test,doc: add tests and docs for addon unloading

Originally from portions of nodejs#23319.

doc/api/addons.md

@@ -234,6 +234,23 @@ NODE_MODULE_INIT(/* exports, module, context */) {
 }
 ```
 
+#### Worker support
+
+In order to support [`Worker`][] threads, addons need to clean up any resources
+they may have allocated when such a thread exists. This can be achieved through
+the usage of the `AddEnvironmentCleanupHook()` function:
+
+```c++
+void AddEnvironmentCleanupHook(v8::Isolate* isolate,
+                               void (*fun)(void* arg),
+                               void* arg);
+```
+
+This function adds a hook that will run before a given Node.js instance shuts
+down. If necessary, such hooks can be removed using
+`RemoveEnvironmentCleanupHook()` before they are run, which has the same
+signature.
+
 ### Building
 
 Once the source code has been written, it must be compiled into the binary
@@ -1349,6 +1366,7 @@ Test in JavaScript by running:
 require('./build/Release/addon');
 ```
 
+[`Worker`]: worker_threads.html#worker_threads_class_worker
 [Electron]: https://electronjs.org/
 [Embedder's Guide]: https://github.com/v8/v8/wiki/Embedder's%20Guide
 [Linking to Node.js' own dependencies]: #addons_linking_to_node_js_own_dependencies

test/addons/hello-world/test-worker.js

@@ -0,0 +1,14 @@
+// Flags: --experimental-worker
+'use strict';
+const common = require('../../common');
+const assert = require('assert');
+const path = require('path');
+const { Worker } = require('worker_threads');
+const binding = path.resolve(__dirname, `./build/${common.buildType}/binding`);
+
+const w = new Worker(`
+require('worker_threads').parentPort.postMessage(
+  require(${JSON.stringify(binding)}).hello());`, { eval: true });
+w.on('message', common.mustCall((message) => {
+  assert.strictEqual(message, 'world');
+}));

test/addons/worker-addon/binding.cc

@@ -0,0 +1,46 @@
+#include <assert.h>
+#include <node.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <v8.h>
+
+using v8::Context;
+using v8::HandleScope;
+using v8::Isolate;
+using v8::Local;
+using v8::Object;
+using v8::Value;
+
+size_t count = 0;
+
+struct statically_allocated {
+  statically_allocated() {
+    assert(count == 0);
+    printf("ctor ");
+  }
+  ~statically_allocated() {
+    assert(count == 0);
+    printf("dtor");
+  }
+} var;
+
+void Dummy(void*) {
+  assert(0);
+}
+
+void Cleanup(void* str) {
+  printf("%s ", static_cast<const char*>(str));
+}
+
+void Initialize(Local<Object> exports,
+                Local<Value> module,
+                Local<Context> context) {
+  node::AddEnvironmentCleanupHook(
+      context->GetIsolate(),
+      Cleanup,
+      const_cast<void*>(static_cast<const void*>("cleanup")));
+  node::AddEnvironmentCleanupHook(context->GetIsolate(), Dummy, nullptr);
+  node::RemoveEnvironmentCleanupHook(context->GetIsolate(), Dummy, nullptr);
+}
+
+NODE_MODULE_CONTEXT_AWARE(NODE_GYP_MODULE_NAME, Initialize)

test/addons/worker-addon/binding.gyp

@@ -0,0 +1,9 @@
+{
+  'targets': [
+    {
+      'target_name': 'binding',
+      'defines': [ 'V8_DEPRECATION_WARNINGS=1' ],
+      'sources': [ 'binding.cc' ]
+    }
+  ]
+}

test/addons/worker-addon/test.js

@@ -0,0 +1,21 @@
+// Flags: --experimental-worker
+'use strict';
+const common = require('../../common');
+const assert = require('assert');
+const child_process = require('child_process');
+const path = require('path');
+const { Worker } = require('worker_threads');
+const binding = path.resolve(__dirname, `./build/${common.buildType}/binding`);
+
+if (process.argv[2] === 'child') {
+  new Worker(`require(${JSON.stringify(binding)});`, { eval: true });
+} else {
+  const proc = child_process.spawnSync(process.execPath, [
+    '--experimental-worker',
+    __filename,
+    'child'
+  ]);
+  assert.strictEqual(proc.stderr.toString(), '');
+  assert.strictEqual(proc.stdout.toString(), 'ctor cleanup dtor');
+  assert.strictEqual(proc.status, 0);
+}