fix(netxlite): resolver _always_ short circuits IP addrs (#458)

We will use this in a moment when we will add support for the
dnstransports that currently are in engine/netx.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/resolver.go

@@ -33,8 +33,10 @@ type ResolverConfig struct {
 func NewResolver(config *ResolverConfig) Resolver {
 	return &resolverIDNA{
 		Resolver: &resolverLogger{
-			Resolver: &resolverSystem{},
-			Logger:   config.Logger,
+			Resolver: &resolverShortCircuitIPAddr{
+				Resolver: &resolverSystem{},
+			},
+			Logger: config.Logger,
 		},
 	}
 }
@@ -143,3 +145,17 @@ func (r *resolverIDNA) LookupHost(ctx context.Context, hostname string) ([]strin
 	}
 	return r.Resolver.LookupHost(ctx, host)
 }
+
+// resolverShortCircuitIPAddr recognizes when the input hostname is an
+// IP address and returns it immediately to the caller.
+type resolverShortCircuitIPAddr struct {
+	Resolver
+}
+
+// LookupHost implements Resolver.LookupHost.
+func (r *resolverShortCircuitIPAddr) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	if net.ParseIP(hostname) != nil {
+		return []string{hostname}, nil
+	}
+	return r.Resolver.LookupHost(ctx, hostname)
+}

internal/netxlite/resolver_test.go

@@ -194,7 +194,47 @@ func TestNewResolverTypeChain(t *testing.T) {
 	if rl.Logger != log.Log {
 		t.Fatal("invalid logger")
 	}
-	if _, ok := rl.Resolver.(*resolverSystem); !ok {
+	scia, ok := rl.Resolver.(*resolverShortCircuitIPAddr)
+	if !ok {
+		t.Fatal("invalid resolver")
+	}
+	if _, ok := scia.Resolver.(*resolverSystem); !ok {
 		t.Fatal("invalid resolver")
 	}
 }
+
+func TestResolverShortCircuitIPAddrWithIPAddr(t *testing.T) {
+	r := &resolverShortCircuitIPAddr{
+		Resolver: &mocks.Resolver{
+			MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+				return nil, errors.New("mocked error")
+			},
+		},
+	}
+	ctx := context.Background()
+	addrs, err := r.LookupHost(ctx, "8.8.8.8")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "8.8.8.8" {
+		t.Fatal("invalid result")
+	}
+}
+
+func TestResolverShortCircuitIPAddrWithDomain(t *testing.T) {
+	r := &resolverShortCircuitIPAddr{
+		Resolver: &mocks.Resolver{
+			MockLookupHost: func(ctx context.Context, domain string) ([]string, error) {
+				return nil, errors.New("mocked error")
+			},
+		},
+	}
+	ctx := context.Background()
+	addrs, err := r.LookupHost(ctx, "dns.google")
+	if err == nil || err.Error() != "mocked error" {
+		t.Fatal("not the error we expected", err)
+	}
+	if addrs != nil {
+		t.Fatal("invalid result")
+	}
+}