lib: include cached modules in module.children

`module.children` is supposed to be the list of modules included by this
module but lib/module.js failed to update the list when the included
module was retrieved from `Module._cache`.

Fixes: #7131
PR-URL: #14132
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

lib/module.js

@@ -46,15 +46,17 @@ function stat(filename) {
 }
 stat.cache = null;
 
+function updateChildren(parent, child, scan) {
+  var children = parent && parent.children;
+  if (children && !(scan && children.includes(child)))
+    children.push(child);
+}
 
 function Module(id, parent) {
   this.id = id;
   this.exports = {};
   this.parent = parent;
-  if (parent && parent.children) {
-    parent.children.push(this);
-  }
-
+  updateChildren(parent, this, false);
   this.filename = null;
   this.loaded = false;
   this.children = [];
@@ -438,6 +440,7 @@ Module._load = function(request, parent, isMain) {
 
   var cachedModule = Module._cache[filename];
   if (cachedModule) {
+    updateChildren(parent, cachedModule, true);
     return cachedModule.exports;
   }
 
@@ -446,6 +449,7 @@ Module._load = function(request, parent, isMain) {
     return NativeModule.require(filename);
   }
 
+  // Don't call updateChildren(), Module constructor already does.
   var module = new Module(filename, parent);
 
   if (isMain) {

test/fixtures/GH-7131/a.js

@@ -0,0 +1,5 @@
+'use strict';
+require('sys');  // Builtin should not show up in module.children array.
+require('./b');  // This should.
+require('./b');  // This should not.
+module.exports = module.children.slice();

test/fixtures/GH-7131/b.js

@@ -0,0 +1,2 @@
+'use strict';
+module.exports = module.children.slice();

test/parallel/test-module-children.js

@@ -0,0 +1,13 @@
+// Flags: --no-deprecation
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+
+const dir = path.join(common.fixturesDir, 'GH-7131');
+const b = require(path.join(dir, 'b'));
+const a = require(path.join(dir, 'a'));
+
+assert.strictEqual(a.length, 1);
+assert.strictEqual(b.length, 0);
+assert.deepStrictEqual(a[0].exports, b);

test/sequential/test-module-loading.js

@@ -237,7 +237,10 @@ try {
   // modules that we've required, and that all of them contain
   // the appropriate children, and so on.
 
+  const visited = new Set();
   const children = module.children.reduce(function red(set, child) {
+    if (visited.has(child)) return set;
+    visited.add(child);
     let id = path.relative(path.dirname(__dirname), child.id);
     id = id.replace(backslash, '/');
     set[id] = child.children.reduce(red, {});