fix : fix a bug in the upstream CommonJSModuleLoader (janus-idp#600)

* fix : fix a bug in the upstream CommonJSLoader

that prevented laoding modules from embedded `node_modules` folders
of private packages located in the plugin `node_modules` folder.

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

* Add a comment, as requested in review.

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

---------

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

.changeset/thirty-bags-kiss.md

@@ -0,0 +1,5 @@
+---
+'backend': patch
+---
+
+Fix a bug in the upstream CommonJSLoader, which prevented laoding modules from embedded node_modules folders of private packages located in the plugin node_modules folder.

packages/backend/src/index.ts

@@ -50,6 +50,13 @@ import {
 } from '@backstage/backend-plugin-manager';
 import { DefaultEventBroker } from '@backstage/plugin-events-backend';
 
+// TODO(davidfestal): The following import is a temporary workaround for a bug
+// in the upstream @backstage/backend-plugin-manager package.
+//
+// It should be removed as soon as the upstream package is fixed and released.
+// see https://github.com/janus-idp/backstage-showcase/pull/600
+import { CommonJSModuleLoader } from './loader/CommonJSModuleLoader';
+
 function makeCreateEnv(config: Config, pluginProvider: BackendPluginProvider) {
   const root = getRootLogger();
   const reader = UrlReaders.default({ logger: root, config });
@@ -161,7 +168,12 @@ async function main() {
     argv: process.argv,
     logger,
   });
-  const pluginManager = await PluginManager.fromConfig(config, logger);
+  const pluginManager = await PluginManager.fromConfig(
+    config,
+    logger,
+    undefined,
+    new CommonJSModuleLoader(logger),
+  );
   const createEnv = makeCreateEnv(config, pluginManager);
 
   const appEnv = useHotMemoize(module, () => createEnv('app'));

packages/backend/src/loader/CommonJSModuleLoader.ts

@@ -0,0 +1,56 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { ModuleLoader } from '@backstage/backend-plugin-manager';
+
+import path from 'path';
+import { Logger } from 'winston';
+
+export class CommonJSModuleLoader implements ModuleLoader {
+  constructor(public readonly logger: Logger) {}
+
+  async bootstrap(
+    backstageRoot: string,
+    dynamicPluginsPaths: string[],
+  ): Promise<void> {
+    const backstageRootNodeModulesPath = `${backstageRoot}/node_modules`;
+    const dynamicNodeModulesPaths = [
+      ...dynamicPluginsPaths.map(p => path.resolve(p, 'node_modules')),
+    ];
+    const Module = require('module');
+    const oldNodeModulePaths = Module._nodeModulePaths;
+    Module._nodeModulePaths = (from: string): string[] => {
+      const result: string[] = oldNodeModulePaths(from);
+      if (!dynamicPluginsPaths.some(p => from.startsWith(p))) {
+        return result;
+      }
+      const filtered = result.filter(nodeModulePath => {
+        return (
+          nodeModulePath === backstageRootNodeModulesPath ||
+          dynamicNodeModulesPaths.some(p => nodeModulePath.startsWith(p))
+        );
+      });
+      this.logger.debug(
+        `Overriding node_modules search path for dynamic plugin ${from} to: ${filtered}`,
+      );
+      return filtered;
+    };
+  }
+
+  async load(packagePath: string): Promise<any> {
+    return await import(/* webpackIgnore: true */ packagePath);
+  }
+}