module: add extra caching layer

This adds an extra modules caching layer that operates on the parent's
`path` property and the current require argument. That together can
be used as unique identifier to speed up loading the same module more
than once. It is a cache on top of the current modules cache.

It has the nice feature that this cache does not only work in the same
file but it works for the whole current directory. So if the same file
is loaded in any other file from the same directory, it will also hit
this cache instead of having to resolve the file again.

To keep it backwards compatible with the old modules cache, it detects
invalidation of that cache.

Author: BridgeAR

lib/internal/modules/cjs/loader.js

@@ -85,6 +85,8 @@ const {
 
 const isWindows = process.platform === 'win32';
 
+const relativeResolveCache = Object.create(null);
+
 let requireDepth = 0;
 let statCache = new Map();
 function stat(filename) {
@@ -598,14 +600,28 @@ Module._resolveLookupPaths = function(request, parent, newReturn) {
 //    Then have it load  the file contents before returning its exports
 //    object.
 Module._load = function(request, parent, isMain) {
+  let relResolveCacheIdentifier;
   if (parent) {
     debug('Module._load REQUEST %s parent: %s', request, parent.id);
+    // Fast path for (lazy loaded) modules in the same directory. The indirect
+    // caching is required to allow cache invalidation without changing the old
+    // cache key names.
+    relResolveCacheIdentifier = `${parent.path}\x00${request}`;
+    const filename = relativeResolveCache[relResolveCacheIdentifier];
+    if (filename !== undefined) {
+      const cachedModule = Module._cache[filename];
+      if (cachedModule !== undefined) {
+        updateChildren(parent, cachedModule, true);
+        return cachedModule.exports;
+      }
+      delete relativeResolveCache[relResolveCacheIdentifier];
+    }
   }
 
   const filename = Module._resolveFilename(request, parent, isMain);
 
   const cachedModule = Module._cache[filename];
-  if (cachedModule) {
+  if (cachedModule !== undefined) {
     updateChildren(parent, cachedModule, true);
     return cachedModule.exports;
   }
@@ -625,6 +641,9 @@ Module._load = function(request, parent, isMain) {
   }
 
   Module._cache[filename] = module;
+  if (parent !== undefined) {
+    relativeResolveCache[relResolveCacheIdentifier] = filename;
+  }
 
   let threw = true;
   try {
@@ -633,6 +652,9 @@ Module._load = function(request, parent, isMain) {
   } finally {
     if (threw) {
       delete Module._cache[filename];
+      if (parent !== undefined) {
+        delete relativeResolveCache[relResolveCacheIdentifier];
+      }
     }
   }