fix memory leak in NsfwWatcher (#12144)

Don't repeatedly call `Promise.race()`.

The code was calling `Promise.race()` repeatedly while waiting for a file
to come into existence. This added a `PromiseReaction` object each time,
which never was collected.

Contributed on behalf of STMicroelectronics

Signed-off-by: Thomas Mäder <[email protected]>
Co-authored-by: Paul Marechal <[email protected]>

packages/filesystem/src/node/nsfw-watcher/nsfw-filesystem-service.ts

@@ -23,7 +23,7 @@ import {
     FileChangeType, FileSystemWatcherService, FileSystemWatcherServiceClient, WatchOptions
 } from '../../common/filesystem-watcher-protocol';
 import { FileChangeCollection } from '../file-change-collection';
-import { Deferred } from '@theia/core/lib/common/promise-util';
+import { Deferred, timeout } from '@theia/core/lib/common/promise-util';
 
 export interface NsfwWatcherOptions {
     ignored: IMinimatch[]
@@ -196,22 +196,34 @@ export class NsfwWatcher {
         return this.refsPerClient.size > 0;
     }
 
+    /**
+     * @throws with {@link WatcherDisposal} if this instance is disposed.
+     */
+    protected assertNotDisposed(): void {
+        if (this.disposed) {
+            throw WatcherDisposal;
+        }
+    }
+
     /**
      * When starting a watcher, we'll first check and wait for the path to exists
      * before running an NSFW watcher.
      */
     protected async start(): Promise<void> {
-        while (await this.orCancel(fsp.stat(this.fsPath).then(() => false, () => true))) {
-            await this.orCancel(new Promise(resolve => setTimeout(resolve, 500)));
+        while (await fsp.stat(this.fsPath).then(() => false, () => true)) {
+            await timeout(500);
+            this.assertNotDisposed();
+        }
+        this.assertNotDisposed();
+        const watcher = await this.createNsfw();
+        this.assertNotDisposed();
+        await watcher.start();
+        this.debug('STARTED', `disposed=${this.disposed}`);
+        // The watcher could be disposed while it was starting, make sure to check for this:
+        if (this.disposed) {
+            await this.stopNsfw(watcher);
+            throw WatcherDisposal;
         }
-        const watcher = await this.orCancel(this.createNsfw());
-        await this.orCancel(watcher.start().then(() => {
-            this.debug('STARTED', `disposed=${this.disposed}`);
-            // The watcher could be disposed while it was starting, make sure to check for this:
-            if (this.disposed) {
-                this.stopNsfw(watcher);
-            }
-        }));
         this.nsfw = watcher;
     }
 
@@ -299,13 +311,6 @@ export class NsfwWatcher {
         });
     }
 
-    /**
-     * Wrap a promise to reject as soon as this handle gets disposed.
-     */
-    protected async orCancel<T>(promise: Promise<T>): Promise<T> {
-        return Promise.race<T>([this.deferredDisposalDeferred.promise, promise]);
-    }
-
     /**
      * When references hit zero, we'll schedule disposal for a bit later.
      *