fix(studio): poll background generation and handle gateway failures

packages/design-studio/src/generation-transport.ts

@@ -0,0 +1,72 @@
+/** Read gateway failures without exposing an HTML error page as a JSON parse error. */
+export async function generationResponse(response: Response): Promise<any> {
+  const text = await response.text();
+  let value: any;
+  try {
+    value = JSON.parse(text);
+  } catch {
+    throw Error(
+      [502, 503, 504].includes(response.status)
+        ? `The deployment gateway could not complete this request (HTTP ${response.status}). Try again shortly.`
+        : `The generation server returned an unreadable response (HTTP ${response.status}). Reload after deploying the latest server.`,
+    );
+  }
+  if (!response.ok)
+    throw Error(
+      typeof value?.error === "string"
+        ? value.error
+        : `Generation request failed (HTTP ${response.status}).`,
+    );
+  return value;
+}
+
+export async function requestGeneration(
+  url: URL,
+  input: unknown,
+  signal: AbortSignal,
+  transport: typeof fetch = fetch,
+) {
+  let jobUrl: URL | undefined;
+  const deadline = AbortSignal.any([signal, AbortSignal.timeout(75_000)]);
+  try {
+    let result = await generationResponse(
+      await transport(url, {
+        method: "POST",
+        headers: { "content-type": "application/json", prefer: "respond-async" },
+        body: JSON.stringify(input),
+        signal: deadline,
+      }),
+    );
+    if (!result.job) return result; // Compatible with local engines and older synchronous servers.
+    if (typeof result.job.id !== "string" || !/^[a-f0-9-]{36}$/.test(result.job.id))
+      throw Error("Invalid generation job response.");
+    jobUrl = new URL(url.href.replace(/\/$/, "") + "/jobs/" + result.job.id);
+    while (true) {
+      await new Promise<void>((resolve, reject) => {
+        const aborted = () => {
+          clearTimeout(timer);
+          reject(deadline.reason);
+        };
+        const timer = setTimeout(() => {
+          deadline.removeEventListener("abort", aborted);
+          resolve();
+        }, 1000);
+        if (deadline.aborted) aborted();
+        else deadline.addEventListener("abort", aborted, { once: true });
+      });
+      result = await generationResponse(
+        await transport(jobUrl, { signal: deadline, cache: "no-store" }),
+      );
+      if (result.status === "complete") return result.result;
+      if (result.status === "failed" || result.status === "cancelled")
+        throw Error(result.error ?? "Generation did not complete.");
+      if (result.status !== "pending") throw Error("Invalid generation job status.");
+    }
+  } finally {
+    if (jobUrl && deadline.aborted) {
+      void transport(jobUrl, { method: "DELETE", signal: AbortSignal.timeout(3000) }).catch(
+        () => {},
+      );
+    }
+  }
+}