derp: add PeerPresentFlags bitmask to Watch messages

Updates tailscale/corp#17816

Change-Id: Ib5baf6c981a6a4c279f8bbfef02048cfbfb3323b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_client.go

@@ -368,6 +368,8 @@ type PeerPresentMessage struct {
 	Key key.NodePublic
 	// IPPort is the remote IP and port of the client.
 	IPPort netip.AddrPort
+	// Flags is a bitmask of info about the client.
+	Flags PeerPresentFlags
 }
 
 func (PeerPresentMessage) msg() {}
@@ -547,18 +549,33 @@ func (c *Client) recvTimeout(timeout time.Duration) (m ReceivedMessage, err erro
 			return pg, nil
 
 		case framePeerPresent:
-			if n < keyLen {
+			remain := b
+			chunk, remain, ok := cutLeadingN(remain, keyLen)
+			if !ok {
 				c.logf("[unexpected] dropping short peerPresent frame from DERP server")
 				continue
 			}
 			var msg PeerPresentMessage
-			msg.Key = key.NodePublicFromRaw32(mem.B(b[:keyLen]))
-			if n >= keyLen+16+2 {
-				msg.IPPort = netip.AddrPortFrom(
-					netip.AddrFrom16([16]byte(b[keyLen:keyLen+16])).Unmap(),
-					binary.BigEndian.Uint16(b[keyLen+16:keyLen+16+2]),
-				)
+			msg.Key = key.NodePublicFromRaw32(mem.B(chunk))
+
+			const ipLen = 16
+			const portLen = 2
+			chunk, remain, ok = cutLeadingN(remain, ipLen+portLen)
+			if !ok {
+				// Older server which didn't send the IP.
+				return msg, nil
 			}
+			msg.IPPort = netip.AddrPortFrom(
+				netip.AddrFrom16([16]byte(chunk[:ipLen])).Unmap(),
+				binary.BigEndian.Uint16(chunk[ipLen:]),
+			)
+
+			chunk, _, ok = cutLeadingN(remain, 1)
+			if !ok {
+				// Older server which doesn't send PeerPresentFlags.
+				return msg, nil
+			}
+			msg.Flags = PeerPresentFlags(chunk[0])
 			return msg, nil
 
 		case frameRecvPacket:
@@ -636,3 +653,10 @@ func (c *Client) LocalAddr() (netip.AddrPort, error) {
 	}
 	return netip.ParseAddrPort(a.String())
 }
+
+func cutLeadingN(b []byte, n int) (chunk, remain []byte, ok bool) {
+	if len(b) >= n {
+		return b[:n], b[n:], true
+	}
+	return nil, b, false
+}