perf(@angular/ssr): prevent potential stampede in entry-points cache

If multiple concurrent requests hit `getEntryPointExports`, all of
them would previously see the cache miss for entry point. With this
change, only the first request will and the others can leverage the
cache.

This can be important when instances are added to a pool under high
traffic.

Author: jkrems

packages/angular/ssr/src/app-engine.ts

@@ -50,7 +50,7 @@ export class AngularAppEngine {
   /**
    * A cache that holds entry points, keyed by their potential locale string.
    */
-  private readonly entryPointsCache = new Map<string, EntryPointExports>();
+  private readonly entryPointsCache = new Map<string, Promise<EntryPointExports>>();
 
   /**
    * Renders a response for the given HTTP request using the server application.
@@ -110,9 +110,7 @@ export class AngularAppEngine {
    * @param potentialLocale - The locale string used to find the corresponding entry point.
    * @returns A promise that resolves to the entry point exports or `undefined` if not found.
    */
-  private async getEntryPointExports(
-    potentialLocale: string,
-  ): Promise<EntryPointExports | undefined> {
+  private getEntryPointExports(potentialLocale: string): Promise<EntryPointExports> | undefined {
     const cachedEntryPoint = this.entryPointsCache.get(potentialLocale);
     if (cachedEntryPoint) {
       return cachedEntryPoint;
@@ -124,7 +122,7 @@ export class AngularAppEngine {
       return undefined;
     }
 
-    const entryPointExports = await entryPoint();
+    const entryPointExports = entryPoint();
     this.entryPointsCache.set(potentialLocale, entryPointExports);
 
     return entryPointExports;
@@ -141,7 +139,7 @@ export class AngularAppEngine {
    * @param url - The URL of the request.
    * @returns A promise that resolves to the entry point exports or `undefined` if not found.
    */
-  private getEntryPointExportsForUrl(url: URL): Promise<EntryPointExports | undefined> {
+  private getEntryPointExportsForUrl(url: URL): Promise<EntryPointExports> | undefined {
     const { entryPoints, basePath } = this.manifest;
     if (entryPoints.size === 1) {
       return this.getEntryPointExports('');