Fixes and improvements

backend/src/sequentialQueue.ts

@@ -0,0 +1,27 @@
+type Job<T> = () => Promise<T>;
+
+let tail = Promise.resolve();
+let pendingJobs = 0;
+
+export function queueSequential<T>(job: Job<T>): Promise<T> {
+  pendingJobs += 1;
+
+  const run = tail.then(async () => {
+    try {
+      return await job();
+    } finally {
+      pendingJobs -= 1;
+    }
+  });
+
+  tail = run.then(
+    () => undefined,
+    () => undefined
+  );
+
+  return run;
+}
+
+export function queuedJobCount() {
+  return pendingJobs;
+}