Error when using runTask in a re-entrant way (#48235)

Summary:
Pull Request resolved: #48235

Recursively calling runTask is not supported, as the inner call will no-op since we're already executing the eventloop.

Currently errors are not correctly propagated, but this at least makes it so that we don't attempt to schedule the task either, which could lead to incorrect assumptions being made.

Changelog: [internal]

Reviewed By: rubennorte

Differential Revision: D67107664

fbshipit-source-id: e665a96671f4812308d87aec3b880ce2009328e2

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';
@@ -70,6 +71,32 @@ describe('Fantom', () => {
 
       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