src/module_wrap.cc

#include "module_wrap.h"

#include "env.h"
#include "memory_tracker-inl.h"
#include "node_contextify.h"
#include "node_errors.h"
#include "node_internals.h"
#include "node_process.h"
#include "node_url.h"
#include "node_watchdog.h"
#include "util-inl.h"

#include <sys/stat.h>  // S_IFDIR

#include <algorithm>

namespace node {
namespace loader {

using errors::TryCatchScope;

using node::contextify::ContextifyContext;
using node::url::URL;
using node::url::URL_FLAGS_FAILED;
using v8::Array;
using v8::ArrayBufferView;
using v8::Context;
using v8::EscapableHandleScope;
using v8::Function;
using v8::FunctionCallbackInfo;
using v8::FunctionTemplate;
using v8::Global;
using v8::HandleScope;
using v8::Integer;
using v8::IntegrityLevel;
using v8::Isolate;
using v8::Just;
using v8::Local;
using v8::Maybe;
using v8::MaybeLocal;
using v8::Module;
using v8::Nothing;
using v8::Number;
using v8::Object;
using v8::PrimitiveArray;
using v8::Promise;
using v8::ScriptCompiler;
using v8::ScriptOrigin;
using v8::ScriptOrModule;
using v8::String;
using v8::UnboundModuleScript;
using v8::Undefined;
using v8::Value;

static const char* const EXTENSIONS[] = {
  ".js",
  ".json",
  ".node",
  ".mjs"
};

ModuleWrap::ModuleWrap(Environment* env,
                       Local<Object> object,
                       Local<Module> module,
                       Local<String> url) :
  BaseObject(env, object),
  id_(env->get_next_module_id()) {
  module_.Reset(env->isolate(), module);
  url_.Reset(env->isolate(), url);
  env->id_to_module_map.emplace(id_, this);
}

ModuleWrap::~ModuleWrap() {
  HandleScope scope(env()->isolate());
  Local<Module> module = module_.Get(env()->isolate());
  env()->id_to_module_map.erase(id_);
  auto range = env()->hash_to_module_map.equal_range(module->GetIdentityHash());
  for (auto it = range.first; it != range.second; ++it) {
    if (it->second == this) {
      env()->hash_to_module_map.erase(it);
      break;
    }
  }
}

ModuleWrap* ModuleWrap::GetFromModule(Environment* env,
                                      Local<Module> module) {
  auto range = env->hash_to_module_map.equal_range(module->GetIdentityHash());
  for (auto it = range.first; it != range.second; ++it) {
    if (it->second->module_ == module) {
      return it->second;
    }
  }
  return nullptr;
}

ModuleWrap* ModuleWrap::GetFromID(Environment* env, uint32_t id) {
  auto module_wrap_it = env->id_to_module_map.find(id);
  if (module_wrap_it == env->id_to_module_map.end()) {
    return nullptr;
  }
  return module_wrap_it->second;
}

// new ModuleWrap(url, context, source, lineOffset, columnOffset)
// new ModuleWrap(url, context, exportNames, syntheticExecutionFunction)
void ModuleWrap::New(const FunctionCallbackInfo<Value>& args) {
  CHECK(args.IsConstructCall());
  CHECK_GE(args.Length(), 3);

  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = env->isolate();

  Local<Object> that = args.This();

  CHECK(args[0]->IsString());
  Local<String> url = args[0].As<String>();

  Local<Context> context;
  if (args[1]->IsUndefined()) {
    context = that->CreationContext();
  } else {
    CHECK(args[1]->IsObject());
    ContextifyContext* sandbox =
        ContextifyContext::ContextFromContextifiedSandbox(
            env, args[1].As<Object>());
    CHECK_NOT_NULL(sandbox);
    context = sandbox->context();
  }

  Local<Integer> line_offset;
  Local<Integer> column_offset;

  bool synthetic = args[2]->IsArray();
  if (synthetic) {
    // new ModuleWrap(url, context, exportNames, syntheticExecutionFunction)
    CHECK(args[3]->IsFunction());
  } else {
    // new ModuleWrap(url, context, source, lineOffset, columOffset, cachedData)
    CHECK(args[2]->IsString());
    CHECK(args[3]->IsNumber());
    line_offset = args[3].As<Integer>();
    CHECK(args[4]->IsNumber());
    column_offset = args[4].As<Integer>();
  }

  Local<PrimitiveArray> host_defined_options =
      PrimitiveArray::New(isolate, HostDefinedOptions::kLength);
  host_defined_options->Set(isolate, HostDefinedOptions::kType,
                            Number::New(isolate, ScriptType::kModule));

  ShouldNotAbortOnUncaughtScope no_abort_scope(env);
  TryCatchScope try_catch(env);

  Local<Module> module;

  {
    Context::Scope context_scope(context);
    if (synthetic) {
      CHECK(args[2]->IsArray());
      Local<Array> export_names_arr = args[2].As<Array>();

      uint32_t len = export_names_arr->Length();
      std::vector<Local<String>> export_names(len);
      for (uint32_t i = 0; i < len; i++) {
        Local<Value> export_name_val =
            export_names_arr->Get(context, i).ToLocalChecked();
        CHECK(export_name_val->IsString());
        export_names[i] = export_name_val.As<String>();
      }

      module = Module::CreateSyntheticModule(isolate, url, export_names,
        SyntheticModuleEvaluationStepsCallback);
    } else {
      ScriptCompiler::CachedData* cached_data = nullptr;
      if (!args[5]->IsUndefined()) {
        CHECK(args[5]->IsArrayBufferView());
        Local<ArrayBufferView> cached_data_buf = args[5].As<ArrayBufferView>();
        uint8_t* data = static_cast<uint8_t*>(
            cached_data_buf->Buffer()->GetBackingStore()->Data());
        cached_data =
            new ScriptCompiler::CachedData(data + cached_data_buf->ByteOffset(),
                                           cached_data_buf->ByteLength());
      }

      Local<String> source_text = args[2].As<String>();
      ScriptOrigin origin(url,
                          line_offset,                      // line offset
                          column_offset,                    // column offset
                          True(isolate),                    // is cross origin
                          Local<Integer>(),                 // script id
                          Local<Value>(),                   // source map URL
                          False(isolate),                   // is opaque (?)
                          False(isolate),                   // is WASM
                          True(isolate),                    // is ES Module
                          host_defined_options);
      ScriptCompiler::Source source(source_text, origin, cached_data);
      ScriptCompiler::CompileOptions options;
      if (source.GetCachedData() == nullptr) {
        options = ScriptCompiler::kNoCompileOptions;
      } else {
        options = ScriptCompiler::kConsumeCodeCache;
      }
      if (!ScriptCompiler::CompileModule(isolate, &source, options)
               .ToLocal(&module)) {
        if (try_catch.HasCaught() && !try_catch.HasTerminated()) {
          CHECK(!try_catch.Message().IsEmpty());
          CHECK(!try_catch.Exception().IsEmpty());
          AppendExceptionLine(env, try_catch.Exception(), try_catch.Message(),
                              ErrorHandlingMode::MODULE_ERROR);
          try_catch.ReThrow();
        }
        return;
      }
      if (options == ScriptCompiler::kConsumeCodeCache &&
          source.GetCachedData()->rejected) {
        THROW_ERR_VM_MODULE_CACHED_DATA_REJECTED(
            env, "cachedData buffer was rejected");
        try_catch.ReThrow();
        return;
      }
    }
  }

  if (!that->Set(context, env->url_string(), url).FromMaybe(false)) {
    return;
  }

  ModuleWrap* obj = new ModuleWrap(env, that, module, url);

  if (synthetic) {
    obj->synthetic_ = true;
    obj->synthetic_evaluation_steps_.Reset(
        env->isolate(), args[3].As<Function>());
  }

  obj->context_.Reset(isolate, context);

  env->hash_to_module_map.emplace(module->GetIdentityHash(), obj);

  host_defined_options->Set(isolate, HostDefinedOptions::kID,
                            Number::New(isolate, obj->id()));

  that->SetIntegrityLevel(context, IntegrityLevel::kFrozen);
  args.GetReturnValue().Set(that);
}

void ModuleWrap::Link(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = args.GetIsolate();

  CHECK_EQ(args.Length(), 1);
  CHECK(args[0]->IsFunction());

  Local<Object> that = args.This();

  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, that);

  if (obj->linked_)
    return;
  obj->linked_ = true;

  Local<Function> resolver_arg = args[0].As<Function>();

  Local<Context> mod_context = obj->context_.Get(isolate);
  Local<Module> module = obj->module_.Get(isolate);

  const int module_requests_length = module->GetModuleRequestsLength();
  MaybeStackBuffer<Local<Value>, 16> promises(module_requests_length);

  // call the dependency resolve callbacks
  for (int i = 0; i < module_requests_length; i++) {
    Local<String> specifier = module->GetModuleRequest(i);
    Utf8Value specifier_utf8(env->isolate(), specifier);
    std::string specifier_std(*specifier_utf8, specifier_utf8.length());

    Local<Value> argv[] = {
      specifier
    };

    MaybeLocal<Value> maybe_resolve_return_value =
        resolver_arg->Call(mod_context, that, 1, argv);
    if (maybe_resolve_return_value.IsEmpty()) {
      return;
    }
    Local<Value> resolve_return_value =
        maybe_resolve_return_value.ToLocalChecked();
    if (!resolve_return_value->IsPromise()) {
      env->ThrowError("linking error, expected resolver to return a promise");
    }
    Local<Promise> resolve_promise = resolve_return_value.As<Promise>();
    obj->resolve_cache_[specifier_std].Reset(env->isolate(), resolve_promise);

    promises[i] = resolve_promise;
  }

  args.GetReturnValue().Set(
      Array::New(isolate, promises.out(), promises.length()));
}

void ModuleWrap::Instantiate(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = args.GetIsolate();
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());
  Local<Context> context = obj->context_.Get(isolate);
  Local<Module> module = obj->module_.Get(isolate);
  TryCatchScope try_catch(env);
  USE(module->InstantiateModule(context, ResolveCallback));

  // clear resolve cache on instantiate
  obj->resolve_cache_.clear();

  if (try_catch.HasCaught() && !try_catch.HasTerminated()) {
    CHECK(!try_catch.Message().IsEmpty());
    CHECK(!try_catch.Exception().IsEmpty());
    AppendExceptionLine(env, try_catch.Exception(), try_catch.Message(),
                        ErrorHandlingMode::MODULE_ERROR);
    try_catch.ReThrow();
    return;
  }
}

void ModuleWrap::Evaluate(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = env->isolate();
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());
  Local<Context> context = obj->context_.Get(isolate);
  Local<Module> module = obj->module_.Get(isolate);

  // module.evaluate(timeout, breakOnSigint)
  CHECK_EQ(args.Length(), 2);

  CHECK(args[0]->IsNumber());
  int64_t timeout = args[0]->IntegerValue(env->context()).FromJust();

  CHECK(args[1]->IsBoolean());
  bool break_on_sigint = args[1]->IsTrue();

  ShouldNotAbortOnUncaughtScope no_abort_scope(env);
  TryCatchScope try_catch(env);

  bool timed_out = false;
  bool received_signal = false;
  MaybeLocal<Value> result;
  if (break_on_sigint && timeout != -1) {
    Watchdog wd(isolate, timeout, &timed_out);
    SigintWatchdog swd(isolate, &received_signal);
    result = module->Evaluate(context);
  } else if (break_on_sigint) {
    SigintWatchdog swd(isolate, &received_signal);
    result = module->Evaluate(context);
  } else if (timeout != -1) {
    Watchdog wd(isolate, timeout, &timed_out);
    result = module->Evaluate(context);
  } else {
    result = module->Evaluate(context);
  }

  if (result.IsEmpty()) {
    CHECK(try_catch.HasCaught());
  }

  // Convert the termination exception into a regular exception.
  if (timed_out || received_signal) {
    if (!env->is_main_thread() && env->is_stopping())
      return;
    env->isolate()->CancelTerminateExecution();
    // It is possible that execution was terminated by another timeout in
    // which this timeout is nested, so check whether one of the watchdogs
    // from this invocation is responsible for termination.
    if (timed_out) {
      THROW_ERR_SCRIPT_EXECUTION_TIMEOUT(env, timeout);
    } else if (received_signal) {
      THROW_ERR_SCRIPT_EXECUTION_INTERRUPTED(env);
    }
  }

  if (try_catch.HasCaught()) {
    if (!try_catch.HasTerminated())
      try_catch.ReThrow();
    return;
  }

  args.GetReturnValue().Set(result.ToLocalChecked());
}

void ModuleWrap::GetNamespace(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = args.GetIsolate();
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());

  Local<Module> module = obj->module_.Get(isolate);

  switch (module->GetStatus()) {
    default:
      return env->ThrowError(
          "cannot get namespace, Module has not been instantiated");
    case v8::Module::Status::kInstantiated:
    case v8::Module::Status::kEvaluating:
    case v8::Module::Status::kEvaluated:
      break;
  }

  Local<Value> result = module->GetModuleNamespace();
  args.GetReturnValue().Set(result);
}

void ModuleWrap::GetStatus(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());

  Local<Module> module = obj->module_.Get(isolate);

  args.GetReturnValue().Set(module->GetStatus());
}

void ModuleWrap::GetStaticDependencySpecifiers(
    const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());

  Local<Module> module = obj->module_.Get(env->isolate());

  int count = module->GetModuleRequestsLength();

  MaybeStackBuffer<Local<Value>, 16> specifiers(count);

  for (int i = 0; i < count; i++)
    specifiers[i] = module->GetModuleRequest(i);

  args.GetReturnValue().Set(
      Array::New(env->isolate(), specifiers.out(), count));
}

void ModuleWrap::GetError(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, args.This());

  Local<Module> module = obj->module_.Get(isolate);
  args.GetReturnValue().Set(module->GetException());
}

MaybeLocal<Module> ModuleWrap::ResolveCallback(Local<Context> context,
                                               Local<String> specifier,
                                               Local<Module> referrer) {
  Environment* env = Environment::GetCurrent(context);
  CHECK_NOT_NULL(env);  // TODO(addaleax): Handle nullptr here.
  Isolate* isolate = env->isolate();

  ModuleWrap* dependent = GetFromModule(env, referrer);
  if (dependent == nullptr) {
    env->ThrowError("linking error, null dep");
    return MaybeLocal<Module>();
  }

  Utf8Value specifier_utf8(isolate, specifier);
  std::string specifier_std(*specifier_utf8, specifier_utf8.length());

  if (dependent->resolve_cache_.count(specifier_std) != 1) {
    env->ThrowError("linking error, not in local cache");
    return MaybeLocal<Module>();
  }

  Local<Promise> resolve_promise =
      dependent->resolve_cache_[specifier_std].Get(isolate);

  if (resolve_promise->State() != Promise::kFulfilled) {
    env->ThrowError("linking error, dependency promises must be resolved on "
                    "instantiate");
    return MaybeLocal<Module>();
  }

  Local<Object> module_object = resolve_promise->Result().As<Object>();
  if (module_object.IsEmpty() || !module_object->IsObject()) {
    env->ThrowError("linking error, expected a valid module object from "
                    "resolver");
    return MaybeLocal<Module>();
  }

  ModuleWrap* module;
  ASSIGN_OR_RETURN_UNWRAP(&module, module_object, MaybeLocal<Module>());
  return module->module_.Get(isolate);
}

namespace {

// Tests whether a path starts with /, ./ or ../
// In WhatWG terminology, the alternative case is called a "bare" specifier
// (e.g. in `import "jquery"`).
inline bool ShouldBeTreatedAsRelativeOrAbsolutePath(
    const std::string& specifier) {
  size_t len = specifier.length();
  if (len == 0)
    return false;
  if (specifier[0] == '/') {
    return true;
  } else if (specifier[0] == '.') {
    if (len == 1 || specifier[1] == '/')
      return true;
    if (specifier[1] == '.') {
      if (len == 2 || specifier[2] == '/')
        return true;
    }
  }
  return false;
}

std::string ReadFile(uv_file file) {
  std::string contents;
  uv_fs_t req;
  char buffer_memory[4096];
  uv_buf_t buf = uv_buf_init(buffer_memory, sizeof(buffer_memory));

  do {
    const int r = uv_fs_read(nullptr,
                             &req,
                             file,
                             &buf,
                             1,
                             contents.length(),  // offset
                             nullptr);
    uv_fs_req_cleanup(&req);

    if (r <= 0)
      break;
    contents.append(buf.base, r);
  } while (true);
  return contents;
}

enum DescriptorType {
  FILE,
  DIRECTORY,
  NONE
};

// When DescriptorType cache is added, this can also return
// Nothing for the "null" cache entries.
inline Maybe<uv_file> OpenDescriptor(const std::string& path) {
  uv_fs_t fs_req;
#ifdef _WIN32
  std::string pth = "\\\\.\\" + path;
  uv_file fd = uv_fs_open(nullptr, &fs_req, pth.c_str(), O_RDONLY, 0, nullptr);
#else
  uv_file fd = uv_fs_open(nullptr, &fs_req, path.c_str(), O_RDONLY, 0, nullptr);
#endif
  uv_fs_req_cleanup(&fs_req);
  if (fd < 0) return Nothing<uv_file>();
  return Just(fd);
}

inline void CloseDescriptor(uv_file fd) {
  uv_fs_t fs_req;
  CHECK_EQ(0, uv_fs_close(nullptr, &fs_req, fd, nullptr));
  uv_fs_req_cleanup(&fs_req);
}

inline DescriptorType CheckDescriptorAtFile(uv_file fd) {
  uv_fs_t fs_req;
  int rc = uv_fs_fstat(nullptr, &fs_req, fd, nullptr);
  if (rc == 0) {
    uint64_t is_directory = fs_req.statbuf.st_mode & S_IFDIR;
    uv_fs_req_cleanup(&fs_req);
    return is_directory ? DIRECTORY : FILE;
  }
  uv_fs_req_cleanup(&fs_req);
  return NONE;
}

// TODO(@guybedford): Add a DescriptorType cache layer here.
// Should be directory based -> if path/to/dir doesn't exist
// then the cache should early-fail any path/to/dir/file check.
DescriptorType CheckDescriptorAtPath(const std::string& path) {
  Maybe<uv_file> fd = OpenDescriptor(path);
  if (fd.IsNothing()) return NONE;
  DescriptorType type = CheckDescriptorAtFile(fd.FromJust());
  CloseDescriptor(fd.FromJust());
  return type;
}

Maybe<std::string> ReadIfFile(const std::string& path) {
  Maybe<uv_file> fd = OpenDescriptor(path);
  if (fd.IsNothing()) return Nothing<std::string>();
  DescriptorType type = CheckDescriptorAtFile(fd.FromJust());
  if (type != FILE) return Nothing<std::string>();
  std::string source = ReadFile(fd.FromJust());
  CloseDescriptor(fd.FromJust());
  return Just(source);
}

using Exists = PackageConfig::Exists;
using IsValid = PackageConfig::IsValid;
using HasMain = PackageConfig::HasMain;
using HasName = PackageConfig::HasName;
using PackageType = PackageConfig::PackageType;

Maybe<const PackageConfig*> GetPackageConfig(Environment* env,
                                             const std::string& path,
                                             const URL& base) {
  auto existing = env->package_json_cache.find(path);
  if (existing != env->package_json_cache.end()) {
    const PackageConfig* pcfg = &existing->second;
    if (pcfg->is_valid == IsValid::No) {
      std::string msg = "Invalid JSON in " + path +
        " imported from " + base.ToFilePath();
      node::THROW_ERR_INVALID_PACKAGE_CONFIG(env, msg.c_str());
      return Nothing<const PackageConfig*>();
    }
    return Just(pcfg);
  }

  Maybe<std::string> source = ReadIfFile(path);

  if (source.IsNothing()) {
    auto entry = env->package_json_cache.emplace(path,
        PackageConfig { Exists::No, IsValid::Yes, HasMain::No, "",
                        HasName::No, "",
                        PackageType::None, Global<Value>() });
    return Just(&entry.first->second);
  }

  std::string pkg_src = source.FromJust();

  Isolate* isolate = env->isolate();
  HandleScope handle_scope(isolate);

  Local<Object> pkg_json;
  {
    Local<Value> src;
    Local<Value> pkg_json_v;
    Local<Context> context = env->context();

    if (!ToV8Value(context, pkg_src).ToLocal(&src) ||
        !v8::JSON::Parse(context, src.As<String>()).ToLocal(&pkg_json_v) ||
        !pkg_json_v->ToObject(context).ToLocal(&pkg_json)) {
      env->package_json_cache.emplace(path,
          PackageConfig { Exists::Yes, IsValid::No, HasMain::No, "",
                          HasName::No, "",
                          PackageType::None, Global<Value>() });
      std::string msg = "Invalid JSON in " + path +
          " imported from " + base.ToFilePath();
      node::THROW_ERR_INVALID_PACKAGE_CONFIG(env, msg.c_str());
      return Nothing<const PackageConfig*>();
    }
  }

  Local<Value> pkg_main;
  HasMain has_main = HasMain::No;
  std::string main_std;
  if (pkg_json->Get(env->context(), env->main_string()).ToLocal(&pkg_main)) {
    if (pkg_main->IsString()) {
      has_main = HasMain::Yes;
    }
    Utf8Value main_utf8(isolate, pkg_main);
    main_std.assign(std::string(*main_utf8, main_utf8.length()));
  }

  Local<Value> pkg_name;
  HasName has_name = HasName::No;
  std::string name_std;
  if (pkg_json->Get(env->context(), env->name_string()).ToLocal(&pkg_name)) {
    if (pkg_name->IsString()) {
      has_name = HasName::Yes;

      Utf8Value name_utf8(isolate, pkg_name);
      name_std.assign(std::string(*name_utf8, name_utf8.length()));
    }
  }

  PackageType pkg_type = PackageType::None;
  Local<Value> type_v;
  if (pkg_json->Get(env->context(), env->type_string()).ToLocal(&type_v)) {
    if (type_v->StrictEquals(env->module_string())) {
      pkg_type = PackageType::Module;
    } else if (type_v->StrictEquals(env->commonjs_string())) {
      pkg_type = PackageType::CommonJS;
    }
    // ignore unknown types for forwards compatibility
  }

  Local<Value> exports_v;
  if (pkg_json->Get(env->context(),
      env->exports_string()).ToLocal(&exports_v) &&
      !exports_v->IsNullOrUndefined()) {
    Global<Value> exports;
    exports.Reset(env->isolate(), exports_v);

    auto entry = env->package_json_cache.emplace(path,
        PackageConfig { Exists::Yes, IsValid::Yes, has_main, main_std,
                        has_name, name_std,
                        pkg_type, std::move(exports) });
    return Just(&entry.first->second);
  }

  auto entry = env->package_json_cache.emplace(path,
      PackageConfig { Exists::Yes, IsValid::Yes, has_main, main_std,
                      has_name, name_std,
                      pkg_type, Global<Value>() });
  return Just(&entry.first->second);
}

Maybe<const PackageConfig*> GetPackageScopeConfig(Environment* env,
                                                  const URL& resolved,
                                                  const URL& base) {
  URL pjson_url("./package.json", &resolved);
  while (true) {
    std::string pjson_url_path = pjson_url.path();
    if (pjson_url_path.length() > 25 &&
        pjson_url_path.substr(pjson_url_path.length() - 25, 25) ==
        "node_modules/package.json") {
      break;
    }
    Maybe<const PackageConfig*> pkg_cfg =
        GetPackageConfig(env, pjson_url.ToFilePath(), base);
    if (pkg_cfg.IsNothing()) return pkg_cfg;
    if (pkg_cfg.FromJust()->exists == Exists::Yes) return pkg_cfg;

    URL last_pjson_url = pjson_url;
    pjson_url = URL("../package.json", pjson_url);

    // Terminates at root where ../package.json equals ../../package.json
    // (can't just check "/package.json" for Windows support).
    if (pjson_url.path() == last_pjson_url.path()) break;
  }
  auto entry = env->package_json_cache.emplace(pjson_url.ToFilePath(),
  PackageConfig { Exists::No, IsValid::Yes, HasMain::No, "",
                  HasName::No, "",
                  PackageType::None, Global<Value>() });
  const PackageConfig* pcfg = &entry.first->second;
  return Just(pcfg);
}

/*
 * Legacy CommonJS main resolution:
 * 1. let M = pkg_url + (json main field)
 * 2. TRY(M, M.js, M.json, M.node)
 * 3. TRY(M/index.js, M/index.json, M/index.node)
 * 4. TRY(pkg_url/index.js, pkg_url/index.json, pkg_url/index.node)
 * 5. NOT_FOUND
 */
inline bool FileExists(const URL& url) {
  return CheckDescriptorAtPath(url.ToFilePath()) == FILE;
}
Maybe<URL> LegacyMainResolve(const URL& pjson_url,
                             const PackageConfig& pcfg) {
  URL guess;
  if (pcfg.has_main == HasMain::Yes) {
    // Note: fs check redundances will be handled by Descriptor cache here.
    if (FileExists(guess = URL("./" + pcfg.main, pjson_url))) {
      return Just(guess);
    }
    if (FileExists(guess = URL("./" + pcfg.main + ".js", pjson_url))) {
      return Just(guess);
    }
    if (FileExists(guess = URL("./" + pcfg.main + ".json", pjson_url))) {
      return Just(guess);
    }
    if (FileExists(guess = URL("./" + pcfg.main + ".node", pjson_url))) {
      return Just(guess);
    }
    if (FileExists(guess = URL("./" + pcfg.main + "/index.js", pjson_url))) {
      return Just(guess);
    }
    // Such stat.
    if (FileExists(guess = URL("./" + pcfg.main + "/index.json", pjson_url))) {
      return Just(guess);
    }
    if (FileExists(guess = URL("./" + pcfg.main + "/index.node", pjson_url))) {
      return Just(guess);
    }
    // Fallthrough.
  }
  if (FileExists(guess = URL("./index.js", pjson_url))) {
    return Just(guess);
  }
  // So fs.
  if (FileExists(guess = URL("./index.json", pjson_url))) {
    return Just(guess);
  }
  if (FileExists(guess = URL("./index.node", pjson_url))) {
    return Just(guess);
  }
  // Not found.
  return Nothing<URL>();
}

enum ResolveExtensionsOptions {
  TRY_EXACT_NAME,
  ONLY_VIA_EXTENSIONS
};

template <ResolveExtensionsOptions options>
Maybe<URL> ResolveExtensions(const URL& search) {
  if (options == TRY_EXACT_NAME) {
    if (FileExists(search)) {
      return Just(search);
    }
  }

  for (const char* extension : EXTENSIONS) {
    URL guess(search.path() + extension, &search);
    if (FileExists(guess)) {
      return Just(guess);
    }
  }

  return Nothing<URL>();
}

inline Maybe<URL> ResolveIndex(const URL& search) {
  return ResolveExtensions<ONLY_VIA_EXTENSIONS>(URL("index", search));
}

Maybe<URL> FinalizeResolution(Environment* env,
                              const URL& resolved,
                              const URL& base) {
  if (env->options()->experimental_specifier_resolution == "node") {
    Maybe<URL> file = ResolveExtensions<TRY_EXACT_NAME>(resolved);
    if (!file.IsNothing()) {
      return file;
    }
    if (resolved.path().back() != '/') {
      file = ResolveIndex(URL(resolved.path() + "/", &base));
    } else {
      file = ResolveIndex(resolved);
    }
    if (!file.IsNothing()) {
      return file;
    }
    std::string msg = "Cannot find module " + resolved.path() +
        " imported from " + base.ToFilePath();
    node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
    return Nothing<URL>();
  }

  if (resolved.path().back() == '/') {
    return Just(resolved);
  }

  const std::string& path = resolved.ToFilePath();
  if (CheckDescriptorAtPath(path) != FILE) {
    std::string msg = "Cannot find module " +
        (path.length() != 0 ? path : resolved.path()) +
        " imported from " + base.ToFilePath();
    node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
    return Nothing<URL>();
  }

  return Just(resolved);
}

void ThrowExportsNotFound(Environment* env,
                          const std::string& subpath,
                          const URL& pjson_url,
                          const URL& base) {
  const std::string msg = "Package subpath '" + subpath + "' is not defined" +
      " by \"exports\" in " + pjson_url.ToFilePath() + " imported from " +
      base.ToFilePath();
  node::THROW_ERR_PACKAGE_PATH_NOT_EXPORTED(env, msg.c_str());
}

void ThrowSubpathInvalid(Environment* env,
                         const std::string& subpath,
                         const URL& pjson_url,
                         const URL& base) {
  const std::string msg = "Package subpath '" + subpath + "' is not a valid " +
      "module request for the \"exports\" resolution of " +
      pjson_url.ToFilePath() + " imported from " + base.ToFilePath();
  node::THROW_ERR_INVALID_MODULE_SPECIFIER(env, msg.c_str());
}

void ThrowExportsInvalid(Environment* env,
                         const std::string& subpath,
                         const std::string& target,
                         const URL& pjson_url,
                         const URL& base) {
  if (subpath.length()) {
    const std::string msg = "Invalid \"exports\" target \"" + target +
        "\" defined for '" + subpath + "' in the package config " +
        pjson_url.ToFilePath() + " imported from " + base.ToFilePath();
    node::THROW_ERR_INVALID_PACKAGE_TARGET(env, msg.c_str());
  } else {
    const std::string msg = "Invalid \"exports\" main target " + target +
        " defined in the package config " + pjson_url.ToFilePath() +
        " imported from " + base.ToFilePath();
    node::THROW_ERR_INVALID_PACKAGE_TARGET(env, msg.c_str());
  }
}

void ThrowExportsInvalid(Environment* env,
                         const std::string& subpath,
                         Local<Value> target,
                         const URL& pjson_url,
                         const URL& base) {
  Local<String> target_string;
  if (target->IsObject()) {
    if (!v8::JSON::Stringify(env->context(), target.As<Object>(),
            v8::String::Empty(env->isolate())).ToLocal(&target_string))
      return;
  } else {
    if (!target->ToString(env->context()).ToLocal(&target_string))
      return;
  }
  Utf8Value target_utf8(env->isolate(), target_string);
  std::string target_str(*target_utf8, target_utf8.length());
  if (target->IsArray()) {
    target_str = '[' + target_str + ']';
  }
  ThrowExportsInvalid(env, subpath, target_str, pjson_url, base);
}

Maybe<URL> ResolveExportsTargetString(Environment* env,
                                      const std::string& target,
                                      const std::string& subpath,
                                      const std::string& match,
                                      const URL& pjson_url,
                                      const URL& base) {
  if (target.substr(0, 2) != "./") {
    ThrowExportsInvalid(env, match, target, pjson_url, base);
    return Nothing<URL>();
  }
  if (subpath.length() > 0 && target.back() != '/') {
    ThrowExportsInvalid(env, match, target, pjson_url, base);
    return Nothing<URL>();
  }
  URL resolved(target, pjson_url);
  std::string resolved_path = resolved.path();
  std::string pkg_path = URL(".", pjson_url).path();
  if (resolved_path.find(pkg_path) != 0 ||
      resolved_path.find("/node_modules/", pkg_path.length() - 1) !=
      std::string::npos) {
    ThrowExportsInvalid(env, match, target, pjson_url, base);
    return Nothing<URL>();
  }
  if (subpath.length() == 0) return Just(resolved);
  URL subpath_resolved(subpath, resolved);
  std::string subpath_resolved_path = subpath_resolved.path();
  if (subpath_resolved_path.find(resolved_path) != 0 ||
      subpath_resolved_path.find("/node_modules/", pkg_path.length() - 1)
      != std::string::npos) {
    ThrowSubpathInvalid(env, match + subpath, pjson_url, base);
    return Nothing<URL>();
  }
  return Just(subpath_resolved);
}

bool IsArrayIndex(Environment* env, Local<Value> p) {
  Local<Context> context = env->context();
  Local<String> p_str = p->ToString(context).ToLocalChecked();
  double n_dbl = static_cast<double>(p_str->NumberValue(context).FromJust());
  Local<Number> n = Number::New(env->isolate(), n_dbl);
  Local<String> cmp_str = n->ToString(context).ToLocalChecked();
  if (!p_str->Equals(context, cmp_str).FromJust()) {
    return false;
  }
  if (n_dbl == 0 && std::signbit(n_dbl) == false) {
    return true;
  }
  Local<Integer> cmp_integer;
  if (!n->ToInteger(context).ToLocal(&cmp_integer)) {
    return false;
  }
  return n_dbl > 0 && n_dbl < (1LL << 32) - 1;
}

Maybe<URL> ResolveExportsTarget(Environment* env,
                                const URL& pjson_url,
                                Local<Value> target,
                                const std::string& subpath,
                                const std::string& pkg_subpath,
                                const URL& base) {
  Isolate* isolate = env->isolate();
  Local<Context> context = env->context();
  if (target->IsString()) {
    Utf8Value target_utf8(isolate, target.As<String>());
    std::string target_str(*target_utf8, target_utf8.length());
    Maybe<URL> resolved = ResolveExportsTargetString(env, target_str, subpath,
        pkg_subpath, pjson_url, base);
    if (resolved.IsNothing()) {
      return Nothing<URL>();
    }
    return FinalizeResolution(env, resolved.FromJust(), base);
  } else if (target->IsArray()) {
      Local<Array> target_arr = target.As<Array>();
      const uint32_t length = target_arr->Length();
      if (length == 0) {
        ThrowExportsInvalid(env, pkg_subpath, target, pjson_url, base);
        return Nothing<URL>();
      }
      for (uint32_t i = 0; i < length; i++) {
        auto target_item = target_arr->Get(context, i).ToLocalChecked();
        {
          TryCatchScope try_catch(env);
          Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url,
              target_item, subpath, pkg_subpath, base);
          if (resolved.IsNothing()) {
            CHECK(try_catch.HasCaught());
            if (try_catch.Exception().IsEmpty()) return Nothing<URL>();
            Local<Object> e;
            if (!try_catch.Exception()->ToObject(context).ToLocal(&e))
              return Nothing<URL>();
            Local<Value> code;
            if (!e->Get(context, env->code_string()).ToLocal(&code))
              return Nothing<URL>();
            Local<String> code_string;
            if (!code->ToString(context).ToLocal(&code_string))
              return Nothing<URL>();
            Utf8Value code_utf8(env->isolate(), code_string);
            if (strcmp(*code_utf8, "ERR_PACKAGE_PATH_NOT_EXPORTED") == 0 ||
                strcmp(*code_utf8, "ERR_INVALID_PACKAGE_TARGET") == 0) {
              continue;
            }
            try_catch.ReThrow();
            return Nothing<URL>();
          }
          CHECK(!try_catch.HasCaught());
          return FinalizeResolution(env, resolved.FromJust(), base);
        }
      }
      auto invalid = target_arr->Get(context, length - 1).ToLocalChecked();
      Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url, invalid,
          subpath, pkg_subpath, base);
      CHECK(resolved.IsNothing());
      return Nothing<URL>();
  } else if (target->IsObject()) {
    Local<Object> target_obj = target.As<Object>();
    Local<Array> target_obj_keys =
        target_obj->GetOwnPropertyNames(context).ToLocalChecked();
    Local<Value> conditionalTarget;
    for (uint32_t i = 0; i < target_obj_keys->Length(); ++i) {
      Local<Value> key =
          target_obj_keys->Get(context, i).ToLocalChecked();
      if (IsArrayIndex(env, key)) {
        const std::string msg = "Invalid package config " +
            pjson_url.ToFilePath() + " imported from " + base.ToFilePath() +
            ". \"exports\" cannot contain numeric property keys.";
        node::THROW_ERR_INVALID_PACKAGE_CONFIG(env, msg.c_str());
        return Nothing<URL>();
      }
    }
    for (uint32_t i = 0; i < target_obj_keys->Length(); ++i) {
      Local<Value> key = target_obj_keys->Get(context, i).ToLocalChecked();
      Utf8Value key_utf8(env->isolate(),
                         key->ToString(context).ToLocalChecked());
      std::string key_str(*key_utf8, key_utf8.length());
      if (key_str == "node" || key_str == "import") {
        conditionalTarget = target_obj->Get(context, key).ToLocalChecked();
        {
          TryCatchScope try_catch(env);
          Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url,
              conditionalTarget, subpath, pkg_subpath, base);
          if (resolved.IsNothing()) {
            CHECK(try_catch.HasCaught());
            if (try_catch.Exception().IsEmpty()) return Nothing<URL>();
            Local<Object> e;
            if (!try_catch.Exception()->ToObject(context).ToLocal(&e))
              return Nothing<URL>();
            Local<Value> code;
            if (!e->Get(context, env->code_string()).ToLocal(&code))
              return Nothing<URL>();
            Local<String> code_string;
            if (!code->ToString(context).ToLocal(&code_string))
              return Nothing<URL>();
            Utf8Value code_utf8(env->isolate(), code_string);
            if (strcmp(*code_utf8, "ERR_PACKAGE_PATH_NOT_EXPORTED") == 0)
              continue;
            try_catch.ReThrow();
            return Nothing<URL>();
          }
          CHECK(!try_catch.HasCaught());
          ProcessEmitExperimentalWarning(env, "Conditional exports");
          return resolved;
        }
      } else if (key_str == "default") {
        conditionalTarget = target_obj->Get(context, key).ToLocalChecked();
        {
          TryCatchScope try_catch(env);
          Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url,
              conditionalTarget, subpath, pkg_subpath, base);
          if (resolved.IsNothing()) {
            CHECK(try_catch.HasCaught() && !try_catch.Exception().IsEmpty());
            auto e = try_catch.Exception()->ToObject(context).ToLocalChecked();
            auto code = e->Get(context, env->code_string()).ToLocalChecked();
            Utf8Value code_utf8(env->isolate(),
                                code->ToString(context).ToLocalChecked());
            std::string code_str(*code_utf8, code_utf8.length());
            if (code_str == "ERR_PACKAGE_PATH_NOT_EXPORTED") continue;
            try_catch.ReThrow();
            return Nothing<URL>();
          }
          CHECK(!try_catch.HasCaught());
          ProcessEmitExperimentalWarning(env, "Conditional exports");
          return resolved;
        }
      }
    }
    ThrowExportsNotFound(env, pkg_subpath, pjson_url, base);
    return Nothing<URL>();
  }
  ThrowExportsInvalid(env, pkg_subpath, target, pjson_url, base);
  return Nothing<URL>();
}

Maybe<bool> IsConditionalExportsMainSugar(Environment* env,
                                          Local<Value> exports,
                                          const URL& pjson_url,
                                          const URL& base) {
  if (exports->IsString() || exports->IsArray()) return Just(true);
  if (!exports->IsObject()) return Just(false);
  Local<Context> context = env->context();
  Local<Object> exports_obj = exports.As<Object>();
  Local<Array> keys =
      exports_obj->GetOwnPropertyNames(context).ToLocalChecked();
  bool isConditionalSugar = false;
  for (uint32_t i = 0; i < keys->Length(); ++i) {
    Local<Value> key = keys->Get(context, i).ToLocalChecked();
    Utf8Value key_utf8(env->isolate(), key->ToString(context).ToLocalChecked());
    bool curIsConditionalSugar = key_utf8.length() == 0 || key_utf8[0] != '.';
    if (i == 0) {
      isConditionalSugar = curIsConditionalSugar;
    } else if (isConditionalSugar != curIsConditionalSugar) {
      const std::string msg = "Invalid package config " + pjson_url.ToFilePath()
        + " imported from " + base.ToFilePath() + ". " +
        "\"exports\" cannot contain some keys starting with '.' and some not." +
        " The exports object must either be an object of package subpath keys" +
        " or an object of main entry condition name keys only.";
      node::THROW_ERR_INVALID_PACKAGE_CONFIG(env, msg.c_str());
      return Nothing<bool>();
    }
  }
  return Just(isConditionalSugar);
}

Maybe<URL> PackageMainResolve(Environment* env,
                              const URL& pjson_url,
                              const PackageConfig& pcfg,
                              const URL& base) {
  if (pcfg.exists == Exists::Yes) {
    Isolate* isolate = env->isolate();

    if (!pcfg.exports.IsEmpty()) {
      Local<Value> exports = pcfg.exports.Get(isolate);
      Maybe<bool> isConditionalExportsMainSugar =
          IsConditionalExportsMainSugar(env, exports, pjson_url, base);
      if (isConditionalExportsMainSugar.IsNothing())
        return Nothing<URL>();
      if (isConditionalExportsMainSugar.FromJust()) {
        return ResolveExportsTarget(env, pjson_url, exports, "", "", base);
      } else if (exports->IsObject()) {
        Local<Object> exports_obj = exports.As<Object>();
        if (exports_obj->HasOwnProperty(env->context(), env->dot_string())
            .FromJust()) {
          Local<Value> target =
              exports_obj->Get(env->context(), env->dot_string())
              .ToLocalChecked();
          return ResolveExportsTarget(env, pjson_url, target, "", "", base);
        }
      }
      std::string msg = "No \"exports\" main resolved in " +
          pjson_url.ToFilePath();
      node::THROW_ERR_PACKAGE_PATH_NOT_EXPORTED(env, msg.c_str());
    }
    if (pcfg.has_main == HasMain::Yes) {
      URL resolved(pcfg.main, pjson_url);
      const std::string& path = resolved.ToFilePath();
      if (CheckDescriptorAtPath(path) == FILE) {
        return Just(resolved);
      }
    }
    if (env->options()->experimental_specifier_resolution == "node") {
      if (pcfg.has_main == HasMain::Yes) {
        return FinalizeResolution(env, URL(pcfg.main, pjson_url), base);
      } else {
        return FinalizeResolution(env, URL("index", pjson_url), base);
      }
    }
    if (pcfg.type != PackageType::Module) {
      Maybe<URL> resolved = LegacyMainResolve(pjson_url, pcfg);
      if (!resolved.IsNothing()) {
        return resolved;
      }
    }
  }
  std::string msg = "Cannot find main entry point for " +
      URL(".", pjson_url).ToFilePath() + " imported from " +
      base.ToFilePath();
  node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
  return Nothing<URL>();
}

Maybe<URL> PackageExportsResolve(Environment* env,
                                 const URL& pjson_url,
                                 const std::string& pkg_subpath,
                                 const PackageConfig& pcfg,
                                 const URL& base) {
  Isolate* isolate = env->isolate();
  Local<Context> context = env->context();
  Local<Value> exports = pcfg.exports.Get(isolate);
  Maybe<bool> isConditionalExportsMainSugar =
      IsConditionalExportsMainSugar(env, exports, pjson_url, base);
  if (isConditionalExportsMainSugar.IsNothing())
    return Nothing<URL>();
  if (!exports->IsObject() || isConditionalExportsMainSugar.FromJust()) {
    ThrowExportsNotFound(env, pkg_subpath, pjson_url, base);
    return Nothing<URL>();
  }
  Local<Object> exports_obj = exports.As<Object>();
  Local<String> subpath = String::NewFromUtf8(isolate,
      pkg_subpath.c_str(), v8::NewStringType::kNormal).ToLocalChecked();

  if (exports_obj->HasOwnProperty(context, subpath).FromJust()) {
    Local<Value> target = exports_obj->Get(context, subpath).ToLocalChecked();
    Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url, target, "",
                                               pkg_subpath, base);
    if (resolved.IsNothing()) {
      return Nothing<URL>();
    }
    return FinalizeResolution(env, resolved.FromJust(), base);
  }

  Local<String> best_match;
  std::string best_match_str = "";
  Local<Array> keys =
      exports_obj->GetOwnPropertyNames(context).ToLocalChecked();
  for (uint32_t i = 0; i < keys->Length(); ++i) {
    Local<Value> key = keys->Get(context, i).ToLocalChecked();
    Utf8Value key_utf8(isolate, key->ToString(context).ToLocalChecked());
    std::string key_str(*key_utf8, key_utf8.length());
    if (key_str.back() != '/') continue;
    if (pkg_subpath.substr(0, key_str.length()) == key_str &&
        key_str.length() > best_match_str.length()) {
      best_match = key->ToString(context).ToLocalChecked();
      best_match_str = key_str;
    }
  }

  if (best_match_str.length() > 0) {
    auto target = exports_obj->Get(context, best_match).ToLocalChecked();
    std::string subpath = pkg_subpath.substr(best_match_str.length());

    Maybe<URL> resolved = ResolveExportsTarget(env, pjson_url, target, subpath,
                                               pkg_subpath, base);
    if (resolved.IsNothing()) {
      return Nothing<URL>();
    }
    return FinalizeResolution(env, resolved.FromJust(), base);
  }

  ThrowExportsNotFound(env, pkg_subpath, pjson_url, base);
  return Nothing<URL>();
}

Maybe<URL> PackageResolve(Environment* env,
                          const std::string& specifier,
                          const URL& base) {
  size_t sep_index = specifier.find('/');
  bool valid_package_name = true;
  bool scope = false;
  if (specifier[0] == '@') {
    scope = true;
    if (sep_index == std::string::npos || specifier.length() == 0) {
      valid_package_name = false;
    } else {
      sep_index = specifier.find('/', sep_index + 1);
    }
  } else if (specifier[0] == '.') {
    valid_package_name = false;
  }
  std::string pkg_name = specifier.substr(0,
      sep_index == std::string::npos ? std::string::npos : sep_index);
  // Package name cannot have leading . and cannot have percent-encoding or
  // separators.
  for (size_t i = 0; i < pkg_name.length(); i++) {
    char c = pkg_name[i];
    if (c == '%' || c == '\\') {
      valid_package_name = false;
      break;
    }
  }
  if (!valid_package_name) {
    std::string msg = "Invalid package name '" + specifier +
      "' imported from " + base.ToFilePath();
    node::THROW_ERR_INVALID_MODULE_SPECIFIER(env, msg.c_str());
    return Nothing<URL>();
  }
  std::string pkg_subpath;
  if (sep_index == std::string::npos) {
    pkg_subpath = "";
  } else {
    pkg_subpath = "." + specifier.substr(sep_index);
  }

  // ResolveSelf
  const PackageConfig* pcfg;
  if (GetPackageScopeConfig(env, base, base).To(&pcfg) &&
      pcfg->exists == Exists::Yes) {
    // TODO(jkrems): Find a way to forward the pair/iterator already generated
    // while executing GetPackageScopeConfig
    URL pjson_url("");
    bool found_pjson = false;
    for (const auto& it : env->package_json_cache) {
      if (&it.second == pcfg) {
        pjson_url = URL::FromFilePath(it.first);
        found_pjson = true;
      }
    }
    if (found_pjson && pcfg->name == pkg_name && !pcfg->exports.IsEmpty()) {
      ProcessEmitExperimentalWarning(env, "Package name self resolution");
      if (pkg_subpath == "./") {
        return Just(URL("./", pjson_url));
      } else if (!pkg_subpath.length()) {
        return PackageMainResolve(env, pjson_url, *pcfg, base);
      } else {
        return PackageExportsResolve(env, pjson_url, pkg_subpath, *pcfg, base);
      }
    }
  }

  URL pjson_url("./node_modules/" + pkg_name + "/package.json", &base);
  std::string pjson_path = pjson_url.ToFilePath();
  std::string last_path;
  do {
    DescriptorType check =
        CheckDescriptorAtPath(pjson_path.substr(0, pjson_path.length() - 13));
    if (check != DIRECTORY) {
      last_path = pjson_path;
      pjson_url = URL((scope ?
          "../../../../node_modules/" : "../../../node_modules/") +
          pkg_name + "/package.json", &pjson_url);
      pjson_path = pjson_url.ToFilePath();
      continue;
    }

    // Package match.
    Maybe<const PackageConfig*> pcfg = GetPackageConfig(env, pjson_path, base);
    // Invalid package configuration error.
    if (pcfg.IsNothing()) return Nothing<URL>();
    if (pkg_subpath == "./") {
      return Just(URL("./", pjson_url));
    } else if (!pkg_subpath.length()) {
      return PackageMainResolve(env, pjson_url, *pcfg.FromJust(), base);
    } else {
      if (!pcfg.FromJust()->exports.IsEmpty()) {
        return PackageExportsResolve(env, pjson_url, pkg_subpath,
                                     *pcfg.FromJust(), base);
      } else {
        return FinalizeResolution(env, URL(pkg_subpath, pjson_url), base);
      }
    }
    CHECK(false);
    // Cross-platform root check.
  } while (pjson_path.length() != last_path.length());

  std::string msg = "Cannot find package '" + pkg_name +
      "' imported from " + base.ToFilePath();
  node::THROW_ERR_MODULE_NOT_FOUND(env, msg.c_str());
  return Nothing<URL>();
}

}  // anonymous namespace

Maybe<URL> Resolve(Environment* env,
                   const std::string& specifier,
                   const URL& base) {
  // Order swapped from spec for minor perf gain.
  // Ok since relative URLs cannot parse as URLs.
  URL resolved;
  if (ShouldBeTreatedAsRelativeOrAbsolutePath(specifier)) {
    resolved = URL(specifier, base);
  } else {
    URL pure_url(specifier);
    if (!(pure_url.flags() & URL_FLAGS_FAILED)) {
      resolved = pure_url;
    } else {
      return PackageResolve(env, specifier, base);
    }
  }
  return FinalizeResolution(env, resolved, base);
}

void ModuleWrap::Resolve(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);

  // module.resolve(specifier, url)
  CHECK_EQ(args.Length(), 2);

  CHECK(args[0]->IsString());
  Utf8Value specifier_utf8(env->isolate(), args[0]);
  std::string specifier_std(*specifier_utf8, specifier_utf8.length());

  CHECK(args[1]->IsString());
  Utf8Value url_utf8(env->isolate(), args[1]);
  URL url(*url_utf8, url_utf8.length());

  if (url.flags() & URL_FLAGS_FAILED) {
    return node::THROW_ERR_INVALID_ARG_TYPE(
        env, "second argument is not a URL string");
  }

  Maybe<URL> result =
      node::loader::Resolve(env,
                            specifier_std,
                            url);
  if (result.IsNothing()) {
    return;
  }

  URL resolution = result.FromJust();
  CHECK(!(resolution.flags() & URL_FLAGS_FAILED));

  Local<Value> resolution_obj;
  if (resolution.ToObject(env).ToLocal(&resolution_obj))
    args.GetReturnValue().Set(resolution_obj);
}

void ModuleWrap::GetPackageType(const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);

  // module.getPackageType(url)
  CHECK_EQ(args.Length(), 1);

  CHECK(args[0]->IsString());
  Utf8Value url_utf8(env->isolate(), args[0]);
  URL url(*url_utf8, url_utf8.length());

  PackageType pkg_type = PackageType::None;
  Maybe<const PackageConfig*> pcfg =
      GetPackageScopeConfig(env, url, url);
  if (!pcfg.IsNothing()) {
    pkg_type = pcfg.FromJust()->type;
  }

  args.GetReturnValue().Set(Integer::New(env->isolate(), pkg_type));
}

static MaybeLocal<Promise> ImportModuleDynamically(
    Local<Context> context,
    Local<ScriptOrModule> referrer,
    Local<String> specifier) {
  Isolate* iso = context->GetIsolate();
  Environment* env = Environment::GetCurrent(context);
  CHECK_NOT_NULL(env);  // TODO(addaleax): Handle nullptr here.
  EscapableHandleScope handle_scope(iso);

  Local<Function> import_callback =
    env->host_import_module_dynamically_callback();

  Local<PrimitiveArray> options = referrer->GetHostDefinedOptions();
  if (options->Length() != HostDefinedOptions::kLength) {
    Local<Promise::Resolver> resolver =
        Promise::Resolver::New(context).ToLocalChecked();
    resolver
        ->Reject(context,
                 v8::Exception::TypeError(FIXED_ONE_BYTE_STRING(
                     context->GetIsolate(), "Invalid host defined options")))
        .ToChecked();
    return handle_scope.Escape(resolver->GetPromise());
  }

  Local<Value> object;

  int type = options->Get(iso, HostDefinedOptions::kType)
                 .As<Number>()
                 ->Int32Value(context)
                 .ToChecked();
  uint32_t id = options->Get(iso, HostDefinedOptions::kID)
                    .As<Number>()
                    ->Uint32Value(context)
                    .ToChecked();
  if (type == ScriptType::kScript) {
    contextify::ContextifyScript* wrap = env->id_to_script_map.find(id)->second;
    object = wrap->object();
  } else if (type == ScriptType::kModule) {
    ModuleWrap* wrap = ModuleWrap::GetFromID(env, id);
    object = wrap->object();
  } else if (type == ScriptType::kFunction) {
    auto it = env->id_to_function_map.find(id);
    CHECK_NE(it, env->id_to_function_map.end());
    object = it->second->object();
  } else {
    UNREACHABLE();
  }

  Local<Value> import_args[] = {
    object,
    Local<Value>(specifier),
  };

  Local<Value> result;
  if (import_callback->Call(
        context,
        Undefined(iso),
        arraysize(import_args),
        import_args).ToLocal(&result)) {
    CHECK(result->IsPromise());
    return handle_scope.Escape(result.As<Promise>());
  }

  return MaybeLocal<Promise>();
}

void ModuleWrap::SetImportModuleDynamicallyCallback(
    const FunctionCallbackInfo<Value>& args) {
  Isolate* iso = args.GetIsolate();
  Environment* env = Environment::GetCurrent(args);
  HandleScope handle_scope(iso);

  CHECK_EQ(args.Length(), 1);
  CHECK(args[0]->IsFunction());
  Local<Function> import_callback = args[0].As<Function>();
  env->set_host_import_module_dynamically_callback(import_callback);

  iso->SetHostImportModuleDynamicallyCallback(ImportModuleDynamically);
}

void ModuleWrap::HostInitializeImportMetaObjectCallback(
    Local<Context> context, Local<Module> module, Local<Object> meta) {
  Environment* env = Environment::GetCurrent(context);
  CHECK_NOT_NULL(env);  // TODO(addaleax): Handle nullptr here.
  ModuleWrap* module_wrap = GetFromModule(env, module);

  if (module_wrap == nullptr) {
    return;
  }

  Local<Object> wrap = module_wrap->object();
  Local<Function> callback =
      env->host_initialize_import_meta_object_callback();
  Local<Value> args[] = { wrap, meta };
  TryCatchScope try_catch(env);
  USE(callback->Call(
        context, Undefined(env->isolate()), arraysize(args), args));
  if (try_catch.HasCaught() && !try_catch.HasTerminated()) {
    try_catch.ReThrow();
  }
}

void ModuleWrap::SetInitializeImportMetaObjectCallback(
    const FunctionCallbackInfo<Value>& args) {
  Environment* env = Environment::GetCurrent(args);
  Isolate* isolate = env->isolate();

  CHECK_EQ(args.Length(), 1);
  CHECK(args[0]->IsFunction());
  Local<Function> import_meta_callback = args[0].As<Function>();
  env->set_host_initialize_import_meta_object_callback(import_meta_callback);

  isolate->SetHostInitializeImportMetaObjectCallback(
      HostInitializeImportMetaObjectCallback);
}

MaybeLocal<Value> ModuleWrap::SyntheticModuleEvaluationStepsCallback(
    Local<Context> context, Local<Module> module) {
  Environment* env = Environment::GetCurrent(context);
  Isolate* isolate = env->isolate();

  ModuleWrap* obj = GetFromModule(env, module);

  TryCatchScope try_catch(env);
  Local<Function> synthetic_evaluation_steps =
      obj->synthetic_evaluation_steps_.Get(isolate);
  MaybeLocal<Value> ret = synthetic_evaluation_steps->Call(context,
      obj->object(), 0, nullptr);
  if (ret.IsEmpty()) {
    CHECK(try_catch.HasCaught());
  }
  obj->synthetic_evaluation_steps_.Reset();
  if (try_catch.HasCaught() && !try_catch.HasTerminated()) {
    CHECK(!try_catch.Message().IsEmpty());
    CHECK(!try_catch.Exception().IsEmpty());
    try_catch.ReThrow();
    return MaybeLocal<Value>();
  }
  return ret;
}

void ModuleWrap::SetSyntheticExport(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Object> that = args.This();

  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, that);

  CHECK(obj->synthetic_);

  CHECK_EQ(args.Length(), 2);

  CHECK(args[0]->IsString());
  Local<String> export_name = args[0].As<String>();

  Local<Value> export_value = args[1];

  Local<Module> module = obj->module_.Get(isolate);
  USE(module->SetSyntheticModuleExport(isolate, export_name, export_value));
}

void ModuleWrap::CreateCachedData(const FunctionCallbackInfo<Value>& args) {
  Isolate* isolate = args.GetIsolate();
  Local<Object> that = args.This();

  ModuleWrap* obj;
  ASSIGN_OR_RETURN_UNWRAP(&obj, that);

  CHECK(!obj->synthetic_);

  Local<Module> module = obj->module_.Get(isolate);

  CHECK_LT(module->GetStatus(), v8::Module::Status::kEvaluating);

  Local<UnboundModuleScript> unbound_module_script =
      module->GetUnboundModuleScript();
  std::unique_ptr<ScriptCompiler::CachedData> cached_data(
      ScriptCompiler::CreateCodeCache(unbound_module_script));
  Environment* env = Environment::GetCurrent(args);
  if (!cached_data) {
    args.GetReturnValue().Set(Buffer::New(env, 0).ToLocalChecked());
  } else {
    MaybeLocal<Object> buf =
        Buffer::Copy(env,
                     reinterpret_cast<const char*>(cached_data->data),
                     cached_data->length);
    args.GetReturnValue().Set(buf.ToLocalChecked());
  }
}

void ModuleWrap::Initialize(Local<Object> target,
                            Local<Value> unused,
                            Local<Context> context,
                            void* priv) {
  Environment* env = Environment::GetCurrent(context);
  Isolate* isolate = env->isolate();

  Local<FunctionTemplate> tpl = env->NewFunctionTemplate(New);
  tpl->SetClassName(FIXED_ONE_BYTE_STRING(isolate, "ModuleWrap"));
  tpl->InstanceTemplate()->SetInternalFieldCount(1);

  env->SetProtoMethod(tpl, "link", Link);
  env->SetProtoMethod(tpl, "instantiate", Instantiate);
  env->SetProtoMethod(tpl, "evaluate", Evaluate);
  env->SetProtoMethod(tpl, "setExport", SetSyntheticExport);
  env->SetProtoMethodNoSideEffect(tpl, "createCachedData", CreateCachedData);
  env->SetProtoMethodNoSideEffect(tpl, "getNamespace", GetNamespace);
  env->SetProtoMethodNoSideEffect(tpl, "getStatus", GetStatus);
  env->SetProtoMethodNoSideEffect(tpl, "getError", GetError);
  env->SetProtoMethodNoSideEffect(tpl, "getStaticDependencySpecifiers",
                                  GetStaticDependencySpecifiers);

  target->Set(env->context(), FIXED_ONE_BYTE_STRING(isolate, "ModuleWrap"),
              tpl->GetFunction(context).ToLocalChecked()).Check();
  env->SetMethod(target, "resolve", Resolve);
  env->SetMethod(target, "getPackageType", GetPackageType);
  env->SetMethod(target,
                 "setImportModuleDynamicallyCallback",
                 SetImportModuleDynamicallyCallback);
  env->SetMethod(target,
                 "setInitializeImportMetaObjectCallback",
                 SetInitializeImportMetaObjectCallback);

#define V(name)                                                                \
    target->Set(context,                                                       \
      FIXED_ONE_BYTE_STRING(env->isolate(), #name),                            \
      Integer::New(env->isolate(), Module::Status::name))                      \
        .FromJust()
    V(kUninstantiated);
    V(kInstantiating);
    V(kInstantiated);
    V(kEvaluating);
    V(kEvaluated);
    V(kErrored);
#undef V
}

}  // namespace loader
}  // namespace node

NODE_MODULE_CONTEXT_AWARE_INTERNAL(module_wrap,
                                   node::loader::ModuleWrap::Initialize)