From 95f172e84945dea68381887dd4a9cbd96390a5e5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Dawid=20Ma=C5=82ecki?= <maleckidawid@meta.com>
Date: Wed, 3 Jun 2026 06:49:33 -0700
Subject: [PATCH] Fix AnimationBackend choreographer deadlock with synchronous
 VirtualView events

Summary:
When the shared animation backend is enabled, starting/stopping a native animation calls `AnimationBackendChoreographer.resume()/pause()` on the thread
that started the animation, which can be the **JS thread**. The `resume()` method posted a frame callback to ReactChoreographer, acquiring the choreographer's callback queue. The UI thread might simultaneously hold that lock while dispatching a synchronous event through `executeSynchronouslyOnSameThread_CAN_DEADLOCK`. That path is chose after invoking the VirtualView `Visible` mode-change event. The `executeSynchronouslyOnSameThread_CAN_DEADLOCK` blocks main thread to access `jsi::Runtime` from the JS thread while holding a lock on the callbacks queue. There is a chance that resuming animation and the synchronous event will collide, as the first one tries to post a callback to the queue from the JS thread, waiting for release while the other holds a lock on that queue and waits for the JS thread to pass the runtime, causing a deadlock.

The fix is to keep the choreographer frame callback permanently registered. It is posted once on the UI thread and re-posts itself every frame, running as a no-op while paused. The `resume` and `pause` methods now only toggle so accessing the lock on the callback queue is not necessary.

Changelog:
[Android][Fixed] - Fix deadlock between the UI and JS threads when native animations start while a synchronous VirtualView mode-change event is dispatched

Differential Revision: D107378627
---
 .../fabric/AnimationBackendChoreographer.kt   | 59 +++++++++----------
 1 file changed, 27 insertions(+), 32 deletions(-)

diff --git a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/AnimationBackendChoreographer.kt b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/AnimationBackendChoreographer.kt
index 49c148abb222..7bdeb518d808 100644
--- a/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/AnimationBackendChoreographer.kt
+++ b/packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/AnimationBackendChoreographer.kt
@@ -9,10 +9,10 @@ package com.facebook.react.fabric
 
 import com.facebook.proguard.annotations.DoNotStripAny
 import com.facebook.react.bridge.ReactApplicationContext
+import com.facebook.react.bridge.UiThreadUtil
 import com.facebook.react.modules.core.ReactChoreographer
 import com.facebook.react.uimanager.GuardedFrameCallback
 import java.util.concurrent.atomic.AtomicBoolean
-import kotlin.synchronized
 
 internal fun interface AnimationFrameCallback {
   fun onAnimationFrame(frameTimeMs: Double)
@@ -32,54 +32,49 @@ internal class AnimationBackendChoreographer(
           executeFrameCallback(frameTimeNanos)
         }
       }
-  private val callbackPosted: AtomicBoolean = AtomicBoolean()
+
+  // When true, the always-registered frame callback runs as a no-op.
+  //
+  // The callback is registered with ReactChoreographer once (on the UI thread)
+  // and re-posts itself every frame regardless of this flag, so it stays
+  // registered for the lifetime of the choreographer.
   private val paused: AtomicBoolean = AtomicBoolean(true)
 
-  /*
-   * resume() and pause() should be called with the same lock to avoid race conditions.
-   */
+  init {
+    // Register the self-reposting callback once, on the UI thread, so the
+    // callback queues are only ever mutated from the UI thread.
+    UiThreadUtil.runOnUiThread { postCallback() }
+  }
 
   fun resume() {
-    if (paused.getAndSet(false)) {
-      scheduleCallback()
-    }
+    paused.set(false)
   }
 
   fun pause() {
-    val shouldRemove: Boolean
-    synchronized(paused) {
-      shouldRemove = !paused.getAndSet(true) && callbackPosted.getAndSet(false)
-    }
-    if (shouldRemove) {
-      reactChoreographer.removeFrameCallback(
-          ReactChoreographer.CallbackType.NATIVE_ANIMATED_MODULE,
-          choreographerCallback,
-      )
-    }
+    paused.set(true)
   }
 
-  private fun scheduleCallback() {
-    val shouldPost: Boolean
-    synchronized(paused) { shouldPost = !paused.get() && !callbackPosted.getAndSet(true) }
-    if (shouldPost) {
-      reactChoreographer.postFrameCallback(
-          ReactChoreographer.CallbackType.NATIVE_ANIMATED_MODULE,
-          choreographerCallback,
-      )
-    }
+  private fun postCallback() {
+    reactChoreographer.postFrameCallback(
+        ReactChoreographer.CallbackType.NATIVE_ANIMATED_MODULE,
+        choreographerCallback,
+    )
   }
 
   private fun executeFrameCallback(frameTimeNanos: Long) {
-    callbackPosted.set(false)
     val currentFrameTimeMs = calculateTimestamp(frameTimeNanos)
-    // It is possible for ChoreographerCallback to be executed twice within the same frame
-    // due to frame drops. If this occurs, the additional callback execution should be ignored.
-    if (currentFrameTimeMs > lastFrameTimeMs) {
+    // Only drive the animation backend while enabled. It is possible for the
+    // ChoreographerCallback to be executed twice within the same frame due to
+    // frame drops; if this occurs, the additional callback execution should be
+    // ignored.
+    if (!paused.get() && currentFrameTimeMs > lastFrameTimeMs) {
       frameCallback?.onAnimationFrame(currentFrameTimeMs)
     }
 
     lastFrameTimeMs = currentFrameTimeMs
-    scheduleCallback()
+    // Always re-post (on the UI thread) so the callback stays registered for the
+    // next frame, whether or not we are currently paused.
+    postCallback()
   }
 
   private fun calculateTimestamp(frameTimeNanos: Long): Double {