Backport to branch(3) : Add AuthAdmin.getRole(roleName) for RBAC (#3245)

Co-authored-by: Mitsunori Komatsu <komamitsu@gmail.com>

Author: feeblefakie

core/src/main/java/com/scalar/db/api/AuthAdmin.java

@@ -197,6 +197,17 @@ default void dropRole(String roleName) throws ExecutionException {
     throw new UnsupportedOperationException(CoreError.AUTH_NOT_ENABLED.buildMessage());
   }
 
+  /**
+   * Retrieves a {@link Role}.
+   *
+   * @param roleName the role name
+   * @return a {@link Role} for the given role name
+   * @throws ExecutionException if the operation fails
+   */
+  default Optional<Role> getRole(String roleName) throws ExecutionException {
+    throw new UnsupportedOperationException(CoreError.AUTH_NOT_ENABLED.buildMessage());
+  }
+
   /**
    * Retrieves a list of {@link Role}s.
    *

core/src/main/java/com/scalar/db/common/DecoratedDistributedTransactionAdmin.java

@@ -382,6 +382,11 @@ public void dropRole(String roleName) throws ExecutionException {
     distributedTransactionAdmin.dropRole(roleName);
   }
 
+  @Override
+  public Optional<Role> getRole(String roleName) throws ExecutionException {
+    return distributedTransactionAdmin.getRole(roleName);
+  }
+
   @Override
   public List<Role> getRoles() throws ExecutionException {
     return distributedTransactionAdmin.getRoles();