actually fix @babel/register

packages/gatsby/src/utils/worker/__tests__/test-helpers/create-test-worker.ts

@@ -12,6 +12,16 @@ export function createTestWorker(): GatsbyTestWorkerPool {
   process.env.FORCE_TEST_DATABASE_ID = process.env.JEST_WORKER_ID
   process.env.GATSBY_WORKER_POOL_WORKER = `true`
 
+  // deleting process.env.JEST_WORKER_ID is a workaround for `@babel/register` used in `./wrapper-for-tests`
+  // creating new Worker without explicitly defining `exposedMethods` result in requiring the module
+  // we need a way to recognize when require module is executed in context of process running test
+  // versus process(es) spawned for worker. jest-worker actually doesn't have a way to pass custom env vars
+  // to worker process without overriding JEST_WORKER_ID (yikes), but it does reuse current process.env
+  // so we temporarily unset JEST_WORKER_ID, so that module loaded in the same process as this code is executed
+  // doesn't use `@babel/register` and we rely on `jest-worker` auto-assigning JEST_WORKER_ID for child processes
+  const tmpJestWorkerId = process.env.JEST_WORKER_ID
+  delete process.env.JEST_WORKER_ID
+
   const worker = new Worker(require.resolve(`./wrapper-for-tests`), {
     numWorkers: 1,
     forkOptions: {
@@ -20,6 +30,7 @@ export function createTestWorker(): GatsbyTestWorkerPool {
     maxRetries: 1,
   }) as GatsbyTestWorkerPool
   delete process.env.GATSBY_WORKER_POOL_WORKER
+  process.env.JEST_WORKER_ID = tmpJestWorkerId
 
   return worker
 }

packages/gatsby/src/utils/worker/__tests__/test-helpers/wrapper-for-tests.js

@@ -1,8 +1,13 @@
-// spawned process won't use jest config to support TS, so we need to add support ourselves
-require(`@babel/register`)({
-  extensions: [`.js`, `.ts`],
-  configFile: require.resolve(`../../../../../babel.config.js`),
-  ignore: [/node_modules/],
-})
+// this IS executed in process that spawn worker to analyze exports and create functions for them on worker pool object
+// so we want to use `@babel/register` ONLY inside actual worker as otherwise it does mess with jest transformation and tools like `rewire`
+// see `./create-test-worker.ts` file for explanation why this env var is used.
+if (process.env.JEST_WORKER_ID) {
+  // spawned process won't use jest config to support TS, so we need to add support ourselves
+  require(`@babel/register`)({
+    extensions: [`.js`, `.ts`],
+    configFile: require.resolve(`../../../../../babel.config.js`),
+    ignore: [/node_modules/],
+  })
+}
 
 module.exports = require(`./child-for-tests`)