Retry transient foreground requests before surfacing errors

frontend/async-guard.js

@@ -35,7 +35,68 @@
     }
   }
 
-  const AsyncGuard = { runExclusive };
+  const wait = ms => new Promise(resolve => setTimeout(resolve, ms));
+  async function retry(task, { attempts = 3, delay = attempt => attempt === 1 ? 350 : 1000,
+    shouldRetry = () => true, sleep = wait } = {}) {
+    let last;
+    for (let attempt = 1; attempt <= Math.max(1, attempts); attempt++) {
+      try { return await task(attempt); }
+      catch (error) {
+        last = error;
+        if (attempt >= attempts || !shouldRetry(error, attempt)) throw error;
+        await sleep(delay(attempt));
+      }
+    }
+    throw last;
+  }
+
+  // Retry only idempotent GETs. Keep the timeout active through JSON parsing so
+  // a stalled response body is treated like a stalled connection.
+  async function getJson(url, options = {}, config = {}) {
+    const fetchFn = config.fetch || root.fetch?.bind(root);
+    if (!fetchFn) throw new Error('fetch is unavailable');
+    if ((options.method || 'GET').toUpperCase() !== 'GET') throw new Error('Retry helper only accepts GET');
+    const attempts = config.attempts ?? 3, timeoutMs = config.timeoutMs ?? 20000;
+    return retry(async () => {
+      const controller = typeof AbortController === 'function' ? new AbortController() : null;
+      let timedOut = false, timer, externalAbort;
+      const external = options.signal;
+      if (controller && external) {
+        if (external.aborted) throw external.reason || new DOMException('Aborted', 'AbortError');
+        externalAbort = () => controller.abort(external.reason);
+        external.addEventListener('abort', externalAbort, { once: true });
+      }
+      if (controller && timeoutMs > 0) timer = setTimeout(() => { timedOut = true; controller.abort(); }, timeoutMs);
+      try {
+        const response = await fetchFn(url, { ...options, ...(controller ? { signal: controller.signal } : {}) });
+        if ([408, 425, 429].includes(response.status) || response.status >= 500) {
+          const error = new Error(`HTTP ${response.status}`); error.retryable = true; throw error;
+        }
+        if (!response.ok) {
+          let message = `HTTP ${response.status}`;
+          try { message = (await response.json()).error || message; } catch {}
+          const error = new Error(message); error.retryable = false; throw error;
+        }
+        const value = await response.json();
+        if (config.retryResult?.(value)) {
+          const error = new Error(value.error || value.message || 'Temporary API failure');
+          error.retryable = true;
+          throw error;
+        }
+        return value;
+      } catch (error) {
+        if (external?.aborted) throw error;
+        if (timedOut) { const timeout = new Error('Request timed out'); timeout.retryable = true; throw timeout; }
+        if (error.retryable === undefined) error.retryable = true;
+        throw error;
+      } finally {
+        if (timer) clearTimeout(timer);
+        if (external && externalAbort) external.removeEventListener('abort', externalAbort);
+      }
+    }, { attempts, shouldRetry: error => error.retryable !== false, sleep: config.sleep || wait });
+  }
+
+  const AsyncGuard = { runExclusive, retry, getJson };
 
   if (typeof module !== 'undefined' && module.exports) {
     module.exports = AsyncGuard;