feat(netxlite): introduce wrapping constructors (#507)

This diff has been extracted from https://github.com/ooni/probe-cli/pull/506.

In it, we introduce wrapping constructors for types and we
update the docs. These new constructures are used by the code
in https://github.com/ooni/probe-cli/pull/506.

In itself, this work is part of https://github.com/ooni/probe/issues/1733.

internal/netxlite/dialer.go

@@ -19,8 +19,14 @@ type Dialer interface {
 	CloseIdleConnections()
 }
 
-// NewDialerWithResolver creates a new Dialer. The returned Dialer
-// has the following properties:
+// NewDialerWithResolver is a convenience factory that calls
+// WrapDialer for a stdlib dialer type.
+func NewDialerWithResolver(logger Logger, resolver Resolver) Dialer {
+	return WrapDialer(logger, resolver, &dialerSystem{})
+}
+
+// WrapDialer creates a new Dialer that wraps the given
+// Dialer. The returned Dialer has the following properties:
 //
 // 1. logs events using the given logger;
 //
@@ -45,12 +51,12 @@ type Dialer interface {
 // 6. if a dialer wraps a resolver, the dialer will forward
 // the CloseIdleConnection call to its resolver (which is
 // instrumental to manage a DoH resolver connections properly).
-func NewDialerWithResolver(logger Logger, resolver Resolver) Dialer {
+func WrapDialer(logger Logger, resolver Resolver, dialer Dialer) Dialer {
 	return &dialerLogger{
 		Dialer: &dialerResolver{
 			Dialer: &dialerLogger{
 				Dialer: &dialerErrWrapper{
-					Dialer: &dialerSystem{},
+					Dialer: dialer,
 				},
 				Logger:          logger,
 				operationSuffix: "_address",

internal/netxlite/http.go

@@ -82,11 +82,15 @@ func (txp *httpTransportConnectionsCloser) CloseIdleConnections() {
 	txp.TLSDialer.CloseIdleConnections()
 }
 
-// NewHTTPTransport creates a new HTTP transport using the given
+// NewHTTPTransport combines NewOOHTTPBaseTransport and
+// WrapHTTPTransport to construct a new HTTPTransport.
+func NewHTTPTransport(logger Logger, dialer Dialer, tlsDialer TLSDialer) HTTPTransport {
+	return WrapHTTPTransport(logger, NewOOHTTPBaseTransport(dialer, tlsDialer))
+}
+
+// NewOOHTTPBaseTransport creates a new HTTP transport using the given
 // dialer and TLS dialer to create connections.
 //
-// The returned transport will use the given Logger for logging.
-//
 // The returned transport will gracefully handle TLS connections
 // created using gitlab.com/yawning/utls.git.
 //
@@ -106,10 +110,7 @@ func (txp *httpTransportConnectionsCloser) CloseIdleConnections() {
 // necessary to perform sane measurements with tracing. We will be
 // able to possibly relax this requirement after we change the
 // way in which we perform measurements.
-//
-// The returned transport will set a default user agent if the
-// request has not already set a user agent.
-func NewHTTPTransport(logger Logger, dialer Dialer, tlsDialer TLSDialer) HTTPTransport {
+func NewOOHTTPBaseTransport(dialer Dialer, tlsDialer TLSDialer) HTTPTransport {
 	// Using oohttp to support any TLS library.
 	txp := oohttp.DefaultTransport.(*oohttp.Transport).Clone()
 
@@ -138,16 +139,24 @@ func NewHTTPTransport(logger Logger, dialer Dialer, tlsDialer TLSDialer) HTTPTra
 	// upon us when we are using TLS parroting).
 	txp.ForceAttemptHTTP2 = true
 
-	// Ensure we correctly forward CloseIdleConnections and compose
-	// with a logging transport thus enabling logging.
+	// Ensure we correctly forward CloseIdleConnections.
+	return &httpTransportConnectionsCloser{
+		HTTPTransport: &oohttp.StdlibTransport{Transport: txp},
+		Dialer:        dialer,
+		TLSDialer:     tlsDialer,
+	}
+}
+
+// WrapHTTPTransport creates a new HTTP transport using
+// the given logger for logging.
+//
+// The returned transport will set a default user agent if the
+// request has not already set a user agent.
+func WrapHTTPTransport(logger Logger, txp HTTPTransport) HTTPTransport {
 	return &httpUserAgentTransport{
 		HTTPTransport: &httpTransportLogger{
-			HTTPTransport: &httpTransportConnectionsCloser{
-				HTTPTransport: &oohttp.StdlibTransport{Transport: txp},
-				Dialer:        dialer,
-				TLSDialer:     tlsDialer,
-			},
-			Logger: logger,
+			HTTPTransport: txp,
+			Logger:        logger,
 		},
 	}
 }

internal/netxlite/resolver.go

@@ -25,10 +25,15 @@ type Resolver interface {
 	CloseIdleConnections()
 }
 
-// NewResolverStdlib creates a new resolver using system
-// facilities for resolving domain names (e.g., getaddrinfo).
-//
-// The resolver will provide the following guarantees:
+// NewResolverStdlib creates a new Resolver by combining
+// WrapResolver with an internal "system" resolver type that
+// adds extra functionality to net.Resolver.
+func NewResolverStdlib(logger Logger) Resolver {
+	return WrapResolver(logger, &resolverSystem{})
+}
+
+// WrapResolver creates a new resolver that wraps an
+// existing resolver to add these properties:
 //
 // 1. handles IDNA;
 //
@@ -41,12 +46,12 @@ type Resolver interface {
 //
 // 5. enforces reasonable timeouts (
 // see https://github.com/ooni/probe/issues/1726).
-func NewResolverStdlib(logger Logger) Resolver {
+func WrapResolver(logger Logger, resolver Resolver) Resolver {
 	return &resolverIDNA{
 		Resolver: &resolverLogger{
 			Resolver: &resolverShortCircuitIPAddr{
 				Resolver: &resolverErrWrapper{
-					Resolver: &resolverSystem{},
+					Resolver: resolver,
 				},
 			},
 			Logger: logger,