Show messages for users if the ROOT_URL is wrong, show JavaScript errors (#18971)

* ROOT_URL issues: some users did wrong to there app.ini config, then:
    * The assets can not be loaded (AppSubUrl != "" and users try to access http://host:3000/)
    *The ROOT_URL is wrong, then many URLs in Gitea are broken.
Now Gitea show enough information to users.

* JavaScript error issues, there are many users affected by JavaScript errors, some are caused by frontend bugs, some are caused by broken customized templates. If these JS errors can be found at first time, then maintainers do not need to ask about how bug occurs again and again.

* Some people like to modify the `head.tmpl`, so we separate the script part to `head_script.tmpl`, then it's much safer.

* use specialized CSS class "js-global-error", end users still have a chance to hide error messages by customized CSS styles.

4 files changed, 65 insertions, 9 deletions

diff --git a/web_src/js/bootstrap.js b/web_src/js/bootstrap.js
new file mode 100644
index 0000000000..cf13b2a559
--- /dev/null
+++ b/web_src/js/bootstrap.js
@@ -0,0 +1,41 @@
+import {joinPaths} from './utils.js';
+
+// DO NOT IMPORT window.config HERE!
+// to make sure the error handler always works, we should never import `window.config`, because some user's custom template breaks it.
+
+// This sets up the URL prefix used in webpack's chunk loading.
+// This file must be imported before any lazy-loading is being attempted.
+__webpack_public_path__ = joinPaths(window?.config?.assetUrlPrefix ?? '/', '/');
+
+export function showGlobalErrorMessage(msg) {
+  const pageContent = document.querySelector('.page-content');
+  if (!pageContent) return;
+  const el = document.createElement('div');
+  el.innerHTML = `<div class="ui container negative message center aligned js-global-error" style="white-space: pre-line;"></div>`;
+  el.childNodes[0].textContent = msg;
+  pageContent.prepend(el.childNodes[0]);
+}
+
+/**
+ * @param {ErrorEvent} e
+ */
+function processWindowErrorEvent(e) {
+  showGlobalErrorMessage(`JavaScript error: ${e.message} (${e.filename} @ ${e.lineno}:${e.colno}). Open browser console to see more details.`);
+}
+
+function initGlobalErrorHandler() {
+  if (!window.config) {
+    showGlobalErrorMessage(`Gitea JavaScript code couldn't run correctly, please check your custom templates`);
+  }
+
+  // we added an event handler for window error at the very beginning of <script> of page head
+  // the handler calls `_globalHandlerErrors.push` (array method) to record all errors occur before this init
+  // then in this init, we can collect all error events and show them
+  for (const e of window._globalHandlerErrors || []) {
+    processWindowErrorEvent(e);
+  }
+  // then, change _globalHandlerErrors to an object with push method, to process further error events directly
+  window._globalHandlerErrors = {'push': (e) => processWindowErrorEvent(e)};
+}
+
+initGlobalErrorHandler();
diff --git a/web_src/js/features/common-global.js b/web_src/js/features/common-global.js
index a9baf9be0c..60af4d0d67 100644
--- a/web_src/js/features/common-global.js
+++ b/web_src/js/features/common-global.js
@@ -3,8 +3,9 @@ import 'jquery.are-you-sure';
 import {mqBinarySearch} from '../utils.js';
 import createDropzone from './dropzone.js';
 import {initCompColorPicker} from './comp/ColorPicker.js';
+import {showGlobalErrorMessage} from '../bootstrap.js';
 
-const {csrfToken} = window.config;
+const {appUrl, csrfToken} = window.config;
 
 export function initGlobalFormDirtyLeaveConfirm() {
   // Warn users that try to leave a page after entering data into a form.
@@ -343,3 +344,20 @@ export function initGlobalButtons() {
     });
   });
 }
+
+/**
+ * Too many users set their ROOT_URL to wrong value, and it causes a lot of problems:
+ *   * Cross-origin API request without correct cookie
+ *   * Incorrect href in <a>
+ *   * ...
+ * So we check whether current URL starts with AppUrl(ROOT_URL).
+ * If they don't match, show a warning to users.
+ */
+export function checkAppUrl() {
+  const curUrl = window.location.href;
+  if (curUrl.startsWith(appUrl)) {
+    return;
+  }
+  showGlobalErrorMessage(`Your ROOT_URL in app.ini is ${appUrl} but you are visiting ${curUrl}
+You should set ROOT_URL correctly, otherwise the web may not work correctly.`);
+}
diff --git a/web_src/js/index.js b/web_src/js/index.js
index b7eba5e664..18b949e4e6 100644
--- a/web_src/js/index.js
+++ b/web_src/js/index.js
@@ -1,4 +1,5 @@
-import './publicpath.js';
+// bootstrap module must be the first one to be imported, it handles webpack lazy-loading and global errors
+import './bootstrap.js';
 
 import $ from 'jquery';
 import {initVueEnv} from './components/VueComponentLoader.js';
@@ -39,6 +40,7 @@ import {
 } from './features/repo-issue.js';
 import {initRepoEllipsisButton, initRepoCommitLastCommitLoader} from './features/repo-commit.js';
 import {
+  checkAppUrl,
   initFootLanguageMenu,
   initGlobalButtonClickOnEnter,
   initGlobalButtons,
@@ -82,7 +84,6 @@ $.fn.tab.settings.silent = true;
 $.fn.checkbox.settings.enableEnterKey = false;
 
 initVueEnv();
-
 $(document).ready(() => {
   initGlobalCommon();
 
@@ -169,4 +170,6 @@ $(document).ready(() => {
   initUserAuthWebAuthn();
   initUserAuthWebAuthnRegister();
   initUserSettings();
+
+  checkAppUrl();
 });
diff --git a/web_src/js/publicpath.js b/web_src/js/publicpath.js
deleted file mode 100644
index 44448a8447..0000000000
--- a/web_src/js/publicpath.js
+++ /dev/null
@@ -1,6 +0,0 @@
-// This sets up the URL prefix used in webpack's chunk loading.
-// This file must be imported before any lazy-loading is being attempted.
-import {joinPaths} from './utils.js';
-const {assetUrlPrefix} = window.config;
-
-__webpack_public_path__ = joinPaths(assetUrlPrefix, '/');