Drain the requests already on the wire before close() and unbind() tear down

src/send-window.ts

@@ -2,6 +2,7 @@
 export class SendWindow {
 	private readonly limit: number;
 	private readonly waiting: (() => void)[] = [];
+	private readonly waitingForIdle: (() => void)[] = [];
 	private inFlight = 0;
 
 	constructor(limit: number) {
@@ -28,5 +29,35 @@ export class SendWindow {
 		}
 
 		this.inFlight--;
+
+		if (this.inFlight > 0) return;
+
+		for (const resolve of this.waitingForIdle.splice(0)) {
+			resolve();
+		}
+	}
+
+	/** Resolves once nothing is in flight, or on the timeout with how many still are. 0 never times out. */
+	idle(timeout: number): Promise<number> {
+		if (this.inFlight === 0) return Promise.resolve(0);
+
+		return new Promise<number>(resolve => {
+			let timer: NodeJS.Timeout | undefined = undefined;
+			const done = (): void => {
+				const index = this.waitingForIdle.indexOf(done);
+
+				if (timer) clearTimeout(timer);
+				if (index !== -1) this.waitingForIdle.splice(index, 1);
+
+				resolve(this.inFlight);
+			};
+
+			if (timeout > 0) {
+				timer = setTimeout(done, timeout);
+				timer.unref();
+			}
+
+			this.waitingForIdle.push(done);
+		});
 	}
 }