctx context.Context, port int, grpcAddr string) error {
+	conn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: port})
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	buf := make([]byte, 256)
+	for {
+		if err := conn.SetReadDeadline(time.Now().Add(time.Second)); err != nil {
+			return err
+		}
+
+		n, remote, err := conn.ReadFromUDP(buf)
+		if err != nil {
+			if ne, ok := err.(net.Error); ok && ne.Timeout() {
+				select {
+				case <-ctx.Done():
+					return nil
+				default:
+					continue
+				}
+			}
+			return err
+		}
+
+		if strings.TrimSpace(string(buf[:n])) != requestMagic {
+			continue
+		}
+
+		_, _ = conn.WriteToUDP([]byte(responsePrefix+grpcAddr), remote)
+	}
+}