feat(netxlite): introduce null dialers (#489)

See https://github.com/ooni/probe/issues/1591

internal/netxlite/dialer.go

@@ -277,3 +277,23 @@ func (c *dialerErrWrapperConn) Close() error {
 	}
 	return nil
 }
+
+// ErrNoDialer indicates that no dialer is configured.
+var ErrNoDialer = errors.New("no configured dialer")
+
+// NewNullDialer returns a dialer that always fails.
+func NewNullDialer() Dialer {
+	return &nullDialer{}
+}
+
+type nullDialer struct{}
+
+var _ Dialer = &nullDialer{}
+
+func (*nullDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return nil, ErrNoDialer
+}
+
+func (*nullDialer) CloseIdleConnections() {
+	// nothing to do
+}

internal/netxlite/dialer_test.go

@@ -462,3 +462,15 @@ func TestDialerErrWrapperConn(t *testing.T) {
 		})
 	})
 }
+
+func TestNewNullDialer(t *testing.T) {
+	dialer := NewNullDialer()
+	conn, err := dialer.DialContext(context.Background(), "", "")
+	if !errors.Is(err, ErrNoDialer) {
+		t.Fatal("unexpected err", err)
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn")
+	}
+	dialer.CloseIdleConnections() // does not crash
+}

internal/netxlite/tls.go

@@ -342,3 +342,23 @@ func (h *tlsHandshakerErrWrapper) Handshake(
 	}
 	return tlsconn, state, nil
 }
+
+// ErrNoTLSDialer indicates that no TLS dialer is configured.
+var ErrNoTLSDialer = errors.New("no configured TLS dialer")
+
+// NewNullTLSDialer returns a TLS dialer that always fails.
+func NewNullTLSDialer() TLSDialer {
+	return &nullTLSDialer{}
+}
+
+type nullTLSDialer struct{}
+
+var _ TLSDialer = &nullTLSDialer{}
+
+func (*nullTLSDialer) DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return nil, ErrNoTLSDialer
+}
+
+func (*nullTLSDialer) CloseIdleConnections() {
+	// nothing to do
+}

internal/netxlite/tls_test.go

@@ -536,3 +536,15 @@ func TestTLSHandshakerErrWrapper(t *testing.T) {
 		})
 	})
 }
+
+func TestNewNullTLSDialer(t *testing.T) {
+	dialer := NewNullTLSDialer()
+	conn, err := dialer.DialTLSContext(context.Background(), "", "")
+	if !errors.Is(err, ErrNoTLSDialer) {
+		t.Fatal("unexpected err", err)
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn")
+	}
+	dialer.CloseIdleConnections() // does not crash
+}