Wait out the messages the application holds before shutting a session down

src/idle-waiters.ts

@@ -0,0 +1,47 @@
+/** What is left of a budget, in the shape a wait takes it: 0 waits forever. */
+export function leftOf(deadline: number): number {
+	return deadline === 0 ? 0 : Math.max(1, deadline - Date.now());
+}
+
+/** Everything waiting for a count to fall to zero, and how such a wait is cut short. */
+export class IdleWaiters {
+	private readonly waiting: (() => void)[] = [];
+
+	/** Wakes everything waiting, whatever the count reads now. */
+	settle(): void {
+		for (const resolve of this.waiting.splice(0)) {
+			resolve();
+		}
+	}
+
+	/**
+	 * Resolves 0 once nothing is left, or with what still is when the timeout or the signal cuts the
+	 * wait short. A timeout of 0 waits forever.
+	 */
+	wait(remaining: () => number, timeout: number, signal?: AbortSignal): Promise<number> {
+		if (remaining() === 0) return Promise.resolve(0);
+
+		if (signal?.aborted === true) return Promise.resolve(remaining());
+
+		return new Promise<number>(resolve => {
+			let timer: NodeJS.Timeout | undefined = undefined;
+			const done = (): void => {
+				const index = this.waiting.indexOf(done);
+
+				if (timer) clearTimeout(timer);
+				if (index !== -1) this.waiting.splice(index, 1);
+
+				signal?.removeEventListener('abort', done);
+				resolve(remaining());
+			};
+
+			if (timeout > 0) {
+				timer = setTimeout(done, timeout);
+				timer.unref();
+			}
+
+			signal?.addEventListener('abort', done, { once: true });
+			this.waiting.push(done);
+		});
+	}
+}