fix: send PostgreSQL v3 StartupMessage in proactive IAM refresh probe for MCP

core/src/main/java/com/google/cloud/sql/core/ConnectionInfoCache.java

@@ -37,4 +37,6 @@ interface ConnectionInfoCache {
   boolean isClosed();
 
   ConnectionConfig getConfig();
+
+  void recordIamPrincipal(String user, String database);
 }

core/src/main/java/com/google/cloud/sql/core/ConnectionInfoRepository.java

@@ -31,11 +31,33 @@ ListenableFuture<ConnectionInfo> getConnectionInfo(
       ListeningScheduledExecutorService executor,
       ListenableFuture<KeyPair> keyPair);
 
+  default ListenableFuture<ConnectionInfo> getConnectionInfo(
+      CloudSqlInstanceName instanceName,
+      AccessTokenSupplier accessTokenSupplier,
+      AuthType authType,
+      ListeningScheduledExecutorService executor,
+      ListenableFuture<KeyPair> keyPair,
+      java.util.List<com.google.cloud.sql.IpType> ipTypes) {
+    return getConnectionInfo(instanceName, accessTokenSupplier, authType, executor, keyPair);
+  }
+
   ConnectionInfo getConnectionInfoSync(
       CloudSqlInstanceName instanceName,
       AccessTokenSupplier accessTokenSupplier,
       AuthType authType,
       KeyPair keyPair);
 
+  default ConnectionInfo getConnectionInfoSync(
+      CloudSqlInstanceName instanceName,
+      AccessTokenSupplier accessTokenSupplier,
+      AuthType authType,
+      KeyPair keyPair,
+      java.util.List<com.google.cloud.sql.IpType> ipTypes) {
+    return getConnectionInfoSync(instanceName, accessTokenSupplier, authType, keyPair);
+  }
+
+  default void recordIamPrincipal(
+      CloudSqlInstanceName instanceName, String user, String database) {}
+
   String resolveConnectionName(String region, String dnsName);
 }

core/src/main/java/com/google/cloud/sql/core/Connector.java

@@ -313,6 +313,9 @@ private Socket connectDirect(ConnectionConfig config, long timeoutMs) throws IOE
           && !Strings.isNullOrEmpty(config.getMdxClientProtocolType())) {
         socket = mdxProtocolHandler.connect(socket, config.getMdxClientProtocolType());
       }
+      if (config.getAuthType() == com.google.cloud.sql.AuthType.IAM) {
+        socket = new PostgresStartupSnifferSocket(socket, instance::recordIamPrincipal);
+      }
 
       logger.debug(String.format("[%s] Connected to instance successfully.", successfulIp));
       instance.addSocket(socket);