feat: add additional checks before using S2A (#2103)

internal/cba.go

@@ -91,16 +91,10 @@ func getTransportConfig(settings *DialSettings) (*transportConfig, error) {
 		s2aMTLSEndpoint:  "",
 	}
 
-	// Check the env to determine whether to use S2A.
-	if !isGoogleS2AEnabled() {
+	if !shouldUseS2A(clientCertSource, settings) {
 		return &defaultTransportConfig, nil
 	}
 
-	// If client cert is found, use that over S2A.
-	// If MTLS is not enabled for the endpoint, skip S2A.
-	if clientCertSource != nil || !mtlsEndpointEnabledForS2A() {
-		return &defaultTransportConfig, nil
-	}
 	s2aMTLSEndpoint := settings.DefaultMTLSEndpoint
 	// If there is endpoint override, honor it.
 	if settings.Endpoint != "" {
@@ -118,10 +112,6 @@ func getTransportConfig(settings *DialSettings) (*transportConfig, error) {
 	}, nil
 }
 
-func isGoogleS2AEnabled() bool {
-	return strings.ToLower(os.Getenv(googleAPIUseS2AEnv)) == "true"
-}
-
 // getClientCertificateSource returns a default client certificate source, if
 // not provided by the user.
 //
@@ -275,8 +265,36 @@ func GetHTTPTransportConfigAndEndpoint(settings *DialSettings) (cert.Source, fun
 	return nil, dialTLSContextFunc, config.s2aMTLSEndpoint, nil
 }
 
+func shouldUseS2A(clientCertSource cert.Source, settings *DialSettings) bool {
+	// If client cert is found, use that over S2A.
+	if clientCertSource != nil {
+		return false
+	}
+	// If EXPERIMENTAL_GOOGLE_API_USE_S2A is not set to true, skip S2A.
+	if !isGoogleS2AEnabled() {
+		return false
+	}
+	// If DefaultMTLSEndpoint is not set, skip S2A.
+	if settings.DefaultMTLSEndpoint == "" {
+		return false
+	}
+	// If MTLS is not enabled for this endpoint, skip S2A.
+	if !mtlsEndpointEnabledForS2A() {
+		return false
+	}
+	// If custom HTTP client is provided, skip S2A.
+	if settings.HTTPClient != nil {
+		return false
+	}
+	return true
+}
+
 // mtlsEndpointEnabledForS2A checks if the endpoint is indeed MTLS-enabled, so that we can use S2A for MTLS connection.
 var mtlsEndpointEnabledForS2A = func() bool {
 	// TODO(xmenxk): determine this via discovery config.
 	return true
 }
+
+func isGoogleS2AEnabled() bool {
+	return strings.ToLower(os.Getenv(googleAPIUseS2AEnv)) == "true"
+}

internal/cba_test.go

@@ -6,6 +6,7 @@ package internal
 
 import (
 	"crypto/tls"
+	"net/http"
 	"os"
 	"testing"
 	"time"
@@ -278,6 +279,29 @@ func TestGetHTTPTransportConfigAndEndpoint(t *testing.T) {
 			testOverrideEndpoint,
 			false,
 		},
+		{
+			"no client cert, S2A address not empty, but DefaultMTLSEndpoint is not set",
+			&DialSettings{
+				DefaultMTLSEndpoint: "",
+				DefaultEndpoint:     testRegularEndpoint,
+			},
+			validConfigResp,
+			func() bool { return true },
+			testRegularEndpoint,
+			true,
+		},
+		{
+			"no client cert, endpoint is MTLS enabled, S2A address not empty, custom HTTP client",
+			&DialSettings{
+				DefaultMTLSEndpoint: testMTLSEndpoint,
+				DefaultEndpoint:     testRegularEndpoint,
+				HTTPClient:          http.DefaultClient,
+			},
+			validConfigResp,
+			func() bool { return true },
+			testRegularEndpoint,
+			true,
+		},
 	}
 
 	defer setupTest()()