From 2163e987857d2f9e7e37051e463455739bc98411 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Rub=C3=A9n=20Norte?= <rubennorte@meta.com>
Date: Mon, 4 Mar 2024 02:59:09 -0800
Subject: [PATCH] Add queueMicrotask method to JSI

Summary:
Changelog: [internal]

## Context

Microtasks are an important aspect of JavaScript and they will become increasingly important in the hosts where we're currently using JSI.

For example, React Native is going to adopt an event loop processing model similar to the one on the Web, which means it would need the ability to schedule and execute microtasks in every iteration of the loop. See https://github.com/react-native-community/discussions-and-proposals/pull/744 for details.

JSI already has a method to execute all pending microtasks (`drainMicrotasks`) but without a method to schedule microtasks this is incomplete.

We're currently testing microtasks with Hermes using an internal method to schedule microtasks (`HermesInternal.enqueueJob`) but we need a method in JSI so this also works in other runtimes like JSC and V8.

## Changes

This adds the `queueMicrotask` to the Runtime API in JSI so we have symmetric API for microtasks and we can implement the necessary functionality.

The expectation for JSI implementations is to queue microtasks from this method and from built-ins like Promises and async functions in the same queue, and not drain that queue until explicitly done via `drainMicrotasks` in JSI.

This also modifies Hermes and JSC to provide stubs for those methods, and the actual implementation will be done in following diffs.

Reviewed By: neildhar

Differential Revision: D54302536
---
 API/jsi/jsi/decorator.h | 7 +++++++
 API/jsi/jsi/jsi.cpp     | 4 ++++
 API/jsi/jsi/jsi.h       | 7 +++++++
 3 files changed, 18 insertions(+)

diff --git a/API/jsi/jsi/decorator.h b/API/jsi/jsi/decorator.h
index 7bddd1fad80..6f4351a420d 100644
--- a/API/jsi/jsi/decorator.h
+++ b/API/jsi/jsi/decorator.h
@@ -126,6 +126,9 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
       const std::shared_ptr<const PreparedJavaScript>& js) override {
     return plain().evaluatePreparedJavaScript(js);
   }
+  void queueMicrotask(const jsi::Function& callback) override {
+    return plain().queueMicrotask(callback);
+  }
   bool drainMicrotasks(int maxMicrotasksHint) override {
     return plain().drainMicrotasks(maxMicrotasksHint);
   }
@@ -544,6 +547,10 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     Around around{with_};
     return RD::evaluatePreparedJavaScript(js);
   }
+  void queueMicrotask(const Function& callback) override {
+    Around around{with_};
+    RD::queueMicrotask(callback);
+  }
   bool drainMicrotasks(int maxMicrotasksHint) override {
     Around around{with_};
     return RD::drainMicrotasks(maxMicrotasksHint);
diff --git a/API/jsi/jsi/jsi.cpp b/API/jsi/jsi/jsi.cpp
index 2d1003f1f83..bc953a55957 100644
--- a/API/jsi/jsi/jsi.cpp
+++ b/API/jsi/jsi/jsi.cpp
@@ -87,6 +87,10 @@ NativeState::~NativeState() {}
 
 Runtime::~Runtime() {}
 
+void Runtime::queueMicrotask(const jsi::Function& /*callback*/) {
+  throw JSINativeException("queueMicrotask is not implemented in this runtime");
+}
+
 Instrumentation& Runtime::instrumentation() {
   class NoInstrumentation : public Instrumentation {
     std::string getRecordedGCStats() override {
diff --git a/API/jsi/jsi/jsi.h b/API/jsi/jsi/jsi.h
index 962dae93609..d86005e4e37 100644
--- a/API/jsi/jsi/jsi.h
+++ b/API/jsi/jsi/jsi.h
@@ -209,6 +209,13 @@ class JSI_EXPORT Runtime {
   virtual Value evaluatePreparedJavaScript(
       const std::shared_ptr<const PreparedJavaScript>& js) = 0;
 
+  /// Queues a microtask in the JavaScript VM internal Microtask (a.k.a. Job in
+  /// ECMA262) queue, to be executed when the host drains microtasks in
+  /// its event loop implementation.
+  ///
+  /// \param callback a function to be executed as a microtask.
+  virtual void queueMicrotask(const jsi::Function& callback);
+
   /// Drain the JavaScript VM internal Microtask (a.k.a. Job in ECMA262) queue.
   ///
   /// \param maxMicrotasksHint a hint to tell an implementation that it should