tsnet: be stricter about arguments to Server.Listen

Fixes #6201

Change-Id: I14b2b8ce9bee838344a3fad4f305c78ab775f72e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tsnet/tsnet_test.go

@@ -4,7 +4,10 @@
 
 package tsnet
 
-import "testing"
+import (
+	"errors"
+	"testing"
+)
 
 // TestListener_Server ensures that the listener type always keeps the Server
 // method, which is used by some external applications to identify a tsnet.Listener
@@ -16,3 +19,29 @@ func TestListener_Server(t *testing.T) {
 		t.Errorf("listener.Server() returned %v, want %v", ln.Server(), s)
 	}
 }
+
+func TestListenerPort(t *testing.T) {
+	errNone := errors.New("sentinel start error")
+
+	tests := []struct {
+		network string
+		addr    string
+		wantErr bool
+	}{
+		{"tcp", ":80", false},
+		{"foo", ":80", true},
+		{"tcp", ":http", false},  // built-in name to Go; doesn't require cgo, /etc/services
+		{"tcp", ":https", false}, // built-in name to Go; doesn't require cgo, /etc/services
+		{"tcp", ":gibberishsdlkfj", true},
+		{"tcp", ":%!d(string=80)", true}, // issue 6201
+	}
+	for _, tt := range tests {
+		s := &Server{}
+		s.initOnce.Do(func() { s.initErr = errNone })
+		_, err := s.Listen(tt.network, tt.addr)
+		gotErr := err != nil && err != errNone
+		if gotErr != tt.wantErr {
+			t.Errorf("Listen(%q, %q) error = %v, want %v", tt.network, tt.addr, gotErr, tt.wantErr)
+		}
+	}
+}