src: restrict unloading addons to Worker threads

Unloading native addons from the main thread was an (presumably
unintended) significant breaking change, because addons may
rely on their memory being available after an `Environment` exits.

This patch only restricts this to Worker threads, at least for the
time being, and thus matches the behaviour from
nodejs#23319.

Refs: nodejs#24861
Refs: nodejs#23319

src/env.cc

@@ -276,9 +276,11 @@ Environment::~Environment() {
   TRACE_EVENT_NESTABLE_ASYNC_END0(
     TRACING_CATEGORY_NODE1(environment), "Environment", this);
 
-  // Dereference all addons that were loaded into this environment.
-  for (binding::DLib& addon : loaded_addons_) {
-    addon.Close();
+  if (!is_main_thread()) {
+    // Dereference all addons that were loaded into this environment.
+    for (binding::DLib& addon : loaded_addons_) {
+      addon.Close();
+    }
   }
 }
 

test/addons/worker-addon/binding.cc

@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <v8.h>
+#include <uv.h>
 
 using v8::Context;
 using v8::HandleScope;
@@ -41,6 +42,16 @@ void Initialize(Local<Object> exports,
       const_cast<void*>(static_cast<const void*>("cleanup")));
   node::AddEnvironmentCleanupHook(context->GetIsolate(), Dummy, nullptr);
   node::RemoveEnvironmentCleanupHook(context->GetIsolate(), Dummy, nullptr);
+
+  if (getenv("addExtraItemToEventLoop") != nullptr) {
+    // Add an item to the event loop that we do not clean up in order to make
+    // sure that for the main thread, this addon's memory persists even after
+    // the Environment instance has been destroyed.
+    static uv_async_t extra_async;
+    uv_loop_t* loop = node::GetCurrentEventLoop(context->GetIsolate());
+    uv_async_init(loop, &extra_async, [](uv_async_t*) {});
+    uv_unref(reinterpret_cast<uv_handle_t*>(&extra_async));
+  }
 }
 
 NODE_MODULE_CONTEXT_AWARE(NODE_GYP_MODULE_NAME, Initialize)

test/addons/worker-addon/test.js

@@ -6,12 +6,19 @@ const path = require('path');
 const { Worker } = require('worker_threads');
 const binding = path.resolve(__dirname, `./build/${common.buildType}/binding`);
 
-if (process.argv[2] === 'child') {
+if (process.argv[2] === 'worker') {
   new Worker(`require(${JSON.stringify(binding)});`, { eval: true });
-} else {
+  return;
+} else if (process.argv[2] === 'main-thread') {
+  process.env.addExtraItemToEventLoop = 'yes';
+  require(binding);
+  return;
+}
+
+for (const test of ['worker', 'main-thread']) {
   const proc = child_process.spawnSync(process.execPath, [
     __filename,
-    'child'
+    test
   ]);
   assert.strictEqual(proc.stderr.toString(), '');
   assert.strictEqual(proc.stdout.toString(), 'ctor cleanup dtor');