Ensure Disposable.NULL always returns a new Disposable

packages/core/src/common/disposable.spec.ts

@@ -0,0 +1,30 @@
+// *****************************************************************************
+// Copyright (C) 2022 Ericsson and others.
+//
+// This program and the accompanying materials are made available under the
+// terms of the Eclipse Public License v. 2.0 which is available at
+// http://www.eclipse.org/legal/epl-2.0.
+//
+// This Source Code may also be made available under the following Secondary
+// Licenses when the conditions for such availability set forth in the Eclipse
+// Public License v. 2.0 are satisfied: GNU General Public License, version 2
+// with the GNU Classpath Exception which is available at
+// https://www.gnu.org/software/classpath/license.html.
+//
+// SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+// *****************************************************************************
+
+import { expect } from 'chai';
+import { DisposableCollection, Disposable } from './disposable';
+
+describe('Disposables', () => {
+    /* eslint-disable no-unused-expressions */
+    it('Is safe to use Disposable.NULL', () => {
+        const collectionA = new DisposableCollection(Disposable.NULL);
+        const collectionB = new DisposableCollection(Disposable.NULL);
+        expect(!collectionA.disposed && !collectionB.disposed, 'Neither should be disposed before either is disposed.').to.be.true;
+        collectionA.dispose();
+        expect(collectionA.disposed, 'A should be disposed after being disposed.').to.be.true;
+        expect(collectionB.disposed, 'B should not be disposed because A was disposed.').to.be.false;
+    });
+});

packages/core/src/common/disposable.ts

@@ -28,13 +28,24 @@ export namespace Disposable {
         return !!arg && typeof arg === 'object' && 'dispose' in arg && typeof arg['dispose'] === 'function';
     }
     export function create(func: () => void): Disposable {
-        return {
-            dispose: func
-        };
+        return { dispose: func };
     }
-    export const NULL = create(() => { });
+    /** Always provides a reference to a new disposable. */
+    export declare const NULL: Disposable;
 }
 
+/**
+ * Ensures that every reference to {@link Disposable.NULL} returns a new object,
+ * as sharing a disposable between multiple {@link DisposableCollection} can have unexpected side effects
+ */
+Object.defineProperty(Disposable, 'NULL', {
+    configurable: false,
+    enumerable: true,
+    get(): Disposable {
+        return { dispose: () => { } };
+    }
+});
+
 export class DisposableCollection implements Disposable {
 
     protected readonly disposables: Disposable[] = [];