perf(fmt): reuse formatter import promise

packages/rstack/src/cachedImport.ts

@@ -0,0 +1,12 @@
+/** Share the pending import while allowing a failed import to be retried. */
+export const cachedImport = <T>(
+  importer: () => Promise<T>,
+): (() => Promise<T>) => {
+  let promise: Promise<T> | undefined;
+
+  return () =>
+    (promise ??= importer().catch((error: unknown) => {
+      promise = undefined;
+      throw error;
+    }));
+};

packages/rstack/src/fmt/worker.ts

@@ -2,6 +2,7 @@
 
 import { hash } from 'node:crypto';
 import { readFileSync, writeFileSync } from 'node:fs';
+import { cachedImport } from '../cachedImport.ts';
 import type { FmtCacheEntry } from './cacheStore.ts';
 import { hasDottedBasename } from './pathHelpers.ts';
 import type { FmtFileCache, FmtFileRequest, FmtWorkerResult } from './types.ts';
@@ -12,6 +13,8 @@ interface FormatFileTask {
   cache?: FmtFileCache;
 }
 
+const getFormatModule = cachedImport(() => import('./format.ts'));
+
 const hashContent = (content: string | Uint8Array): string =>
   hash('sha256', content, 'base64url').slice(0, 16);
 
@@ -65,7 +68,7 @@ const formatFile = async ({
     }
   }
 
-  const { formatFmtSource } = await import('./format.ts');
+  const { formatFmtSource } = await getFormatModule();
   const result = await formatFmtSource(file, () => (source ??= readSource()));
   if (result.status === 'unsupported') {
     return cache

packages/rstack/tests/cachedImport.test.ts

@@ -0,0 +1,30 @@
+import { expect, rs, test } from 'rstack/test';
+import { cachedImport } from '../src/cachedImport.ts';
+
+test('loads lazily and shares pending and fulfilled imports', async () => {
+  const module = { value: 1 };
+  const importer = rs.fn(() => Promise.resolve(module));
+  const load = cachedImport(importer);
+
+  expect(importer).not.toHaveBeenCalled();
+  const pending = load();
+  expect(load()).toBe(pending);
+  await expect(pending).resolves.toBe(module);
+  await expect(load()).resolves.toBe(module);
+  expect(importer).toHaveBeenCalledTimes(1);
+});
+
+test('allows retrying a failed import', async () => {
+  const error = new Error('Failed to load module');
+  const module = { value: 1 };
+  const importer = rs
+    .fn<() => Promise<typeof module>>()
+    .mockRejectedValueOnce(error)
+    .mockResolvedValue(module);
+  const load = cachedImport(importer);
+
+  await expect(load()).rejects.toBe(error);
+  await expect(load()).resolves.toBe(module);
+  await expect(load()).resolves.toBe(module);
+  expect(importer).toHaveBeenCalledTimes(2);
+});