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

src/held-messages.ts

@@ -0,0 +1,29 @@
+import type { PduObject } from './pdu.ts';
+import { IdleWaiters } from './idle-waiters.ts';
+
+/** The messages handed to the application that it has not answered yet, held by their segments. */
+export class HeldMessages {
+	private readonly held = new Set<PduObject[]>();
+	private readonly idleWaiters = new IdleWaiters();
+
+	hold(pduObjs: PduObject[]): void {
+		this.held.add(pduObjs);
+	}
+
+	release(pduObjs: PduObject[]): void {
+		if (!this.held.delete(pduObjs)) return;
+
+		if (this.held.size === 0) this.idleWaiters.settle();
+	}
+
+	/** Drops every message: their segments went with the link, so no answer of ours correlates now. */
+	clear(): void {
+		this.held.clear();
+		this.idleWaiters.settle();
+	}
+
+	/** Resolves 0 once every message has been answered, or with how many have not. */
+	idle(timeout: number, signal?: AbortSignal): Promise<number> {
+		return this.idleWaiters.wait(() => this.held.size, timeout, signal);
+	}
+}