fix: JSON parsing of S2A addresses. (#1589)

* fix: JSON parsing of S2A addresses.

* extract extra parsing logic to calling method.

* cast once + use defined constant.

Author: rmehta19

oauth2_http/java/com/google/auth/oauth2/SecureSessionAgent.java

@@ -45,6 +45,7 @@
 import java.io.InputStream;
 import java.util.Arrays;
 import java.util.HashSet;
+import java.util.Map;
 import java.util.ServiceLoader;
 import java.util.Set;
 import javax.annotation.concurrent.ThreadSafe;
@@ -59,6 +60,7 @@
  */
 @ThreadSafe
 public class SecureSessionAgent {
+  static final String S2A_JSON_KEY = "s2a";
   static final String S2A_PLAINTEXT_ADDRESS_JSON_KEY = "plaintext_address";
   static final String S2A_MTLS_ADDRESS_JSON_KEY = "mtls_address";
   static final String S2A_CONFIG_ENDPOINT_POSTFIX =
@@ -188,17 +190,25 @@ private SecureSessionAgentConfig getSecureSessionAgentConfigFromMDS() {
 
     String plaintextS2AAddress = "";
     String mtlsS2AAddress = "";
+    Map<String, Object> s2aAddressConfig = (Map<String, Object>) responseData.get(S2A_JSON_KEY);
+    if (s2aAddressConfig == null) {
+      /*
+       * Return empty addresses in {@link SecureSessionAgentConfig} if endpoint doesn't return anything.
+       */
+      return SecureSessionAgentConfig.createBuilder().build();
+    }
     try {
       plaintextS2AAddress =
-          OAuth2Utils.validateString(responseData, S2A_PLAINTEXT_ADDRESS_JSON_KEY, PARSE_ERROR_S2A);
+          OAuth2Utils.validateString(
+              s2aAddressConfig, S2A_PLAINTEXT_ADDRESS_JSON_KEY, PARSE_ERROR_S2A);
     } catch (IOException ignore) {
       /*
        * Do not throw error because of parsing error, just leave the address as empty in {@link SecureSessionAgentConfig}.
        */
     }
     try {
       mtlsS2AAddress =
-          OAuth2Utils.validateString(responseData, S2A_MTLS_ADDRESS_JSON_KEY, PARSE_ERROR_S2A);
+          OAuth2Utils.validateString(s2aAddressConfig, S2A_MTLS_ADDRESS_JSON_KEY, PARSE_ERROR_S2A);
     } catch (IOException ignore) {
       /*
        * Do not throw error because of parsing error, just leave the address as empty in {@link SecureSessionAgentConfig}.

oauth2_http/javatests/com/google/auth/oauth2/MockMetadataServerTransport.java

@@ -300,9 +300,7 @@ public LowLevelHttpResponse execute() throws IOException {
         GenericJson content = new GenericJson();
         content.setFactory(OAuth2Utils.JSON_FACTORY);
         if (requestStatusCode == 200) {
-          for (Map.Entry<String, String> entrySet : s2aContentMap.entrySet()) {
-            content.put(entrySet.getKey(), entrySet.getValue());
-          }
+          content.put(SecureSessionAgent.S2A_JSON_KEY, s2aContentMap);
         }
         String contentText = content.toPrettyString();