src: make process.dlopen() load well-known symbol

Look for symbol `node_register_module_v${NODE_MODULE_VERSION}` if the
add-on didn't self-register.  This can be used to create add-ons that
support multiple Node.js versions from a single shared object.

PR-URL: #18934
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: bnoordhuis

src/node.cc

@@ -2483,6 +2483,7 @@ struct DLib {
 
   inline bool Open();
   inline void Close();
+  inline void* GetSymbolAddress(const char* name);
 
   const std::string filename_;
   const int flags_;
@@ -2510,6 +2511,10 @@ void DLib::Close() {
   dlclose(handle_);
   handle_ = nullptr;
 }
+
+void* DLib::GetSymbolAddress(const char* name) {
+  return dlsym(handle_, name);
+}
 #else  // !__POSIX__
 bool DLib::Open() {
   int ret = uv_dlopen(filename_.c_str(), &lib_);
@@ -2527,8 +2532,23 @@ void DLib::Close() {
   uv_dlclose(&lib_);
   handle_ = nullptr;
 }
+
+void* DLib::GetSymbolAddress(const char* name) {
+  void* address;
+  if (0 == uv_dlsym(&lib_, name, &address)) return address;
+  return nullptr;
+}
 #endif  // !__POSIX__
 
+using InitializerCallback = void (*)(Local<Object> exports,
+                                     Local<Value> module,
+                                     Local<Context> context);
+
+inline InitializerCallback GetInitializerCallback(DLib* dlib) {
+  const char* name = "node_register_module_v" STRINGIFY(NODE_MODULE_VERSION);
+  return reinterpret_cast<InitializerCallback>(dlib->GetSymbolAddress(name));
+}
+
 // DLOpen is process.dlopen(module, filename, flags).
 // Used to load 'module.node' dynamically shared objects.
 //
@@ -2583,10 +2603,15 @@ static void DLOpen(const FunctionCallbackInfo<Value>& args) {
   }
 
   if (mp == nullptr) {
-    dlib.Close();
-    env->ThrowError("Module did not self-register.");
+    if (auto callback = GetInitializerCallback(&dlib)) {
+      callback(exports, module, context);
+    } else {
+      dlib.Close();
+      env->ThrowError("Module did not self-register.");
+    }
     return;
   }
+
   if (mp->nm_version == -1) {
     if (env->EmitNapiWarning()) {
       if (ProcessEmitWarning(env, "N-API is an experimental feature and could "

src/node.h

@@ -532,6 +532,9 @@ extern "C" NODE_EXTERN void node_module_register(void* mod);
     }                                                                 \
   }
 
+// Usage: `NODE_MODULE(NODE_GYP_MODULE_NAME, InitializerFunction)`
+// If no NODE_MODULE is declared, Node.js looks for the well-known
+// symbol `node_register_module_v${NODE_MODULE_VERSION}`.
 #define NODE_MODULE(modname, regfunc)                                 \
   NODE_MODULE_X(modname, regfunc, NULL, 0)  // NOLINT (readability/null_usage)
 

test/addons/hello-world/binding.cc

@@ -6,8 +6,12 @@ void Method(const v8::FunctionCallbackInfo<v8::Value>& args) {
   args.GetReturnValue().Set(v8::String::NewFromUtf8(isolate, "world"));
 }
 
-void init(v8::Local<v8::Object> exports) {
+#define CONCAT(a, b) CONCAT_HELPER(a, b)
+#define CONCAT_HELPER(a, b) a##b
+#define INITIALIZER CONCAT(node_register_module_v, NODE_MODULE_VERSION)
+
+extern "C" NODE_MODULE_EXPORT void INITIALIZER(v8::Local<v8::Object> exports,
+                                               v8::Local<v8::Value> module,
+                                               v8::Local<v8::Context> context) {
   NODE_SET_METHOD(exports, "hello", Method);
 }
-
-NODE_MODULE(NODE_GYP_MODULE_NAME, init)