Give the held-message protocol one home in MessageHold

src/held-messages.ts

@@ -25,6 +25,41 @@ type Held = {
 	pduObjs: PduObject[];
 };
 
+/** One message offered to the application, held until it is answered or every listener gives up. */
+export class MessageHold {
+	private readonly held: HeldMessages;
+	private readonly pduObjs: PduObject[];
+	private working: number;
+
+	constructor(held: HeldMessages, pduObjs: PduObject[], listeners: number) {
+		this.held = held;
+		this.pduObjs = pduObjs;
+		this.working = listeners;
+	}
+
+	/** Whether a drain is still waiting for this message to be answered. */
+	isHeld(): boolean {
+		return this.held.has(this.pduObjs);
+	}
+
+	/** A turn later, so a listener sending its receipt straight after the response still holds. */
+	answered(): void {
+		setImmediate(() => { this.held.release(this.pduObjs); });
+	}
+
+	/** A rejection leaves the other listeners running, so only the last one to fail gives the message up. */
+	listenerGaveUp(): void {
+		this.working--;
+
+		if (this.working <= 0) this.answered();
+	}
+
+	/** A message nobody took is not work a shutdown can wait for. */
+	untaken(): void {
+		this.held.release(this.pduObjs);
+	}
+}
+
 /** The messages handed to the application that it has not answered yet, held by their segments. */
 export class HeldMessages {
 	private readonly held: ExpiringGroups<Held>;
@@ -59,10 +94,12 @@ export class HeldMessages {
 		return this.held.full || this.octets >= this.maxOctets;
 	}
 
-	hold(pduObjs: PduObject[]): void {
+	/** Holds a message about to be offered to `listeners` listeners. */
+	hold(pduObjs: PduObject[], listeners = 1): MessageHold {
+		const hold = new MessageHold(this, pduObjs, listeners);
 		const key = keyOf(pduObjs);
 
-		if (key === undefined) return;
+		if (key === undefined) return hold;
 
 		this.sweep();
 
@@ -77,6 +114,8 @@ export class HeldMessages {
 
 		this.held.set(key, { octets, pduObjs });
 		this.octets += octets;
+
+		return hold;
 	}
 
 	/** Whether a drain is still waiting for this message to be answered. */