src: unregister Isolate with platform before disposing

I previously thought the order of these calls was no longer
relevant. I was wrong.

This commit undoes the changes from 312c02d, adds a comment
explaining why I was wrong, and flips the order of the calls
elsewhere for consistency, the latter having been the goal
of 312c02d.

Fixes: #30846
Refs: #30181

PR-URL: #30909
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

src/node.h

@@ -286,6 +286,7 @@ class NODE_EXTERN MultiIsolatePlatform : public v8::Platform {
 
   // This function may only be called once per `Isolate`, and discard any
   // pending delayed tasks scheduled for that isolate.
+  // This needs to be called right before calling `Isolate::Dispose()`.
   virtual void UnregisterIsolate(v8::Isolate* isolate) = 0;
 
   // The platform should call the passed function once all state associated

src/node_main_instance.cc

@@ -99,8 +99,8 @@ NodeMainInstance::~NodeMainInstance() {
   if (!owns_isolate_) {
     return;
   }
-  isolate_->Dispose();
   platform_->UnregisterIsolate(isolate_);
+  isolate_->Dispose();
 }
 
 int NodeMainInstance::Run() {

src/node_worker.cc

@@ -190,8 +190,13 @@ class WorkerThreadData {
         *static_cast<bool*>(data) = true;
       }, &platform_finished);
 
-      isolate->Dispose();
+      // The order of these calls is important; if the Isolate is first disposed
+      // and then unregistered, there is a race condition window in which no
+      // new Isolate at the same address can successfully be registered with
+      // the platform.
+      // (Refs: https://github.com/nodejs/node/issues/30846)
       w_->platform_->UnregisterIsolate(isolate);
+      isolate->Dispose();
 
       // Wait until the platform has cleaned up all relevant resources.
       while (!platform_finished)

test/cctest/node_test_fixture.h

@@ -107,8 +107,8 @@ class NodeTestFixture : public ::testing::Test {
   void TearDown() override {
     platform->DrainTasks(isolate_);
     isolate_->Exit();
-    isolate_->Dispose();
     platform->UnregisterIsolate(isolate_);
+    isolate_->Dispose();
     isolate_ = nullptr;
   }
 };