Refuse a message past the held-message bound with ESME_RTHROTTLED, and answer a full reassembly buffer the same way

src/held-messages.ts

@@ -30,7 +30,6 @@ export class HeldMessages {
 	private readonly held: ExpiringGroups<Held>;
 	private readonly idleWaiters = new IdleWaiters();
 	private readonly log: SmppLog;
-	private readonly max: number;
 	private readonly maxOctets: number;
 	private octets = 0;
 
@@ -42,7 +41,6 @@ export class HeldMessages {
 			timeout: options.timeout,
 		});
 		this.log = options.log;
-		this.max = options.max;
 		this.maxOctets = options.maxOctets;
 	}
 
@@ -50,7 +48,13 @@ export class HeldMessages {
 		return this.held.size;
 	}
 
-	/** An application that answers no message at all may not grow this without end. */
+	/** Whether a message arriving now is past the bound, once the expired are swept. */
+	full(): boolean {
+		this.sweep();
+
+		return this.held.full || this.octets >= this.maxOctets;
+	}
+
 	hold(pduObjs: PduObject[]): void {
 		const key = keyOf(pduObjs);
 
@@ -63,17 +67,10 @@ export class HeldMessages {
 		if (replaced) {
 			this.log.warn('heldMessages - replacing a message on a re-used sequence number', { seqNr: Number(key) });
 			this.delete(key, replaced);
-		} else if (this.held.full) {
-			this.dropOldest();
 		}
 
 		const octets = pduObjs.reduce((sum, pduObj) => sum + retainedOctets(pduObj), 0);
 
-		// The message just held stays even alone past the cap: the peer is still owed its answer.
-		while (this.held.size > 0 && this.octets + octets > this.maxOctets) {
-			this.dropOldest();
-		}
-
 		this.held.set(key, { octets, pduObjs });
 		this.octets += octets;
 	}
@@ -109,22 +106,6 @@ export class HeldMessages {
 		return this.idleWaiters.wait(() => this.held.size, timeout, signal);
 	}
 
-	private dropOldest(): void {
-		const oldest = this.held.takeOldest();
-
-		if (!oldest) return;
-
-		const [seqNr, held] = oldest;
-
-		this.octets -= held.octets;
-		this.log.warn('heldMessages - buffer full, dropping the oldest message', {
-			max: this.max,
-			maxOctets: this.maxOctets,
-			octets: this.octets,
-			seqNr: Number(seqNr),
-		});
-	}
-
 	/** Drops every message past its deadline. Runs before each hold and on its own timer. */
 	sweep(): void {
 		const expired = this.held.takeExpired();

src/incoming-requests.ts

@@ -19,7 +19,11 @@ import { paramText } from './defs/types.ts';
 import { respIdParams, segmentId } from './sms-id.ts';
 import { respNameFor } from './defs/commands.ts';
 
-/** SMPP 3.4 lists ESME_RMSGQFUL under submit_sm_resp only; 4.6.2's retryable code is another. */
+/** Asks the peer to keep the message and retry: RTHROTTLED is the SMSC's to send, so an ESME's is another. */
+function throttledStatus(carriedAs: string): ErrorName {
+	return carriedAs === 'submit_sm' ? 'ESME_RTHROTTLED' : 'ESME_RX_T_APPN';
+}
+
 export function refusedSegmentStatus(
 	carriedAs: string,
 	refusal: Refusal,
@@ -30,7 +34,7 @@ export function refusedSegmentStatus(
 		return spelling === 'sar' ? 'ESME_RINVTLVVAL' : 'ESME_RINVESMCLASS';
 	}
 
-	return carriedAs === 'submit_sm' ? 'ESME_RMSGQFUL' : 'ESME_RX_T_APPN';
+	return throttledStatus(carriedAs);
 }
 
 const lostReasons: Record<LostGroup['reason'], string> = {
@@ -212,6 +216,16 @@ export class IncomingRequests {
 	 * one request at a time never sends the second segment until the first has been answered.
 	 */
 	private async onMessage(pduObj: PduObject): Promise<void> {
+		if (this.held.full()) {
+			this.log.info('session - unanswered messages at their bound, asking the peer to retry', {
+				cmdName: pduObj.cmdName,
+				seqNr: pduObj.seqNr,
+			});
+			await this.session.sendReturn(pduObj, throttledStatus(this.carriedAs(pduObj)));
+
+			return;
+		}
+
 		const concat = concatOf(pduObj);
 
 		if (!concat) {