Enable synchronous void method invocations via feature flag - RFC

[christophpurrer]

Summary:
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

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104331837.