Fix CI failure and add bundler validation improvements

Critical Fixes:
- Fixed RSpec test mutation issue by deep copying config with YAML.load(YAML.dump())
- Prevents test contamination from mutating original_config snapshot

Improvements:
1. Added bundler validation with helpful error messages
   - Validates assets_bundler is 'webpack' or 'rspack'
   - Provides clear error with valid options

2. Added memoization to getBundler()
   - Caches bundler module for performance
   - Documented that config requires restart to change

3. Enhanced edge case test coverage (+3 tests, now 17 total)
   - Tests undefined bundler (defaults to webpack)
   - Tests invalid bundler (throws clear error)
   - Tests memoization (returns cached instance)

4. Improved SWC config documentation
   - Made React runtime comment version-agnostic
   - Added TODO for React 19+ migration consideration

All tests passing: 17 tests across 4 suites.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

client/__tests__/webpack/bundlerUtils.spec.js

@@ -104,4 +104,38 @@ describe('bundlerUtils', () => {
       expect(plugin.name).toBe('MockCssExtractRspackPlugin');
     });
   });
+
+  describe('Edge cases and error handling', () => {
+    it('defaults to webpack when assets_bundler is undefined', () => {
+      mockConfig.assets_bundler = undefined;
+      jest.doMock('shakapacker', () => ({ config: mockConfig }));
+      const utils = require('../../../config/webpack/bundlerUtils');
+
+      const bundler = utils.getBundler();
+
+      expect(bundler).toBeDefined();
+      expect(bundler.ProvidePlugin.name).toBe('MockProvidePlugin');
+    });
+
+    it('throws error for invalid bundler type', () => {
+      mockConfig.assets_bundler = 'invalid-bundler';
+      jest.doMock('shakapacker', () => ({ config: mockConfig }));
+      const utils = require('../../../config/webpack/bundlerUtils');
+
+      expect(() => utils.getBundler()).toThrow('Invalid assets_bundler: "invalid-bundler"');
+      expect(() => utils.getBundler()).toThrow('Must be one of: webpack, rspack');
+    });
+
+    it('returns cached bundler on subsequent calls', () => {
+      mockConfig.assets_bundler = 'webpack';
+      jest.doMock('shakapacker', () => ({ config: mockConfig }));
+      const utils = require('../../../config/webpack/bundlerUtils');
+
+      const bundler1 = utils.getBundler();
+      const bundler2 = utils.getBundler();
+
+      // Should return same instance (memoized)
+      expect(bundler1).toBe(bundler2);
+    });
+  });
 });

config/swc.config.js

@@ -13,8 +13,8 @@ const customConfig = {
           // Use classic runtime for SSR compatibility with React on Rails
           // This ensures React is explicitly imported in each component file, which
           // provides better compatibility with server-side rendering in Rails.
-          // Note: React 19 supports automatic runtime with SSR, but classic runtime
-          // is more explicit and avoids potential issues with different React versions.
+          // Classic runtime is more explicit and works reliably across all React versions.
+          // TODO: Consider switching to 'automatic' runtime when fully on React 19+
           runtime: 'classic',
           // Enable React Fast Refresh in development
           refresh: env.isDevelopment && env.runningWebpackDevServer,

config/webpack/bundlerUtils.js

@@ -8,15 +8,45 @@
 
 const { config } = require('shakapacker');
 
+const VALID_BUNDLERS = ['webpack', 'rspack'];
+
+// Cache for bundler module (config is read at startup and cannot change without restart)
+let _cachedBundler = null;
+let _cachedBundlerType = null;
+
 /**
  * Gets the appropriate bundler module based on shakapacker.yml configuration.
  *
+ * Note: The bundler configuration is read from shakapacker.yml at startup.
+ * Changing the config requires restarting the Node process. This function
+ * memoizes the result for performance.
+ *
  * @returns {Object} Either webpack or @rspack/core module
+ * @throws {Error} If assets_bundler is not 'webpack' or 'rspack'
  */
 const getBundler = () => {
-  return config.assets_bundler === 'rspack'
+  // Return cached bundler if config hasn't changed
+  if (_cachedBundler && _cachedBundlerType === config.assets_bundler) {
+    return _cachedBundler;
+  }
+
+  // Validate bundler configuration
+  const bundlerType = config.assets_bundler || 'webpack'; // Default to webpack
+  if (!VALID_BUNDLERS.includes(bundlerType)) {
+    throw new Error(
+      `Invalid assets_bundler: "${bundlerType}". ` +
+      `Must be one of: ${VALID_BUNDLERS.join(', ')}. ` +
+      `Check config/shakapacker.yml`,
+    );
+  }
+
+  // Load and cache the bundler module
+  _cachedBundlerType = bundlerType;
+  _cachedBundler = bundlerType === 'rspack'
     ? require('@rspack/core')
     : require('webpack');
+
+  return _cachedBundler;
 };
 
 /**

spec/webpack/bundler_switching_spec.rb

@@ -14,8 +14,8 @@
 
   describe 'switching between webpack and rspack' do
     it 'successfully builds with webpack' do
-      # Set bundler to webpack
-      config = original_config
+      # Set bundler to webpack (deep copy to avoid mutating original_config)
+      config = YAML.load(YAML.dump(original_config))
       config['default']['assets_bundler'] = 'webpack'
       File.write(shakapacker_config_path, YAML.dump(config))
 
@@ -28,8 +28,8 @@
     end
 
     it 'successfully builds with rspack' do
-      # Set bundler to rspack
-      config = original_config
+      # Set bundler to rspack (deep copy to avoid mutating original_config)
+      config = YAML.load(YAML.dump(original_config))
       config['default']['assets_bundler'] = 'rspack'
       File.write(shakapacker_config_path, YAML.dump(config))
 
@@ -45,8 +45,8 @@
       bundlers = %w[webpack rspack]
 
       bundlers.each do |bundler|
-        # Set bundler
-        config = original_config
+        # Set bundler (deep copy to avoid mutating original_config)
+        config = YAML.load(YAML.dump(original_config))
         config['default']['assets_bundler'] = bundler
         File.write(shakapacker_config_path, YAML.dump(config))