fs,module: add module-loader-only realpath cache

lib/fs.js

@@ -1560,6 +1560,10 @@ function encodeRealpathResult(result, options, err) {
   }
 }
 
+// This is removed from the fs exports in lib/module.js in order to make
+// sure that this stays internal.
+const realpathCacheKey = fs.realpathCacheKey = Symbol('realpathCacheKey');
+
 fs.realpathSync = function realpathSync(p, options) {
   if (!options)
     options = {};
@@ -1574,6 +1578,13 @@ fs.realpathSync = function realpathSync(p, options) {
 
   const seenLinks = {};
   const knownHard = {};
+  const cache = options[realpathCacheKey];
+  const original = p;
+
+  const maybeCachedResult = cache && cache.get(p);
+  if (maybeCachedResult) {
+    return maybeCachedResult;
+  }
 
   // current character position in p
   var pos;
@@ -1614,39 +1625,47 @@ fs.realpathSync = function realpathSync(p, options) {
     pos = nextPartRe.lastIndex;
 
     // continue if not a symlink
-    if (knownHard[base]) {
+    if (knownHard[base] || (cache && cache.get(base) === base)) {
       continue;
     }
 
     var resolvedLink;
-    var stat = fs.lstatSync(base);
-    if (!stat.isSymbolicLink()) {
-      knownHard[base] = true;
-      continue;
-    }
+    const maybeCachedResolved = cache && cache.get(base);
+    if (maybeCachedResolved) {
+      resolvedLink = maybeCachedResolved;
+    } else {
+      var stat = fs.lstatSync(base);
+      if (!stat.isSymbolicLink()) {
+        knownHard[base] = true;
+        continue;
+      }
 
-    // read the link if it wasn't read before
-    // dev/ino always return 0 on windows, so skip the check.
-    var linkTarget = null;
-    if (!isWindows) {
-      var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
-      if (seenLinks.hasOwnProperty(id)) {
-        linkTarget = seenLinks[id];
+      // read the link if it wasn't read before
+      // dev/ino always return 0 on windows, so skip the check.
+      let linkTarget = null;
+      let id;
+      if (!isWindows) {
+        id = `${stat.dev.toString(32)}:${stat.ino.toString(32)}`;
+        if (seenLinks.hasOwnProperty(id)) {
+          linkTarget = seenLinks[id];
+        }
       }
-    }
-    if (linkTarget === null) {
-      fs.statSync(base);
-      linkTarget = fs.readlinkSync(base);
-    }
-    resolvedLink = pathModule.resolve(previous, linkTarget);
+      if (linkTarget === null) {
+        fs.statSync(base);
+        linkTarget = fs.readlinkSync(base);
+      }
+      resolvedLink = pathModule.resolve(previous, linkTarget);
 
-    if (!isWindows) seenLinks[id] = linkTarget;
+      if (cache) cache.set(base, resolvedLink);
+      if (!isWindows) seenLinks[id] = linkTarget;
+    }
 
     // resolve the link, then start over
     p = pathModule.resolve(resolvedLink, p.slice(pos));
     start();
   }
 
+  if (cache) cache.set(original, p);
   return encodeRealpathResult(p, options);
 };
 
@@ -1742,8 +1761,9 @@ fs.realpath = function realpath(p, options, callback) {
     // stat & read the link if not read before
     // call gotTarget as soon as the link target is known
     // dev/ino always return 0 on windows, so skip the check.
+    let id;
     if (!isWindows) {
-      var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);
+      id = `${stat.dev.toString(32)}:${stat.ino.toString(32)}`;
       if (seenLinks.hasOwnProperty(id)) {
         return gotTarget(null, seenLinks[id], base);
       }

lib/module.js

@@ -109,6 +109,14 @@ function tryPackage(requestPath, exts, isMain) {
          tryExtensions(path.resolve(filename, 'index'), exts, isMain);
 }
 
+// In order to minimize unnecessary lstat() calls,
+// this cache is a list of known-real paths.
+// Set to an empty Map to reset.
+const realpathCache = new Map();
+
+const realpathCacheKey = fs.realpathCacheKey;
+delete fs.realpathCacheKey;
+
 // check if the file exists and is not a directory
 // if using --preserve-symlinks and isMain is false,
 // keep symlinks intact, otherwise resolve to the
@@ -118,7 +126,13 @@ function tryFile(requestPath, isMain) {
   if (preserveSymlinks && !isMain) {
     return rc === 0 && path.resolve(requestPath);
   }
-  return rc === 0 && fs.realpathSync(requestPath);
+  return rc === 0 && toRealPath(requestPath);
+}
+
+function toRealPath(requestPath) {
+  return fs.realpathSync(requestPath, {
+    [realpathCacheKey]: realpathCache
+  });
 }
 
 // given a path check a the file exists with any of the set extensions
@@ -164,7 +178,7 @@ Module._findPath = function(request, paths, isMain) {
         if (preserveSymlinks && !isMain) {
           filename = path.resolve(basePath);
         } else {
-          filename = fs.realpathSync(basePath);
+          filename = toRealPath(basePath);
         }
       } else if (rc === 1) {  // Directory.
         if (exts === undefined)