Fix: let the config export report a failure

CHANGELOG.md

@@ -36,6 +36,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Exporting the config now reports a failure instead of saving an error response
+  as `stackyard-config.json`.
+
 - A session that expires while you are working in Admin now raises the sign-in
   box over the page, and the interrupted save completes once you are back in,
   instead of failing as "Unauthorised" until the page is reloaded.

ui/js/admin.js

@@ -10,7 +10,7 @@ import {
   snapshotItems,
   saveWithRevert,
 } from '/js/admin-save-logic.js?v=77cac1d1';
-import { API, toast, ag, ap, initInlineEdit, setReauthHandler } from '/js/admin-shared.js?v=bb36dbb3';
+import { toast, ag, ap, initInlineEdit, setReauthHandler } from '/js/admin-shared.js?v=bb36dbb3';
 import { checkAuth, requireLogin, wirePasswordStrength } from '/js/admin-auth.js?v=dd849d4c';
 import { state } from '/js/admin-state.js?v=3f9ad806';
 import { buildWidgetForm } from '/js/admin-widget-form.js?v=653071f0';
@@ -1232,16 +1232,29 @@ function initLanguage() {
 const dashSaveEl = el('dash-save');
 if (dashSaveEl) dashSaveEl.onclick = () => save();
 
+/* Fetched rather than reached by navigating a link. A link hands the request to
+   the browser, so the page never learns the outcome: a refusal or an expired
+   session saved an error body under the backup's own filename, and the catch
+   here could not see it. Going through `ag` also means an expired session
+   raises the sign-in box and the export finishes afterwards. */
 el('btn-exp').onclick = async () => {
+  let url;
   try {
+    const config = await ag('/api/config/export');
+    const blob = new Blob([JSON.stringify(config, null, 2)], { type: 'application/json' });
+    url = URL.createObjectURL(blob);
     const a = document.createElement('a');
-    a.href = API + '/api/config/export';
+    a.href = url;
     a.download = 'stackyard-config.json';
     document.body.appendChild(a);
     a.click();
     document.body.removeChild(a);
   } catch (e) {
     toast(t('toast.exportFailed', { err: e.message }), 'err');
+  } finally {
+    /* Revoked on the next frame: revoking it in this one races the download the
+       click just started, and the browser keeps nothing to save. */
+    if (url) requestAnimationFrame(() => URL.revokeObjectURL(url));
   }
 };
 el('imp').onchange = async e => {

ui/test/config-export.test.mjs

@@ -0,0 +1,65 @@
+/* Regression test for the config export, which could not report a failure.
+
+   The export set an anchor's href to /api/config/export and clicked it, which
+   hands the request to the browser. The page then never learns the outcome: a
+   refusal, a server error or an expired session was written to disk as an error
+   body under the backup's own filename, and the try/catch around the click
+   could not observe any of it. The failure surfaces when the backup is needed,
+   which is the worst moment to find out.
+
+   Fetching it instead means the response is checked before anything is saved,
+   and going through `ag` means an expired session raises the sign-in box and
+   the export finishes afterwards.
+
+   Scanned from the source rather than driven, because the download itself is
+   browser machinery: what matters is which of the two routes the code takes. */
+
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
+const admin = fs.readFileSync(path.join(root, 'js/admin.js'), 'utf8');
+
+/* The handler only, so an unrelated mention of the endpoint elsewhere in the
+   file cannot pass or fail this. */
+function exportHandler() {
+  const start = admin.indexOf("el('btn-exp').onclick");
+  assert.notEqual(start, -1, 'the export handler moved; this test needs updating');
+  const end = admin.indexOf("el('imp').onchange", start);
+  assert.notEqual(end, -1, 'the export handler moved; this test needs updating');
+  return admin.slice(start, end);
+}
+
+test('the export never reaches the endpoint by navigating to it', () => {
+  const body = exportHandler();
+  assert.ok(
+    !/\.href\s*=\s*[^;]*\/api\/config\/export/.test(body),
+    'assigning the endpoint to an href hands the request to the browser, which cannot report a failure',
+  );
+});
+
+test('the export goes through the shared helper, so a failure is visible', () => {
+  const body = exportHandler();
+  assert.match(
+    body,
+    /await ag\(\s*'\/api\/config\/export'\s*\)/,
+    'the response has to be checked before anything is saved',
+  );
+});
+
+test('the export reports a failure it can now see', () => {
+  assert.match(exportHandler(), /toast\(\s*t\('toast\.exportFailed'/);
+});
+
+test('the object URL is released rather than leaked', () => {
+  const body = exportHandler();
+  assert.match(body, /createObjectURL/);
+  assert.match(body, /revokeObjectURL/, 'a blob URL held for the page lifetime pins the whole config in memory');
+});
+
+test('the download keeps the filename users already know', () => {
+  assert.match(exportHandler(), /download\s*=\s*'stackyard-config\.json'/);
+});