feat(netxmocks): implement mocks for netxlite.Resolver (#398)

While there, make sure we require using &netxmocks.Dialer.

Still part of https://github.com/ooni/probe/issues/1505

internal/netxmocks/dialer.go

@@ -16,8 +16,8 @@ type Dialer struct {
 }
 
 // DialContext implements Dialer.DialContext.
-func (d Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+func (d *Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	return d.MockDialContext(ctx, network, address)
 }
 
-var _ dialer = Dialer{}
+var _ dialer = &Dialer{}

internal/netxmocks/resolver.go

@@ -0,0 +1,34 @@
+package netxmocks
+
+import "context"
+
+// resolver is the interface we expect from a resolver
+type resolver interface {
+	LookupHost(ctx context.Context, domain string) ([]string, error)
+	Network() string
+	Address() string
+}
+
+// Resolver is a mockable Resolver.
+type Resolver struct {
+	MockLookupHost func(ctx context.Context, domain string) ([]string, error)
+	MockNetwork    func() string
+	MockAddress    func() string
+}
+
+// LookupHost implements Resolver.LookupHost.
+func (r *Resolver) LookupHost(ctx context.Context, domain string) ([]string, error) {
+	return r.MockLookupHost(ctx, domain)
+}
+
+// Address implements Resolver.Address.
+func (r *Resolver) Address() string {
+	return r.MockAddress()
+}
+
+// Network implements Resolver.Network.
+func (r *Resolver) Network() string {
+	return r.MockNetwork()
+}
+
+var _ resolver = &Resolver{}

internal/netxmocks/resolver_test.go

@@ -0,0 +1,46 @@
+package netxmocks
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+func TestResolverLookupHost(t *testing.T) {
+	expected := errors.New("mocked error")
+	r := &Resolver{
+		MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+			return nil, expected
+		},
+	}
+	ctx := context.Background()
+	addrs, err := r.LookupHost(ctx, "dns.google")
+	if !errors.Is(err, expected) {
+		t.Fatal("unexpected error", err)
+	}
+	if addrs != nil {
+		t.Fatal("expected nil addr")
+	}
+}
+
+func TestResolverNetwork(t *testing.T) {
+	r := &Resolver{
+		MockNetwork: func() string {
+			return "antani"
+		},
+	}
+	if v := r.Network(); v != "antani" {
+		t.Fatal("unexpected network", v)
+	}
+}
+
+func TestResolverAddress(t *testing.T) {
+	r := &Resolver{
+		MockAddress: func() string {
+			return "1.1.1.1"
+		},
+	}
+	if v := r.Address(); v != "1.1.1.1" {
+		t.Fatal("unexpected address", v)
+	}
+}