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>

spec/unit/utils.spec.js

@@ -282,4 +282,30 @@ describe("utils", function() {
             expect(target.nonenumerableProp).toBe(undefined);
         });
     });
+
+    describe("chunkPromises", function() {
+        it("should execute promises in chunks", async function() {
+            let promiseCount = 0;
+
+            function fn1() {
+                return new Promise(async function(resolve, reject) {
+                    await utils.sleep(1);
+                    expect(promiseCount).toEqual(0);
+                    ++promiseCount;
+                    resolve();
+                });
+            }
+
+            function fn2() {
+                return new Promise(function(resolve, reject) {
+                    expect(promiseCount).toEqual(1);
+                    ++promiseCount;
+                    resolve();
+                });
+            }
+
+            await utils.chunkPromises([fn1, fn2], 1);
+            expect(promiseCount).toEqual(2);
+        });
+    });
 });