Drain the requests already on the wire before close() and unbind() tear down

AGENTS.md

@@ -252,6 +252,15 @@ exactly 140.
   one's. Every segment still reaches the application as a `dlr`. The rule covers the bases the
   merger opened — `expect()` ignores a lone id, so a single-part message never claims one.
 
+- **A deliberate shutdown drains; an unusable link does not.** `close()` and `unbind()` refuse
+  further sends and wait for the send window to empty, not the pending map — the map misses a
+  segment still queued behind a full window, and finishing a half-sent multipart message is the
+  point. What `shutdownTimeout` leaves is torn down and reported as an `err`, and `0` waits forever
+  like every other timeout here. A stream the framer or the codec cannot read takes `abort()`
+  instead: nothing on that link can answer, so draining it would only hold a dead socket open for
+  the timeout. `unbind()` sends its own PDU past the window, since the drain already waited for
+  every slot.
+
 - **The TLS tests build their own self-signed certificate in DER** (`test/tls.test.ts`) instead of
   adding a devDependency or shelling out to openssl. Maintainer's call, 2026-08-26: the dev image
   `node:24.18.0-bookworm-slim` ships no openssl binary, so a shelled-out fixture would pass in CI

README.md

@@ -21,6 +21,7 @@ by hand on top of a library; it is built in here.
 | **Submit window** | `maxOutstanding` holds requests in flight at 10; further sends queue instead of overrunning the SMSC. |
 | **Delivery receipts** | Correlated by `receipted_message_id`/`message_state` where the SMSC sends them, falling back to parsing the receipt text — what Kannel and several others send. |
 | **Multipart** | Long messages split on send; concatenated `deliver_sm` reassembled into one `sms`. |
+| **Graceful shutdown** | `close()` and `unbind()` wait out the requests already on the wire, so a submit the SMSC accepted is not reported as a failure. |
 | **Never throws** | Everything fallible resolves to `{ err?, … }`, the codec included. |
 
 Throughput throttling is deliberately absent: an operator's rate limit is scoped to the account, and
@@ -100,6 +101,7 @@ Every one is optional.
 | `enquireLinkInterval` | `20000` | How often to send `enquire_link` on a quiet link. |
 | `idleTimeout` | `2 × enquireLinkInterval` | Give up on a link the peer has stopped answering; with `reconnect` set, it re-binds. |
 | `responseTimeout` | `30000` | How long to wait for a response before giving up on it; `0` waits forever. |
+| `shutdownTimeout` | `5000` | How long `close()` and `unbind()` wait for the requests already on the wire; `0` waits forever. |
 | `maxOutstanding` | `10` | Requests allowed on the wire at once; further sends queue. |
 | `reconnect` | off | `{ minDelay, maxDelay }` to re-bind automatically after a drop or an idle timeout, with exponential backoff. |
 | `log` | silent | Any object with `debug`, `error`, `info`, `verbose` and `warn` methods — see [Logging](#logging). |
@@ -206,7 +208,7 @@ smpp.on('session', session => {
 });
 
 console.log(smpp.port);  // the port actually bound, useful when 0 was requested
-await smpp.close();      // stop listening and close every live session
+await smpp.close();      // drain and close every live session, then stop listening
 ```
 
 `sendDlr` accepts `SCHEDULED`, `ENROUTE`, `DELIVERED`, `EXPIRED`, `DELETED`, `UNDELIVERABLE`,
@@ -228,7 +230,7 @@ A message whose `data_coding` says 8-bit binary arrives as Latin-1, so `Buffer.f
 | `maxReassembly` | `1000` | Incomplete multipart messages held per session. |
 | `maxOctets` | `67108864` | Bytes of incomplete multipart messages held per session. |
 | `reassemblyTimeout` | `300000` | How long a late segment can still join an incomplete message. |
-| `responseTimeout`, `maxOutstanding`, `log`, `signal` | as for the client | |
+| `responseTimeout`, `shutdownTimeout`, `maxOutstanding`, `log`, `signal` | as for the client | |
 
 ### Bind direction
 
@@ -307,8 +309,11 @@ TypeScript users can import `SmppLog` to have the compiler check one.
 
 ### Methods
 
-`sendSms()`, `send()`, `sendReturn()`, `unbind()` and `close()`. `send()` reaches any of the 33 SMPP
+`sendSms()`, `send()`, `sendReturn()`, `unbind()` and `close()`. `close()` and `unbind()` both
-commands the codec knows, not just the four the session handles natively:
+refuse further sends, wait out the requests already on the wire up to `shutdownTimeout`, and then
+tear down whatever is left; each resolves to an `err` naming how many requests it gave up on.
+`send()` reaches any of the 33 SMPP commands the codec knows, not just the four the session handles
+natively:
 
 ```javascript
 const { err, pduObj } = await session.send({

src/client.ts

@@ -27,6 +27,7 @@ export type ClientOptions = {
 	port?: number;
 	reconnect?: { maxDelay?: number; minDelay?: number };
 	responseTimeout?: number;
+	shutdownTimeout?: number;
 	signal?: AbortSignal;
 	systemType?: string;
 	tls?: ConnectionOptions | boolean;
@@ -145,6 +146,7 @@ function createSession(options: ClientOptions, log: SmppLog, sock: Socket): Sess
 		log,
 		maxOutstanding: options.maxOutstanding,
 		responseTimeout: options.responseTimeout,
+		shutdownTimeout: options.shutdownTimeout,
 		sock,
 		...(options.reconnect
 			? {
@@ -194,18 +196,18 @@ export async function client(options: ClientOptions = {}): Promise<Result<{ sess
 	const signal = options.signal;
 
 	if (signal?.aborted === true) {
-		session.close();
+		void session.close();
 
 		return { err: new Error('Aborted before binding') };
 	}
 
 	// Registered before the bind: an abort landing while it is in flight has to close the session.
-	signal?.addEventListener('abort', () => { session.close(); }, { once: true });
+	signal?.addEventListener('abort', () => { void session.close(); }, { once: true });
 
 	const bound = await bind(session, options);
 
 	if (bound.err) {
-		session.close();
+		void session.close();
 
 		return { err: bound.err };
 	}

src/incoming-requests.ts

@@ -70,7 +70,7 @@ export class IncomingRequests {
 				break;
 			case 'unbind':
 				await this.session.sendReturn(pduObj);
-				this.session.close();
+				await this.session.close();
 				break;
 			default:
 				await this.unhandled(pduObj);

src/send-window.ts

@@ -2,6 +2,7 @@
 export class SendWindow {
 	private readonly limit: number;
 	private readonly waiting: (() => void)[] = [];
+	private readonly waitingForIdle: (() => void)[] = [];
 	private inFlight = 0;
 
 	constructor(limit: number) {
@@ -28,5 +29,35 @@ export class SendWindow {
 		}
 
 		this.inFlight--;
+
+		if (this.inFlight > 0) return;
+
+		for (const resolve of this.waitingForIdle.splice(0)) {
+			resolve();
+		}
+	}
+
+	/** Resolves once nothing is in flight, or on the timeout with how many still are. 0 never times out. */
+	idle(timeout: number): Promise<number> {
+		if (this.inFlight === 0) return Promise.resolve(0);
+
+		return new Promise<number>(resolve => {
+			let timer: NodeJS.Timeout | undefined = undefined;
+			const done = (): void => {
+				const index = this.waitingForIdle.indexOf(done);
+
+				if (timer) clearTimeout(timer);
+				if (index !== -1) this.waitingForIdle.splice(index, 1);
+
+				resolve(this.inFlight);
+			};
+
+			if (timeout > 0) {
+				timer = setTimeout(done, timeout);
+				timer.unref();
+			}
+
+			this.waitingForIdle.push(done);
+		});
 	}
 }

src/server.ts

@@ -34,6 +34,7 @@ export type ServerOptions = {
 	port?: number;
 	reassemblyTimeout?: number;
 	responseTimeout?: number;
+	shutdownTimeout?: number;
 	signal?: AbortSignal;
 	systemId?: string;
 	tls?: TlsOptions | boolean;
@@ -114,20 +115,19 @@ export class SmppServer extends EventEmitter<ServerEvents> {
 		if (event !== 'serverError') this.emit('serverError', error);
 	}
 
-	/** Stops listening and closes every live session. */
+	/** Stops listening and closes every live session, draining each one first. */
-	close(): Promise<void> {
+	async close(): Promise<void> {
-		return new Promise(resolve => {
+		const live = [...this.sessions];
-			for (const session of this.sessions) {
-				try {
-					session.close();
-				} catch (thrown: unknown) {
-					this.emit('serverError', errorFrom(thrown));
-				}
-			}
 
 		this.sessions.clear();
-			this.server.close(() => { resolve(); });
+
-		});
+		await Promise.all(live.map(async session => {
+			const closed = await session.close().catch((thrown: unknown) => ({ err: errorFrom(thrown) }));
+
+			if (closed.err) this.emit('serverError', closed.err);
+		}));
+
+		return new Promise(resolve => { this.server.close(() => { resolve(); }); });
 	}
 }
 
@@ -233,6 +233,7 @@ function onConnection(sock: Socket, options: ServerOptions, server: SmppServer):
 		onRequest: (bound, pduObj) => onRequest(bound, pduObj, options),
 		reassemblyTimeout: options.reassemblyTimeout,
 		responseTimeout: options.responseTimeout,
+		shutdownTimeout: options.shutdownTimeout,
 		sock,
 		systemId: options.systemId ?? defaults.systemId,
 	});

src/session-options.ts

@@ -78,6 +78,8 @@ export type SessionOptions = {
 	reassemblyTimeout?: number | undefined;
 	reconnect?: ReconnectOptions | undefined;
 	responseTimeout?: number | undefined;
+	/** How long a drain waits for the requests already on the wire. 0 waits forever. */
+	shutdownTimeout?: number | undefined;
 	sock: Socket;
 	/** This end's own identity, answered to the peer in place of the one it sent. */
 	systemId?: string | undefined;
@@ -98,6 +100,7 @@ export const defaults = {
 	minDelay: 1000,
 	reassemblyTimeout: 300_000,
 	responseTimeout: 30_000,
+	shutdownTimeout: 5000,
 	systemId: defaultSystemId,
 };
 
@@ -112,6 +115,7 @@ export function checkSessionOptions(options: SessionCounts): VoidResult {
 		['maxReassembly', options.maxReassembly ?? defaults.maxReassembly, 1],
 		['reassemblyTimeout', options.reassemblyTimeout ?? defaults.reassemblyTimeout, 0],
 		['responseTimeout', options.responseTimeout ?? defaults.responseTimeout, 0],
+		['shutdownTimeout', options.shutdownTimeout ?? defaults.shutdownTimeout, 0],
 	];
 
 	for (const [name, value, min] of limits) {
@@ -129,4 +133,5 @@ export type SessionCounts = {
 	maxReassembly?: number | undefined;
 	reassemblyTimeout?: number | undefined;
 	responseTimeout?: number | undefined;
+	shutdownTimeout?: number | undefined;
 };

src/session.ts

@@ -67,6 +67,7 @@ export class Session extends EventEmitter<SessionEvents> {
 
 	private closed = false;
 	private concatReference = 0;
+	private draining = false;
 	private framer = new PduFramer();
 
 	/** A listener that throws is the application's bug; it must not become ours. Hard rule 1. */
@@ -160,6 +161,8 @@ export class Session extends EventEmitter<SessionEvents> {
 
 		if (this.closed) return { err: new Error('Session is closed') };
 
+		if (this.draining) return { err: new Error('Session is shutting down') };
+
 		// Before the window, or a full window makes an aborted call wait for a slot it will not use.
 		if (options.signal?.aborted === true) {
 			return { err: new Error('Aborted before the request was sent') };
@@ -213,22 +216,38 @@ export class Session extends EventEmitter<SessionEvents> {
 		return sent;
 	}
 
-	/** Unbinds politely, then closes. Many SMSCs drop the link instead of answering, which is fine. */
+	/**
+	 * Drains, unbinds politely, then closes. Many SMSCs drop the link instead of answering the
+	 * unbind, which is fine. Reports the unbind's own failure ahead of an unfinished drain.
+	 */
 	async unbind(): Promise<VoidResult> {
+		this.reconnectLoop?.stop();
+
+		const drained = await this.drain();
 		const wasOpen = !this.closed;
-		const sent = await this.send({ cmdName: 'unbind' });
+		// request(), not send(): the drain gate would refuse it, and the window is empty by now.
+		const sent = wasOpen
+			? await this.request({ cmdName: 'unbind' }, {})
+			: { err: new Error('Session is closed') };
 		const closedOnUnbind = wasOpen && this.closed;
 
-		this.close();
+		this.shutdown();
 
-		return sent.err && !closedOnUnbind ? { err: sent.err } : {};
+		return sent.err && !closedOnUnbind ? { err: sent.err } : drained;
 	}
 
-	/** Closes for good. A session closed this way never reconnects. */
+	/**
-	close(): void {
+	 * Closes for good: refuses new sends, waits out the requests already on the wire up to
+	 * `shutdownTimeout`, then tears down whatever is left. A session closed this way never reconnects.
+	 */
+	async close(): Promise<VoidResult> {
 		this.reconnectLoop?.stop();
-		this.teardown();
+
-		this.dlrMerger.clear();
+		const drained = await this.drain();
+
+		this.shutdown();
+
+		return drained;
 	}
 
 	private loopFor(reconnect: ReconnectOptions | undefined): ReconnectLoop | undefined {
@@ -311,6 +330,34 @@ export class Session extends EventEmitter<SessionEvents> {
 		return response;
 	}
 
+	/** Stops new sends and waits out the ones already issued. A dead link has nothing to wait for. */
+	private async drain(): Promise<VoidResult> {
+		this.draining = true;
+		this.timers.clear();
+
+		if (this.closed || this.sock.destroyed) return {};
+
+		const timeout = this.options.shutdownTimeout ?? defaults.shutdownTimeout;
+		const inFlight = await this.window.idle(timeout);
+
+		if (inFlight === 0) return {};
+
+		this.log.warn('session - shutting down with requests still in flight', { inFlight, timeout });
+
+		return { err: new Error(`Shut down with ${String(inFlight)} request(s) still in flight`) };
+	}
+
+	private shutdown(): void {
+		this.teardown();
+		this.dlrMerger.clear();
+	}
+
+	/** The link is unusable, so nothing can answer and there is nothing to drain. */
+	private abort(): void {
+		this.reconnectLoop?.stop();
+		this.shutdown();
+	}
+
 	private teardown(): void {
 		if (this.closed) return;
 
@@ -348,7 +395,7 @@ export class Session extends EventEmitter<SessionEvents> {
 		if (framed.err) {
 			this.log.warn('session - unusable stream, closing', { message: framed.err.message });
 			this.emit('sessionError', framed.err);
-			this.close();
+			this.abort();
 
 			return;
 		}
@@ -369,7 +416,7 @@ export class Session extends EventEmitter<SessionEvents> {
 				message: parsed.err.message,
 			});
 			this.emit('sessionError', parsed.err);
-			this.close();
+			this.abort();
 
 			return false;
 		}

test/interop.test.ts

@@ -232,7 +232,7 @@ describe('a live session against the reference implementation', () => {
 		const port = refServer.address()?.port ?? 0;
 		const { err, session } = await client({ port });
 
-		t.after(() => { session?.close(); });
+		t.after(() => session?.close());
 
 		assert.equal(err, undefined);
 		assert.ok(session);

test/readme.test.ts

@@ -77,7 +77,7 @@ describe('README: Client', () => {
 		});
 		if (err) throw err;
 
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		const reported = once<Dlr>(resolve => { session.on('dlr', resolve); });
 		const { err: sendErr, smsIds } = await session.sendSms({
@@ -100,7 +100,7 @@ describe('README: Client', () => {
 		const { err, session } = await client();
 		if (err) throw err;
 
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		const { signal } = new AbortController();
 		const [sms, sent] = await Promise.all([
@@ -128,7 +128,7 @@ describe('README: Client', () => {
 		const { err, session } = await client();
 		if (err) throw err;
 
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		const incoming = once<Sms>(resolve => { session.on('sms', resolve); });
 		const peer = await bound;

test/session-extras.test.ts

@@ -83,7 +83,7 @@ describe('merged delivery reports', () => {
 		assert.equal(report.statusMsg, 'DELIVERED');
 		assert.deepEqual(perSegment, ['merge-me-1', 'merge-me-2', 'merge-me-3']);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -119,7 +119,7 @@ describe('merged delivery reports', () => {
 		assert.equal(report.statusMsg, 'UNDELIVERABLE');
 		assert.equal(report.segments.length, 3);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 });
@@ -203,7 +203,7 @@ describe('sendSms()', () => {
 		assert.ok(sent.err instanceof Error);
 		assert.match(sent.err.message, /ESME_RMSGQFUL/);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -324,7 +324,7 @@ describe('reconnect', () => {
 
 		// Drop the connection from the server's side, as a peer restart would.
 		for (const serverSession of smpp.sessions) {
-			serverSession.close();
+			await serverSession.close();
 		}
 
 		await reconnected;
@@ -341,7 +341,7 @@ describe('reconnect', () => {
 		assert.equal(sent.err, undefined);
 		assert.deepEqual(messages, ['after reconnect']);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -373,7 +373,7 @@ describe('reconnect', () => {
 
 		const reconnected = once<true>(resolve => { session.on('reconnected', () => { resolve(true); }); });
 
-		peerOf(smpp).close();
+		await peerOf(smpp).close();
 		await reconnected;
 
 		const merged = once<MessageDlr>(resolve => { session.on('messageDlr', resolve); });
@@ -386,7 +386,7 @@ describe('reconnect', () => {
 		assert.equal(report.smsId, 'across-the-drop');
 		assert.equal(report.segments.length, 3);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -402,7 +402,7 @@ describe('reconnect', () => {
 		let reconnects = 0;
 
 		session.on('reconnected', () => { reconnects++; });
-		session.close();
+		await session.close();
 
 		await new Promise(resolve => setTimeout(resolve, 150));
 
@@ -586,10 +586,73 @@ describe('AbortSignal on a send', () => {
 
 		assert.ok(sent.err instanceof Error);
 
-		session.close();
+		await session.close();
 
 		for (const sock of accepted) sock.destroy();
 
 		await new Promise<void>(resolve => silent.close(() => { resolve(); }));
 	});
 });
+
+describe('graceful shutdown', () => {
+	async function submitInFlight(options: Parameters<typeof client>[0] = {}) {
+		const smpp = await startServer();
+		const incoming = once<Sms>(resolve => {
+			smpp.on('session', bound => bound.on('sms', resolve));
+		});
+		const { session } = await client({ port: smpp.port, ...options });
+
+		assert.ok(session);
+
+		const sent = session.sendSms({ from: '46701113311', message: 'answer me', to: '46709771337' });
+
+		return { sent, session, sms: await incoming, smpp };
+	}
+
+	test('close() waits out a submit already on the wire and refuses new ones', async () => {
+		const { sent, session, sms, smpp } = await submitInFlight();
+		const closed = session.close();
+		const refused = await session.sendSms({
+			from: '46701113311',
+			message: 'too late',
+			to: '46709771337',
+		});
+
+		assert.ok(refused.err instanceof Error);
+		assert.equal(refused.err.message, 'Session is shutting down');
+
+		await sms.sendResp({ smsId: 'answered-while-draining' });
+
+		assert.deepEqual((await sent).smsIds, ['answered-while-draining']);
+		assert.deepEqual(await closed, {});
+
+		await smpp.close();
+	});
+
+	test('unbind() waits out a submit already on the wire before it unbinds', async () => {
+		const { sent, session, sms, smpp } = await submitInFlight();
+		const unbound = session.unbind();
+
+		await sms.sendResp({ smsId: 'answered-before-unbind' });
+
+		assert.deepEqual((await sent).smsIds, ['answered-before-unbind']);
+		assert.deepEqual(await unbound, {});
+
+		await smpp.close();
+	});
+
+	test('gives up on a request that outlasts shutdownTimeout', async () => {
+		const { sent, session, smpp } = await submitInFlight({ shutdownTimeout: 50 });
+		const closed = await session.close();
+
+		assert.ok(closed.err instanceof Error);
+		assert.match(closed.err.message, /still in flight/);
+
+		const result = await sent;
+
+		assert.ok(result.err instanceof Error);
+		assert.equal(result.err.message, 'Session closed before a response arrived');
+
+		await smpp.close();
+	});
+});

test/session.test.ts

@@ -220,7 +220,7 @@ describe('bind', () => {
 		assert.ok(sent.err instanceof Error);
 		assert.equal(sent.err.message, 'Session closed before a response arrived');
 
-		session.close();
+		await session.close();
 		await peer.close();
 	});
 
@@ -268,7 +268,7 @@ describe('bind', () => {
 
 		assert.deepEqual(bound.userData, { userId: 123 });
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -404,7 +404,7 @@ describe('bind', () => {
 		const { session } = await connect(smpp);
 
 		assert.ok(session);
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 		assert.equal(session.peerInterfaceVersion, 0x50);
 		assert.ok(session.acceptsOptionalParams());
 	});
@@ -418,7 +418,7 @@ describe('bind', () => {
 		const { session } = await client({ port: peer.port });
 
 		assert.ok(session);
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 		assert.equal(session.peerInterfaceVersion, 0x00);
 		assert.equal(session.acceptsOptionalParams(), false);
 	});
@@ -440,7 +440,7 @@ describe('bind direction', () => {
 		assert.ok(sent.pduObj);
 		assert.equal(sent.pduObj.cmdStatus, 'ESME_RINVBNDSTS');
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -461,7 +461,7 @@ describe('bind direction', () => {
 		assert.deepEqual(sent.smsIds, []);
 		assert.equal(arrived.length, 0);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -481,7 +481,7 @@ describe('bind direction', () => {
 		assert.ok(sent.pduObj);
 		assert.equal(sent.pduObj.cmdStatus, 'ESME_RINVBNDSTS');
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -506,7 +506,7 @@ describe('bind direction', () => {
 		assert.ok(report.err instanceof Error);
 		assert.match(report.err.message, /transmitter-bound/);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 });
@@ -548,7 +548,7 @@ describe('sending', () => {
 		assert.equal(submitted.params.source_addr_ton, 5);
 		assert.equal(submitted.params.dest_addr_ton, 1);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -577,7 +577,7 @@ describe('sending', () => {
 		assert.equal(sent.pduObjs.length, 4);
 		assert.deepEqual(sent.smsIds, ['long-id-1', 'long-id-2', 'long-id-3', 'long-id-4']);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -603,7 +603,7 @@ describe('sending', () => {
 		assert.equal(sms.message, message);
 		assert.match(sms.smsId, /^[0-9a-f]{8}-[0-9a-f]{4}-7[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -630,7 +630,7 @@ describe('sending', () => {
 		assert.equal(sms.message, 'تست');
 		assert.ok(sms.flash);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -664,7 +664,7 @@ describe('sending', () => {
 		assert.equal(params.source_addr_npi, consts.NPI.PRIVATE);
 		assert.equal(params.source_addr_ton, consts.TON.ABBREVIATED);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 });
@@ -679,7 +679,7 @@ describe('receiving', () => {
 		const { session } = await connect(smpp);
 
 		assert.ok(session);
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		return { peer: await bound, session };
 	}
@@ -808,7 +808,7 @@ describe('delivery reports', () => {
 		assert.equal(receipt.tlvs.receipted_message_id?.tagValue, 'dlr-id');
 		assert.equal(receipt.tlvs.message_state?.tagValue, 2);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -845,7 +845,7 @@ describe('delivery reports', () => {
 		// 0.4.0 wrote stat:UNDELIVERABLE, which is not the spec's seven-character field.
 		assert.match(await raw, /stat:UNDELIV /);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -918,7 +918,7 @@ describe('delivery reports', () => {
 		assert.deepEqual(perSegment, ['unrequested-1', 'unrequested-2', 'unrequested-3']);
 		assert.equal(merged, 0);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 });
@@ -1044,7 +1044,7 @@ describe('robustness', () => {
 
 		assert.ok(peak <= 2, `peak was ${String(peak)}`);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1062,7 +1062,7 @@ describe('robustness', () => {
 		assert.ok(reported instanceof Error, 'a response that never reached the wire should be reported');
 		assert.equal(reported.message, sent.err.message);
 
-		session.close();
+		await session.close();
 	});
 
 	test('ignores events from the socket it left behind on a reconnect', async () => {
@@ -1080,7 +1080,7 @@ describe('robustness', () => {
 		const dead = session.sock;
 
 		for (const serverSession of smpp.sessions) {
-			serverSession.close();
+			await serverSession.close();
 		}
 
 		await reconnected;
@@ -1099,7 +1099,7 @@ describe('robustness', () => {
 		assert.equal(closes, 0);
 		assert.equal(sent.err, undefined);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1129,7 +1129,7 @@ describe('robustness', () => {
 		const { session } = await connect(smpp);
 
 		assert.ok(session);
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		const peer = await bound;
 		const controller = new AbortController();
@@ -1161,7 +1161,7 @@ describe('robustness', () => {
 		const { session } = await connect(smpp, { maxOutstanding: 1, responseTimeout: 5000 });
 
 		assert.ok(session);
-		t.after(() => { session.close(); });
+		t.after(() => session.close());
 
 		const held = session.sendSms({ from: '46701113311', message: 'holds the slot', to: '46709771337' });
 		const controller = new AbortController();
@@ -1177,7 +1177,7 @@ describe('robustness', () => {
 		assert.notEqual(aborted, false, 'an aborted send should not wait for the window');
 		assert.ok(aborted !== false && aborted.err instanceof Error);
 
-		session.close();
+		await session.close();
 		await held;
 	});
 
@@ -1262,7 +1262,7 @@ describe('application hooks that throw or reject', () => {
 		assert.ok(reported instanceof Error, 'a throwing sms listener should reach the session');
 		assert.equal(reported.message, 'listener exploded');
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1287,7 +1287,7 @@ describe('application hooks that throw or reject', () => {
 
 		assert.ok(sent.err instanceof Error);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1319,7 +1319,7 @@ describe('application hooks that throw or reject', () => {
 		assert.ok(reported instanceof Error, 'a rejecting sms listener should reach the session');
 		assert.equal(reported.message, 'null');
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1343,7 +1343,7 @@ describe('application hooks that throw or reject', () => {
 
 		assert.ok(sent.err instanceof Error);
 
-		session.close();
+		await session.close();
 		await smpp.close();
 	});
 
@@ -1359,7 +1359,7 @@ describe('application hooks that throw or reject', () => {
 		assert.ok(reported instanceof Error, 'a rejecting session listener should reach the server');
 		assert.equal(reported.message, 'session listener rejected');
 
-		session?.close();
+		await session?.close();
 		await smpp.close();
 	});
 
@@ -1376,7 +1376,7 @@ describe('application hooks that throw or reject', () => {
 		await smpp.close();
 		assert.equal(smpp.sessions.size, 0);
 
-		session.close();
+		await session.close();
 	});
 
 	test('refuses a send window that can never free a slot', async () => {
@@ -1387,6 +1387,12 @@ describe('application hooks that throw or reject', () => {
 		assert.match(err.message, /maxOutstanding/);
 		assert.equal(session, undefined);
 
+		const negative = await connect(smpp, { shutdownTimeout: -1 });
+
+		assert.ok(negative.err instanceof Error);
+		assert.match(negative.err.message, /shutdownTimeout/);
+		assert.equal(negative.session, undefined);
+
 		await smpp.close();
 	});
 
@@ -1492,7 +1498,7 @@ describe('link timers', () => {
 			'a peer that answers nothing should time the link out',
 		);
 
-		session.close();
+		await session.close();
 		await peer.close();
 	});
 
@@ -1510,7 +1516,7 @@ describe('link timers', () => {
 
 		assert.ok(await raceWithin(2000, back), 'a link that timed out should be reconnected');
 
-		session.close();
+		await session.close();
 		await peer.close();
 	});
 });

todo.md

@@ -5,7 +5,7 @@ rules there constrain every item below.
 
 ## Status
 
-The rewrite is **feature complete and green**: 236 tests, lint and typecheck clean, verified on Node
+The rewrite is **feature complete and green**: 242 tests, lint and typecheck clean, verified on Node
 18, 20, 22 and 24. What is left is release work and a few things worth adding before or after 1.0.0.
 
 ```bash
@@ -55,6 +55,7 @@ Rules the API follows:
 | Delivery receipt parsing, TLV and text | `test/dlr.test.ts` |
 | Session, client, server: bind, auth, send, reassembly, DLRs, timeouts, abort, send window | `test/session.test.ts` |
 | Merged multipart DLRs including across a reconnect, reassembly bounds, per-send abort, the segment cap | `test/session-extras.test.ts` |
+| A draining `close()` and `unbind()`, bounded by `shutdownTimeout` | `test/session-extras.test.ts` |
 | Every runnable README example | `test/readme.test.ts` |
 | Receipt-versus-message classification by `esm_class` | `test/dlr.test.ts`, `test/session.test.ts` |
 | A listener that throws, or rejects, reaching `sessionError`/`serverError` rather than the process | `test/session.test.ts`, `test/error-from.test.ts` |
@@ -118,14 +119,7 @@ session message is a change to every call site.
       loses every incomplete group, and a peer has no reason to resend a receipt it already had
       answered. Surviving one means exposing the merge state for the application to persist and hand
       back, which is a public-surface decision.
-- [ ] **`close()` and `unbind()` drop in-flight requests instead of draining them.** `teardown()`
+- [ ] **`session.ts` is 465 lines.** The one seam left in it is a socket-to-PDU transport, which
-      settles every pending request with "Session closed before a response arrived" and destroys the
-      socket in the same tick, so a submit the SMSC has already accepted is reported to the caller
-      as a failure — the ambiguous outcome that produces a duplicate on retry. Give both a drain:
-      refuse new sends, wait out the pending responses up to a `shutdownTimeout`, then tear down
-      whatever is left. Distinct from re-queueing across a reconnect above — this is the deliberate
-      shutdown path, where there is nothing to come back to.
-- [ ] **`session.ts` is 386 lines.** The one seam left in it is a socket-to-PDU transport, which
       would move the deliberately public `sock` field out of `Session` or turn it into a getter —
       a public-surface change, so it waits for a decision.
 - [ ] **Group the session's collaborators under `src/session/`.** Only `session.ts` imports