Enable synchronous void method invocations via feature flag - RFC (#56740)

Summary:
Pull Request resolved: #56740

Introduce a feature flag `enableSyncVoidMethods` that allows TurboModule methods with void return types to be invoked synchronously on the JS thread instead of being dispatched asynchronously.

The flag only applies to pure TurboModules (not interop modules) to avoid impacting backward compatibility with legacy Native Modules. When enabled, void methods behave like other synchronous methods, improving performance for operations that don't need async dispatch.

# Behavior mismatch
Currently:
- C++ Turbo Modules run void methods `sync`
- Java / ObjC Turbo Modules run void methods `async`

# Rationale
JavaScript code like this
```
      const buffer = new ArrayBuffer(4);
      const view = new Uint8Array(buffer);
      view[0] = 1;
      view[1] = 2;
      view[2] = 3;
      view[3] = 4;
      const result = NativeCxxModuleExample.passArrayBuffer(buffer);
      /** In native code do something like:
      * std::span<uint8_t> bytes(arg.data(rt), arg.size(rt));
      * std::reverse(bytes.begin(), bytes.end());
      **/
      // ArrayBuffer was Shared with native code and value updates are reflected to JS code
      // view[0] = 4;
      // view[1] = 3;
      // view[2] = 2;
      // view[3] = 1;
```
for a TM spec as
```
 +passArrayBuffer: (arg: ArrayBuffer) => void;
```
works for C++ Turbo Modules
see: #56690

but currently fails for Java / Objc Turbo Modules as calling
```
const result = NativeCxxModuleExample.passArrayBuffer(buffer); //
```
is done `async` for Java/ObjC Modules and hence we would need to copy the `ArrayBuffer` in native code.

Changelog: [Internal]

Differential Revision: D104331837

Author: christophpurrer

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<<4342ccb696b4123b9d463a31c024b9bc>>
+ * @generated SignedSource<<4b2133f0c7c9b0a7ef37e9b928faad64>>
  */
 
 /**
@@ -300,6 +300,12 @@ public object ReactNativeFeatureFlags {
   @JvmStatic
   public fun enableSwiftUIBasedFilters(): Boolean = accessor.enableSwiftUIBasedFilters()
 
+  /**
+   * When enabled, TurboModule methods with void return type will be invoked synchronously on the JS thread instead of being dispatched asynchronously. Only applies to pure TurboModules, not interop modules.
+   */
+  @JvmStatic
+  public fun enableSyncVoidMethods(): Boolean = accessor.enableSyncVoidMethods()
+
   /**
    * Enables View Culling: as soon as a view goes off screen, it can be reused anywhere in the UI and pieced together with other items to create new UI elements.
    */

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<<ec375fbc64b2b2b8995c1640a0369923>>
+ * @generated SignedSource<<65e4bd35f7b92d56d1b26176b8c08b90>>
  */
 
 /**
@@ -65,6 +65,7 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
   private var enablePropsUpdateReconciliationAndroidCache: Boolean? = null
   private var enableSchedulerDelegateInvalidationCache: Boolean? = null
   private var enableSwiftUIBasedFiltersCache: Boolean? = null
+  private var enableSyncVoidMethodsCache: Boolean? = null
   private var enableViewCullingCache: Boolean? = null
   private var enableViewRecyclingCache: Boolean? = null
   private var enableViewRecyclingForImageCache: Boolean? = null
@@ -518,6 +519,15 @@ internal class ReactNativeFeatureFlagsCxxAccessor : ReactNativeFeatureFlagsAcces
     return cached
   }
 
+  override fun enableSyncVoidMethods(): Boolean {
+    var cached = enableSyncVoidMethodsCache
+    if (cached == null) {
+      cached = ReactNativeFeatureFlagsCxxInterop.enableSyncVoidMethods()
+      enableSyncVoidMethodsCache = cached
+    }
+    return cached
+  }
+
   override fun enableViewCulling(): Boolean {
     var cached = enableViewCullingCache
     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<<81989dbed82df7bd963d8023c595ff44>>
+ * @generated SignedSource<<9cc7f5ffd7c0951db9bbf397e96aa83d>>
  */
 
 /**
@@ -118,6 +118,8 @@ public object ReactNativeFeatureFlagsCxxInterop {
 
   @DoNotStrip @JvmStatic public external fun enableSwiftUIBasedFilters(): Boolean
 
+  @DoNotStrip @JvmStatic public external fun enableSyncVoidMethods(): Boolean
+
   @DoNotStrip @JvmStatic public external fun enableViewCulling(): Boolean
 
   @DoNotStrip @JvmStatic public external fun enableViewRecycling(): 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<<93e83fb3dc9577acd9678803321e0fe1>>
+ * @generated SignedSource<<2efb60076f46168bacb0cf064cc574b9>>
  */
 
 /**
@@ -113,6 +113,8 @@ public open class ReactNativeFeatureFlagsDefaults : ReactNativeFeatureFlagsProvi
 
   override fun enableSwiftUIBasedFilters(): Boolean = false
 
+  override fun enableSyncVoidMethods(): Boolean = false
+
   override fun enableViewCulling(): Boolean = false
 
   override fun enableViewRecycling(): 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<<9d07e8adeee69583b788069649306d8e>>
+ * @generated SignedSource<<9481147f7c9af5ba0c2c3c3f609f7050>>
  */
 
 /**
@@ -69,6 +69,7 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
   private var enablePropsUpdateReconciliationAndroidCache: Boolean? = null
   private var enableSchedulerDelegateInvalidationCache: Boolean? = null
   private var enableSwiftUIBasedFiltersCache: Boolean? = null
+  private var enableSyncVoidMethodsCache: Boolean? = null
   private var enableViewCullingCache: Boolean? = null
   private var enableViewRecyclingCache: Boolean? = null
   private var enableViewRecyclingForImageCache: Boolean? = null
@@ -567,6 +568,16 @@ internal class ReactNativeFeatureFlagsLocalAccessor : ReactNativeFeatureFlagsAcc
     return cached
   }
 
+  override fun enableSyncVoidMethods(): Boolean {
+    var cached = enableSyncVoidMethodsCache
+    if (cached == null) {
+      cached = currentProvider.enableSyncVoidMethods()
+      accessedFeatureFlags.add("enableSyncVoidMethods")
+      enableSyncVoidMethodsCache = cached
+    }
+    return cached
+  }
+
   override fun enableViewCulling(): Boolean {
     var cached = enableViewCullingCache
     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<<bcc599e04e3672d4bff293e267b3efa2>>
+ * @generated SignedSource<<ccd35df7079700d8e7dc95e8fac53039>>
  */
 
 /**
@@ -113,6 +113,8 @@ public interface ReactNativeFeatureFlagsProvider {
 
   @DoNotStrip public fun enableSwiftUIBasedFilters(): Boolean
 
+  @DoNotStrip public fun enableSyncVoidMethods(): Boolean
+
   @DoNotStrip public fun enableViewCulling(): Boolean
 
   @DoNotStrip public fun enableViewRecycling(): 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<<32c8c18771d6690f50d60dbc8e3d07e2>>
+ * @generated SignedSource<<f8fe31da669d498efb3d3b580ee57853>>
  */
 
 /**
@@ -309,6 +309,12 @@ class ReactNativeFeatureFlagsJavaProvider
     return method(javaProvider_);
   }
 
+  bool enableSyncVoidMethods() override {
+    static const auto method =
+        getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("enableSyncVoidMethods");
+    return method(javaProvider_);
+  }
+
   bool enableViewCulling() override {
     static const auto method =
         getReactNativeFeatureFlagsProviderJavaClass()->getMethod<jboolean()>("enableViewCulling");
@@ -820,6 +826,11 @@ bool JReactNativeFeatureFlagsCxxInterop::enableSwiftUIBasedFilters(
   return ReactNativeFeatureFlags::enableSwiftUIBasedFilters();
 }
 
+bool JReactNativeFeatureFlagsCxxInterop::enableSyncVoidMethods(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
+  return ReactNativeFeatureFlags::enableSyncVoidMethods();
+}
+
 bool JReactNativeFeatureFlagsCxxInterop::enableViewCulling(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop> /*unused*/) {
   return ReactNativeFeatureFlags::enableViewCulling();
@@ -1221,6 +1232,9 @@ void JReactNativeFeatureFlagsCxxInterop::registerNatives() {
       makeNativeMethod(
         "enableSwiftUIBasedFilters",
         JReactNativeFeatureFlagsCxxInterop::enableSwiftUIBasedFilters),
+      makeNativeMethod(
+        "enableSyncVoidMethods",
+        JReactNativeFeatureFlagsCxxInterop::enableSyncVoidMethods),
       makeNativeMethod(
         "enableViewCulling",
         JReactNativeFeatureFlagsCxxInterop::enableViewCulling),

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<<aacbc520b40625dbdba328590cc78112>>
+ * @generated SignedSource<<ea2ed97e3e835c1fc96dd4d50daef879>>
  */
 
 /**
@@ -165,6 +165,9 @@ class JReactNativeFeatureFlagsCxxInterop
   static bool enableSwiftUIBasedFilters(
     facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
 
+  static bool enableSyncVoidMethods(
+    facebook::jni::alias_ref<JReactNativeFeatureFlagsCxxInterop>);
+
   static bool enableViewCulling(
     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<<483ac339c0d636c85f4639c60982c504>>
+ * @generated SignedSource<<e32e9b6fc8c8872a64a2a5f53ce20a60>>
  */
 
 /**
@@ -206,6 +206,10 @@ bool ReactNativeFeatureFlags::enableSwiftUIBasedFilters() {
   return getAccessor().enableSwiftUIBasedFilters();
 }
 
+bool ReactNativeFeatureFlags::enableSyncVoidMethods() {
+  return getAccessor().enableSyncVoidMethods();
+}
+
 bool ReactNativeFeatureFlags::enableViewCulling() {
   return getAccessor().enableViewCulling();
 }

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<<771a335070649f38cb559a95d80947aa>>
+ * @generated SignedSource<<7d77e761aea48d71ac2f19195bbedced>>
  */
 
 /**
@@ -264,6 +264,11 @@ class ReactNativeFeatureFlags {
    */
   RN_EXPORT static bool enableSwiftUIBasedFilters();
 
+  /**
+   * When enabled, TurboModule methods with void return type will be invoked synchronously on the JS thread instead of being dispatched asynchronously. Only applies to pure TurboModules, not interop modules.
+   */
+  RN_EXPORT static bool enableSyncVoidMethods();
+
   /**
    * Enables View Culling: as soon as a view goes off screen, it can be reused anywhere in the UI and pieced together with other items to create new UI elements.
    */