fix: prevent preflight from running in loader

src/preflight.cts

@@ -1,21 +1,9 @@
+import { isMainThread } from 'node:worker_threads';
 import { constants as osConstants } from 'os';
 import './suppress-warnings.cts';
 
 type BaseEventListener = () => void;
 
-/**
- * Hook require() to transform to CJS
- *
- * This needs to be loaded via --require flag so subsequent --require
- * flags can support TypeScript.
- *
- * This is also added in loader.ts for the loader API.
- * Although it is required twice, it's not executed twice because
- * it's cached.
- */
-// eslint-disable-next-line import/no-unresolved
-require('./cjs/index.cjs');
-
 const bindHiddenSignalsHandler = (
 	signals: NodeJS.Signals[],
 	handler: NodeJS.SignalsListener,
@@ -47,20 +35,39 @@ const bindHiddenSignalsHandler = (
 	};
 };
 
-// If a parent process is detected
-if (process.send) {
-	bindHiddenSignalsHandler(['SIGINT', 'SIGTERM'], (signal: NodeJS.Signals) => {
-		process.send!({
-			type: 'kill',
-			signal,
-		});
+/**
+ * Seems module.register() calls the loader with the same Node arguments
+ * which causes this preflight to be loaded in the loader thread
+ */
+if (isMainThread) {
+	/**
+	 * Hook require() to transform to CJS
+	 *
+	 * This needs to be loaded via --require flag so subsequent --require
+	 * flags can support TypeScript.
+	 *
+	 * This is also added in loader.ts for the loader API.
+	 * Although it is required twice, it's not executed twice because
+	 * it's cached.
+	 */
+	// eslint-disable-next-line import/no-unresolved
+	require('./cjs/index.cjs');
 
-		/**
-		 * Since we're setting a custom signal handler, we need to emulate the
-		 * default behavior when there are no other handlers set
-		 */
-		if (process.listenerCount(signal) === 0) {
-			process.exit(128 + osConstants.signals[signal]);
-		}
-	});
+	// If a parent process is detected
+	if (process.send) {
+		bindHiddenSignalsHandler(['SIGINT', 'SIGTERM'], (signal: NodeJS.Signals) => {
+			process.send!({
+				type: 'kill',
+				signal,
+			});
+
+			/**
+			 * Since we're setting a custom signal handler, we need to emulate the
+			 * default behavior when there are no other handlers set
+			 */
+			if (process.listenerCount(signal) === 0) {
+				process.exit(128 + osConstants.signals[signal]);
+			}
+		});
+	}
 }