transport: replace manual signal linking with AbortSignal.any()

The external signal support added in 46744a4 manually managed an abort
event listener and cleanup function to link req.signal to the internal
controller. AbortSignal.any() does this in a single line and the
platform handles lifetime — no listener, no cleanup, no
cleanupExternalSignal field on RegisteredTurn.

Removes two tests that asserted removeEventListener was called, since
there is no manual listener to remove. The five behavioral tests
(signal fires, already-aborted, start rejection, in-flight stream
cancellation, Ably cancel independence) are unchanged.

Author: mschristensen

src/core/transport/server-transport.ts

@@ -50,10 +50,10 @@ interface RegisteredTurn {
   turnId: string;
   clientId: string;
   controller: AbortController;
+  /** Composite signal that fires when either the internal controller or the external signal aborts. */
+  signal: AbortSignal;
   onCancel?: (request: CancelRequest) => Promise<boolean>;
   onError?: (error: Ably.ErrorInfo) => void;
-  /** Removes the listener from the external signal, if one was provided. */
-  cleanupExternalSignal?: () => void;
 }
 
 // ---------------------------------------------------------------------------
@@ -118,7 +118,6 @@ class DefaultServerTransport<TEvent, TMessage> implements ServerTransport<TEvent
     this._logger?.trace('DefaultServerTransport.close();');
     this._channel.unsubscribe(EVENT_CANCEL, this._channelListener);
     for (const reg of this._registeredTurns.values()) {
-      reg.cleanupExternalSignal?.();
       reg.controller.abort();
     }
     this._registeredTurns.clear();
@@ -259,37 +258,19 @@ class DefaultServerTransport<TEvent, TMessage> implements ServerTransport<TEvent
     let started = false;
     let ended = false;
 
-    // Link external signal (e.g. req.signal) to the internal controller so
-    // platform-level cancellation (client disconnect, function timeout) aborts
-    // the turn through the same path as Ably cancel messages.
-    let cleanupExternalSignal: (() => void) | undefined;
-    if (externalSignal) {
-      if (externalSignal.aborted) {
-        this._logger?.debug('DefaultServerTransport._createTurn(); external signal already aborted', { turnId });
-        controller.abort();
-      } else {
-        const listener = () => {
-          this._logger?.debug('DefaultServerTransport._createTurn(); external signal fired, aborting turn', {
-            turnId,
-          });
-          controller.abort();
-        };
-        externalSignal.addEventListener('abort', listener, { once: true });
-        cleanupExternalSignal = () => {
-          externalSignal.removeEventListener('abort', listener);
-        };
-        this._logger?.debug('DefaultServerTransport._createTurn(); linked external signal', { turnId });
-      }
-    }
+    // Compose the internal controller signal with the external signal (e.g.
+    // req.signal) so platform-level cancellation (client disconnect, function
+    // timeout) aborts the turn through the same path as Ably cancel messages.
+    const signal = externalSignal ? AbortSignal.any([controller.signal, externalSignal]) : controller.signal;
 
     // Spec: AIT-ST3a — register immediately so early cancels can fire the abort signal.
     const registration: RegisteredTurn = {
       turnId,
       clientId: turnClientId ?? '',
       controller,
+      signal,
       onCancel,
       onError: turnOnError,
-      cleanupExternalSignal,
     };
     this._registeredTurns.set(turnId, registration);
 
@@ -307,15 +288,15 @@ class DefaultServerTransport<TEvent, TMessage> implements ServerTransport<TEvent
         return turnId;
       },
       get abortSignal() {
-        return controller.signal;
+        return signal;
       },
 
       // Spec: AIT-ST4, AIT-ST4a, AIT-ST4b
       start: async (): Promise<void> => {
         logger?.trace('Turn.start();', { turnId });
 
         // Spec: AIT-ST4a
-        if (controller.signal.aborted) {
+        if (signal.aborted) {
           throw new Ably.ErrorInfo(
             `unable to start turn; turn ${turnId} was cancelled before start()`,
             ErrorCode.InvalidArgument,
@@ -443,7 +424,6 @@ class DefaultServerTransport<TEvent, TMessage> implements ServerTransport<TEvent
         }
         await attachPromise;
 
-        const signal = turnManager.getSignal(turnId);
         const turnOwnerClientId = turnManager.getClientId(turnId);
 
         // Per-operation parent overrides the turn-level default.
@@ -496,7 +476,6 @@ class DefaultServerTransport<TEvent, TMessage> implements ServerTransport<TEvent
           throw errInfo;
         } finally {
           registeredTurns.delete(turnId);
-          cleanupExternalSignal?.();
         }
 
         logger?.debug('Turn.end(); turn ended', { turnId, reason });

test/core/transport/server-transport.test.ts

@@ -616,18 +616,6 @@ describe('ServerTransport', () => {
       expect(result.reason).toBe('cancelled');
     });
 
-    it('removes the external signal listener when the turn ends', async () => {
-      const externalController = new AbortController();
-      const signal = externalController.signal;
-      const removeSpy = vi.spyOn(signal, 'removeEventListener');
-
-      const turn = transport.newTurn({ turnId: 'turn-1', signal });
-      await turn.start();
-      await turn.end('complete');
-
-      expect(removeSpy).toHaveBeenCalledWith('abort', expect.any(Function));
-    });
-
     it('does not interfere with Ably cancel routing', async () => {
       const externalController = new AbortController();
       const turn = transport.newTurn({ turnId: 'turn-1', clientId: 'user-a', signal: externalController.signal });
@@ -661,15 +649,5 @@ describe('ServerTransport', () => {
       expect(channel.unsubscribe).toHaveBeenCalledWith(EVENT_CANCEL, expect.any(Function));
     });
 
-    it('removes external signal listeners on close', () => {
-      const externalController = new AbortController();
-      const removeSpy = vi.spyOn(externalController.signal, 'removeEventListener');
-
-      transport.newTurn({ turnId: 'turn-1', signal: externalController.signal });
-
-      transport.close();
-
-      expect(removeSpy).toHaveBeenCalledWith('abort', expect.any(Function));
-    });
   });
 });