From 47b45df65c104411339090d2b9c307402a8eb679 Mon Sep 17 00:00:00 2001
From: Nicola Corti <ncor@meta.com>
Date: Mon, 22 May 2023 11:56:13 -0700
Subject: [PATCH] Make sure the Native RuntimeScheduler is initialized on Old
 Arch (#37517)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37517

Fixes #35778

We got reports of regressions on `useEffect` starting from 0.69+ when on Hermes.

The issue seems to be caused by a bump of the `scheduler` package from 0.20 to 0.21.
In scheduler@0.21, the method `setImmediate` gets called if available
(see https://github.com/facebook/react/pull/20834). This causes React Native to use Microtasks
which ends up in changing the semantic of useEffect.

The solution is to use the Native RuntimeScheduler properly.
On Paper specifically, we never initialized it as it's effectively initialized by the
TurboModuleManagerDelegate. Here I trigger the initialization of it on Paper as well.

Changelog:
[Android] [Fixed] - Make sure the Native RuntimeScheduler is initialized on Old Arch

Reviewed By: sammy-SC

Differential Revision: D46024807

fbshipit-source-id: d72cd774df58410467644cddeaaf37e3c227b505
---
 .../java/com/facebook/react/ReactInstanceManager.java    | 9 +++++++++
 .../com/facebook/react/config/ReactFeatureFlags.java     | 7 +++++++
 2 files changed, 16 insertions(+)

diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
index 844050e739eb1f..1df76cc02afe21 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java
@@ -1360,6 +1360,15 @@ private ReactApplicationContext createReactContext(
 
     reactContext.initializeWithInstance(catalystInstance);
 
+    if (ReactFeatureFlags.unstable_useRuntimeSchedulerAlways) {
+      // On Old Architecture, we need to initialize the Native Runtime Scheduler so that
+      // the `nativeRuntimeScheduler` object is registered on JS.
+      // On New Architecture, this is normally triggered by instantiate a TurboModuleManager.
+      // Here we invoke getRuntimeScheduler() to trigger the creation of it regardless of the
+      // architecture so it will always be there.
+      catalystInstance.getRuntimeScheduler();
+    }
+
     if (ReactFeatureFlags.useTurboModules && mTMMDelegateBuilder != null) {
       TurboModuleManagerDelegate tmmDelegate =
           mTMMDelegateBuilder
diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
index 4a4bf687b6a90b..eb0d76fe005065 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java
@@ -42,6 +42,13 @@ public class ReactFeatureFlags {
    */
   public static volatile boolean unstable_useFabricInterop = false;
 
+  /**
+   * Should this application always use the Native RuntimeScheduler? If yes, we'll be instantiating
+   * it over all the architectures (both Old and New). This is intentionally set to true as we want
+   * to use it more as a kill-switch to turn off this feature to potentially debug issues.
+   */
+  public static volatile boolean unstable_useRuntimeSchedulerAlways = true;
+
   /**
    * Feature flag to enable the new bridgeless architecture. Note: Enabling this will force enable
    * the following flags: `useTurboModules` & `enableFabricRenderer`.