CLI: take in account package exports in module federation exposed modules

Signed-off-by: David Festal <dfestal@redhat.com>

.changeset/shaggy-tomatoes-travel.md

@@ -0,0 +1,7 @@
+---
+'@backstage/cli': patch
+---
+
+Enhance the behavior of the experimental support for module federation in the backstage CLI,
+by using the `package.json` exports (when present) to complete the list of exposed modules.
+This allows, for example, using exported `alpha` definitions through module federation.

packages/cli/src/commands/build/buildFrontend.ts

@@ -19,6 +19,7 @@ import { resolve as resolvePath } from 'path';
 import { buildBundle, getModuleFederationOptions } from '../../lib/bundler';
 import { getEnvironmentParallelism } from '../../lib/parallel';
 import { loadCliConfig } from '../../modules/config/lib/config';
+import { BackstagePackageJson } from '@backstage/cli-node';
 
 interface BuildAppOptions {
   targetDir: string;
@@ -30,20 +31,21 @@ interface BuildAppOptions {
 
 export async function buildFrontend(options: BuildAppOptions) {
   const { targetDir, writeStats, configPaths, rspack } = options;
-  const { name } = await fs.readJson(resolvePath(targetDir, 'package.json'));
-
+  const packageJson = (await fs.readJson(
+    resolvePath(targetDir, 'package.json'),
+  )) as BackstagePackageJson;
   await buildBundle({
     targetDir,
     entry: 'src/index',
     parallelism: getEnvironmentParallelism(),
     statsJsonEnabled: writeStats,
     moduleFederation: getModuleFederationOptions(
-      name,
+      packageJson,
       options.isModuleFederationRemote,
     ),
     ...(await loadCliConfig({
       args: configPaths,
-      fromPackage: name,
+      fromPackage: packageJson.name,
     })),
     rspack,
   });

packages/cli/src/commands/start/startFrontend.ts

@@ -17,6 +17,7 @@
 import { readJson } from 'fs-extra';
 import { getModuleFederationOptions, serveBundle } from '../../lib/bundler';
 import { paths } from '../../lib/paths';
+import { BackstagePackageJson } from '@backstage/cli-node';
 
 interface StartAppOptions {
   verifyVersions?: boolean;
@@ -30,7 +31,9 @@ interface StartAppOptions {
 }
 
 export async function startFrontend(options: StartAppOptions) {
-  const { name } = await readJson(paths.resolveTarget('package.json'));
+  const packageJson = (await readJson(
+    paths.resolveTarget('package.json'),
+  )) as BackstagePackageJson;
 
   const waitForExit = await serveBundle({
     entry: options.entry,
@@ -40,7 +43,7 @@ export async function startFrontend(options: StartAppOptions) {
     skipOpenBrowser: options.skipOpenBrowser,
     linkedWorkspace: options.linkedWorkspace,
     moduleFederation: getModuleFederationOptions(
-      name,
+      packageJson,
       options.isModuleFederationRemote,
     ),
   });

packages/cli/src/lib/bundler/config.ts

@@ -235,13 +235,22 @@ export async function createConfig(
           .ModuleFederationPlugin as unknown as typeof ModuleFederationPlugin)
       : ModuleFederationPlugin;
 
+    const exposes = options.moduleFederation?.exposes
+      ? Object.fromEntries(
+          Object.entries(options.moduleFederation?.exposes).map(([k, v]) => [
+            k,
+            resolvePath(paths.targetPath, v),
+          ]),
+        )
+      : {
+          '.': paths.targetEntry,
+        };
+
     plugins.push(
       new AdaptedModuleFederationPlugin({
         ...(isRemote && {
           filename: 'remoteEntry.js',
-          exposes: {
-            '.': paths.targetEntry,
-          },
+          exposes,
         }),
         name: options.moduleFederation.name,
         runtime: false,

packages/cli/src/lib/bundler/moduleFederation.ts

@@ -16,9 +16,11 @@
 
 import chalk from 'chalk';
 import { ModuleFederationOptions } from './types';
+import { BackstagePackageJson } from '@backstage/cli-node';
+import { readEntryPoints } from '../entryPoints';
 
 export function getModuleFederationOptions(
-  name: string,
+  packageJson: BackstagePackageJson,
   isModuleFederationRemote?: boolean,
 ): ModuleFederationOptions | undefined {
   if (
@@ -39,6 +41,17 @@ export function getModuleFederationOptions(
     // The default output mode requires the name to be a usable as a code
     // symbol, there might be better options here but for now we need to
     // sanitize the name.
-    name: name.replaceAll('@', '').replaceAll('/', '__').replaceAll('-', '_'),
+    name: packageJson.name
+      .replaceAll('@', '')
+      .replaceAll('/', '__')
+      .replaceAll('-', '_'),
+    exposes:
+      isModuleFederationRemote && packageJson.exports
+        ? Object.fromEntries(
+            readEntryPoints(packageJson)
+              .filter(ep => ep.mount !== './package.json')
+              .map(ep => [ep.mount, ep.path]),
+          )
+        : undefined,
   };
 }

packages/cli/src/lib/bundler/types.ts

@@ -23,6 +23,12 @@ export type ModuleFederationOptions = {
   name: string;
   // Whether this is a host or remote bundle
   mode: 'host' | 'remote';
+  exposes?: {
+    /**
+     * Modules that should be exposed by this container.
+     */
+    [k: string]: string;
+  };
 };
 
 export type BundlingOptions = {