Send 'Terminate' to the worker when destroy races the load-time handshake (bug 1942304)

If `PDFDocumentLoadingTask.destroy` ran while `workerIdPromise` was
pending, the inner `.then` in `getDocument` threw "Loading aborted"
before `WorkerTransport` was constructed, so `_transport` was never set
and the "Terminate" message was never posted.

Author: calixteman

src/display/api.js

@@ -424,9 +424,6 @@ function getDocument(src = {}) {
 
   Promise.all([worker.promise, gpuPromise])
     .then(function ([, hasGPU]) {
-      if (task.destroyed) {
-        throw new Error("Loading aborted");
-      }
       if (worker.destroyed) {
         throw new Error("Worker was destroyed");
       }
@@ -469,9 +466,6 @@ function getDocument(src = {}) {
       }
 
       return workerIdPromise.then(workerId => {
-        if (task.destroyed) {
-          throw new Error("Loading aborted");
-        }
         if (worker.destroyed) {
           throw new Error("Worker was destroyed");
         }
@@ -486,10 +480,18 @@ function getDocument(src = {}) {
           pagesMapper
         );
         task._transport = transport;
+
+        if (task.destroyed) {
+          // `destroy()` was called during the worker handshake; the orderly
+          // shutdown (including the "Terminate" message) will be issued
+          // through the transport once destroy resumes.
+          throw new Error("Loading aborted");
+        }
         messageHandler.send("Ready", null);
       });
     })
-    .catch(task._capability.reject);
+    .catch(task._capability.reject)
+    .finally(task._setupCapability.resolve);
 
   return task;
 }
@@ -515,6 +517,14 @@ class PDFDocumentLoadingTask {
    */
   _capability = Promise.withResolvers();
 
+  /**
+   * Resolves once the load-time setup chain has settled, regardless of
+   * outcome; used by `destroy()` to wait until `_transport` is either set
+   * or definitely never going to be.
+   * @private
+   */
+  _setupCapability = Promise.withResolvers();
+
   /**
    * @private
    */
@@ -568,11 +578,25 @@ class PDFDocumentLoadingTask {
    */
   async destroy() {
     this.destroyed = true;
+    // The setup chain rejects `_capability` with "Loading aborted" once the
+    // load-time chain unwinds (see `getDocument`). Claim that rejection
+    // here so it isn't reported as unhandled during the awaits below;
+    // callers awaiting `task.promise` still see it.
+    this._capability.promise.catch(() => {});
 
     try {
+      // `_pendingDestroy` must be set synchronously, before any `await`,
+      // so subsequent `PDFWorker.create()` calls on the shared `workerPort`
+      // observe it and throw (see issue 16777).
       if (this._worker?.port) {
         this._worker._pendingDestroy = true;
       }
+      // Wait for the load-time setup chain to settle so `_transport` is set
+      // (when applicable) before we tear down. This is what guarantees the
+      // "Terminate" message gets sent through `WorkerTransport.destroy` if
+      // `destroy` races with the initial worker handshake.
+      await this._setupCapability.promise;
+
       await this._transport?.destroy();
     } catch (ex) {
       if (this._worker?.port) {