packages/cli: add support for runtime configuration of the webpack public path

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

packages/cli/config/webpack-public-path.js

@@ -0,0 +1,31 @@
+/*
+ * 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.
+ */
+
+// This script is used to pick up and set the public path of the Webpack bundle
+// at runtime. The meta tag is injected by the app build, but only present in
+// the `index.html.tmpl` file. The runtime value of the meta tag is populated by
+// the app backend, when it templates the final `index.html` file.
+//
+// This is needed for additional chunks to use the correct public path, and it
+// is not possible to set the `__webpack_public_path__` variable outside of the
+// build itself. The Webpack output also does not read any <base> tags or
+// similar, this seems to be the only way to dynamically configure the public
+// path at runtime.
+const el = document.querySelector('meta[name="backstage-public-path"]');
+const path = el?.getAttribute('content');
+if (path) {
+  __webpack_public_path__ = path;
+}