src: permit native modules to be loaded in more than one context

Maintain metadata on loaded native modules so that if additional
contexts load the same module, the metadata will be reused and the
registration function dispatched again.

Fix race condition where if multiple contexts load modules at the same
time, it's possible one of them could fail to load.

src/node.cc

@@ -183,7 +183,9 @@ static int v8_thread_pool_size = v8_default_thread_pool_size;
 static bool prof_process = false;
 static bool v8_is_profiling = false;
 static bool node_is_initialized = false;
+static Mutex module_mutex;
 static node_module* modpending;
+static std::unordered_map<std::string, node_module *> modmap_metadata;
 static node_module* modlist_builtin;
 static node_module* modlist_internal;
 static node_module* modlist_linked;
@@ -1266,8 +1268,6 @@ static void DLOpen(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
   auto context = env->context();
 
-  CHECK_NULL(modpending);
-
   if (args.Length() < 2) {
     env->ThrowError("process.dlopen needs at least 2 arguments.");
     return;
@@ -1289,13 +1289,27 @@ static void DLOpen(const FunctionCallbackInfo<Value>& args) {
 
   node::Utf8Value filename(env->isolate(), args[1]);  // Cast
   DLib dlib(*filename, flags);
-  bool is_opened = dlib.Open();
-
-  // Objects containing v14 or later modules will have registered themselves
-  // on the pending list.  Activate all of them now.  At present, only one
-  // module per object is supported.
-  node_module* const mp = modpending;
-  modpending = nullptr;
+  bool is_opened;
+  node_module* mp;
+  {
+    Mutex::ScopedLock lock(module_mutex);
+    CHECK_NULL(modpending);
+    is_opened = dlib.Open();
+    // Objects containing v14 or later modules will have registered themselves
+    // on the pending list.  Activate all of them now.  At present, only one
+    // module per object is supported.
+    std::string key(*filename, filename.length());
+    mp = modpending;
+    if (mp) {
+      modpending = nullptr;
+      modmap_metadata.emplace(std::move(key), mp);
+    } else {
+      auto existing = modmap_metadata.find(key);
+      if (existing != modmap_metadata.end()) {
+        mp = existing->second;
+      }
+    }
+  }
 
   if (!is_opened) {
     Local<String> errmsg = OneByteString(env->isolate(), dlib.errmsg_.c_str());