Download device keys in chunks of 250

Depending on the number of users in the request, the server might
overload. To prevent this, the download is broken into chunks of
250 users each. Additionally, no more than 3 requests are kicked off
at the same time to avoid running into rate limiting. Responses are
processed once all chunks have been downloaded.

Fixes: #1619

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

src/utils.ts

@@ -745,6 +745,15 @@ export function promiseTry<T>(fn: () => T): Promise<T> {
     return new Promise((resolve) => resolve(fn()));
 }
 
+// Creates and awaits all promises, running no more than `chunkSize` at the same time
+export async function chunkPromises<T>(fns: (() => Promise<T>)[], chunkSize: number): Promise<T[]> {
+    const results: T[] = [];
+    for (let i = 0; i < fns.length; i += chunkSize) {
+        results.push(...(await Promise.all(fns.slice(i, i + chunkSize).map(fn => fn()))));
+    }
+    return results;
+}
+
 // We need to be able to access the Node.js crypto library from within the
 // Matrix SDK without needing to `require("crypto")`, which will fail in
 // browsers.  So `index.ts` will call `setCrypto` to store it, and when we need