From 2167afc5e57bb65103467748d71bf3e04130e7e6 Mon Sep 17 00:00:00 2001
From: Patrik Oldsberg <poldsberg@gmail.com>
Date: Wed, 29 Jan 2025 15:07:09 +0100
Subject: [PATCH] cli: treat static file assets as free from side-effects

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>
---
 .changeset/fifty-trains-attend.md       |  5 ++++
 packages/cli/src/lib/builder/plugins.ts | 40 +++++++++++++++++++++++--
 2 files changed, 43 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/fifty-trains-attend.md

diff --git a/.changeset/fifty-trains-attend.md b/.changeset/fifty-trains-attend.md
new file mode 100644
index 0000000000..04318409fd
--- /dev/null
+++ b/.changeset/fifty-trains-attend.md
@@ -0,0 +1,5 @@
+---
+'@backstage/cli': patch
+---
+
+Treat static file assets as always being free from side effects in package builds.
diff --git a/packages/cli/src/lib/builder/plugins.ts b/packages/cli/src/lib/builder/plugins.ts
index 1c1dd1b1e2..ace2c048a9 100644
--- a/packages/cli/src/lib/builder/plugins.ts
+++ b/packages/cli/src/lib/builder/plugins.ts
@@ -21,7 +21,12 @@ import {
   relative as relativePath,
 } from 'path';
 import { createFilter } from 'rollup-pluginutils';
-import { Plugin, InputOptions, OutputChunk } from 'rollup';
+import {
+  Plugin,
+  InputOptions,
+  OutputChunk,
+  HasModuleSideEffects,
+} from 'rollup';
 
 type ForwardFileImportsOptions = {
   include: Array<string | RegExp> | string | RegExp | null;
@@ -91,6 +96,33 @@ export function forwardFileImports(options: ForwardFileImportsOptions) {
       }
     },
     options(inputOptions) {
+      // We're in control of the config ourselves, so these are just checks to
+      // make sure we don't update the config but forget about the config
+      // overrides here
+      const treeshake = inputOptions.treeshake;
+      if (treeshake !== undefined && typeof treeshake !== 'object') {
+        throw new Error(
+          'Expected treeshake input config to be an object or not set',
+        );
+      }
+      if (treeshake?.moduleSideEffects) {
+        throw new Error('treeshake.moduleSideEffects must not be set');
+      }
+
+      // All external assets are treated as being side-effect free.
+      //
+      // This also works around an apparent bug in rollup where the
+      // `makeAbsoluteExternalsRelative: false` option sometimes caused relative
+      // asset paths to be rewritten with an incorrect path. They are rewritten
+      // in the first place because they are being treated as external by this
+      // plugin, but that seems to be the best way to handle asset files.
+      const moduleSideEffects: HasModuleSideEffects = id => {
+        if (filter(id)) {
+          return false;
+        }
+        return true;
+      };
+
       const origExternal = inputOptions.external;
 
       // We decorate any existing `external` option with our own way of determining
@@ -129,7 +161,11 @@ export function forwardFileImports(options: ForwardFileImportsOptions) {
         return true;
       };
 
-      return { ...inputOptions, external };
+      return {
+        ...inputOptions,
+        external,
+        treeshake: { ...treeshake, moduleSideEffects },
+      };
     },
   } satisfies Plugin;
 }