Fix fs.realpath to work on Windows

1. Make the isRoot check valid
2. Don't cache results based on dev/ino, since those are alwasy 0 on
windows.

lib/fs.js

@@ -897,6 +897,11 @@ var normalize = pathModule.normalize;
 // result is [base_with_slash, base], e.g. ['somedir/', 'somedir']
 var nextPartRe = /(.*?)(?:[\/]+|$)/g;
 
+// Regex to split a windows path into three parts: [*, device, slash,
+// tail] windows-only
+var splitDeviceRe =
+    /^([a-zA-Z]:|[\\\/]{2}[^\\\/]+[\\\/][^\\\/]+)?([\\\/])?([\s\S]*?)$/;
+
 fs.realpathSync = function realpathSync(p, cache) {
   // make p is absolute
   p = pathModule.resolve(p);
@@ -931,7 +936,15 @@ fs.realpathSync = function realpathSync(p, cache) {
     pos = nextPartRe.lastIndex;
 
     // continue if not a symlink, or if root
-    if (!base || knownHard[base] || (cache && cache[base] === base)) {
+    var isRoot = !base;
+    if (isWindows) {
+      // if it doens't have a tail, then it's the root.
+      var split = base.match(splitDeviceRe);
+      if (split) {
+        isRoot = !split[2];
+      }
+    }
+    if (isRoot || knownHard[base] || (cache && cache[base] === base)) {
       continue;
     }
 
@@ -948,13 +961,21 @@ fs.realpathSync = function realpathSync(p, cache) {
       }
 
       // read the link if it wasn't read before
-      var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
-      if (!seenLinks[id]) {
+      // dev/ino always return 0 on windows, so skip the check.
+      var linkTarget;
+      if (!isWindows) {
+        var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
+        if (seenLinks[id]) {
+          linkTarget = seenLinks[id];
+        }
+      }
+      if (!linkTarget) {
         fs.statSync(base);
-        seenLinks[id] = fs.readlinkSync(base);
-        resolvedLink = pathModule.resolve(previous, seenLinks[id]);
+        linkTarget = fs.readlinkSync(base);
+        resolvedLink = pathModule.resolve(previous, linkTarget);
         // track this, if given a cache.
         if (cache) cache[base] = resolvedLink;
+        if (!isWindows) seenLinks[id] = linkTarget;
       }
     }
 
@@ -1014,8 +1035,16 @@ fs.realpath = function realpath(p, cache, cb) {
     base = previous + result[1];
     pos = nextPartRe.lastIndex;
 
-    // continue if known to be hard or if root or in cache already.
-    if (!base || knownHard[base] || (cache && cache[base] === base)) {
+    // continue if not a symlink, or if root
+    var isRoot = !base;
+    if (isWindows) {
+      // if it doens't have a tail, then it's the root.
+      var split = base.match(splitDeviceRe);
+      if (split) {
+        isRoot = !split[2];
+      }
+    }
+    if (isRoot || knownHard[base] || (cache && cache[base] === base)) {
       return process.nextTick(LOOP);
     }
 
@@ -1039,15 +1068,19 @@ fs.realpath = function realpath(p, cache, cb) {
 
     // stat & read the link if not read before
     // call gotTarget as soon as the link target is known
-    var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
-    if (seenLinks[id]) {
-      return gotTarget(null, seenLinks[id], base);
+    // dev/ino always return 0 on windows, so skip the check.
+    if (!isWindows) {
+      var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
+      if (seenLinks[id]) {
+        return gotTarget(null, seenLinks[id], base);
+      }
     }
     fs.stat(base, function(err) {
       if (err) return cb(err);
 
       fs.readlink(base, function(err, target) {
-        gotTarget(err, seenLinks[id] = target);
+        if (!isWindows) seenLinks[id] = target;
+        gotTarget(err, target);
       });
     });
   }