Error when using runTask in a re-entrant way

packages/react-native-fantom/src/__tests__/Fantom-itest.js

@@ -10,6 +10,7 @@
  */
 
 import 'react-native/Libraries/Core/InitializeCore';
+
 import {createRoot, runTask} from '..';
 import * as React from 'react';
 import {Text, View} from 'react-native';
@@ -25,7 +26,7 @@
     });
 
     // TODO: fix error handling and make this pass
     it.skip('should re-throw errors from the task synchronously', () => {
       expect(() => {
         runTask(() => {
           throw new Error('test error');
@@ -50,7 +51,7 @@
     });
 
     // TODO: fix error handling and make this pass
     it.skip('should re-throw errors from microtasks synchronously', () => {
       expect(() => {
         runTask(() => {
           queueMicrotask(() => {
@@ -70,6 +71,32 @@
 
       expect(completed).toBe(true);
     });
+
+    // TODO: when error handling is fixed, this should verify using `toThrow`
+    it('should throw when running a task inside another task', () => {
+      let lastCallbackExecuted = 0;
+      runTask(() => {
+        lastCallbackExecuted = 1;
+        runTask(() => {
+          lastCallbackExecuted = 2;
+          throw new Error('Recursive runTask should be unreachable');
+        });
+      });
+      expect(lastCallbackExecuted).toBe(1);
+
+      runTask(() => {
+        queueMicrotask(() => {
+          lastCallbackExecuted = 3;
+          runTask(() => {
+            lastCallbackExecuted = 4;
+            throw new Error(
+              'Recursive runTask from micro-task should be unreachable',
+            );
+          });
+        });
+      });
+      expect(lastCallbackExecuted).toBe(3);
+    });
   });
 
   describe('getRenderedOutput', () => {

packages/react-native-fantom/src/index.js

@@ -60,17 +60,31 @@ class Root {
   // TODO: add an API to check if all surfaces were deallocated when tests are finished.
 }
 
+let flushingQueue = false;
+
 /*
  * Runs a task on on the event loop. To be used together with root.render.
  *
  * React must run inside of event loop to ensure scheduling environment is closer to production.
  */
 export function runTask(task: () => void | Promise<void>) {
+  if (flushingQueue) {
+    throw new Error(
+      'Nested runTask calls are not allowed. If you want to schedule a task from inside another task, use scheduleTask instead.',
+    );
+  }
+
   nativeRuntimeScheduler.unstable_scheduleCallback(
     schedulerPriorityImmediate,
     task,
   );
-  global.$$JSTesterModuleName$$.flushMessageQueue();
+
+  try {
+    flushingQueue = true;
+    global.$$JSTesterModuleName$$.flushMessageQueue();
+  } finally {
+    flushingQueue = false;
+  }
 }
 
 // TODO: Add option to define surface props and pass it to startSurface