Give the held-message flow one place a reader can follow it

src/held-messages.ts

@@ -25,7 +25,10 @@ type HoldEntry = {
 	release: () => void;
 };
 
-/** One message offered to the application, held until it is answered or every listener gives up. */
+/**
+ * One message offered to the application. A drain waits on it until the first of: `answered()`,
+ * every listener that took it rejecting, no listener taking it, its deadline, or the link going.
+ */
 export class MessageHold {
 	private readonly entry: HoldEntry;
 	private working: number;
@@ -64,6 +67,8 @@ export class HeldMessages {
 	private readonly idleWaiters = new IdleWaiters();
 	private readonly log: SmppLog;
 	private readonly maxOctets: number;
+	/** A rejecting listener hands the message back as an `unknown`, so its hold is found by identity. */
+	private readonly offered = new WeakMap<object, MessageHold>();
 
 	constructor(options: HeldMessagesOptions) {
 		this.held = new ExpiringGroups({
@@ -112,6 +117,28 @@ export class HeldMessages {
 		return hold;
 	}
 
+	/** Holds a message, builds what the application is handed, and emits it. */
+	offer<T extends object>(
+		pduObjs: PduObject[],
+		listeners: number,
+		build: (hold: MessageHold) => T,
+		emit: (message: T) => boolean,
+	): void {
+		const hold = this.hold(pduObjs, listeners);
+		const message = build(hold);
+
+		this.offered.set(message, hold);
+
+		if (!emit(message)) hold.release();
+	}
+
+	/** One listener gave up on a message; the last one to do so is what releases it. */
+	listenerRejected(message: unknown): void {
+		if (typeof message !== 'object' || message === null) return;
+
+		this.offered.get(message)?.listenerGaveUp();
+	}
+
 	private has(pduObjs: PduObject[]): boolean {
 		const key = keyOf(pduObjs);