Initial support for job queue

ts/util/AsyncQueue.ts

@@ -0,0 +1,48 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { once, noop } from 'lodash';
+
+/**
+ * You can do two things with an async queue:
+ *
+ * 1. Put values in.
+ * 2. Consume values out in the order they were added.
+ *
+ * Values are removed from the queue when they're consumed.
+ *
+ * There can only be one consumer, though this could be changed.
+ *
+ * See the tests to see how this works.
+ */
+export class AsyncQueue<T> implements AsyncIterable<T> {
+  private onAdd: () => void = noop;
+
+  private queue: Array<T> = [];
+
+  private isReading = false;
+
+  add(value: Readonly<T>): void {
+    this.queue.push(value);
+    this.onAdd();
+  }
+
+  async *[Symbol.asyncIterator](): AsyncIterator<T> {
+    if (this.isReading) {
+      throw new Error('Cannot iterate over a queue more than once');
+    }
+    this.isReading = true;
+
+    while (true) {
+      yield* this.queue;
+
+      this.queue = [];
+
+      // We want to iterate over the queue in series.
+      // eslint-disable-next-line no-await-in-loop
+      await new Promise<void>(resolve => {
+        this.onAdd = once(resolve);
+      });
+    }
+  }
+}