fix(netxlite): prefer composition over embedding (#733)

This diff has been extracted and adapted from 8848c8c516

The reason to prefer composition over embedding is that we want the
build to break if we add new methods to interfaces we define. If the build
does not break, we may forget about wrapping methods we should
actually be wrapping. I noticed this issue inside netxlite when I was working
on websteps-illustrated and I added support for NS and PTR queries.

See https://github.com/ooni/probe/issues/2096

While there, perform comprehensive netxlite code review
and apply minor changes and improve the docs.

internal/netxlite/tls.go

@@ -1,5 +1,9 @@
 package netxlite
 
+//
+// TLS implementation
+//
+
 import (
 	"context"
 	"crypto/tls"
@@ -11,8 +15,12 @@ import (
 
 	oohttp "github.com/ooni/oohttp"
 	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
 )
 
+// TODO(bassosimone): check whether there's now equivalent functionality
+// inside the standard library allowing us to map numbers to names.
+
 var (
 	tlsVersionString = map[uint16]string{
 		tls.VersionTLS10: "TLSv1",
@@ -81,7 +89,8 @@ func NewDefaultCertPool() *x509.CertPool {
 	pool := x509.NewCertPool()
 	// Assumption: AppendCertsFromPEM cannot fail because we
 	// have a test in certify_test.go that guarantees that
-	pool.AppendCertsFromPEM([]byte(pemcerts))
+	ok := pool.AppendCertsFromPEM([]byte(pemcerts))
+	runtimex.PanicIfFalse(ok, "pool.AppendCertsFromPEM failed")
 	return pool
 }
 
@@ -201,8 +210,8 @@ var defaultTLSHandshaker = &tlsHandshakerConfigurable{}
 
 // tlsHandshakerLogger is a TLSHandshaker with logging.
 type tlsHandshakerLogger struct {
-	model.TLSHandshaker
-	model.DebugLogger
+	TLSHandshaker model.TLSHandshaker
+	DebugLogger   model.DebugLogger
 }
 
 var _ model.TLSHandshaker = &tlsHandshakerLogger{}
@@ -313,7 +322,7 @@ func NewSingleUseTLSDialer(conn TLSConn) model.TLSDialer {
 // tlsDialerSingleUseAdapter adapts dialerSingleUse to
 // be a TLSDialer type rather than a Dialer type.
 type tlsDialerSingleUseAdapter struct {
-	model.Dialer
+	Dialer model.Dialer
 }
 
 var _ model.TLSDialer = &tlsDialerSingleUseAdapter{}
@@ -323,9 +332,13 @@ func (d *tlsDialerSingleUseAdapter) DialTLSContext(ctx context.Context, network,
 	return d.Dialer.DialContext(ctx, network, address)
 }
 
+func (d *tlsDialerSingleUseAdapter) CloseIdleConnections() {
+	d.Dialer.CloseIdleConnections()
+}
+
 // tlsHandshakerErrWrapper wraps the returned error to be an OONI error
 type tlsHandshakerErrWrapper struct {
-	model.TLSHandshaker
+	TLSHandshaker model.TLSHandshaker
 }
 
 // Handshake implements TLSHandshaker.Handshake
@@ -334,7 +347,7 @@ func (h *tlsHandshakerErrWrapper) Handshake(
 ) (net.Conn, tls.ConnectionState, error) {
 	tlsconn, state, err := h.TLSHandshaker.Handshake(ctx, conn, config)
 	if err != nil {
-		return nil, tls.ConnectionState{}, NewErrWrapper(
+		return nil, tls.ConnectionState{}, newErrWrapper(
 			classifyTLSHandshakeError, TLSHandshakeOperation, err)
 	}
 	return tlsconn, state, nil