portlist: ignore ports bound to localhost

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

portlist/portlist_test.go

@@ -4,7 +4,10 @@
 
 package portlist
 
-import "testing"
+import (
+	"net"
+	"testing"
+)
 
 func TestGetList(t *testing.T) {
 	pl, err := GetList(nil)
@@ -17,6 +20,25 @@ func TestGetList(t *testing.T) {
 	t.Logf("As String: %v", pl.String())
 }
 
+func TestIgnoreLocallyBoundPorts(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Skipf("failed to bind: %v", err)
+	}
+	defer ln.Close()
+	ta := ln.Addr().(*net.TCPAddr)
+	port := ta.Port
+	pl, err := GetList(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, p := range pl {
+		if p.Proto == "tcp" && int(p.Port) == port {
+			t.Fatal("didn't expect to find test's localhost ephemeral port")
+		}
+	}
+}
+
 func BenchmarkGetList(b *testing.B) {
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {