win, fs: fix realpath behavior on substed drives

Before v6 on drives created with subst and network mapped drives realpath used to return filename on the mapped drive. With v6 it now returns filename on the original device or on the network share. This restores the old behavior by using old javascript implementation when path returned by new implementation is on a different device than the original path. Fixes: 7294
nodejs · bzoz · Jul 6, 2016 · Jul 6, 2016 · Jul 6, 2016 · Jul 6, 2016
commit 288cd95c63e8565e9c4c76040687c47f808b12a9
diff --git a/lib/fs.js b/lib/fs.js
@@ -1562,6 +1562,8 @@ fs.unwatchFile = function(filename, listener) {
   }
 };
 
+// Cache for JS real path 
+var realpathCache = {};
 // Regexp that finds the next partion of a (partial) path
 // result is [base_with_slash, base], e.g. ['somedir/', 'somedir']
 const nextPartRe = isWindows ?
@@ -1669,6 +1671,25 @@ function realpathSyncJS(p, cache) {
   return p;
 };
 
+function realpathJSNeeded(resolvedPath, result, err) {
+  if (!isWindows || err)
+    return false;
+  const resultStr = result.toString('utf8');
+  return resultStr.length > 0 && resolvedPath.length > 0 &&
+         resultStr[0].toUpperCase() !== resolvedPath[0].toUpperCase();
+};
+
+function convertRealpathJSResult(result, encoding, err) {
+  if (!encoding || encoding === 'utf8' || err)
+    return result;
+  const asBuffer = Buffer.from(result);
+  if (encoding === 'buffer') {
+    return asBuffer;
+  } else {
+    return asBuffer.toString(encoding);
+  }
+};
+
 fs.realpathSync = function realpathSync(path, options) {
   if (!options)
     options = {};
@@ -1677,7 +1698,16 @@ fs.realpathSync = function realpathSync(path, options) {
   else if (typeof options !== 'object')
     throw new TypeError('"options" must be a string or an object');
   nullCheck(path);
-  return binding.realpath(pathModule._makeLong(path), options.encoding);
+  const resolvedPath = pathModule.resolve(path.toString('utf8'));
+  const uvResult = binding.realpath(pathModule._makeLong(resolvedPath),
+                                    options.encoding);
+  if (realpathJSNeeded(resolvedPath, uvResult))
+  {
+    const jsResult = realpathSyncJS(resolvedPath, realpathCache);
+    return convertRealpathJSResult(jsResult, options.encoding);
+  } else {
+    return uvResult;
+  }
 };
 
 function realpathJS(p, cache, cb) {
@@ -1813,11 +1843,25 @@ fs.realpath = function realpath(path, options, callback) {
   } else if (typeof options !== 'object') {
     throw new TypeError('"options" must be a string or an object');
   }
-  callback = makeCallback(callback);
   if (!nullCheck(path, callback))
     return;
+
+  const resolvedPath = pathModule.resolve(path.toString('utf8'));
+  const win32callback = function(err, result) {
+    if (realpathJSNeeded(resolvedPath, result, err)) {
+      realpathJS(resolvedPath, realpathCache, function(err, result) {
+        result = convertRealpathJSResult(result, options.encoding, err);
+        return callback(err, result);
+      });
+    } else {
+      return callback(err, result);
+    }
+  };
+
+  const use_callback = makeCallback(isWindows ? win32callback : callback);
+
   var req = new FSReqWrap();
-  req.oncomplete = callback;
+  req.oncomplete = use_callback;
   binding.realpath(pathModule._makeLong(path), options.encoding, req);
   return;
 };

diff --git a/test/parallel/test-fs-realpath-subst-drive.js b/test/parallel/test-fs-realpath-subst-drive.js
@@ -0,0 +1,58 @@
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const spawnSync = require('child_process').spawnSync;
+
+if (!common.isWindows) {
+  common.skip("Test for Windows only");
+  return;
+}
+let result;
+
+// create a subst drive
+const driveLetters = "ABCDEFGHIJKLMNOPQRSTUWXYZ";
+let driveLetter;
+for (var i = 0; i < driveLetters.length; ++i) {
+  driveLetter = `${driveLetters[i]}:`
+   result = spawnSync('subst', [driveLetter, common.fixturesDir]);
+  if (result.status === 0)
+    break;
+}
+if (i === driveLetters.length) {
+  common.skip("Cannot create subst drive");
+  return;
+}
+
+var asyncCompleted = 0;
+// schedule cleanup (and check if all callbacks where called)
+process.on('exit', function() {
+    spawnSync('subst', ['/d', driveLetter]);
+    assert.equal(asyncCompleted, 2);
+});
+
+
+// test:
+const filename = `${driveLetter}\\empty.js`;
+const filenameBuffer = Buffer.from(filename);
+
+result = fs.realpathSync(filename);
+assert.equal(typeof result, 'string');
+assert.equal(result, filename);
+
+result = fs.realpathSync(filename, 'buffer');
+assert(Buffer.isBuffer(result));
+assert(result.equals(filenameBuffer));
+
+fs.realpath(filename, function(err, result) {
+  ++asyncCompleted;
+  assert(!err);
+  assert.equal(typeof result, 'string');
+  assert.equal(result, filename);
+});
+
+fs.realpath(filename, 'buffer', function(err, result) {
+  ++asyncCompleted;
+  assert(!err);
+  assert(Buffer.isBuffer(result));
+  assert(result.equals(filenameBuffer));
+});