feat(netxlite): implement single use {,tls} dialer (#464)

This basically adapts already existing code inside websteps to
instead be into the netxlite package, where it belongs.

In the process, abstract the TLSDialer but keep a reference to the
previous name to avoid refactoring existing code (just for now).

While there, notice that the right name is CloseIdleConnections (i.e.,
plural not singular) and change the name.

While there, since we abstracted TLSDialer to be an interface, create
suitable factories for making a TLSDialer type from a Dialer and a
TLSHandshaker.

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

internal/netxlite/tls.go

@@ -216,8 +216,29 @@ func (h *tlsHandshakerLogger) Handshake(
 	return tlsconn, state, nil
 }
 
-// TLSDialer is the TLS dialer
-type TLSDialer struct {
+// TLSDialer is a Dialer dialing TLS connections.
+type TLSDialer interface {
+	// CloseIdleConnections closes idle connections, if any.
+	CloseIdleConnections()
+
+	// DialTLSContext dials a TLS connection.
+	DialTLSContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// NewTLSDialer creates a new TLS dialer using the given dialer
+// and TLS handshaker to establish TLS connections.
+func NewTLSDialer(dialer Dialer, handshaker TLSHandshaker) TLSDialer {
+	return NewTLSDialerWithConfig(dialer, handshaker, &tls.Config{})
+}
+
+// NewTLSDialerWithConfig is like NewTLSDialer but takes an optional config
+// parameter containing your desired TLS configuration.
+func NewTLSDialerWithConfig(d Dialer, h TLSHandshaker, c *tls.Config) TLSDialer {
+	return &tlsDialer{Config: c, Dialer: d, TLSHandshaker: h}
+}
+
+// tlsDialer is the TLS dialer
+type tlsDialer struct {
 	// Config is the OPTIONAL tls config.
 	Config *tls.Config
 
@@ -228,13 +249,15 @@ type TLSDialer struct {
 	TLSHandshaker TLSHandshaker
 }
 
-// CloseIdleConnection closes idle connections, if any.
-func (d *TLSDialer) CloseIdleConnection() {
+var _ TLSDialer = &tlsDialer{}
+
+// CloseIdleConnections implements TLSDialer.CloseIdleConnections.
+func (d *tlsDialer) CloseIdleConnections() {
 	d.Dialer.CloseIdleConnections()
 }
 
-// DialTLSContext dials a TLS connection.
-func (d *TLSDialer) DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
+// DialTLSContext implements TLSDialer.DialTLSContext.
+func (d *tlsDialer) DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
 	host, port, err := net.SplitHostPort(address)
 	if err != nil {
 		return nil, err
@@ -258,7 +281,7 @@ func (d *TLSDialer) DialTLSContext(ctx context.Context, network, address string)
 // We set the ServerName field if not already set.
 //
 // We set the ALPN if the port is 443 or 853, if not already set.
-func (d *TLSDialer) config(host, port string) *tls.Config {
+func (d *tlsDialer) config(host, port string) *tls.Config {
 	config := d.Config
 	if config == nil {
 		config = &tls.Config{}
@@ -277,3 +300,22 @@ func (d *TLSDialer) config(host, port string) *tls.Config {
 	}
 	return config
 }
+
+// NewSingleUseTLSDialer is like NewSingleUseDialer but takes
+// in input a TLSConn rather than a net.Conn.
+func NewSingleUseTLSDialer(conn TLSConn) TLSDialer {
+	return &tlsDialerSingleUseAdapter{NewSingleUseDialer(conn)}
+}
+
+// tlsDialerSingleUseAdapter adapts dialerSingleUse to
+// be a TLSDialer type rather than a Dialer type.
+type tlsDialerSingleUseAdapter struct {
+	Dialer
+}
+
+var _ TLSDialer = &tlsDialerSingleUseAdapter{}
+
+// DialTLSContext implements TLSDialer.DialTLSContext.
+func (d *tlsDialerSingleUseAdapter) DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return d.Dialer.DialContext(ctx, network, address)
+}