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Synthetic Modules Design Document

Contact emails

daniec@microsoft.com, sasebree@microsoft.com, travil@microsoft.com, pcupp@microsoft.com, gwhit@microsoft.com

Introduction

The following document proposes an implementation of Synthetic Module Records (originally proposed by here by Domenic) in Chromium. Also proposed is an implementation of CSS Modules and JSON Modules using Synthetic Module Records. It is expected that Synthetic Module Record will be used to support additional future module types as well, e.g. WebIDL Modules.

At a high level, the changes are:

  • Introduce a SyntheticModule type in V8 to implement Synthetic Module Records in the ES module graph.
  • Introduce the ValueWrapperSyntheticModuleScript type in Blink to support the creation of a Synthetic Module record that wraps a single value as the module's default export.
  • Enable ModuleScriptLoader to create CSS and JSON modules using ValueWrapperSyntheticModuleScript.

V8 changes

Currently the v8::internal::Module class is V8's representation of a Source Text Module Record, which represents information about a Script Module such as its RequestedModules, its status ("uninstantiated", "instantiating", etc), and its imports/exports. We will introduce two subclasses, JSModule and SyntheticModule. JSModule will contain the functionality specific to JavaScript Modules that currently resides in Module, and SyntheticModule will contain the new Synthetic Module code. Common functionality will remain in Module, which can now be considered as roughly equivalent to the spec's Abstract Module Record base type.

Since Structs in V8 don't allow VTables for performance reasons, Module will contain the following new field to distinguish whether it is a JSModule or SyntheticModule:

  DECL_INT_ACCESSORS(type)
  enum Type {
    kJS,
    kSynthetic
  };

All existing fields on Module that are specific to JS or other Cyclic Module types will be pushed down to JSModule. The following fields will be used for all module types and will therefore remain on Module:

  // The complete export table, mapping an export name to its cell.
  // TODO(neis): We may want to remove the regular exports from the table.
  DECL_ACCESSORS(exports, ObjectHashTable)

  // Hash for this object (a random non-zero Smi).
  DECL_INT_ACCESSORS(hash)

  // Status.
  DECL_INT_ACCESSORS(status)

  // The namespace object (or undefined).
  DECL_ACCESSORS(module_namespace, HeapObject)

SyntheticModule will contain the following new fields in addition to those inherited from Module:

  // A list of all names exported by the module
  DECL_ACCESSORS(export_names, FixedArray)

  // A callback defined by the embedder that is executed upon evaluation of the module.
  // QUESTION: evaluation_steps is just a function pointer to a Blink function.  Is
  // it optimal to store it as a Foreign or should we just stash the function pointer
  // directly as a uintptr_t?  Looking around the codebase I see some business about
  // V8_COMPRESS_POINTERS that I'd rather not have to handle in the module code, and
  // Foreign takes care of this for us.
  DECL_ACCESSORS(evaluation_steps, Foreign)

SyntheticModule methods

SyntheticModule will implement the following methods:

  static void SetExport(
      Isolate* isolate, Handle<SyntheticModule> module,
      Handle<String> export_name,
      Handle<Object> export_value);

  static V8_WARN_UNUSED_RESULT MaybeHandle<Cell> ResolveExport(
      Isolate* isolate, Handle<SyntheticModule> module,
      Handle<String> module_specifier, Handle<String> export_name,
      MessageLocation loc, bool must_resolve, ResolveSet* resolve_set);

  static V8_WARN_UNUSED_RESULT bool PrepareInstantiate(
      Isolate* isolate, Handle<SyntheticModule> module,
      v8::Local<v8::Context> context, v8::Module::ResolveCallback callback);
  static V8_WARN_UNUSED_RESULT bool FinishInstantiate(
      Isolate* isolate, Handle<SyntheticModule> module, unsigned* dfs_index);

  static V8_WARN_UNUSED_RESULT MaybeHandle<Object> Evaluate(
      Isolate* isolate, Handle<SyntheticModule> module,
      ZoneForwardList<Handle<Module>>* stack, unsigned* dfs_index);

  static void Reset(Isolate* isolate, Handle<SyntheticModule> module);

V8 Synthetic Module Record API

The v8::Module export class in v8\src\include\v8.h will have the following additional items:

  // Callback defined in the embedder
  typedef v8::MaybeLocal<v8::Value> (*SyntheticModuleEvaluationSteps)(
      Local<Context> context, Local<Module> module);

  static Local<Module> CreateSyntheticModule(
      Isolate* isolate, const std::vector<Local<v8::String>>& export_names,
      SyntheticModuleEvaluationSteps evaluation_steps);

  void SetSyntheticModuleExport(Local<String> export_name,
                                Local<v8::Value> export_value);

v8::Module::GetModuleRequestsLength() will be updated to return 0 for a SyntheticModule.

SyntheticModule Construction:

v8::Module::CreateSyntheticModule will be implemented with a call to the following new factory method:

Handle<SyntheticModule> Factory::NewSyntheticModule(
    i::Handle<i::FixedArray> export_names,
    v8::Module::SyntheticModuleEvaluationSteps evaluation_steps);

SyntheticModule Instantiation:

SyntheticModule::PrepareInstantiate() and SyntheticModule::FinishInstantiate() together implement the corresponding Synthetic Module Record concrete method Instantiate(). PrepareInstantiate will loop over the module's export_names, calling Module::CreateExport on each to create a binding of the name to undefined. FinishInstantiate simply changes the module's state to kInstantiated.

SyntheticModule Execution:

SyntheticModule::Evaluate() implements the corresponding Synthetic Module Record concrete method Evaluate(). The implementation will call the evaluation_steps callback, which is itself expected to make calls back into v8::Module::SetSyntheticModuleExport() to set the synthetic module's export bindings to non-undefined values.

SyntheticModule Export Resolution:

SyntheticModule::ResolveExport() implements the corresponding Synthetic Module Record concrete method ResolveExport(). The implementation will do the following:

  1. Look up the cell for the export name in the Module::exports ObjectHashTable and return it if it exists.
  2. If no such entry exists and must_resolve is true, call Factory::NewSyntaxError().

Resolution of namespace imports (import * from "./my-synthetic-module"), will work the same as for JSModules with the exception that FetchStarExports will now early-return if its module parameter is a SyntheticModule, as these have no requested modules and no star exports.

Open question: should each SyntheticModule have a corresponding Context object?

Should each instance of the new SyntheticModule class have a v8::internal::Context of its own? Each instance of the existing Module class (which will end up being called JSModule) has a corresponding Context (of MODULE_CONTEXT_TYPE), since each JavaScript module needs a script context where its module-local variables etc. are stored. Similarly, in the Synthetic Modules Specification, the Instantiate step calls NewModuleEnvironment to create a new Lexical Environment and a new module Environment Record for the Synthetic Module. This is necessary because the Synthetic MR needs a place to set the bindings for its [[ExportNames]]. In my understanding so far, V8’s Context roughly corresponds to a combination of the spec’s Lexical Environment and Environment Record. But, I am thinking that Synthetic Modules when implemented in V8 can operate without being paired with a Context. This is based on the following two observations:

  1. The consumers of Synthetic MR currently being discussed (JSON and CSS modules) do not run any JavaScript of their own.
  2. In V8’s implementation of FinishInstantiate/ResolveImport/ResolveExport, any JS modules importing values from a Synthetic module will lift each imported value’s Cell into the JS module’s own regular_imports FixedArray during PrepareInstantiate/FinishInstantiate, where they will then be directly accessible from the JS module’s Context when the JS module executes. At no time does the Synthetic Module need to store the Cells for its exports anywhere but in its exports ObjectHashTable (where they would be placed during calls to SetSyntheticModuleExport and then accessed during ResolveExport calls triggered by JS modules).

Implementing Synthetic modules without a Context would significantly simplify their design in various ways; for example they wouldn’t need to manage their own regular_exports array, which comes with additional baggage for mapping a single local name to multiple export names – something that is not needed by Synthetic modules. On the other hand, if future consumers of Synthetic Modules wanted to run JavaScript code aside from just calling SetSyntheticModuleExport, then a need for Synthetic Modules to have their own execution Context might arise. However I’m not currently aware of any concrete proposals that would require this. My tendency is towards simplicity, rather than introducing anticipatory codepaths that won’t yet have anything to exercise them. So unless there are objections we’ll move forward with the design that does not create a new Context for each SyntheticModule.

Blink Changes

We will introduce the new classes blink::JSModuleScript and blink::ValueWrapperSyntheticModuleScript, both deriving from the existing blink::ModuleScript.

Class Diagram

The JavaScript-specific bits of ModuleScript will be pushed down to the derived JSModuleScript, but ModuleScript will otherwise remain the same, containing data and functionality common to both module types. We anticipate that as other module types are introduced, further corresponding derivations of ModuleScript will be added.

Existing code in Modulator, ModuleMap, and ModuleTreeLinker will continue to work with ModuleScript with minimal-to-no modification.

ValueWrapperSyntheticModuleScript

The new ValueWrapperSyntheticModuleScript class is a ModuleScript that exports a single v8::Value as the default export via a V8 Synthetic Module.

It is worth noting that Synthetic Module Record is capable of supporting more sophisticated kinds of behavior, but current module proposals (JSON and CSS) only require this relatively simple functionality. Additional types of ModuleScript with more varied and complex uses of Synthetic Modules could be introduced in the future.

ValueWrapperSyntheticModuleScript has the following members in addition to those inherited from ModuleScript:

public:
ValueWrapperSyntheticModuleScript::ValueWrapperSyntheticModuleScript(
    Modulator* modulator,
    v8::Local<v8::Value> value,
    const KURL& source_url,
    const KURL& base_url,
    const ScriptFetchOptions& options,
    const TextPosition& start_position = TextPosition::MinimumPosition());

private:
static v8::MaybeLocal<v8::Value> EvaluationSteps(v8::Local<v8::Context> context, v8::Local<v8::Module> module);

TraceWrapperV8Reference<v8::Value> export_value;

The ValueWrapperSyntheticModuleScript constructor will do the following:

  1. Stash the value parameter in this->export_value.
  2. Call v8::Module::CreateSyntheticModule() with a List containing the single name "default" as the export_names and ValueWrapperSyntheticModuleScript::EvaluationSteps as the evaluation_steps callback.
  3. Create a blink::ModuleRecord using the result of CreateSyntheticModule() and stash it in this->record_.

EvaluationSteps will call v8::Module::SetSyntheticModuleExport("default", this->export_value) and return an empty MaybeLocal<Value>.

ModuleScriptLoader

ModuleScriptLoader and related classes will be modified to support CSS and JSON modules via ValueWrapperSyntheticModuleScript.

  • DocumentModuleScriptFetcher::NotifyFinished() and ModuleScriptFetcher::WasModuleLoadSuccessful() will be modified to let CSS and JSON MIME types through in addition to JavaScript.

  • A MIME type field will be added to ModuleScriptCreationParams for use in ModuleScriptLoader.

  • ModuleScriptLoader::NotifyFetchFinished will key off of the MIME type in ModuleScriptCreationParams to determine what type of module to create:

    • For a JavaScript MIME type, a JSModuleScript will be created as before.
    • For a CSS MIME type, the CSS parser will be applied to params->GetSourceText() and the resulting CSSStyleSheet will be used to construct a ValueWrapperSyntheticModuleScript with the CSSStyleSheet as the wrapped value.
    • For a JSON MIME type, the JSON parser will be applied to params->GetSourceText() and the resulting JSON v8::Value will be used to construct a ValueWrapperSyntheticModuleScript with the JSON value as the wrapped value.

Security considerations for new module types

As we add module types other than JavaScript to the module system, we must be aware that future changes to the module infrastructure may have security implications that, even if benign to JavaScript, may be harmful for other resource types.

For example, even though all modules are loaded with CORS, JavaScript code on the web still must be aware that it could be loaded cross-origin e.g. via <script src="otherOrigin/victim.js">. This is not the case for JSON as there are no web APIs that allow no-CORS loads of JSON resources (in some cases these too could be loaded via a <script> but there would be no way to reach in and access the actual JSON data).

If a 'no-cors' mode was added to the module system, then there would now be a way to load JSON content cross-origin -- which would not have been the case if the hypothetical 'no-cors' module system was limited to JavaScript.

Thus, as we add additional module types we must be exceedingly wary of any changes to the security properties of the module infrastructure overall as there are now security implications for more than just JavaScript resources.

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