refactor-for-testability (#3)

Co-authored-by: jdl <jdl@desktop>
Co-authored-by: jdl <jdl@crumpington.com>
Reviewed-on: #3

peer/mcwriter.go

@@ -0,0 +1,53 @@
+package peer
+
+import (
+	"log"
+	"net"
+	"time"
+
+	"golang.org/x/crypto/nacl/sign"
+)
+
+func createLocalDiscoveryPacket(localIP byte, signingKey []byte) []byte {
+	h := header{
+		SourceIP: localIP,
+		DestIP:   255,
+	}
+	buf := make([]byte, headerSize)
+	h.Marshal(buf)
+	out := make([]byte, headerSize+signOverhead)
+	return sign.Sign(out[:0], buf, (*[64]byte)(signingKey))
+}
+
+func headerFromLocalDiscoveryPacket(pkt []byte) (h header, ok bool) {
+	if len(pkt) != headerSize+signOverhead {
+		return
+	}
+
+	h.Parse(pkt[signOverhead:])
+	ok = true
+	return
+}
+
+func verifyLocalDiscoveryPacket(pkt, buf []byte, pubSignKey []byte) bool {
+	_, ok := sign.Open(buf[:0], pkt, (*[32]byte)(pubSignKey))
+	return ok
+}
+
+// ----------------------------------------------------------------------------
+
+func runMCWriter(localIP byte, signingKey []byte) {
+	discoveryPacket := createLocalDiscoveryPacket(localIP, signingKey)
+
+	conn, err := net.ListenMulticastUDP("udp", nil, multicastAddr)
+	if err != nil {
+		log.Fatalf("[MCWriter] Failed to bind to multicast address: %v", err)
+	}
+
+	for range time.Tick(broadcastInterval) {
+		_, err := conn.WriteToUDP(discoveryPacket, multicastAddr)
+		if err != nil {
+			log.Printf("[MCWriter] Failed to write multicast: %v", err)
+		}
+	}
+}