module: fix inconsistency between load and _findPath

Files with multiple extensions are not handled by require-module system
therefore if you have file 'file.foo.bar' and require('./file') it won't
be found even while using require.extensions['foo.bar'] but before this
commit if you have require.extensions['foo.bar'] and
require.extensions['bar'] set then the latter will be called if you do
require('./file') but if you remove the former the former ('foo.bar')
property it will fail.
This commit makes it always fail in such cases.

Fixes: nodejs#4778

lib/internal/modules/cjs/loader.js

@@ -216,6 +216,13 @@ function tryExtensions(p, exts, isMain) {
   return false;
 }
 
+function readExtensions() {
+  const exts = Object.keys(Module._extensions);
+  for (var i = 0; i < exts.length; ++i)
+    exts[i] = path.extname(exts[i]) || exts[i];
+  return exts;
+}
+
 var warned = false;
 Module._findPath = function(request, paths, isMain) {
   if (path.isAbsolute(request)) {
@@ -272,15 +279,15 @@ Module._findPath = function(request, paths, isMain) {
       if (!filename) {
         // try it with each of the extensions
         if (exts === undefined)
-          exts = Object.keys(Module._extensions);
+          exts = readExtensions();
         filename = tryExtensions(basePath, exts, isMain);
       }
     }
 
     if (!filename && rc === 1) {  // Directory.
       // try it with each of the extensions at "index"
       if (exts === undefined)
-        exts = Object.keys(Module._extensions);
+        exts = readExtensions();
       filename = tryPackage(basePath, exts, isMain);
       if (!filename) {
         filename = tryExtensions(path.resolve(basePath, 'index'), exts, isMain);

test/parallel/test-module-deleted-extensions.js

@@ -0,0 +1,43 @@
+'use strict';
+
+// Refs: https://github.com/nodejs/node/issues/4778
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const tmpdir = require('../common/tmpdir');
+const file = path.join(tmpdir.path, 'test-extensions.foo.bar');
+
+tmpdir.refresh();
+fs.writeFileSync(file, '', 'utf8');
+
+{
+  require.extensions['.bar'] = common.mustNotCall();
+  require.extensions['.foo.bar'] = common.mustNotCall();
+  const modulePath = path.join(tmpdir.path, 'test-extensions');
+  assert.throws(
+    () => require(modulePath),
+    new Error(`Cannot find module '${modulePath}'`)
+  );
+}
+
+{
+  require.extensions['.foo.bar'] = common.mustNotCall();
+  delete require.extensions['.foo.bar'];
+  const modulePath = path.join(tmpdir.path, 'test-extensions');
+  assert.throws(
+    () => require(modulePath),
+    new Error(`Cannot find module '${modulePath}'`)
+  );
+}
+
+{
+  require.extensions['.bar'] = common.mustCall((module, path) => {
+    assert.strictEqual(module.id, file);
+    assert.strictEqual(path, file);
+  });
+
+  const modulePath = path.join(tmpdir.path, 'test-extensions.foo');
+  require(modulePath);
+}