Fix wrong alpha support in dynamic plugins support

The `alpha` sub-package should not be required for the dynamic plugins to be loaded under the new backend system.

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

.changeset/calm-cups-rule.md

@@ -0,0 +1,5 @@
+---
+'@backstage/backend-dynamic-feature-service': patch
+---
+
+Fix wrong `alpha` support in dynamic plugins support: the `alpha` sub-package should not be required for the dynamic plugins to be loaded under the new backend system.

packages/backend-dynamic-feature-service/src/scanner/plugin-scanner.test.ts

@@ -484,6 +484,37 @@ Please add '${mockDir.resolve(
           ],
         },
       },
+      {
+        name: "alpha manifest preferred but skipped because the `alpha` sub-directory doesn't exist",
+        preferAlpha: true,
+        fileSystem: {
+          backstageRoot: {
+            'dist-dynamic': {
+              'test-backend-plugin': {
+                'package.json': JSON.stringify({
+                  name: 'test-backend-plugin-dynamic',
+                  version: '0.0.0',
+                  main: 'dist/index.cjs.js',
+                  backstage: { role: 'backend-plugin' },
+                }),
+              },
+            },
+          },
+        },
+        expectedPluginPackages: [
+          {
+            location: url.pathToFileURL(
+              mockDir.resolve('backstageRoot/dist-dynamic/test-backend-plugin'),
+            ),
+            manifest: {
+              name: 'test-backend-plugin-dynamic',
+              version: '0.0.0',
+              main: 'dist/index.cjs.js',
+              backstage: { role: 'backend-plugin' },
+            },
+          },
+        ],
+      },
       {
         name: 'invalid alpha package.json',
         preferAlpha: true,

packages/backend-dynamic-feature-service/src/scanner/plugin-scanner.ts

@@ -16,7 +16,7 @@
 import { Config } from '@backstage/config';
 import { ScannedPluginPackage, ScannedPluginManifest } from './types';
 import * as fs from 'fs/promises';
-import { Stats, lstatSync } from 'fs';
+import { Stats, lstatSync, existsSync } from 'fs';
 import * as chokidar from 'chokidar';
 import * as path from 'path';
 import * as url from 'url';
@@ -178,22 +178,24 @@ export class PluginScanner {
       if (platform === 'node') {
         if (this.preferAlpha) {
           const pluginHomeAlpha = path.resolve(pluginHome, 'alpha');
-          if ((await fs.lstat(pluginHomeAlpha)).isDirectory()) {
-            const backstage = scannedPlugin.manifest.backstage;
-            try {
-              scannedPlugin = await this.scanDir(pluginHomeAlpha);
-            } catch (e) {
-              this.logger.error(
-                `failed to load dynamic plugin manifest from '${pluginHomeAlpha}'`,
-                e,
+          if (existsSync(pluginHomeAlpha)) {
+            if ((await fs.lstat(pluginHomeAlpha)).isDirectory()) {
+              const backstage = scannedPlugin.manifest.backstage;
+              try {
+                scannedPlugin = await this.scanDir(pluginHomeAlpha);
+              } catch (e) {
+                this.logger.error(
+                  `failed to load dynamic plugin manifest from '${pluginHomeAlpha}'`,
+                  e,
+                );
+                continue;
+              }
+              scannedPlugin.manifest.backstage = backstage;
+            } else {
+              this.logger.warn(
+                `skipping '${pluginHomeAlpha}' since it is not a directory`,
               );
-              continue;
             }
-            scannedPlugin.manifest.backstage = backstage;
-          } else {
-            this.logger.warn(
-              `skipping '${pluginHomeAlpha}' since it is not a directory`,
-            );
           }
         }
       }