Fix MdDocumentInfoCache computing values twice (#152799)

* Fix MdDocumentInfoCache computing values twice

Fixes a race where values could be computed twice before being cached

* Remove only

extensions/markdown-language-features/src/test/documentInfoCache.test.ts

@@ -0,0 +1,33 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as assert from 'assert';
+import 'mocha';
+import { InMemoryDocument } from '../util/inMemoryDocument';
+import { MdDocumentInfoCache } from '../util/workspaceCache';
+import { InMemoryWorkspaceMarkdownDocuments } from './inMemoryWorkspace';
+import { workspacePath } from './util';
+
+suite('DocumentInfoCache', () => {
+	test('Repeated calls should only compute value once', async () => {
+		const doc = workspacePath('doc.md');
+		const workspace = new InMemoryWorkspaceMarkdownDocuments([
+			new InMemoryDocument(doc, '')
+		]);
+
+		let i = 0;
+		const cache = new MdDocumentInfoCache<number>(workspace, async () => {
+			return ++i;
+		});
+
+		const a = cache.get(doc);
+		const b = cache.get(doc);
+
+		assert.strictEqual(await a, 1);
+		assert.strictEqual(i, 1);
+		assert.strictEqual(await b, 1);
+		assert.strictEqual(i, 1);
+	});
+});