src: unregister Isolate with platform before disposing

addaleax · addaleax · commit 27d48ed7ef78 · 2019-12-11T21:52:28.000-05:00
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: nodejs#30846 Refs: nodejs#30181
diff --git a/src/node.h b/src/node.h
@@ -298,6 +298,7 @@ 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
diff --git a/src/node_main_instance.cc b/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() {
diff --git a/src/node_worker.cc b/src/node_worker.cc
@@ -189,8 +189,13 @@ 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)
diff --git a/test/cctest/node_test_fixture.h b/test/cctest/node_test_fixture.h
@@ -116,8 +116,8 @@ class NodeTestFixture : public NodeZeroIsolateTestFixture {
   void TearDown() override {
     platform->DrainTasks(isolate_);
     isolate_->Exit();
-    isolate_->Dispose();
     platform->UnregisterIsolate(isolate_);
+    isolate_->Dispose();
     isolate_ = nullptr;
     NodeZeroIsolateTestFixture::TearDown();
   }
diff --git a/test/cctest/test_platform.cc b/test/cctest/test_platform.cc
@@ -101,6 +101,6 @@ TEST_F(NodeZeroIsolateTestFixture, IsolatePlatformDelegateTest) {
 
   // Graceful shutdown
   delegate->Shutdown();
-  isolate->Dispose();
   platform->UnregisterIsolate(isolate);
+  isolate->Dispose();
 }

Original file line number	Diff line number	Diff line change
`@@ -99,8 +99,8 @@ NodeMainInstance::~NodeMainInstance() {`
`99`	`99`	`if (!owns_isolate_) {`
`100`	`100`	`return;`
`101`	`101`	`}`
`102`		`- isolate_->Dispose();`
`103`	`102`	`platform_->UnregisterIsolate(isolate_);`
	`103`	`+ isolate_->Dispose();`
`104`	`104`	`}`
`105`	`105`
`106`	`106`	`int NodeMainInstance::Run() {`
Original file line number	Diff line number	Diff line change
`@@ -101,6 +101,6 @@ TEST_F(NodeZeroIsolateTestFixture, IsolatePlatformDelegateTest) {`
`101`	`101`
`102`	`102`	`// Graceful shutdown`
`103`	`103`	`delegate->Shutdown();`
`104`		`- isolate->Dispose();`
`105`	`104`	`platform->UnregisterIsolate(isolate);`
	`105`	`+ isolate->Dispose();`
`106`	`106`	`}`