wgengine/magicsock: prefer IPv6 transport if roughly equivalent latency

Fixes #1566

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

wgengine/magicsock/magicsock.go

@@ -3670,6 +3670,17 @@ func betterAddr(a, b addrLatency) bool {
 	if a.IsZero() {
 		return false
 	}
+	if a.IP.Is6() && b.IP.Is4() {
+		// Prefer IPv6 for being a bit more robust, as long as
+		// the latencies are roughly equivalent.
+		if a.latency/10*9 < b.latency {
+			return true
+		}
+	} else if a.IP.Is4() && b.IP.Is6() {
+		if betterAddr(b, a) {
+			return false
+		}
+	}
 	return a.latency < b.latency
 }
 

wgengine/magicsock/magicsock_test.go

@@ -1867,6 +1867,29 @@ func TestBetterAddr(t *testing.T) {
 		{a: zero, b: al("10.0.0.2:123", 5*ms), want: false},
 		{a: al("10.0.0.2:123", 5*ms), b: al("1.2.3.4:555", 6*ms), want: true},
 		{a: al("10.0.0.2:123", 5*ms), b: al("10.0.0.2:123", 10*ms), want: false}, // same IPPort
+
+		// Prefer IPv6 if roughly equivalent:
+		{
+			a:    al("[2001::5]:123", 100*ms),
+			b:    al("1.2.3.4:555", 91*ms),
+			want: true,
+		},
+		{
+			a:    al("1.2.3.4:555", 91*ms),
+			b:    al("[2001::5]:123", 100*ms),
+			want: false,
+		},
+		// But not if IPv4 is much faster:
+		{
+			a:    al("[2001::5]:123", 100*ms),
+			b:    al("1.2.3.4:555", 30*ms),
+			want: false,
+		},
+		{
+			a:    al("1.2.3.4:555", 30*ms),
+			b:    al("[2001::5]:123", 100*ms),
+			want: true,
+		},
 	}
 	for _, tt := range tests {
 		got := betterAddr(tt.a, tt.b)