fs: expose realpath(3) bindings

Make the `uv_fs_realpath()` binding (which calls the libc `realpath()`
on UNIX and `GetFinalPathNameByHandle()` on Windows) available as the
`fs.realpath.native()` and `fs.realpathSync.native()` functions.

The binding was already available as `process.binding('fs').realpath`
but was not exposed or tested - and partly broken as a result.

Fixes: #8715
PR-URL: #15776
Refs: #7899
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

lib/fs.js

@@ -1722,6 +1722,14 @@ fs.realpathSync = function realpathSync(p, options) {
 };
 
 
+fs.realpathSync.native = function(path, options) {
+  options = getOptions(options, {});
+  handleError((path = getPathFromURL(path)));
+  nullCheck(path);
+  return binding.realpath(path, options.encoding);
+};
+
+
 fs.realpath = function realpath(p, options, callback) {
   callback = maybeCallback(typeof options === 'function' ? options : callback);
   if (!options)
@@ -1858,6 +1866,18 @@ fs.realpath = function realpath(p, options, callback) {
   }
 };
 
+
+fs.realpath.native = function(path, options, callback) {
+  callback = maybeCallback(callback || options);
+  options = getOptions(options, {});
+  if (handleError((path = getPathFromURL(path)), callback)) return;
+  if (!nullCheck(path, callback)) return;
+  const req = new FSReqWrap();
+  req.oncomplete = callback;
+  return binding.realpath(path, options.encoding, req);
+};
+
+
 fs.mkdtemp = function(prefix, options, callback) {
   callback = makeCallback(typeof options === 'function' ? options : callback);
   options = getOptions(options, {});

src/node_file.cc

@@ -827,24 +827,15 @@ static void MKDir(const FunctionCallbackInfo<Value>& args) {
 }
 
 static void RealPath(const FunctionCallbackInfo<Value>& args) {
+  CHECK_GE(args.Length(), 2);
   Environment* env = Environment::GetCurrent(args);
-
-  const int argc = args.Length();
-
-  if (argc < 1)
-    return TYPE_ERROR("path required");
-
   BufferValue path(env->isolate(), args[0]);
   ASSERT_PATH(path)
 
   const enum encoding encoding = ParseEncoding(env->isolate(), args[1], UTF8);
 
-  Local<Value> callback = Null(env->isolate());
-  if (argc == 3)
-    callback = args[2];
-
-  if (callback->IsObject()) {
-    ASYNC_CALL(realpath, callback, encoding, *path);
+  if (args[2]->IsObject()) {
+    ASYNC_CALL(realpath, args[2], encoding, *path);
   } else {
     SYNC_CALL(realpath, *path, *path);
     const char* link_path = static_cast<const char*>(SYNC_REQ.ptr);

test/parallel/test-fs-realpath-native.js

@@ -0,0 +1,12 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+
+if (!common.isOSX) common.skip('MacOS-only test.');
+
+assert.strictEqual(fs.realpathSync.native('/users'), '/Users');
+fs.realpath.native('/users', common.mustCall((err, res) => {
+  assert.ifError(err);
+  assert.strictEqual(res, '/Users');
+}));