Make RCTUnsafeExecuteOnMainQueueSync safer

packages/react-native/React/Base/RCTUtils.h

@@ -49,7 +49,7 @@ RCT_EXTERN void RCTExecuteOnMainQueue(dispatch_block_t block);
 // Legacy function to execute the specified block on the main queue synchronously.
 // Please do not use this unless you know what you're doing.
 RCT_EXTERN void RCTUnsafeExecuteOnMainQueueSync(dispatch_block_t block);
-RCT_EXTERN void RCTUnsafeExecuteOnMainQueueSyncWithError(dispatch_block_t block, NSString *context);
+RCT_EXTERN void RCTUnsafeExecuteOnMainQueueSyncWithError(dispatch_block_t block, NSString *_Nullable context);
 
 // Get screen metrics in a thread-safe way
 RCT_EXTERN CGFloat RCTScreenScale(void);

packages/react-native/React/Base/RCTUtils.mm

@@ -19,6 +19,7 @@
 #import <CommonCrypto/CommonCrypto.h>
 
 #import <React/RCTUtilsUIOverride.h>
+#import <ReactCommon/RuntimeExecutorSyncUIThreadUtils.h>
 #import "RCTAssert.h"
 #import "RCTLog.h"
 
@@ -295,56 +296,71 @@ void RCTExecuteOnMainQueue(dispatch_block_t block)
   }
 }
 
+static BOOL RCTIsJSThread()
+{
+  return [[NSThread currentThread].name containsString:@"JavaScript"];
+}
+
 // Please do not use this method
 // unless you know what you are doing.
 void RCTUnsafeExecuteOnMainQueueSync(dispatch_block_t block)
 {
-  if (RCTIsMainQueue()) {
-    block();
-  } else {
-    if (facebook::react::ReactNativeFeatureFlags::disableMainQueueSyncDispatchIOS()) {
-      RCTLogError(@"RCTUnsafeExecuteOnMainQueueSync: Sync dispatches to the main queue can deadlock React Native.");
-    }
-    dispatch_sync(dispatch_get_main_queue(), ^{
-      block();
-    });
-  }
+  RCTUnsafeExecuteOnMainQueueSyncWithError(block, nil);
 }
 
 // Please do not use this method
 // unless you know what you are doing.
-void RCTUnsafeExecuteOnMainQueueSyncWithError(dispatch_block_t block, NSString *context)
+void RCTUnsafeExecuteOnMainQueueSyncWithError(dispatch_block_t block, NSString *_Nullable context)
 {
   if (RCTIsMainQueue()) {
     block();
-  } else {
-    if (facebook::react::ReactNativeFeatureFlags::disableMainQueueSyncDispatchIOS()) {
-      RCTLogError(@"RCTUnsafeExecuteOnMainQueueSync: %@", context);
+    return;
+  }
+
+  if (facebook::react::ReactNativeFeatureFlags::enableSaferMainQueueSyncDispatchOnIOS()) {
+    if (RCTIsJSThread()) {
+      facebook::react::unsafeExecuteOnMainThreadSync(block);
+      return;
     }
-    dispatch_sync(dispatch_get_main_queue(), ^{
-      block();
-    });
+  } else if (facebook::react::ReactNativeFeatureFlags::disableMainQueueSyncDispatchIOS()) {
+    RCTLogError(
+        @"RCTUnsafeExecuteOnMainQueueSync: %@",
+        context ?: @"Sync dispatches to the main queue can deadlock React Native.");
   }
+
+  dispatch_sync(dispatch_get_main_queue(), ^{
+    block();
+  });
 }
 
 static void RCTUnsafeExecuteOnMainQueueOnceSync(dispatch_once_t *onceToken, dispatch_block_t block)
 {
   // The solution was borrowed from a post by Sophie Alpert:
   // https://sophiebits.com/2014/04/02/dispatch-once-initialization-on-the-main-thread
   // See also: https://www.mikeash.com/pyblog/friday-qa-2014-06-06-secrets-of-dispatch_once.html
-  if (RCTIsMainQueue()) {
+  auto executeOnce = ^{
     dispatch_once(onceToken, block);
-  } else {
-    if (DISPATCH_EXPECT(*onceToken == 0L, NO)) {
-      if (facebook::react::ReactNativeFeatureFlags::disableMainQueueSyncDispatchIOS()) {
-        RCTLogError(
-            @"RCTUnsafeExecuteOnMainQueueOnceSync: Sync dispatches to the main queue can deadlock React Native.");
-      }
-      dispatch_sync(dispatch_get_main_queue(), ^{
-        dispatch_once(onceToken, block);
-      });
+  };
+
+  if (RCTIsMainQueue()) {
+    executeOnce();
+    return;
+  }
+
+  if (!DISPATCH_EXPECT(*onceToken == 0L, NO)) {
+    return;
+  }
+
+  if (facebook::react::ReactNativeFeatureFlags::enableSaferMainQueueSyncDispatchOnIOS()) {
+    if (RCTIsJSThread()) {
+      facebook::react::unsafeExecuteOnMainThreadSync(block);
+      return;
     }
+  } else if (facebook::react::ReactNativeFeatureFlags::disableMainQueueSyncDispatchIOS()) {
+    RCTLogError(@"RCTUnsafeExecuteOnMainQueueOnceSync: Sync dispatches to the main queue can deadlock React Native.");
   }
+
+  dispatch_sync(dispatch_get_main_queue(), executeOnce);
 }
 
 CGFloat RCTScreenScale(void)

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlags.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<e9a109fd77667dd0cb945ef6ef9737f2>>
+ * @generated SignedSource<<24415ceea0dad0fec42af0b77ceec393>>
  */
 
 /**
@@ -198,6 +198,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun enableResourceTimingAPI(): Boolean = accessor.enableResourceTimingAPI()
 
+  /**
+   * Make RCTUnsafeExecuteOnMainQueueSync less likely to deadlock, when used in conjuction with sync rendering/events.
+   */
+  @JvmStatic
+  public fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean = accessor.enableSaferMainQueueSyncDispatchOnIOS()
+
   /**
    * Dispatches state updates synchronously in Fabric (e.g.: updates the scroll position in the shadow tree synchronously from the main thread).
    */

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<1a5cd689229d4e0c31070123045e84da>>
+ * @generated SignedSource<<07f351d4f8971f7bd7d2eb0115760565>>
  */
 
 /**
@@ -48,6 +48,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var enablePreparedTextLayoutCache: Boolean? = null
   private var enablePropsUpdateReconciliationAndroidCache: Boolean? = null
   private var enableResourceTimingAPICache: Boolean? = null
+  private var enableSaferMainQueueSyncDispatchOnIOSCache: Boolean? = null
   private var enableSynchronousStateUpdatesCache: Boolean? = null
   private var enableViewCullingCache: Boolean? = null
   private var enableViewRecyclingCache: Boolean? = null
@@ -321,6 +322,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean {
+    var cached = enableSaferMainQueueSyncDispatchOnIOSCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.enableSaferMainQueueSyncDispatchOnIOS()
+      enableSaferMainQueueSyncDispatchOnIOSCache = cached
+    }
+    return cached
+  }
+
   override fun enableSynchronousStateUpdates(): Boolean {
     var cached = enableSynchronousStateUpdatesCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsCxxInterop.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<c39d0c1834797b3309b4fc5ce814280c>>
+ * @generated SignedSource<<db670241f820223af2783a44ca452d98>>
  */
 
 /**
@@ -84,6 +84,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun enableResourceTimingAPI(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean
+
   @DoNotStrip @JvmStatic public external fun enableSynchronousStateUpdates(): Boolean
 
   @DoNotStrip @JvmStatic public external fun enableViewCulling(): Boolean

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsDefaults.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<7f357475254104729cc7910c14e1c1fb>>
+ * @generated SignedSource<<c7599024bfc8c04b5d53145d16a06c20>>
  */
 
 /**
@@ -79,6 +79,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun enableResourceTimingAPI(): Boolean = false
 
+  override fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean = false
+
   override fun enableSynchronousStateUpdates(): Boolean = false
 
   override fun enableViewCulling(): Boolean = false

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsLocalAccessor.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<14cd1a58bd153dedda045a72c1494caa>>
+ * @generated SignedSource<<52f72deb1601f741295b22caa9755749>>
  */
 
 /**
@@ -52,6 +52,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var enablePreparedTextLayoutCache: Boolean? = null
   private var enablePropsUpdateReconciliationAndroidCache: Boolean? = null
   private var enableResourceTimingAPICache: Boolean? = null
+  private var enableSaferMainQueueSyncDispatchOnIOSCache: Boolean? = null
   private var enableSynchronousStateUpdatesCache: Boolean? = null
   private var enableViewCullingCache: Boolean? = null
   private var enableViewRecyclingCache: Boolean? = null
@@ -353,6 +354,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean {
+    var cached = enableSaferMainQueueSyncDispatchOnIOSCache
+    if (cached == null) {
+      cached = currentProvider.enableSaferMainQueueSyncDispatchOnIOS()
+      accessedFeatureFlags.add("enableSaferMainQueueSyncDispatchOnIOS")
+      enableSaferMainQueueSyncDispatchOnIOSCache = cached
+    }
+    return cached
+  }
+
   override fun enableSynchronousStateUpdates(): Boolean {
     var cached = enableSynchronousStateUpdatesCache
     if (cached == null) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/internal/featureflags/ReactNativeFeatureFlagsProvider.kt

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<593b1d64dc31038140032a6b0a439700>>
+ * @generated SignedSource<<8c2ffe69841478213407e92e44519157>>
  */
 
 /**
@@ -79,6 +79,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun enableResourceTimingAPI(): Boolean
 
+  @DoNotStrip public fun enableSaferMainQueueSyncDispatchOnIOS(): Boolean
+
   @DoNotStrip public fun enableSynchronousStateUpdates(): Boolean
 
   @DoNotStrip public fun enableViewCulling(): Boolean

packages/react-native/ReactAndroid/src/main/jni/react/featureflags/JReactNativeFeatureFlagsCxxInterop.cpp

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<d74dc8182a14fddbd7118149298515bd>>
+ * @generated SignedSource<<e197b252b246d3c777ce1850d365220e>>
  */
 
 /**
@@ -207,6 +207,12 @@ class ReactNativeFeatureFlagsJavaProvider
     return method(javaProvider_);
   }
 
+  bool enableSaferMainQueueSyncDispatchOnIOS() override {
+    static const auto method =
+        getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("enableSaferMainQueueSyncDispatchOnIOS");
+    return method(javaProvider_);
+  }
+
   bool enableSynchronousStateUpdates() override {
     static const auto method =
         getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("enableSynchronousStateUpdates");
@@ -471,6 +477,11 @@ bool JReactNativeFeatureFlagsCxxInterop::enableResourceTimingAPI(
   return ReactNativeFeatureFlags::enableResourceTimingAPI();
 }
 
+bool JReactNativeFeatureFlagsCxxInterop::enableSaferMainQueueSyncDispatchOnIOS(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
+  return ReactNativeFeatureFlags::enableSaferMainQueueSyncDispatchOnIOS();
+}
+
 bool JReactNativeFeatureFlagsCxxInterop::enableSynchronousStateUpdates(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
   return ReactNativeFeatureFlags::enableSynchronousStateUpdates();
@@ -686,6 +697,9 @@ void JReactNativeFeatureFlagsCxxInterop::registerNatives() {
       makeNativeMethod(
         "enableResourceTimingAPI",
         JReactNativeFeatureFlagsCxxInterop::enableResourceTimingAPI),
+      makeNativeMethod(
+        "enableSaferMainQueueSyncDispatchOnIOS",
+        JReactNativeFeatureFlagsCxxInterop::enableSaferMainQueueSyncDispatchOnIOS),
       makeNativeMethod(
         "enableSynchronousStateUpdates",
         JReactNativeFeatureFlagsCxxInterop::enableSynchronousStateUpdates),

packages/react-native/ReactAndroid/src/main/jni/react/featureflags/JReactNativeFeatureFlagsCxxInterop.h

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<a5ab0022f6a01bd6e929d36d9d87db10>>
+ * @generated SignedSource<<7f021510ca3f485b75e231d18cb58db9>>
  */
 
 /**
@@ -114,6 +114,9 @@ class JReactNativeFeatureFlagsCxxInterop
   static bool enableResourceTimingAPI(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 
+  static bool enableSaferMainQueueSyncDispatchOnIOS(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
+
   static bool enableSynchronousStateUpdates(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlags.cpp

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<88fdbea2f97f628187164a47a9737da0>>
+ * @generated SignedSource<<997a6c9e018988ae1e4b7f02bad8f433>>
  */
 
 /**
@@ -138,6 +138,10 @@ bool ReactNativeFeatureFlags::enableResourceTimingAPI() {
   return getAccessor().enableResourceTimingAPI();
 }
 
+bool ReactNativeFeatureFlags::enableSaferMainQueueSyncDispatchOnIOS() {
+  return getAccessor().enableSaferMainQueueSyncDispatchOnIOS();
+}
+
 bool ReactNativeFeatureFlags::enableSynchronousStateUpdates() {
   return getAccessor().enableSynchronousStateUpdates();
 }

packages/react-native/ReactCommon/react/featureflags/ReactNativeFeatureFlags.h

@@ -4,7 +4,7 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @generated SignedSource<<a4123bc6f44835c022a1a5238908674c>>
+ * @generated SignedSource<<b4febb028accc1620853ee78403a707a>>
  */
 
 /**
@@ -179,6 +179,11 @@ class ReactNativeFeatureFlags {
    */
   RN_EXPORT static bool enableResourceTimingAPI();
 
+  /**
+   * Make RCTUnsafeExecuteOnMainQueueSync less likely to deadlock, when used in conjuction with sync rendering/events.
+   */
+  RN_EXPORT static bool enableSaferMainQueueSyncDispatchOnIOS();
+
   /**
    * Dispatches state updates synchronously in Fabric (e.g.: updates the scroll position in the shadow tree synchronously from the main thread).
    */