added app-node with a static fallback handler extension point

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

plugins/app-node/src/extensions.ts

@@ -0,0 +1,52 @@
+/*
+ * 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 { createExtensionPoint } from '@backstage/backend-plugin-api';
+import { Handler } from 'express';
+
+/**
+ * The interface for {@link staticFallbackHandlerExtensionPoint}.
+ *
+ * @public
+ */
+export interface StaticFallbackHandlerExtensionPoint {
+  /**
+   * Sets the static fallback handler. This can only be done once.
+   */
+  setStaticFallbackHandler(handler: Handler): void;
+}
+
+/**
+ * An extension point the exposes the ability to configure a static fallback handler for the app backend.
+ *
+ * The static fallback handler is a request handler to handle requests for static content that is not
+ * present in the app bundle.
+ *
+ * This can be used to avoid issues with clients on older deployment versions trying to access lazy
+ * loaded content that is no longer present. Typically the requests would fall back to a long-term
+ * object store where all recently deployed versions of the app are present.
+ *
+ * Another option is to provide a `database` that will take care of storing the static assets instead.
+ *
+ * If both `database` and `staticFallbackHandler` are provided, the `database` will attempt to serve
+ * static assets first, and if they are not found, the `staticFallbackHandler` will be called.
+ *
+ * @public
+ */
+export const staticFallbackHandlerExtensionPoint =
+  createExtensionPoint<StaticFallbackHandlerExtensionPoint>({
+    id: 'app.staticFallbackHandler',
+  });

plugins/app-node/src/index.ts

@@ -0,0 +1,26 @@
+/*
+ * 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.
+ */
+
+/**
+ * Node.js library for the app plugin.
+ *
+ * @packageDocumentation
+ */
+
+export {
+  staticFallbackHandlerExtensionPoint,
+  type StaticFallbackHandlerExtensionPoint,
+} from './extensions';

plugins/app-node/src/setupTests.ts

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+/*
+ * 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.
+ */
+export {};