fs: more realpath*() optimizations

Including:

* Avoid regexp on non-Windows platforms when parsing the root of a path

Backport-PR-URL: nodejs#11665

lib/fs.js

@@ -1622,10 +1622,23 @@ fs.unwatchFile = function(filename, listener) {
 };
 
 
-// Regex to find the device root, including trailing slash. E.g. 'c:\\'.
-const splitRootRe = isWindows ?
-  /^(?:[a-zA-Z]:|[\\/]{2}[^\\/]+[\\/][^\\/]+)?[\\/]*/ :
-  /^[/]*/;
+var splitRoot;
+if (isWindows) {
+  // Regex to find the device root on Windows (e.g. 'c:\\'), including trailing
+  // slash.
+  const splitRootRe = /^(?:[a-zA-Z]:|[\\/]{2}[^\\/]+[\\/][^\\/]+)?[\\/]*/;
+  splitRoot = function splitRoot(str) {
+    return splitRootRe.exec(str)[0];
+  };
+} else {
+  splitRoot = function splitRoot(str) {
+    for (var i = 0; i < str.length; ++i) {
+      if (str.charCodeAt(i) !== 47/*'/'*/)
+        return str.slice(0, i);
+    }
+    return str;
+  };
+}
 
 function encodeRealpathResult(result, options, err) {
   if (!options || !options.encoding || options.encoding === 'utf8' || err)
@@ -1688,17 +1701,15 @@ fs.realpathSync = function realpathSync(p, options) {
   // the partial path scanned in the previous round, with slash
   var previous;
 
-    // Skip over roots
-    var m = splitRootRe.exec(p);
-    pos = m[0].length;
-    current = m[0];
-    base = m[0];
+  // Skip over roots
+  current = base = splitRoot(p);
+  pos = current.length;
 
-    // On windows, check that the root exists. On unix there is no need.
-    if (isWindows && !knownHard[base]) {
+  // On windows, check that the root exists. On unix there is no need.
+  if (isWindows && !knownHard[base]) {
     binding.lstat(pathModule._makeLong(base));
-      knownHard[base] = true;
-    }
+    knownHard[base] = true;
+  }
 
   // walk down the path, swapping out linked path parts for their real
   // values
@@ -1731,7 +1742,8 @@ fs.realpathSync = function realpathSync(p, options) {
       // Use stats array directly to avoid creating an fs.Stats instance just
       // for our internal use.
 
-      binding.lstat(pathModule._makeLong(base));
+      var baseLong = pathModule._makeLong(base);
+      binding.lstat(baseLong);
 
       if ((statValues[1/*mode*/] & S_IFMT) !== S_IFLNK) {
         knownHard[base] = true;
@@ -1752,8 +1764,8 @@ fs.realpathSync = function realpathSync(p, options) {
         }
       }
       if (linkTarget === null) {
-        binding.stat(pathModule._makeLong(base));
-        linkTarget = binding.readlink(pathModule._makeLong(base));
+        binding.stat(baseLong);
+        linkTarget = binding.readlink(baseLong);
       }
       resolvedLink = pathModule.resolve(previous, linkTarget);
 
@@ -1765,10 +1777,8 @@ fs.realpathSync = function realpathSync(p, options) {
     p = pathModule.resolve(resolvedLink, p.slice(pos));
 
     // Skip over roots
-    m = splitRootRe.exec(p);
-    pos = m[0].length;
-    current = m[0];
-    base = m[0];
+    current = base = splitRoot(p);
+    pos = current.length;
 
     // On windows, check that the root exists. On unix there is no need.
     if (isWindows && !knownHard[base]) {
@@ -1815,11 +1825,8 @@ fs.realpath = function realpath(p, options, callback) {
   // the partial path scanned in the previous round, with slash
   var previous;
 
-  var m = splitRootRe.exec(p);
-  pos = m[0].length;
-  current = m[0];
-  base = m[0];
-  previous = '';
+  current = base = splitRoot(p);
+  pos = current.length;
 
     // On windows, check that the root exists. On unix there is no need.
   if (isWindows && !knownHard[base]) {
@@ -1906,11 +1913,8 @@ fs.realpath = function realpath(p, options, callback) {
   function gotResolvedLink(resolvedLink) {
     // resolve the link, then start over
     p = pathModule.resolve(resolvedLink, p.slice(pos));
-    var m = splitRootRe.exec(p);
-    pos = m[0].length;
-    current = m[0];
-    base = m[0];
-    previous = '';
+    current = base = splitRoot(p);
+    pos = current.length;
 
     // On windows, check that the root exists. On unix there is no need.
     if (isWindows && !knownHard[base]) {