n-api: initialize a module via a special symbol

Much like regular modules, N-API modules can also benefit from having
a special symbol which they can expose.

Fixes: https://github.com/nodejs/node/issues/19845
PR-URL: https://github.com/nodejs/node/pull/20161
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/n-api.md

@@ -982,6 +982,32 @@ napi_value Init(napi_env env, napi_value exports) {
 }
 ```
 
+If you expect that your module will be loaded multiple times during the lifetime
+of the Node.js process, you can use the `NAPI_MODULE_INIT` macro to initialize
+your module:
+
+```C
+NAPI_MODULE_INIT() {
+  napi_value answer;
+  napi_status result;
+
+  status = napi_create_int64(env, 42, &answer);
+  if (status != napi_ok) return NULL;
+
+  status = napi_set_named_property(env, exports, "answer", answer);
+  if (status != napi_ok) return NULL;
+
+  return exports;
+}
+```
+
+This macro includes `NAPI_MODULE`, and declares an `Init` function with a
+special name and with visibility beyond the addon. This will allow Node.js to
+initialize the module even if it is loaded multiple times.
+
+The variables `env` and `exports` will be available inside the function body
+following the macro invocation.
+
 For more details on setting properties on objects, see the section on
 [Working with JavaScript Properties][].
 

src/node.cc

@@ -2229,6 +2229,13 @@ inline InitializerCallback GetInitializerCallback(DLib* dlib) {
   return reinterpret_cast<InitializerCallback>(dlib->GetSymbolAddress(name));
 }
 
+inline napi_addon_register_func GetNapiInitializerCallback(DLib* dlib) {
+  const char* name =
+      STRINGIFY(NAPI_MODULE_INITIALIZER_BASE) STRINGIFY(NAPI_MODULE_VERSION);
+  return
+      reinterpret_cast<napi_addon_register_func>(dlib->GetSymbolAddress(name));
+}
+
 // DLOpen is process.dlopen(module, filename, flags).
 // Used to load 'module.node' dynamically shared objects.
 //
@@ -2285,6 +2292,8 @@ static void DLOpen(const FunctionCallbackInfo<Value>& args) {
   if (mp == nullptr) {
     if (auto callback = GetInitializerCallback(&dlib)) {
       callback(exports, module, context);
+    } else if (auto napi_callback = GetNapiInitializerCallback(&dlib)) {
+      napi_module_register_by_symbol(exports, module, context, napi_callback);
     } else {
       dlib.Close();
       env->ThrowError("Module did not self-register.");

src/node_api.cc

@@ -858,16 +858,23 @@ void napi_module_register_cb(v8::Local<v8::Object> exports,
                              v8::Local<v8::Value> module,
                              v8::Local<v8::Context> context,
                              void* priv) {
-  napi_module* mod = static_cast<napi_module*>(priv);
+  napi_module_register_by_symbol(exports, module, context,
+      static_cast<napi_module*>(priv)->nm_register_func);
+}
 
+}  // end of anonymous namespace
+
+void napi_module_register_by_symbol(v8::Local<v8::Object> exports,
+                                    v8::Local<v8::Value> module,
+                                    v8::Local<v8::Context> context,
+                                    napi_addon_register_func init) {
   // 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_value _exports;
   NAPI_CALL_INTO_MODULE_THROW(env,
-      _exports = mod->nm_register_func(env,
-          v8impl::JsValueFromV8LocalValue(exports)));
+      _exports = init(env, v8impl::JsValueFromV8LocalValue(exports)));
 
   // If register function returned a non-null exports object different from
   // the exports object we passed it, set that as the "exports" property of
@@ -879,8 +886,6 @@ void napi_module_register_cb(v8::Local<v8::Object> exports,
   }
 }
 
-}  // end of anonymous namespace
-
 // Registers a NAPI module.
 void napi_module_register(napi_module* mod) {
   node::node_module* nm = new node::node_module {

src/node_api.h

@@ -90,9 +90,28 @@ typedef struct {
     }                                                                 \
   EXTERN_C_END
 
-#define NAPI_MODULE(modname, regfunc) \
+#define NAPI_MODULE(modname, regfunc)                                 \
   NAPI_MODULE_X(modname, regfunc, NULL, 0)  // NOLINT (readability/null_usage)
 
+#define NAPI_MODULE_INITIALIZER_BASE napi_register_module_v
+
+#define NAPI_MODULE_INITIALIZER_X(base, version)                      \
+    NAPI_MODULE_INITIALIZER_X_HELPER(base, version)
+#define NAPI_MODULE_INITIALIZER_X_HELPER(base, version) base##version
+
+#define NAPI_MODULE_INITIALIZER                                       \
+  NAPI_MODULE_INITIALIZER_X(NAPI_MODULE_INITIALIZER_BASE,             \
+      NAPI_MODULE_VERSION)
+
+#define NAPI_MODULE_INIT()                                            \
+  EXTERN_C_START                                                      \
+  NAPI_MODULE_EXPORT napi_value                                       \
+  NAPI_MODULE_INITIALIZER(napi_env env, napi_value exports);          \
+  EXTERN_C_END                                                        \
+  NAPI_MODULE(NODE_GYP_MODULE_NAME, NAPI_MODULE_INITIALIZER)          \
+  napi_value NAPI_MODULE_INITIALIZER(napi_env env,                    \
+                                     napi_value exports)
+
 #define NAPI_AUTO_LENGTH SIZE_MAX
 
 EXTERN_C_START

src/node_internals.h

@@ -33,6 +33,7 @@
 #include "tracing/trace_event.h"
 #include "node_perf_common.h"
 #include "node_debug_options.h"
+#include "node_api.h"
 
 #include <stdint.h>
 #include <stdlib.h>
@@ -840,6 +841,10 @@ static inline const char *errno_string(int errorno) {
 
 }  // namespace node
 
+void napi_module_register_by_symbol(v8::Local<v8::Object> exports,
+                                    v8::Local<v8::Value> module,
+                                    v8::Local<v8::Context> context,
+                                    napi_addon_register_func init);
 
 #endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
 

test/addons-napi/1_hello_world/binding.c

@@ -10,10 +10,8 @@ napi_value Method(napi_env env, napi_callback_info info) {
   return world;
 }
 
-napi_value Init(napi_env env, napi_value exports) {
+NAPI_MODULE_INIT() {
   napi_property_descriptor desc = DECLARE_NAPI_PROPERTY("hello", Method);
   NAPI_CALL(env, napi_define_properties(env, exports, 1, &desc));
   return exports;
 }
-
-NAPI_MODULE(NODE_GYP_MODULE_NAME, Init)

test/addons-napi/1_hello_world/test.js

@@ -1,6 +1,13 @@
 'use strict';
 const common = require('../../common');
 const assert = require('assert');
-const addon = require(`./build/${common.buildType}/binding`);
+const bindingPath = require.resolve(`./build/${common.buildType}/binding`);
+const binding = require(bindingPath);
+assert.strictEqual(binding.hello(), 'world');
+console.log('binding.hello() =', binding.hello());
 
-assert.strictEqual(addon.hello(), 'world');
+// Test multiple loading of the same module.
+delete require.cache[bindingPath];
+const rebinding = require(bindingPath);
+assert.strictEqual(rebinding.hello(), 'world');
+assert.notStrictEqual(binding.hello, rebinding.hello);