net/tsaddr: simplify TailscaleServiceIP

netaddr allocated at the time this was written. No longer.

name                    old time/op  new time/op  delta
TailscaleServiceAddr-4  5.46ns ± 4%  1.83ns ± 3%  -66.52%  (p=0.008 n=5+5)

A bunch of the others can probably be simplified too, but this
was the only one with just an IP and not an IPPrefix.

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

net/tsaddr/tsaddr.go

@@ -41,12 +41,9 @@ var (
 // TailscaleServiceIP returns the listen address of services
 // provided by Tailscale itself such as the MagicDNS proxy.
 func TailscaleServiceIP() netaddr.IP {
-	serviceIP.Do(func() { mustIP(&serviceIP.v, "100.100.100.100") })
-	return serviceIP.v
+	return netaddr.IPv4(100, 100, 100, 100) // "100.100.100.100" for those grepping
 }
 
-var serviceIP onceIP
-
 // IsTailscaleIP reports whether ip is an IP address in a range that
 // Tailscale assigns from.
 func IsTailscaleIP(ip netaddr.IP) bool {
@@ -126,19 +123,6 @@ type oncePrefix struct {
 	v netaddr.IPPrefix
 }
 
-func mustIP(v *netaddr.IP, ip string) {
-	var err error
-	*v, err = netaddr.ParseIP(ip)
-	if err != nil {
-		panic(err)
-	}
-}
-
-type onceIP struct {
-	sync.Once
-	v netaddr.IP
-}
-
 // NewContainsIPFunc returns a func that reports whether ip is in addrs.
 //
 // It's optimized for the cases of addrs being empty and addrs

net/tsaddr/tsaddr_test.go

@@ -93,3 +93,11 @@ func TestNewContainsIPFunc(t *testing.T) {
 		t.Fatal("bad")
 	}
 }
+
+var sinkIP netaddr.IP
+
+func BenchmarkTailscaleServiceAddr(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		sinkIP = TailscaleServiceIP()
+	}
+}