work towards fixing [1239]

[1239]: https://issues.apache.org/jira/browse/GUACAMOLE-1239

guacamole-ext/src/main/java/org/apache/guacamole/environment/Environment.java

@@ -68,6 +68,14 @@ public interface Environment {
 
     };
 
+
+    public static final StringGuacamoleProperty USERNAME_LETTER_CASE = new StringGuacamoleProperty() {
+
+        @Override
+        public String getName() { return "username-letter-case"; }
+
+    };
+
     /**
      * Returns the Guacamole home directory as determined when this Environment
      * object was created. The Guacamole home directory is found by checking, in

guacamole/src/main/java/org/apache/guacamole/rest/auth/AuthenticationService.java

@@ -21,6 +21,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Optional;
 import java.util.regex.Pattern;
 import javax.inject.Inject;
 import javax.servlet.http.HttpServletRequest;
@@ -54,6 +55,7 @@ public class AuthenticationService {
      * Logger for this class.
      */
     private static final Logger logger = LoggerFactory.getLogger(AuthenticationService.class);
+    private static final String DEFAULT_LETTER_CASE = "mixed-case";
 
     /**
      * The Guacamole server environment.
@@ -462,6 +464,8 @@ public String authenticate(Credentials credentials, String token)
         else
             existingSession = null;
 
+        adaptUsernameLetterCase(credentials);
+
         // Get up-to-date AuthenticatedUser and associated UserContexts
         AuthenticatedUser authenticatedUser = getAuthenticatedUser(existingSession, credentials);
         List<DecoratedUserContext> userContexts = getUserContexts(existingSession, authenticatedUser, credentials);
@@ -485,6 +489,35 @@ public String authenticate(Credentials credentials, String token)
 
     }
 
+    /**
+     * Alters letter casing of username, if configured.
+     * Configuration options:
+     * <ul>
+     *     <li>mixed-case (default)</li>
+     *     <li>lower-case</li>
+     *     <li>upper-case</li>
+     * </ul>
+     * @param credentials
+     *     The Credentials provided by the user in the most recent
+     *     authentication attempt.
+     * @throws GuacamoleException If an error occurs while parsing the value
+     *                            for the given property in
+     *                            guacamole.properties, or if the property is
+     *                            not specified.
+     */
+    private void adaptUsernameLetterCase(Credentials credentials) throws GuacamoleException {
+        switch (environment.getProperty(Environment.USERNAME_LETTER_CASE,DEFAULT_LETTER_CASE)){
+            case "lower-case":
+                Optional.ofNullable(credentials.getUsername()).map(String::toLowerCase).ifPresent(credentials::setUsername);
+                break;
+            case "upper-case":
+                Optional.ofNullable(credentials.getUsername()).filter(un -> !"guacadmin".equals(un)).map(String::toUpperCase).ifPresent(credentials::setUsername);
+                break;
+            default:
+                /* don't change */
+        }
+    }
+
     /**
      * Finds the Guacamole session for a given auth token, if the auth token
      * represents a currently logged in user. Throws an unauthorized error