fix(core): reuse single module instance in jiti config loading on Windows

packages/api/core/spec/fast/util/forge-config.spec.ts

@@ -4,6 +4,7 @@ import { ResolvedForgeConfig } from '@electron-forge/shared-types';
 import { describe, expect, it, vi } from 'vitest';
 
 import findConfig, {
+  createCaseInsensitiveModuleCache,
   forgeConfigIsValidFilePath,
   registerForgeConfigForDirectory,
   renderConfigTemplate,
@@ -411,3 +412,79 @@ describe('renderConfigTemplate', () => {
     });
   });
 });
+
+// Regression coverage for electron/forge#3949. On Windows jiti's resolved
+// module filenames aren't case-canonicalized against Node's realpath-keyed
+// require.cache, so jiti's dedup misses an already-loaded module and evaluates
+// a duplicate (e.g. webpack, whose Compilation stage constants then come back
+// undefined). createCaseInsensitiveModuleCache wraps the cache so lookups that
+// only differ by case still resolve to the single loaded copy.
+describe('createCaseInsensitiveModuleCache', () => {
+  // NodeModule is heavy; the cache logic only cares about object identity.
+  const fakeModule = (id: string) =>
+    ({ id, exports: {} }) as unknown as NodeModule;
+
+  it('resolves a key that differs only by case to the same module object', () => {
+    const realKey =
+      'C:\\Users\\Niklas\\project\\node_modules\\webpack\\lib\\index.js';
+    const mod = fakeModule(realKey);
+    const cache = createCaseInsensitiveModuleCache({ [realKey]: mod });
+
+    // jiti asks with a differently-cased drive/segment casing than Node stored.
+    const requestedKey = realKey.toLowerCase();
+    expect(cache[requestedKey]).toBe(mod);
+    expect(requestedKey in cache).toBe(true);
+  });
+
+  it('uses the exact key fast-path without a case fallback when present', () => {
+    const realKey = '/abs/path/webpack/lib/index.js';
+    const mod = fakeModule(realKey);
+    const cache = createCaseInsensitiveModuleCache({ [realKey]: mod });
+
+    expect(cache[realKey]).toBe(mod);
+    expect(realKey in cache).toBe(true);
+  });
+
+  it('returns undefined / false for keys with no case-insensitive match', () => {
+    const cache = createCaseInsensitiveModuleCache({
+      '/abs/webpack/lib/index.js': fakeModule('/abs/webpack/lib/index.js'),
+    });
+
+    expect(cache['/abs/other/lib/index.js']).toBeUndefined();
+    expect('/abs/other/lib/index.js' in cache).toBe(false);
+  });
+
+  it('reflects entries added after construction (index invalidation)', () => {
+    const cache = createCaseInsensitiveModuleCache({});
+
+    const lateKey = '/Abs/Late/module.js';
+    cache[lateKey] = fakeModule(lateKey);
+
+    // A later case-insensitive lookup must see the newly-set entry.
+    expect(cache[lateKey.toLowerCase()]).toBe(cache[lateKey]);
+    expect(lateKey.toLowerCase() in cache).toBe(true);
+  });
+
+  it('reflects deletions (index invalidation)', () => {
+    const key = '/Abs/module.js';
+    const cache = createCaseInsensitiveModuleCache({ [key]: fakeModule(key) });
+    expect(key.toLowerCase() in cache).toBe(true);
+
+    delete cache[key];
+
+    expect(cache[key.toLowerCase()]).toBeUndefined();
+    expect(key.toLowerCase() in cache).toBe(false);
+  });
+
+  it('writes through to the underlying real cache object', () => {
+    const realCache: Record<string, NodeModule | undefined> = {};
+    const cache = createCaseInsensitiveModuleCache(realCache);
+
+    const key = '/Abs/written.js';
+    const mod = fakeModule(key);
+    cache[key] = mod;
+
+    // Mutations must land on the same object Node hands around, not a copy.
+    expect(realCache[key]).toBe(mod);
+  });
+});

packages/api/core/src/util/forge-config.ts

@@ -1,10 +1,10 @@
+import Module from 'node:module';
 import path from 'node:path';
 
 import { ForgeConfig, ResolvedForgeConfig } from '@electron-forge/shared-types';
 import { Eta } from 'eta';
 import fs from 'fs-extra';
 import * as interpret from 'interpret';
-import { createJiti } from 'jiti';
 import * as rechoir from 'rechoir';
 
 // eslint-disable-next-line n/no-missing-import
@@ -14,6 +14,96 @@ import { runMutatingHook } from './hook';
 import PluginInterface from './plugin-interface';
 import { readRawPackageJson } from './read-package-json';
 
+import type { createJiti } from 'jiti';
+
+// Node's require.cache is keyed by realpath'd, case-preserving filenames. On
+// Windows (a case-insensitive filesystem) jiti's internally-resolved module
+// filenames are not case-canonicalized against those keys, so jiti's dedup
+// (a lookup into its require's `.cache`) misses a module Node has already
+// loaded and jiti evaluates a *second* copy of it. When that module is webpack,
+// the duplicate copy's `Compilation.PROCESS_ASSETS_STAGE_*` constants come back
+// undefined and plugins tap `processAssets` at the wrong stage, silently
+// dropping assets like index.html from the packaged app. See electron/forge#3949.
+//
+// This wraps the raw require.cache in a Proxy that also resolves lookups by a
+// case-insensitive fallback, so jiti's dedup finds the already-loaded copy.
+type ModuleCache = Record<string, NodeModule | undefined>;
+
+export function createCaseInsensitiveModuleCache(
+  realCache: ModuleCache,
+): ModuleCache {
+  // Lazily-built lower-cased-key -> real-key index, invalidated on mutation.
+  let lowerIndex: Map<string, string> | null = null;
+  const buildIndex = () => {
+    lowerIndex = new Map();
+    for (const key of Object.keys(realCache)) {
+      lowerIndex.set(key.toLowerCase(), key);
+    }
+  };
+  // Returns the real (case-preserving) cache key for a requested key when it
+  // only differs by case, or undefined when the exact key already exists or no
+  // case-insensitive match is found.
+  const canonicalKey = (prop: string | symbol): string | undefined => {
+    if (typeof prop !== 'string' || prop in realCache) return undefined;
+    if (lowerIndex === null) buildIndex();
+    return lowerIndex!.get(prop.toLowerCase());
+  };
+  return new Proxy(realCache, {
+    get(target, prop, receiver) {
+      const key = canonicalKey(prop);
+      if (key !== undefined && key in target) return target[key];
+      return Reflect.get(target, prop, receiver);
+    },
+    has(target, prop) {
+      if (canonicalKey(prop) !== undefined) return true;
+      return Reflect.has(target, prop);
+    },
+    set(target, prop, value, receiver) {
+      lowerIndex = null;
+      return Reflect.set(target, prop, value, receiver);
+    },
+    deleteProperty(target, prop) {
+      lowerIndex = null;
+      return Reflect.deleteProperty(target, prop);
+    },
+  });
+}
+
+// jiti destructures `createRequire` from `node:module` at its own load time and
+// forge-config.ts is jiti's sole importer, so we can scope the fix to exactly
+// the moment jiti is loaded: temporarily wrap `Module.createRequire` so the
+// require jiti builds for itself gets a case-insensitive view over the same
+// module cache, then restore it in a `finally`. Only jiti's own require view is
+// affected; the global require.cache is never written or replaced. Off-win32,
+// jiti is loaded untouched. Must use CJS `require('jiti')` (not dynamic import)
+// so the wrapped `createRequire` is the one jiti captures.
+let cachedCreateJiti: typeof createJiti | undefined;
+function loadCreateJiti(): typeof createJiti {
+  if (cachedCreateJiti) return cachedCreateJiti;
+  if (process.platform !== 'win32') {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    cachedCreateJiti = require('jiti').createJiti;
+    return cachedCreateJiti!;
+  }
+  const original = Module.createRequire;
+  Module.createRequire = function patchedCreateRequire(filename) {
+    const req = original.call(this, filename);
+    try {
+      req.cache = createCaseInsensitiveModuleCache(req.cache);
+    } catch {
+      // req.cache non-writable in some runtime: fall back to the untouched cache
+    }
+    return req;
+  };
+  try {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    cachedCreateJiti = require('jiti').createJiti;
+  } finally {
+    Module.createRequire = original;
+  }
+  return cachedCreateJiti!;
+}
+
 const underscoreCase = (str: string) =>
   str
     .replace(/(.)([A-Z][a-z]+)/g, '$1_$2')
@@ -198,7 +288,7 @@ export default async (dir: string): Promise<ResolvedForgeConfig> => {
     try {
       let loadFn;
       if (['.cts', '.mts', '.ts'].includes(path.extname(forgeConfigPath))) {
-        const jiti = createJiti(__filename);
+        const jiti = loadCreateJiti()(__filename);
         loadFn = jiti.import;
       } else {
         loadFn = dynamicImportMaybe;