Refuse to answer a message whose link went, instead of reporting it delivered

src/dlr-merger.ts

@@ -120,8 +120,8 @@ export class DlrMerger {
 	}
 
 	clear(): void {
-		this.groups.clear();
-		this.spent.clear();
+		this.groups.takeAll();
+		this.spent.takeAll();
 	}
 
 	/** Drops every group past its deadline. Runs before each collect and on its own timer. */

src/expiring-groups.ts

@@ -58,9 +58,14 @@ export class ExpiringGroups<T> {
 		this.idle();
 	}
 
-	clear(): void {
+	/** Removes every group and hands them over. */
+	takeAll(): [string, T][] {
+		const taken: [string, T][] = [...this.entries].map(([key, entry]) => [key, entry.group]);
+
 		this.entries.clear();
 		this.idle();
+
+		return taken;
 	}
 
 	/** Removes every group past its deadline and hands them over. */

src/held-messages.ts

@@ -20,6 +20,7 @@ function keyOf(pduObjs: PduObject[]): string | undefined {
 
 /** The messages handed to the application that it has not answered yet, held by their segments. */
 export class HeldMessages {
+	private readonly droppedWithLink = new WeakSet<PduObject[]>();
 	private readonly held: ExpiringGroups<PduObject[]>;
 	private readonly idleWaiters = new IdleWaiters();
 	private readonly log: SmppLog;
@@ -64,6 +65,11 @@ export class HeldMessages {
 		return key !== undefined && this.held.get(key) === pduObjs;
 	}
 
+	/** Whether this message's link went before it was answered, so no answer of ours correlates. */
+	lostLink(pduObjs: PduObject[]): boolean {
+		return this.droppedWithLink.has(pduObjs);
+	}
+
 	release(pduObjs: PduObject[]): void {
 		const key = keyOf(pduObjs);
 
@@ -76,7 +82,10 @@ export class HeldMessages {
 
 	/** Drops every message: their segments went with the link, so no answer of ours correlates now. */
 	clear(): void {
-		this.held.clear();
+		for (const [, pduObjs] of this.held.takeAll()) {
+			this.droppedWithLink.add(pduObjs);
+		}
+
 		this.idleWaiters.settle();
 	}
 

src/incoming-requests.ts

@@ -170,6 +170,7 @@ export class IncomingRequests {
 			session: this.session,
 			to: paramText(first.params.destination_addr),
 		}, {
+			lostLink: () => this.held.lostLink(pduObjs),
 			// A turn later, so a listener sending its receipt straight after the response still holds.
 			onAnswered: () => { setImmediate(() => { this.held.release(pduObjs); }); },
 			// Past the refusal only while a drain is still waiting for this message; an ordinary send after.

src/reassembly.ts

@@ -163,7 +163,7 @@ export class Reassembler {
 	}
 
 	clear(): void {
-		this.groups.clear();
+		this.groups.takeAll();
 		this.octets = 0;
 	}
 

src/sms.ts

@@ -54,6 +54,8 @@ export type SmsInput = {
 
 /** What the session's incoming side gives a message so it can be answered and accounted for. */
 export type SmsHandlers = {
+	/** Whether the link this message arrived on is gone, so no answer of ours correlates. */
+	lostLink: () => boolean;
 	onAnswered: () => void;
 	send: (input: PduObjectInput) => Promise<Result<{ pduObj: PduObject }>>;
 };
@@ -76,7 +78,8 @@ export function createSms(input: SmsInput, handlers: SmsHandlers): Sms {
 		message: input.message,
 		pduObjs: input.pduObjs,
 		sendDlr: status => sendDlr(sms, handlers.send, status),
-		sendResp: options => sendResp(sms, answered, options ?? {}).finally(handlers.onAnswered),
+		sendResp: options => sendResp(sms, answered, options ?? {}, handlers.lostLink)
+			.finally(handlers.onAnswered),
 		session: input.session,
 		get smsId(): string {
 			return answered.smsId;
@@ -92,6 +95,7 @@ async function sendResp(
 	sms: Sms,
 	answered: { smsId: string },
 	options: SendRespOptions,
+	lostLink: () => boolean,
 ): Promise<VoidResult> {
 	const total = sms.pduObjs.length;
 
@@ -103,6 +107,11 @@ async function sendResp(
 		return { err: new Error('smsId must not be empty') };
 	}
 
+	// A response carries the sequence number it was asked on, which the next link knows nothing about.
+	if (lostLink()) {
+		return { err: new Error('The link this message arrived on is gone, so nothing would correlate the response') };
+	}
+
 	if (options.smsId !== undefined) answered.smsId = options.smsId;
 
 	const results = await Promise.all(sms.pduObjs.map((pduObj, index) => sms.session.sendReturn(