refactor-for-testability (#3)

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

peer/ifreader.go

@@ -0,0 +1,103 @@
+package peer
+
+import (
+	"io"
+	"log"
+	"net/netip"
+	"sync/atomic"
+)
+
+type ifReader struct {
+	iface              io.Reader
+	writeToUDPAddrPort func([]byte, netip.AddrPort) (int, error)
+	rt                 *atomic.Pointer[routingTable]
+	buf1               []byte
+	buf2               []byte
+}
+
+func newIFReader(
+	iface io.Reader,
+	writeToUDPAddrPort func([]byte, netip.AddrPort) (int, error),
+	rt *atomic.Pointer[routingTable],
+) *ifReader {
+	return &ifReader{iface, writeToUDPAddrPort, rt, newBuf(), newBuf()}
+}
+
+func (r *ifReader) Run() {
+	packet := newBuf()
+	for {
+		r.handleNextPacket(packet)
+	}
+}
+
+func (r *ifReader) handleNextPacket(packet []byte) {
+	packet = r.readNextPacket(packet)
+	remoteIP, ok := r.parsePacket(packet)
+	if !ok {
+		return
+	}
+
+	rt := r.rt.Load()
+	peer := rt.Peers[remoteIP]
+	if !peer.Up {
+		r.logf("Peer %d not up.", peer.IP)
+		return
+	}
+
+	enc := peer.EncryptDataPacket(peer.IP, packet, r.buf1)
+	if peer.Direct {
+		r.writeToUDPAddrPort(enc, peer.DirectAddr)
+		return
+	}
+
+	relay, ok := rt.GetRelay()
+	if !ok {
+		r.logf("Relay not available for peer %d.", peer.IP)
+		return
+	}
+
+	enc = relay.EncryptDataPacket(peer.IP, enc, r.buf2)
+	r.writeToUDPAddrPort(enc, relay.DirectAddr)
+}
+
+func (r *ifReader) readNextPacket(buf []byte) []byte {
+	n, err := r.iface.Read(buf[:cap(buf)])
+	if err != nil {
+		log.Fatalf("Failed to read from interface: %v", err)
+	}
+
+	return buf[:n]
+}
+
+func (r *ifReader) parsePacket(buf []byte) (byte, bool) {
+	n := len(buf)
+	if n == 0 {
+		return 0, false
+	}
+
+	version := buf[0] >> 4
+
+	switch version {
+	case 4:
+		if n < 20 {
+			r.logf("Short IPv4 packet: %d", len(buf))
+			return 0, false
+		}
+		return buf[19], true
+
+	case 6:
+		if len(buf) < 40 {
+			r.logf("Short IPv6 packet: %d", len(buf))
+			return 0, false
+		}
+		return buf[39], true
+
+	default:
+		r.logf("Invalid IP packet version: %v", version)
+		return 0, false
+	}
+}
+
+func (*ifReader) logf(s string, args ...any) {
+	log.Printf("[IFReader] "+s, args...)
+}