n-api: add APIs for per-instance state management

Adds `napi_set_instance_data()` and `napi_get_instance_data()`, which
allow native addons to store their data on and retrieve their data from
`napi_env`. `napi_set_instance_data()` accepts a finalizer which is
called when the `node::Environment()` is destroyed.

This entails rendering the `napi_env` local to each add-on.

Fixes: nodejs/abi-stable-node#378
PR-URL: #28682
Reviewed-By: Ben Noordhuis <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: Gabriel Schulhof

doc/api/n-api.md

@@ -251,6 +251,82 @@ NAPI_MODULE_INIT() {
 }
 ```
 
+## Environment Life Cycle APIs
+
+> Stability: 1 - Experimental
+
+[Section 8.7][] of the [ECMAScript Language Specification][] defines the concept
+of an "Agent" as a self-contained environment in which JavaScript code runs.
+Multiple such Agents may be started and terminated either concurrently or in
+sequence by the process.
+
+A Node.js environment corresponds to an ECMAScript Agent. In the main process,
+an environment is created at startup, and additional environments can be created
+on separate threads to serve as [worker threads][]. When Node.js is embedded in
+another application, the main thread of the application may also construct and
+destroy a Node.js environment multiple times during the life cycle of the
+application process such that each Node.js environment created by the
+application may, in turn, during its life cycle create and destroy additional
+environments as worker threads.
+
+From the perspective of a native addon this means that the bindings it provides
+may be called multiple times, from multiple contexts, and even concurrently from
+multiple threads.
+
+Native addons may need to allocate global state of which they make use during
+their entire life cycle such that the state must be unique to each instance of
+the addon.
+
+To this env, N-API provides a way to allocate data such that its life cycle is
+tied to the life cycle of the Agent.
+
+### napi_set_instance_data
+<!-- YAML
+added: REPLACEME
+-->
+
+```C
+napi_status napi_set_instance_data(napi_env env,
+                                   void* data,
+                                   napi_finalize finalize_cb,
+                                   void* finalize_hint);
+```
+
+- `[in] env`: The environment that the N-API call is invoked under.
+- `[in] data`: The data item to make available to bindings of this instance.
+- `[in] finalize_cb`: The function to call when the environment is being torn
+down. The function receives `data` so that it might free it.
+- `[in] finalize_hint`: Optional hint to pass to the finalize callback
+during collection.
+
+Returns `napi_ok` if the API succeeded.
+
+This API associates `data` with the currently running Agent. `data` can later
+be retrieved using `napi_get_instance_data()`. Any existing data associated with
+the currently running Agent which was set by means of a previous call to
+`napi_set_instance_data()` will be overwritten. If a `finalize_cb` was provided
+by the previous call, it will not be called.
+
+### napi_get_instance_data
+<!-- YAML
+added: REPLACEME
+-->
+
+```C
+napi_status napi_get_instance_data(napi_env env,
+                                   void** data);
+```
+
+- `[in] env`: The environment that the N-API call is invoked under.
+- `[out] data`: The data item that was previously associated with the currently
+running Agent by a call to `napi_set_instance_data()`.
+
+Returns `napi_ok` if the API succeeded.
+
+This API retrieves data that was previously associated with the currently
+running Agent via `napi_set_instance_data()`. If no data is set, the call will
+succeed and `data` will be set to `NULL`.
+
 ## Basic N-API Data Types
 
 N-API exposes the following fundamental datatypes as abstractions that are
@@ -4876,6 +4952,7 @@ This API may only be called from the main thread.
 [Section 6.1.4]: https://tc39.github.io/ecma262/#sec-ecmascript-language-types-string-type
 [Section 6.1.6]: https://tc39.github.io/ecma262/#sec-ecmascript-language-types-number-type
 [Section 6.1.7.1]: https://tc39.github.io/ecma262/#table-2
+[Section 8.7]: https://tc39.es/ecma262/#sec-agents
 [Section 9.1.6]: https://tc39.github.io/ecma262/#sec-ordinary-object-internal-methods-and-internal-slots-defineownproperty-p-desc
 [Working with JavaScript Functions]: #n_api_working_with_javascript_functions
 [Working with JavaScript Properties]: #n_api_working_with_javascript_properties
@@ -4930,3 +5007,4 @@ This API may only be called from the main thread.
 [`uv_unref`]: http://docs.libuv.org/en/v1.x/handle.html#c.uv_unref
 [async_hooks `type`]: async_hooks.html#async_hooks_type
 [context-aware addons]: addons.html#addons_context_aware_addons
+[worker threads]: https://nodejs.org/api/worker_threads.html

src/env.h

@@ -130,7 +130,6 @@ constexpr size_t kFsStatsBufferLength = kFsStatsFieldsNumber * 2;
   V(contextify_context_private_symbol, "node:contextify:context")             \
   V(contextify_global_private_symbol, "node:contextify:global")               \
   V(decorated_private_symbol, "node:decorated")                               \
-  V(napi_env, "node:napi:env")                                                \
   V(napi_wrapper, "node:napi:wrapper")                                        \
   V(sab_lifetimepartner_symbol, "node:sharedArrayBufferLifetimePartner")      \
 

src/js_native_api.h

@@ -499,6 +499,15 @@ NAPI_EXTERN napi_status napi_add_finalizer(napi_env env,
                                            napi_finalize finalize_cb,
                                            void* finalize_hint,
                                            napi_ref* result);
+
+// Instance data
+NAPI_EXTERN napi_status napi_set_instance_data(napi_env env,
+                                               void* data,
+                                               napi_finalize finalize_cb,
+                                               void* finalize_hint);
+
+NAPI_EXTERN napi_status napi_get_instance_data(napi_env env,
+                                               void** data);
 #endif  // NAPI_EXPERIMENTAL
 
 EXTERN_C_END

src/js_native_api_v8.cc

@@ -305,12 +305,10 @@ class Reference : private Finalizer {
   static void SecondPassCallback(const v8::WeakCallbackInfo<Reference>& data) {
     Reference* reference = data.GetParameter();
 
-    napi_env env = reference->_env;
-
     if (reference->_finalize_callback != nullptr) {
-      NapiCallIntoModuleThrow(env, [&]() {
+      reference->_env->CallIntoModuleThrow([&](napi_env env) {
         reference->_finalize_callback(
-            reference->_env,
+            env,
             reference->_finalize_data,
             reference->_finalize_hint);
       });
@@ -452,7 +450,9 @@ class CallbackWrapperBase : public CallbackWrapper {
     napi_callback cb = _bundle->*FunctionField;
 
     napi_value result;
-    NapiCallIntoModuleThrow(env, [&]() { result = cb(env, cbinfo_wrapper); });
+    env->CallIntoModuleThrow([&](napi_env env) {
+      result = cb(env, cbinfo_wrapper);
+    });
 
     if (result != nullptr) {
       this->SetReturnValue(result);
@@ -2986,3 +2986,26 @@ napi_status napi_adjust_external_memory(napi_env env,
 
   return napi_clear_last_error(env);
 }
+
+napi_status napi_set_instance_data(napi_env env,
+                                   void* data,
+                                   napi_finalize finalize_cb,
+                                   void* finalize_hint) {
+  CHECK_ENV(env);
+
+  env->instance_data.data = data;
+  env->instance_data.finalize_cb = finalize_cb;
+  env->instance_data.hint = finalize_hint;
+
+  return napi_clear_last_error(env);
+}
+
+napi_status napi_get_instance_data(napi_env env,
+                                   void** data) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, data);
+
+  *data = env->instance_data.data;
+
+  return napi_clear_last_error(env);
+}

src/js_native_api_v8.h

@@ -6,13 +6,21 @@
 #include "js_native_api_types.h"
 #include "js_native_api_v8_internals.h"
 
+static napi_status napi_clear_last_error(napi_env env);
+
 struct napi_env__ {
   explicit napi_env__(v8::Local<v8::Context> context)
       : isolate(context->GetIsolate()),
         context_persistent(isolate, context) {
     CHECK_EQ(isolate, context->GetIsolate());
   }
-  virtual ~napi_env__() = default;
+  virtual ~napi_env__() {
+    if (instance_data.finalize_cb != nullptr) {
+      CallIntoModuleThrow([&](napi_env env) {
+        instance_data.finalize_cb(env, instance_data.data, instance_data.hint);
+      });
+    }
+  }
   v8::Isolate* const isolate;  // Shortcut for context()->GetIsolate()
   v8impl::Persistent<v8::Context> context_persistent;
 
@@ -25,11 +33,37 @@ struct napi_env__ {
 
   virtual bool can_call_into_js() const { return true; }
 
+  template <typename T, typename U>
+  void CallIntoModule(T&& call, U&& handle_exception) {
+    int open_handle_scopes_before = open_handle_scopes;
+    int open_callback_scopes_before = open_callback_scopes;
+    napi_clear_last_error(this);
+    call(this);
+    CHECK_EQ(open_handle_scopes, open_handle_scopes_before);
+    CHECK_EQ(open_callback_scopes, open_callback_scopes_before);
+    if (!last_exception.IsEmpty()) {
+      handle_exception(this, last_exception.Get(this->isolate));
+      last_exception.Reset();
+    }
+  }
+
+  template <typename T>
+  void CallIntoModuleThrow(T&& call) {
+    CallIntoModule(call, [&](napi_env env, v8::Local<v8::Value> value) {
+      env->isolate->ThrowException(value);
+    });
+  }
+
   v8impl::Persistent<v8::Value> last_exception;
   napi_extended_error_info last_error;
   int open_handle_scopes = 0;
   int open_callback_scopes = 0;
   int refs = 1;
+  struct {
+    void* data = nullptr;
+    void* hint = nullptr;
+    napi_finalize finalize_cb = nullptr;
+  } instance_data;
 };
 
 static inline napi_status napi_clear_last_error(napi_env env) {
@@ -114,27 +148,6 @@ napi_status napi_set_last_error(napi_env env, napi_status error_code,
     }                                                              \
   } while (0)
 
-template <typename T, typename U>
-void NapiCallIntoModule(napi_env env, T&& call, U&& handle_exception) {
-  int open_handle_scopes = env->open_handle_scopes;
-  int open_callback_scopes = env->open_callback_scopes;
-  napi_clear_last_error(env);
-  call();
-  CHECK_EQ(env->open_handle_scopes, open_handle_scopes);
-  CHECK_EQ(env->open_callback_scopes, open_callback_scopes);
-  if (!env->last_exception.IsEmpty()) {
-    handle_exception(env->last_exception.Get(env->isolate));
-    env->last_exception.Reset();
-  }
-}
-
-template <typename T>
-void NapiCallIntoModuleThrow(napi_env env, T&& call) {
-  NapiCallIntoModule(env, call, [&](v8::Local<v8::Value> value) {
-    env->isolate->ThrowException(value);
-  });
-}
-
 namespace v8impl {
 
 //=== Conversion between V8 Handles and napi_value ========================

src/node_api.cc

@@ -46,9 +46,9 @@ class BufferFinalizer : private Finalizer {
         v8::HandleScope handle_scope(finalizer->_env->isolate);
         v8::Context::Scope context_scope(finalizer->_env->context());
 
-        NapiCallIntoModuleThrow(finalizer->_env, [&]() {
+        finalizer->_env->CallIntoModuleThrow([&](napi_env env) {
           finalizer->_finalize_callback(
-              finalizer->_env,
+              env,
               finalizer->_finalize_data,
               finalizer->_finalize_hint);
         });
@@ -59,44 +59,22 @@ class BufferFinalizer : private Finalizer {
   }
 };
 
-static inline napi_env GetEnv(v8::Local<v8::Context> context) {
+static inline napi_env NewEnv(v8::Local<v8::Context> context) {
   node_napi_env result;
 
-  auto isolate = context->GetIsolate();
-  auto global = context->Global();
-
-  // In the case of the string for which we grab the private and the value of
-  // the private on the global object we can call .ToLocalChecked() directly
-  // because we need to stop hard if either of them is empty.
-  //
-  // Re https://github.com./nodejs/node/pull/14217#discussion_r128775149
-  auto value = global->GetPrivate(context, NAPI_PRIVATE_KEY(context, env))
-      .ToLocalChecked();
-
-  if (value->IsExternal()) {
-    result = static_cast<node_napi_env>(value.As<v8::External>()->Value());
-  } else {
-    result = new node_napi_env__(context);
-    auto external = v8::External::New(isolate, result);
-
-    // We must also stop hard if the result of assigning the env to the global
-    // is either nothing or false.
-    CHECK(global->SetPrivate(context, NAPI_PRIVATE_KEY(context, env), external)
-        .FromJust());
-
-    // TODO(addaleax): There was previously code that tried to delete the
-    // napi_env when its v8::Context was garbage collected;
-    // However, as long as N-API addons using this napi_env are in place,
-    // the Context needs to be accessible and alive.
-    // Ideally, we'd want an on-addon-unload hook that takes care of this
-    // once all N-API addons using this napi_env are unloaded.
-    // For now, a per-Environment cleanup hook is the best we can do.
-    result->node_env()->AddCleanupHook(
-        [](void* arg) {
-          static_cast<napi_env>(arg)->Unref();
-        },
-        static_cast<void*>(result));
-  }
+  result = new node_napi_env__(context);
+  // TODO(addaleax): There was previously code that tried to delete the
+  // napi_env when its v8::Context was garbage collected;
+  // However, as long as N-API addons using this napi_env are in place,
+  // the Context needs to be accessible and alive.
+  // Ideally, we'd want an on-addon-unload hook that takes care of this
+  // once all N-API addons using this napi_env are unloaded.
+  // For now, a per-Environment cleanup hook is the best we can do.
+  result->node_env()->AddCleanupHook(
+      [](void* arg) {
+        static_cast<napi_env>(arg)->Unref();
+      },
+      static_cast<void*>(result));
 
   return result;
 }
@@ -325,7 +303,7 @@ class ThreadSafeFunction : public node::AsyncResource {
             v8::Local<v8::Function>::New(env->isolate, ref);
           js_callback = v8impl::JsValueFromV8LocalValue(js_cb);
         }
-        NapiCallIntoModuleThrow(env, [&]() {
+        env->CallIntoModuleThrow([&](napi_env env) {
           call_js_cb(env, js_callback, context, data);
         });
       }
@@ -346,7 +324,7 @@ class ThreadSafeFunction : public node::AsyncResource {
     v8::HandleScope scope(env->isolate);
     if (finalize_cb) {
       CallbackScope cb_scope(this);
-      NapiCallIntoModuleThrow(env, [&]() {
+      env->CallIntoModuleThrow([&](napi_env env) {
         finalize_cb(env, finalize_data, context);
       });
     }
@@ -481,10 +459,10 @@ void napi_module_register_by_symbol(v8::Local<v8::Object> exports,
 
   // Create a new napi_env for this module or reference one if a pre-existing
   // one is found.
-  napi_env env = v8impl::GetEnv(context);
+  napi_env env = v8impl::NewEnv(context);
 
   napi_value _exports;
-  NapiCallIntoModuleThrow(env, [&]() {
+  env->CallIntoModuleThrow([&](napi_env env) {
     _exports = init(env, v8impl::JsValueFromV8LocalValue(exports));
   });
 
@@ -889,15 +867,9 @@ class Work : public node::AsyncResource, public node::ThreadPoolWork {
 
     CallbackScope callback_scope(this);
 
-    // We have to back up the env here because the `NAPI_CALL_INTO_MODULE` macro
-    // makes use of it after the call into the module completes, but the module
-    // may have deallocated **this**, and along with it the place where _env is
-    // stored.
-    napi_env env = _env;
-
-    NapiCallIntoModule(env, [&]() {
-      _complete(_env, ConvertUVErrorCode(status), _data);
-    }, [env](v8::Local<v8::Value> local_err) {
+    _env->CallIntoModule([&](napi_env env) {
+      _complete(env, ConvertUVErrorCode(status), _data);
+    }, [](napi_env env, v8::Local<v8::Value> local_err) {
       // If there was an unhandled exception in the complete callback,
       // report it as a fatal exception. (There is no JavaScript on the
       // callstack that can possibly handle it.)