Implement queueMicrotask and drainMicrotasks in JSC

Summary:
Changelog: [internal]

## Context

We want to enable the new React Native event loop by default for all users on the new RN architecture (on the bridgeless initialization path more concretely), which requires support for microtasks in all the JS engines that the support (Hermes already has it, JSC doesn't).

## Changes

This adds initial support for microtasks in JSC, so we can schedule and execute microtasks in this runtime.

One limitation about this approach is that, AFAIK, the public API for JSC doesn't allow us to customize its internal microtask queue or specify the method to be used by its built-in `Promise` or native `async function`, so we're forced to continue using a polyfill in that case (which uses `setImmediate` that will be mapped to `queueMicrotask`).

Reviewed By: NickGerleman

Differential Revision: D54302534

fbshipit-source-id: 47f71620344a81bc6624917f77452106ffbf55a3

packages/react-native/ReactCommon/jsc/JSCRuntime.cpp

@@ -13,8 +13,8 @@
 #include <atomic>
 #include <condition_variable>
 #include <cstdlib>
+#include <deque>
 #include <mutex>
-#include <queue>
 #include <sstream>
 #include <thread>
 
@@ -51,6 +51,12 @@ class JSCRuntime : public jsi::Runtime {
       const std::shared_ptr<const jsi::Buffer>& buffer,
       const std::string& sourceURL) override;
 
+  // If we use this interface to implement microtasks in the host we need to
+  // polyfill `Promise` to use these methods, because JSC doesn't currently
+  // support providing a custom queue for its built-in implementation.
+  // Not doing this would result in a non-compliant behavior, as microtasks
+  // wouldn't execute in the order in which they were queued.
+  void queueMicrotask(const jsi::Function& callback) override;
   bool drainMicrotasks(int maxMicrotasksHint = -1) override;
 
   jsi::Object global() override;
@@ -265,6 +271,7 @@ class JSCRuntime : public jsi::Runtime {
   std::atomic<bool> ctxInvalid_;
   std::string desc_;
   JSValueRef nativeStateSymbol_ = nullptr;
+  std::deque<jsi::Function> microtaskQueue_;
 #ifndef NDEBUG
   mutable std::atomic<intptr_t> objectCounter_;
   mutable std::atomic<intptr_t> symbolCounter_;
@@ -380,6 +387,10 @@ JSCRuntime::JSCRuntime(JSGlobalContextRef ctx)
 }
 
 JSCRuntime::~JSCRuntime() {
+  // We need to clear the microtask queue to remove all references to the
+  // callbacks, so objectCounter_ would be 0 below.
+  microtaskQueue_.clear();
+
   // On shutting down and cleaning up: when JSC is actually torn down,
   // it calls JSC::Heap::lastChanceToFinalize internally which
   // finalizes anything left over.  But at this point,
@@ -436,7 +447,24 @@ jsi::Value JSCRuntime::evaluateJavaScript(
   return createValue(res);
 }
 
-bool JSCRuntime::drainMicrotasks(int maxMicrotasksHint) {
+void JSCRuntime::queueMicrotask(const jsi::Function& callback) {
+  microtaskQueue_.emplace_back(
+      jsi::Value(*this, callback).asObject(*this).asFunction(*this));
+}
+
+bool JSCRuntime::drainMicrotasks(int /*maxMicrotasksHint*/) {
+  // Note that new jobs can be enqueued during the draining.
+  while (!microtaskQueue_.empty()) {
+    jsi::Function callback = std::move(microtaskQueue_.front());
+
+    // We need to pop before calling the callback because that might throw.
+    // When that happens, the host will call `drainMicrotasks` again to execute
+    // the remaining microtasks, and this one shouldn't run again.
+    microtaskQueue_.pop_front();
+
+    callback.call(*this);
+  }
+
   return true;
 }