refactor(@angular-devkit/build-angular): lazily initialize JS transformer worker pool

The Worker thread pool that is used by the JavaScript Transformer within the application
builder is now only initialized if there is JavaScript that must be transformed. This
removes any overhead associated with the worker pool if no work is required. This is
particularly useful when using the development server with caching enabled and all the
dependencies have already been prebundled.

Author: clydin

packages/angular_devkit/build_angular/src/tools/esbuild/javascript-transformer.ts

@@ -26,18 +26,13 @@ export interface JavaScriptTransformerOptions {
  * and advanced optimizations.
  */
 export class JavaScriptTransformer {
-  #workerPool: Piscina;
+  #workerPool: Piscina | undefined;
   #commonOptions: Required<JavaScriptTransformerOptions>;
 
-  constructor(options: JavaScriptTransformerOptions, maxThreads: number) {
-    this.#workerPool = new Piscina({
-      filename: require.resolve('./javascript-transformer-worker'),
-      minThreads: 1,
-      maxThreads,
-      // Shutdown idle threads after 1 second of inactivity
-      idleTimeout: 1000,
-    });
-
+  constructor(
+    options: JavaScriptTransformerOptions,
+    readonly maxThreads: number,
+  ) {
     // Extract options to ensure only the named options are serialized and sent to the worker
     const {
       sourcemap,
@@ -53,6 +48,18 @@ export class JavaScriptTransformer {
     };
   }
 
+  #ensureWorkerPool(): Piscina {
+    this.#workerPool ??= new Piscina({
+      filename: require.resolve('./javascript-transformer-worker'),
+      minThreads: 1,
+      maxThreads: this.maxThreads,
+      // Shutdown idle threads after 1 second of inactivity
+      idleTimeout: 1000,
+    });
+
+    return this.#workerPool;
+  }
+
   /**
    * Performs JavaScript transformations on a file from the filesystem.
    * If no transformations are required, the data for the original file will be returned.
@@ -63,7 +70,7 @@ export class JavaScriptTransformer {
   transformFile(filename: string, skipLinker?: boolean): Promise<Uint8Array> {
     // Always send the request to a worker. Files are almost always from node modules which means
     // they may need linking. The data is also not yet available to perform most transformation checks.
-    return this.#workerPool.run({
+    return this.#ensureWorkerPool().run({
       filename,
       skipLinker,
       ...this.#commonOptions,
@@ -92,7 +99,7 @@ export class JavaScriptTransformer {
       );
     }
 
-    return this.#workerPool.run({
+    return this.#ensureWorkerPool().run({
       filename,
       data,
       skipLinker,
@@ -104,10 +111,16 @@ export class JavaScriptTransformer {
    * Stops all active transformation tasks and shuts down all workers.
    * @returns A void promise that resolves when closing is complete.
    */
-  close(): Promise<void> {
-    // Workaround piscina bug where a worker thread will be recreated after destroy to meet the minimum.
-    this.#workerPool.options.minThreads = 0;
+  async close(): Promise<void> {
+    if (this.#workerPool) {
+      // Workaround piscina bug where a worker thread will be recreated after destroy to meet the minimum.
+      this.#workerPool.options.minThreads = 0;
 
-    return this.#workerPool.destroy();
+      try {
+        await this.#workerPool.destroy();
+      } finally {
+        this.#workerPool = undefined;
+      }
+    }
   }
 }