esm: implement flagged resolution algo

src/module_wrap.cc

@@ -43,6 +43,8 @@ using v8::String;
 using v8::Undefined;
 using v8::Value;
 
+static const char* const EXTENSIONS[] = {".mjs", ".js", ".json", ".node"};
+
 ModuleWrap::ModuleWrap(Environment* env,
                        Local<Object> object,
                        Local<Module> module,
@@ -683,6 +685,36 @@ Maybe<URL> FinalizeResolution(Environment* env,
   return Just(resolved);
 }
 
+enum ResolveExtensionsOptions {
+  TRY_EXACT_NAME,
+  ONLY_VIA_EXTENSIONS
+};
+
+template <ResolveExtensionsOptions options>
+Maybe<URL> ResolveExtensions(const URL& search) {
+  if (options == TRY_EXACT_NAME) {
+    std::string filePath = search.ToFilePath();
+    Maybe<uv_file> fd = OpenDescriptor(filePath);
+    if (!fd.IsNothing()) {
+      return Just(search);
+    }
+  }
+
+  for (const char* extension : EXTENSIONS) {
+    URL guess(search.path() + extension, &search);
+    Maybe<uv_file> fd = OpenDescriptor(guess.ToFilePath());
+    if (!fd.IsNothing()) {
+      return Just(guess);
+    }
+  }
+
+  return Nothing<URL>();
+}
+
+inline Maybe<URL> ResolveIndex(const URL& search) {
+  return ResolveExtensions<ONLY_VIA_EXTENSIONS>(URL("index", search));
+}
+
 Maybe<URL> PackageMainResolve(Environment* env,
                                            const URL& pjson_url,
                                            const PackageConfig& pcfg,
@@ -778,6 +810,16 @@ Maybe<URL> Resolve(Environment* env,
   URL resolved;
   if (ShouldBeTreatedAsRelativeOrAbsolutePath(specifier)) {
     resolved = URL(specifier, base);
+    if (env->options()->legacy_resolution) {
+      Maybe<URL> file = ResolveExtensions<TRY_EXACT_NAME>(resolved);
+      if (!file.IsNothing()) {
+        return file;
+      }
+      if (specifier.back() != '/') {
+        resolved = URL(specifier + "/", base);
+      }
+      return ResolveIndex(resolved);
+    }
   } else {
     URL pure_url(specifier);
     if (!(pure_url.flags() & URL_FLAGS_FAILED)) {