app-backend: validate env config against schema

Signed-off-by: Vincenzo Scamporlino <vincenzos@spotify.com>

.changeset/friendly-coins-approve.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-app-backend': patch
+---
+
+Fixed unexpected behaviour where configuration supplied with `APP_CONFIG_*` environment variables where not filtered by the configuration schema.

plugins/app-backend/src/lib/config/readFrontendConfig.test.ts

@@ -0,0 +1,99 @@
+/*
+ * Copyright 2024 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 { createMockDirectory } from '@backstage/backend-test-utils';
+import { readFrontendConfig } from './readFrontendConfig';
+import { ConfigReader } from '@backstage/config';
+
+describe('readFrontendConfig', () => {
+  const mockDir = createMockDirectory();
+
+  afterEach(() => {
+    mockDir.clear();
+  });
+
+  it('should validate env config', async () => {
+    mockDir.setContent({
+      'appDir/.config-schema.json': JSON.stringify({
+        schemas: [
+          {
+            value: {
+              type: 'object',
+
+              properties: {
+                app: {
+                  type: 'object',
+                  properties: {
+                    secretOfLife: {
+                      type: 'string',
+                      visibility: 'secret',
+                    },
+                    backendConfig: {
+                      type: 'string',
+                      visibility: 'backend',
+                    },
+                    publicValue: {
+                      type: 'string',
+                      visibility: 'frontend',
+                    },
+                  },
+                },
+              },
+            },
+          },
+        ],
+        backstageConfigSchemaVersion: 1,
+      }),
+    });
+
+    const config = new ConfigReader({
+      app: {
+        secretOfLife: '42',
+        backendConfig: 'backend',
+        publicValue: 'public',
+      },
+    });
+
+    const frontendConfig = await readFrontendConfig({
+      env: {
+        APP_CONFIG_app_secretOfLife: 'ignored',
+        APP_CONFIG_app_backendConfig: 'ignored',
+        APP_CONFIG_app_publicValue: 'injected',
+      },
+      appDistDir: `${mockDir.path}/appDir`,
+      config,
+    });
+
+    expect(frontendConfig).toEqual([
+      {
+        context: 'env',
+        data: {
+          app: {
+            publicValue: 'injected',
+          },
+        },
+        deprecatedKeys: [],
+        filteredKeys: undefined,
+      },
+      {
+        context: 'app',
+        data: { app: { publicValue: 'public' } },
+        deprecatedKeys: [],
+        filteredKeys: undefined,
+      },
+    ]);
+  });
+});

plugins/app-backend/src/lib/config/readFrontendConfig.ts

@@ -36,10 +36,9 @@ export async function readFrontendConfig(options: {
 }): Promise<AppConfig[]> {
   const { env, appDistDir, config } = options;
 
-  const appConfigs = readEnvConfig(env);
-
   const schemaPath = resolvePath(appDistDir, '.config-schema.json');
   if (await fs.pathExists(schemaPath)) {
+    const envConfigs = readEnvConfig(env);
     const serializedSchema = await fs.readJson(schemaPath);
 
     try {
@@ -49,11 +48,10 @@ export async function readFrontendConfig(options: {
           serialized: serializedSchema,
         }));
 
-      const frontendConfigs = await schema.process(
-        [{ data: config.get() as JsonObject, context: 'app' }],
+      return await schema.process(
+        [...envConfigs, { data: config.get() as JsonObject, context: 'app' }],
         { visibility: ['frontend'], withDeprecatedKeys: true },
       );
-      appConfigs.push(...frontendConfigs);
     } catch (error) {
       throw new Error(
         'Invalid app bundle schema. If this error is unexpected you need to run `yarn build` in the app. ' +
@@ -63,5 +61,5 @@ export async function readFrontendConfig(options: {
     }
   }
 
-  return appConfigs;
+  return [];
 }