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/resolver.go

@@ -1,5 +1,9 @@
 package netxlite
 
+//
+// DNS resolver
+//
+
 import (
 	"context"
 	"errors"
@@ -134,8 +138,8 @@ func (r *resolverSystem) LookupHTTPS(
 
 // resolverLogger is a resolver that emits events
 type resolverLogger struct {
-	model.Resolver
-	Logger model.DebugLogger
+	Resolver model.Resolver
+	Logger   model.DebugLogger
 }
 
 var _ model.Resolver = &resolverLogger{}
@@ -172,13 +176,27 @@ func (r *resolverLogger) LookupHTTPS(
 	return https, nil
 }
 
+func (r *resolverLogger) Address() string {
+	return r.Resolver.Address()
+}
+
+func (r *resolverLogger) Network() string {
+	return r.Resolver.Network()
+}
+
+func (r *resolverLogger) CloseIdleConnections() {
+	r.Resolver.CloseIdleConnections()
+}
+
 // resolverIDNA supports resolving Internationalized Domain Names.
 //
 // See RFC3492 for more information.
 type resolverIDNA struct {
-	model.Resolver
+	Resolver model.Resolver
 }
 
+var _ model.Resolver = &resolverIDNA{}
+
 func (r *resolverIDNA) LookupHost(ctx context.Context, hostname string) ([]string, error) {
 	host, err := idna.ToASCII(hostname)
 	if err != nil {
@@ -196,12 +214,26 @@ func (r *resolverIDNA) LookupHTTPS(
 	return r.Resolver.LookupHTTPS(ctx, host)
 }
 
+func (r *resolverIDNA) Network() string {
+	return r.Resolver.Network()
+}
+
+func (r *resolverIDNA) Address() string {
+	return r.Resolver.Address()
+}
+
+func (r *resolverIDNA) CloseIdleConnections() {
+	r.Resolver.CloseIdleConnections()
+}
+
 // resolverShortCircuitIPAddr recognizes when the input hostname is an
 // IP address and returns it immediately to the caller.
 type resolverShortCircuitIPAddr struct {
-	model.Resolver
+	Resolver model.Resolver
 }
 
+var _ model.Resolver = &resolverShortCircuitIPAddr{}
+
 func (r *resolverShortCircuitIPAddr) LookupHost(ctx context.Context, hostname string) ([]string, error) {
 	if net.ParseIP(hostname) != nil {
 		return []string{hostname}, nil
@@ -222,6 +254,18 @@ func (r *resolverShortCircuitIPAddr) LookupHTTPS(ctx context.Context, hostname s
 	return r.Resolver.LookupHTTPS(ctx, hostname)
 }
 
+func (r *resolverShortCircuitIPAddr) Network() string {
+	return r.Resolver.Network()
+}
+
+func (r *resolverShortCircuitIPAddr) Address() string {
+	return r.Resolver.Address()
+}
+
+func (r *resolverShortCircuitIPAddr) CloseIdleConnections() {
+	r.Resolver.CloseIdleConnections()
+}
+
 // IsIPv6 returns true if the given candidate is a valid IP address
 // representation and such representation is IPv6.
 func IsIPv6(candidate string) (bool, error) {
@@ -271,7 +315,7 @@ func (r *nullResolver) LookupHTTPS(
 
 // resolverErrWrapper is a Resolver that knows about wrapping errors.
 type resolverErrWrapper struct {
-	model.Resolver
+	Resolver model.Resolver
 }
 
 var _ model.Resolver = &resolverErrWrapper{}
@@ -279,7 +323,7 @@ var _ model.Resolver = &resolverErrWrapper{}
 func (r *resolverErrWrapper) LookupHost(ctx context.Context, hostname string) ([]string, error) {
 	addrs, err := r.Resolver.LookupHost(ctx, hostname)
 	if err != nil {
-		return nil, NewErrWrapper(classifyResolverError, ResolveOperation, err)
+		return nil, newErrWrapper(classifyResolverError, ResolveOperation, err)
 	}
 	return addrs, nil
 }
@@ -288,7 +332,19 @@ func (r *resolverErrWrapper) LookupHTTPS(
 	ctx context.Context, domain string) (*model.HTTPSSvc, error) {
 	out, err := r.Resolver.LookupHTTPS(ctx, domain)
 	if err != nil {
-		return nil, NewErrWrapper(classifyResolverError, ResolveOperation, err)
+		return nil, newErrWrapper(classifyResolverError, ResolveOperation, err)
 	}
 	return out, nil
 }
+
+func (r *resolverErrWrapper) Network() string {
+	return r.Resolver.Network()
+}
+
+func (r *resolverErrWrapper) Address() string {
+	return r.Resolver.Address()
+}
+
+func (r *resolverErrWrapper) CloseIdleConnections() {
+	r.Resolver.CloseIdleConnections()
+}