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: https://github.com/nodejs/node/pull/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>

2 files changed, 30 insertions, 2 deletions

diff --git a/src/node.cc b/src/node.cc
index 6d082caacf..dbf9314273 100644
--- a/src/node.cc
+++ b/src/node.cc
@@ -2158,6 +2158,7 @@ struct DLib {
 
   inline bool Open();
   inline void Close();
+  inline void* GetSymbolAddress(const char* name);
 
   const std::string filename_;
   const int flags_;
@@ -2185,6 +2186,10 @@ 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_);
@@ -2202,8 +2207,23 @@ 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.
 //
@@ -2258,10 +2278,15 @@ 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 "
diff --git a/src/node.h b/src/node.h
index cb43468930..ab5d1c120f 100644
--- a/src/node.h
+++ b/src/node.h
@@ -535,6 +535,9 @@ 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)