refactor(netxlite): introduce the getaddrinfo transport (#775)

This diff modifies the system resolver to use a getaddrinf transport.

Obviously the transport is a fake, but its existence will allow us
to observe DNS events more naturally.

A lookup using the system resolver would be a ANY lookup that will
contain all the resolved IP addresses into the same response.

This change was also part of websteps-illustrated, albeit the way in
which I did it there was less clean than what we have here.

Ref issue: https://github.com/ooni/probe/issues/2096

internal/engine/netx/netx.go

@@ -78,7 +78,7 @@ var defaultCertPool *x509.CertPool = netxlite.NewDefaultCertPool()
 // NewResolver creates a new resolver from the specified config
 func NewResolver(config Config) model.Resolver {
 	if config.BaseResolver == nil {
-		config.BaseResolver = &netxlite.ResolverSystem{}
+		config.BaseResolver = netxlite.NewResolverSystem()
 	}
 	var r model.Resolver = config.BaseResolver
 	r = &netxlite.AddressResolver{
@@ -260,7 +260,7 @@ func NewDNSClientWithOverrides(config Config, URL, hostOverride, SNIOverride,
 	}
 	switch resolverURL.Scheme {
 	case "system":
-		return &netxlite.ResolverSystem{}, nil
+		return netxlite.NewResolverSystem(), nil
 	case "https":
 		config.TLSConfig.NextProtos = []string{"h2", "http/1.1"}
 		httpClient := &http.Client{Transport: NewHTTPTransport(config)}

internal/engine/netx/netx_test.go

@@ -32,7 +32,7 @@ func TestNewResolverVanilla(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -82,7 +82,7 @@ func TestNewResolverWithBogonFilter(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -111,7 +111,7 @@ func TestNewResolverWithLogging(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -141,7 +141,7 @@ func TestNewResolverWithSaver(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -170,7 +170,7 @@ func TestNewResolverWithReadWriteCache(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -204,7 +204,7 @@ func TestNewResolverWithPrefilledReadonlyCache(t *testing.T) {
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
-	_, ok = ar.Resolver.(*netxlite.ResolverSystem)
+	_, ok = ar.Resolver.(*netxlite.ResolverSystemDoNotInstantiate)
 	if !ok {
 		t.Fatal("not the resolver we expected")
 	}
@@ -556,7 +556,7 @@ func TestNewDNSClientSystemResolver(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if _, ok := dnsclient.(*netxlite.ResolverSystem); !ok {
+	if _, ok := dnsclient.(*netxlite.ResolverSystemDoNotInstantiate); !ok {
 		t.Fatal("not the resolver we expected")
 	}
 	dnsclient.CloseIdleConnections()
@@ -568,7 +568,7 @@ func TestNewDNSClientEmpty(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if _, ok := dnsclient.(*netxlite.ResolverSystem); !ok {
+	if _, ok := dnsclient.(*netxlite.ResolverSystemDoNotInstantiate); !ok {
 		t.Fatal("not the resolver we expected")
 	}
 	dnsclient.CloseIdleConnections()

internal/engine/netx/resolver/integration_test.go

@@ -64,7 +64,7 @@ func testresolverquickidna(t *testing.T, reso model.Resolver) {
 }
 
 func TestNewResolverSystem(t *testing.T) {
-	reso := &netxlite.ResolverSystem{}
+	reso := netxlite.NewResolverSystem()
 	testresolverquick(t, reso)
 	testresolverquickidna(t, reso)
 }