module: cache stat() results more aggressively

Reduce the number of stat() system calls that require() makes by caching
the results more aggressively.

To avoid unbounded growth without implementing a LRU cache, scope the
cache to the lifetime of the first call to require().  Recursive calls
(i.e. require() calls in the included code) transparently profit from
the cache.

The benchmarked application is the loopback-sample-app[0] and it sees
the number of stat calls at start-up go down by 40%, from 4736 to 2810.

[0] https://github.com/strongloop/loopback-sample-app

PR-URL: #4575
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/module.js

@@ -1,6 +1,8 @@
 'use strict';
 
-module.exports = { makeRequireFunction, stripBOM };
+exports = module.exports = { makeRequireFunction, stripBOM };
+
+exports.requireDepth = 0;
 
 // Invoke with makeRequireFunction.call(module) where |module| is the
 // Module object to use as the context for the require() function.
@@ -9,7 +11,12 @@ function makeRequireFunction() {
   const self = this;
 
   function require(path) {
-    return self.require(path);
+    try {
+      exports.requireDepth += 1;
+      return self.require(path);
+    } finally {
+      exports.requireDepth -= 1;
+    }
   }
 
   require.resolve = function(request) {

lib/module.js

@@ -33,6 +33,20 @@ function tryWrapper(wrapper, opts) {
 }
 
 
+function stat(filename) {
+  filename = path._makeLong(filename);
+  const cache = stat.cache;
+  if (cache !== null) {
+    const result = cache.get(filename);
+    if (result !== undefined) return result;
+  }
+  const result = internalModuleStat(filename);
+  if (cache !== null) cache.set(filename, result);
+  return result;
+}
+stat.cache = null;
+
+
 function Module(id, parent) {
   this.id = id;
   this.exports = {};
@@ -114,7 +128,7 @@ Module._realpathCache = {};
 
 // check if the file exists and is not a directory
 function tryFile(requestPath) {
-  const rc = internalModuleStat(path._makeLong(requestPath));
+  const rc = stat(requestPath);
   return rc === 0 && toRealPath(requestPath);
 }
 
@@ -151,12 +165,12 @@ Module._findPath = function(request, paths) {
   // For each path
   for (var i = 0, PL = paths.length; i < PL; i++) {
     // Don't search further if path doesn't exist
-    if (paths[i] && internalModuleStat(path._makeLong(paths[i])) < 1) continue;
+    if (paths[i] && stat(paths[i]) < 1) continue;
     var basePath = path.resolve(paths[i], request);
     var filename;
 
     if (!trailingSlash) {
-      const rc = internalModuleStat(path._makeLong(basePath));
+      const rc = stat(basePath);
       if (rc === 0) {  // File.
         filename = toRealPath(basePath);
       } else if (rc === 1) {  // Directory.
@@ -404,7 +418,11 @@ Module.prototype._compile = function(content, filename) {
   const dirname = path.dirname(filename);
   const require = internalModule.makeRequireFunction.call(this);
   const args = [this.exports, require, this, filename, dirname];
-  return compiledWrapper.apply(this.exports, args);
+  const depth = internalModule.requireDepth;
+  if (depth === 0) stat.cache = new Map();
+  const result = compiledWrapper.apply(this.exports, args);
+  if (depth === 0) stat.cache = null;
+  return result;
 };
 
 

test/fixtures/module-require-depth/one.js

@@ -0,0 +1,9 @@
+// Flags: --expose_internals
+'use strict';
+const assert = require('assert');
+const internalModule = require('internal/module');
+
+exports.requireDepth = internalModule.requireDepth;
+assert.strictEqual(internalModule.requireDepth, 1);
+assert.deepStrictEqual(require('./two'), { requireDepth: 2 });
+assert.strictEqual(internalModule.requireDepth, 1);

test/fixtures/module-require-depth/two.js

@@ -0,0 +1,9 @@
+// Flags: --expose_internals
+'use strict';
+const assert = require('assert');
+const internalModule = require('internal/module');
+
+exports.requireDepth = internalModule.requireDepth;
+assert.strictEqual(internalModule.requireDepth, 2);
+assert.deepStrictEqual(require('./one'), { requireDepth: 1 });
+assert.strictEqual(internalModule.requireDepth, 2);

test/parallel/test-module-require-depth.js

@@ -0,0 +1,13 @@
+// Flags: --expose_internals
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const internalModule = require('internal/module');
+
+// Module one loads two too so the expected depth for two is, well, two.
+assert.strictEqual(internalModule.requireDepth, 0);
+const one = require(common.fixturesDir + '/module-require-depth/one');
+const two = require(common.fixturesDir + '/module-require-depth/two');
+assert.deepStrictEqual(one, { requireDepth: 1 });
+assert.deepStrictEqual(two, { requireDepth: 2 });
+assert.strictEqual(internalModule.requireDepth, 0);