node-api: allow retrieval of add-on file name

Unlike JS-only modules, native add-ons are always associated with a
dynamic shared object from which they are loaded. Being able to
retrieve its absolute path is important to native-only add-ons, i.e.
add-ons that are not themselves being loaded from a JS-only module
located in the same package as the native add-on itself.

Currently, the file name is obtained at environment construction time
from the JS `module.filename`. Nevertheless, the presence of `module`
is not required, because the file name could also be passed in via a
private property added onto `exports` from the `process.dlopen`
binding.

As an attempt at future-proofing, the file name is provided as a URL,
i.e. prefixed with the `file://` protocol.

Fixes: https://github.com/nodejs/node-addon-api/issues/449
PR-URL: https://github.com/nodejs/node/pull/37195
Co-authored-by: Michael Dawson <mdawson@devrus.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>

1 files changed, 39 insertions, 6 deletions

diff --git a/src/node_api.cc b/src/node_api.cc
index f1a5265b6a7..8dbf48d466d 100644
--- a/src/node_api.cc
+++ b/src/node_api.cc
@@ -15,8 +15,9 @@
 #include <memory>
 
 struct node_napi_env__ : public napi_env__ {
-  explicit node_napi_env__(v8::Local<v8::Context> context):
-      napi_env__(context) {
+  explicit node_napi_env__(v8::Local<v8::Context> context,
+                           const std::string& module_filename):
+      napi_env__(context), filename(module_filename) {
     CHECK_NOT_NULL(node_env());
   }
 
@@ -46,6 +47,10 @@ struct node_napi_env__ : public napi_env__ {
       });
     });
   }
+
+  const char* GetFilename() const { return filename.c_str(); }
+
+  std::string filename;
 };
 
 typedef node_napi_env__* node_napi_env;
@@ -87,10 +92,11 @@ class BufferFinalizer : private Finalizer {
   };
 };
 
-static inline napi_env NewEnv(v8::Local<v8::Context> context) {
+static inline napi_env
+NewEnv(v8::Local<v8::Context> context, const std::string& module_filename) {
   node_napi_env result;
 
-  result = new node_napi_env__(context);
+  result = new node_napi_env__(context, module_filename);
   // 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,
@@ -552,16 +558,35 @@ 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) {
+  node::Environment* node_env = node::Environment::GetCurrent(context);
+  std::string module_filename = "";
   if (init == nullptr) {
-    node::Environment* node_env = node::Environment::GetCurrent(context);
     CHECK_NOT_NULL(node_env);
     node_env->ThrowError(
         "Module has no declared entry point.");
     return;
   }
 
+  // We set `env->filename` from `module.filename` here, but we could just as
+  // easily add a private property to `exports` in `process.dlopen`, which
+  // receives the file name from JS, and retrieve *that* here. Thus, we are not
+  // endorsing commonjs here by making use of `module.filename`.
+  v8::Local<v8::Value> filename_js;
+  v8::Local<v8::Object> modobj;
+  if (module->ToObject(context).ToLocal(&modobj) &&
+      modobj->Get(context, node_env->filename_string()).ToLocal(&filename_js) &&
+      filename_js->IsString()) {
+    node::Utf8Value filename(node_env->isolate(), filename_js);  // Cast
+
+    // Turn the absolute path into a URL. Currently the absolute path is always
+    // a file system path.
+    // TODO(gabrielschulhof): Pass the `filename` through unchanged if/when we
+    // receive it as a URL already.
+    module_filename = std::string("file://") + (*filename);
+  }
+
   // Create a new napi_env for this specific module.
-  napi_env env = v8impl::NewEnv(context);
+  napi_env env = v8impl::NewEnv(context, module_filename);
 
   napi_value _exports;
   env->CallIntoModule([&](napi_env env) {
@@ -1257,3 +1282,11 @@ napi_ref_threadsafe_function(napi_env env, napi_threadsafe_function func) {
   CHECK_NOT_NULL(func);
   return reinterpret_cast<v8impl::ThreadSafeFunction*>(func)->Ref();
 }
+
+napi_status node_api_get_module_file_name(napi_env env, const char** result) {
+  CHECK_ENV(env);
+  CHECK_ARG(env, result);
+
+  *result = static_cast<node_napi_env>(env)->GetFilename();
+  return napi_clear_last_error(env);
+}