wgengine/magicsock: add side-effect-free function for netcheck UDP sends (#13487)

Updates #13484
Updates tailscale/corp#17879

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -493,13 +493,7 @@ func NewConn(opts Options) (*Conn, error) {
 	c.netChecker = &netcheck.Client{
 		Logf:                logger.WithPrefix(c.logf, "netcheck: "),
 		NetMon:              c.netMon,
-		SendPacket: func(b []byte, ap netip.AddrPort) (int, error) {
-			ok, err := c.sendUDP(ap, b)
-			if !ok {
-				return 0, err
-			}
-			return len(b), err
-		},
+		SendPacket:          c.sendUDPNetcheck,
 		SkipExternalNetwork: inTest(),
 		PortMapper:          c.portMapper,
 		UseDNSCache:         true,
@@ -582,7 +576,7 @@ func (c *Conn) updateEndpoints(why string) {
 		c.muCond.Broadcast()
 	}()
 	c.dlogf("[v1] magicsock: starting endpoint update (%s)", why)
-	if c.noV4Send.Load() && runtime.GOOS != "js" {
+	if c.noV4Send.Load() && runtime.GOOS != "js" && !c.onlyTCP443.Load() {
 		c.mu.Lock()
 		closed := c.closed
 		c.mu.Unlock()
@@ -1175,6 +1169,24 @@ func (c *Conn) maybeRebindOnError(os string, err error) bool {
 	return false
 }
 
+// sendUDPNetcheck sends b via UDP to addr. It is used exclusively by netcheck.
+// It returns the number of bytes sent along with any error encountered. It
+// returns errors.ErrUnsupported if the client is explicitly configured to only
+// send data over TCP port 443 and/or we're running on wasm.
+func (c *Conn) sendUDPNetcheck(b []byte, addr netip.AddrPort) (int, error) {
+	if c.onlyTCP443.Load() || runtime.GOOS == "js" {
+		return 0, errors.ErrUnsupported
+	}
+	switch {
+	case addr.Addr().Is4():
+		return c.pconn4.WriteToUDPAddrPort(b, addr)
+	case addr.Addr().Is6():
+		return c.pconn6.WriteToUDPAddrPort(b, addr)
+	default:
+		panic("bogus sendUDPNetcheck addr type")
+	}
+}
+
 // sendUDP sends UDP packet b to addr.
 // See sendAddr's docs on the return value meanings.
 func (c *Conn) sendUDPStd(addr netip.AddrPort, b []byte) (sent bool, err error) {