fix: allow Node.js to manage microtasks queue (#28972)

* fix: allow Node.js to manage microtasks queue

When `uv_run()` resulted in invocation of JS functions the microtask
queue checkpoint in Node's CallbackScope was a no-op because the
expected microtask queue policy was `kExplicit` and Electron ran under
`kScoped` policy. This change switches policy to `kExplicit` right
before `uv_run()` and reverts it back to original value after `uv_run()`
completes to provide better compatibility with Node.

* add comment

Co-authored-by: Fedor Indutny <[email protected]>

shell/common/node_bindings.cc

@@ -575,8 +575,13 @@ void NodeBindings::UvRunOnce() {
   // Enter node context while dealing with uv events.
   v8::Context::Scope context_scope(env->context());
 
-  // Perform microtask checkpoint after running JavaScript.
-  gin_helper::MicrotasksScope microtasks_scope(env->isolate());
+  // Node.js expects `kExplicit` microtasks policy and will run microtasks
+  // checkpoints after every call into JavaScript. Since we use a different
+  // policy in the renderer - switch to `kExplicit` and then drop back to the
+  // previous policy value.
+  auto old_policy = env->isolate()->GetMicrotasksPolicy();
+  DCHECK_EQ(v8::MicrotasksScope::GetCurrentDepth(env->isolate()), 0);
+  env->isolate()->SetMicrotasksPolicy(v8::MicrotasksPolicy::kExplicit);
 
   if (browser_env_ != BrowserEnvironment::kBrowser)
     TRACE_EVENT_BEGIN0("devtools.timeline", "FunctionCall");
@@ -587,6 +592,8 @@ void NodeBindings::UvRunOnce() {
   if (browser_env_ != BrowserEnvironment::kBrowser)
     TRACE_EVENT_END0("devtools.timeline", "FunctionCall");
 
+  env->isolate()->SetMicrotasksPolicy(old_policy);
+
   if (r == 0)
     base::RunLoop().QuitWhenIdle();  // Quit from uv.