feat: make maintenance-related 503s more intuitive (#5018)

This makes maintenance-related 503s more intuitive on our UI by
mentioning that maintenance banner is currently enabled.


![image](https://github.com/Unleash/unleash/assets/14320932/43142c58-6b87-4b2d-9239-50f2bb1409e6)

frontend/src/constants/statusCodes.ts

@@ -4,3 +4,4 @@ export const CREATED = 201;
 export const NOT_FOUND = 404;
 export const FORBIDDEN = 403;
 export const UNAUTHORIZED = 401;
+export const UNAVAILABLE = 503;

frontend/src/hooks/api/actions/useApi/useApi.ts

@@ -5,6 +5,7 @@ import {
     NOT_FOUND,
     OK,
     UNAUTHORIZED,
+    UNAVAILABLE,
 } from 'constants/statusCodes';
 import {
     AuthenticationError,
@@ -12,6 +13,7 @@ import {
     ForbiddenError,
     headers,
     NotFoundError,
+    UnavailableError,
 } from 'utils/apiUtils';
 import { formatApiPath } from 'utils/formatPath';
 import { ACCESS_DENIED_TEXT } from 'utils/formatAccessText';
@@ -27,6 +29,7 @@ interface IUseAPI {
     handleNotFound?: ApiErrorHandler;
     handleUnauthorized?: ApiErrorHandler;
     handleForbidden?: ApiErrorHandler;
+    handleUnavailable?: ApiErrorHandler;
     propagateErrors?: boolean;
 }
 
@@ -35,6 +38,7 @@ const useAPI = ({
     handleNotFound,
     handleForbidden,
     handleUnauthorized,
+    handleUnavailable,
     propagateErrors = false,
 }: IUseAPI) => {
     const [errors, setErrors] = useState<Record<string, string>>({});
@@ -104,6 +108,22 @@ const useAPI = ({
                 }
             }
 
+            if (res.status === UNAVAILABLE) {
+                if (handleUnavailable) {
+                    return handleUnavailable(setErrors, res, requestId);
+                } else {
+                    setErrors((prev) => ({
+                        ...prev,
+                        unavailable: 'This operation is unavailable',
+                    }));
+                }
+
+                if (propagateErrors) {
+                    const response = await res.json();
+                    throw new UnavailableError(res.status, response);
+                }
+            }
+
             if (res.status > 399) {
                 const response = await res.json();
                 if (response?.details?.length > 0 && propagateErrors) {

frontend/src/utils/apiUtils.ts

@@ -3,12 +3,17 @@ import {
     FORBIDDEN,
     NOT_FOUND,
     UNAUTHORIZED,
+    UNAVAILABLE,
 } from 'constants/statusCodes';
 
 export interface IErrorBody {
+    message?: string;
     details?: { message: string }[];
 }
 
+const getErrorMessage = (body: IErrorBody) =>
+    body.details?.[0]?.message || body.message;
+
 export class AuthenticationError extends Error {
     statusCode: number;
 
@@ -24,23 +29,31 @@ export class ForbiddenError extends Error {
     body: IErrorBody;
 
     constructor(statusCode: number = FORBIDDEN, body: IErrorBody = {}) {
-        super(
-            body.details?.length
-                ? body.details[0].message
-                : 'You cannot perform this action',
-        );
+        super(getErrorMessage(body) || 'You cannot perform this action');
         this.name = 'ForbiddenError';
         this.statusCode = statusCode;
         this.body = body;
     }
 }
 
+export class UnavailableError extends Error {
+    statusCode: number;
+    body: IErrorBody;
+
+    constructor(statusCode: number = UNAVAILABLE, body: IErrorBody = {}) {
+        super(getErrorMessage(body) || 'This operation is unavailable');
+        this.name = 'UnavailableError';
+        this.statusCode = statusCode;
+        this.body = body;
+    }
+}
+
 export class BadRequestError extends Error {
     statusCode: number;
     body: IErrorBody;
 
     constructor(statusCode: number = BAD_REQUEST, body: IErrorBody = {}) {
-        super(body.details?.length ? body.details[0].message : 'Bad request');
+        super(getErrorMessage(body) || 'Bad request');
         this.name = 'BadRequestError';
         this.statusCode = statusCode;
         this.body = body;

src/lib/middleware/maintenance-middleware.ts

@@ -2,6 +2,9 @@ import { IUnleashConfig } from '../types';
 import MaintenanceService from '../services/maintenance-service';
 import { IAuthRequest } from '../routes/unleash-types';
 
+export const MAINTENANCE_MODE_ENABLED =
+    'Unleash is currently in maintenance mode.';
+
 const maintenanceMiddleware = (
     { getLogger }: Pick<IUnleashConfig, 'getLogger' | 'flagResolver'>,
     maintenanceService: MaintenanceService,
@@ -17,7 +20,9 @@ const maintenanceMiddleware = (
             writeMethod &&
             (await maintenanceService.isMaintenanceMode())
         ) {
-            res.status(503).send({});
+            res.status(503).send({
+                message: MAINTENANCE_MODE_ENABLED,
+            });
         } else {
             next();
         }