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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
* Options specified when creating a realm to determine its behavior, immutable
* options determining the behavior of an existing realm, and mutable options on
* an existing realm that may be changed when desired.
#ifndef js_RealmOptions_h
#define js_RealmOptions_h
#include "mozilla/Assertions.h" // MOZ_ASSERT
#include "mozilla/Maybe.h"
#include "jstypes.h" // JS_PUBLIC_API
#include "js/Class.h" // JSTraceOp
#include "js/RefCounted.h"
struct JS_PUBLIC_API JSContext;
class JS_PUBLIC_API JSObject;
namespace JS {
class JS_PUBLIC_API Compartment;
class JS_PUBLIC_API Realm;
class JS_PUBLIC_API Zone;
} // namespace JS
namespace JS {
* Specification for which compartment/zone a newly created realm should use.
enum class CompartmentSpecifier {
// Create a new realm and compartment in the single runtime wide system
// zone. The meaning of this zone is left to the embedder.
// Create a new realm and compartment in a particular existing zone.
// Create a new zone/compartment.
// Create a new realm in an existing compartment.
struct LocaleString : js::RefCounted<LocaleString> {
const char* chars_;
explicit LocaleString(const char* chars) : chars_(chars) {}
auto chars() const { return chars_; }
* RealmCreationOptions specifies options relevant to creating a new realm, that
* are either immutable characteristics of that realm or that are discarded
* after the realm has been created.
* Access to these options on an existing realm is read-only: if you need
* particular selections, you must make them before you create the realm.
class JS_PUBLIC_API RealmCreationOptions {
RealmCreationOptions() : comp_(nullptr) {}
JSTraceOp getTrace() const { return traceGlobal_; }
RealmCreationOptions& setTrace(JSTraceOp op) {
traceGlobal_ = op;
return *this;
Zone* zone() const {
MOZ_ASSERT(compSpec_ == CompartmentSpecifier::NewCompartmentInExistingZone);
return zone_;
Compartment* compartment() const {
MOZ_ASSERT(compSpec_ == CompartmentSpecifier::ExistingCompartment);
return comp_;
CompartmentSpecifier compartmentSpecifier() const { return compSpec_; }
// Set the compartment/zone to use for the realm. See CompartmentSpecifier
// above.
RealmCreationOptions& setNewCompartmentInSystemZone();
RealmCreationOptions& setNewCompartmentInExistingZone(JSObject* obj);
RealmCreationOptions& setNewCompartmentAndZone();
RealmCreationOptions& setExistingCompartment(JSObject* obj);
RealmCreationOptions& setExistingCompartment(Compartment* compartment);
// Certain compartments are implementation details of the embedding, and
// references to them should never leak out to script. This flag causes this
// realm to skip firing onNewGlobalObject and makes addDebuggee a no-op for
// this global.
// Debugger visibility is per-compartment, not per-realm (it's only practical
// to enforce visibility on compartment boundaries), so if a realm is being
// created in an extant compartment, its requested visibility must match that
// of the compartment.
bool invisibleToDebugger() const { return invisibleToDebugger_; }
RealmCreationOptions& setInvisibleToDebugger(bool flag) {
invisibleToDebugger_ = flag;
return *this;
// Determines whether this realm should preserve JIT code on non-shrinking
// GCs.
bool preserveJitCode() const { return preserveJitCode_; }
RealmCreationOptions& setPreserveJitCode(bool flag) {
preserveJitCode_ = flag;
return *this;
// Determines whether 1) the global Atomic property is defined and atomic
// operations are supported, and 2) whether shared-memory operations are
// supported.
bool getSharedMemoryAndAtomicsEnabled() const;
RealmCreationOptions& setSharedMemoryAndAtomicsEnabled(bool flag);
// Determines (if getSharedMemoryAndAtomicsEnabled() is true) whether the
// global SharedArrayBuffer property is defined. If the property is not
// defined, shared array buffer functionality can only be invoked if the
// host/embedding specifically acts to expose it.
// This option defaults to true: embeddings unable to tolerate a global
// SharedAraryBuffer property must opt out of it.
bool defineSharedArrayBufferConstructor() const {
return defineSharedArrayBufferConstructor_;
RealmCreationOptions& setDefineSharedArrayBufferConstructor(bool flag) {
defineSharedArrayBufferConstructor_ = flag;
return *this;
// Structured clone operations support the cloning of shared memory objects
// (SharedArrayBuffer or or a shared WASM Memory object) *optionally* -- at
// the discretion of the embedder code that performs the cloning. When a
// structured clone operation encounters a shared memory object and cloning
// shared memory objects has not been enabled, the clone fails and an
// error is thrown.
// In the web embedding context, shared memory object cloning is disabled
// either because
// 1) *no* way of supporting it is available (because the
// Cross-Origin-Opener-Policy and Cross-Origin-Embedder-Policy HTTP
// headers are not respected to force the page into its own process), or
// 2) the aforementioned HTTP headers don't specify that the page should be
// opened in its own process.
// These two scenarios demand different error messages, and this option can be
// used to specify which scenario is in play.
// In the former case, if COOP/COEP support is not enabled, set this option to
// false. (This is the default.)
// In the latter case, if COOP/COEP weren't used to force this page into its
// own process, set this option to true.
// (Embeddings that are not the web and do not wish to support structured
// cloning of shared memory objects will get a "bad" web-centric error message
// no matter what. At present, SpiderMonkey does not offer a way for such
// embeddings to use an embedding-specific error message.)
bool getCoopAndCoepEnabled() const;
RealmCreationOptions& setCoopAndCoepEnabled(bool flag);
bool getToSourceEnabled() const { return toSource_; }
RealmCreationOptions& setToSourceEnabled(bool flag) {
toSource_ = flag;
return *this;
// This flag doesn't affect JS engine behavior. It is used by Gecko to
// mark whether content windows and workers are "Secure Context"s. See
bool secureContext() const { return secureContext_; }
RealmCreationOptions& setSecureContext(bool flag) {
secureContext_ = flag;
return *this;
// Non-standard option to freeze certain builtin constructors and seal their
// prototypes. Also defines these constructors on the global as non-writable
// and non-configurable.
bool freezeBuiltins() const { return freezeBuiltins_; }
RealmCreationOptions& setFreezeBuiltins(bool flag) {
freezeBuiltins_ = flag;
return *this;
// Force all date/time methods in JavaScript to use the UTC timezone for
// fingerprinting protection.
bool forceUTC() const { return forceUTC_; }
RealmCreationOptions& setForceUTC(bool flag) {
forceUTC_ = flag;
return *this;
RefPtr<LocaleString> locale() const { return locale_; }
RealmCreationOptions& setLocaleCopyZ(const char* locale);
// Always use the fdlibm implementation of math functions instead of the
// platform native libc implementations. Useful for fingerprinting protection
// and cross-platform consistency.
bool alwaysUseFdlibm() const { return alwaysUseFdlibm_; }
RealmCreationOptions& setAlwaysUseFdlibm(bool flag) {
alwaysUseFdlibm_ = flag;
return *this;
uint64_t profilerRealmID() const { return profilerRealmID_; }
RealmCreationOptions& setProfilerRealmID(uint64_t id) {
profilerRealmID_ = id;
return *this;
JSTraceOp traceGlobal_ = nullptr;
CompartmentSpecifier compSpec_ = CompartmentSpecifier::NewCompartmentAndZone;
union {
Compartment* comp_;
Zone* zone_;
uint64_t profilerRealmID_ = 0;
RefPtr<LocaleString> locale_;
bool invisibleToDebugger_ = false;
bool preserveJitCode_ = false;
bool sharedMemoryAndAtomics_ = false;
bool defineSharedArrayBufferConstructor_ = true;
bool coopAndCoep_ = false;
bool toSource_ = false;
bool secureContext_ = false;
bool freezeBuiltins_ = false;
bool forceUTC_ = false;
bool alwaysUseFdlibm_ = false;
// This is a wrapper for mozilla::RTPCallerType, that can't easily
// be exposed to the JS engine for layering reasons.
struct RTPCallerTypeToken {
uint8_t value;
* RealmBehaviors specifies behaviors of a realm that can be changed after the
* realm's been created.
class JS_PUBLIC_API RealmBehaviors {
RealmBehaviors() = default;
// When a JS::ReduceMicrosecondTimePrecisionCallback callback is defined via
// JS::SetReduceMicrosecondTimePrecisionCallback, a JS::RTPCallerTypeToken (a
// wrapper for mozilla::RTPCallerType) needs to be set for every Realm.
mozilla::Maybe<RTPCallerTypeToken> reduceTimerPrecisionCallerType() const {
return rtpCallerType;
RealmBehaviors& setReduceTimerPrecisionCallerType(RTPCallerTypeToken type) {
rtpCallerType = mozilla::Some(type);
return *this;
// For certain globals, we know enough about the code that will run in them
// that we can discard script source entirely.
bool discardSource() const { return discardSource_; }
RealmBehaviors& setDiscardSource(bool flag) {
discardSource_ = flag;
return *this;
bool clampAndJitterTime() const { return clampAndJitterTime_; }
RealmBehaviors& setClampAndJitterTime(bool flag) {
clampAndJitterTime_ = flag;
return *this;
// A Realm can stop being "live" in all the ways that matter before its global
// is actually GCed. Consumers that tear down parts of a Realm or its global
// before that point should set isNonLive accordingly.
bool isNonLive() const { return isNonLive_; }
RealmBehaviors& setNonLive() {
isNonLive_ = true;
return *this;
mozilla::Maybe<RTPCallerTypeToken> rtpCallerType;
bool discardSource_ = false;
bool clampAndJitterTime_ = true;
bool isNonLive_ = false;
* RealmOptions specifies realm characteristics: both those that can't be
* changed on a realm once it's been created (RealmCreationOptions), and those
* that can be changed on an existing realm (RealmBehaviors).
class JS_PUBLIC_API RealmOptions {
explicit RealmOptions() : creationOptions_(), behaviors_() {}
RealmOptions(const RealmCreationOptions& realmCreation,
const RealmBehaviors& realmBehaviors)
: creationOptions_(realmCreation), behaviors_(realmBehaviors) {}
// RealmCreationOptions specify fundamental realm characteristics that must
// be specified when the realm is created, that can't be changed after the
// realm is created.
RealmCreationOptions& creationOptions() { return creationOptions_; }
const RealmCreationOptions& creationOptions() const {
return creationOptions_;
// RealmBehaviors specify realm characteristics that can be changed after
// the realm is created.
RealmBehaviors& behaviors() { return behaviors_; }
const RealmBehaviors& behaviors() const { return behaviors_; }
RealmCreationOptions creationOptions_;
RealmBehaviors behaviors_;
extern JS_PUBLIC_API const RealmCreationOptions& RealmCreationOptionsRef(
Realm* realm);
extern JS_PUBLIC_API const RealmCreationOptions& RealmCreationOptionsRef(
JSContext* cx);
extern JS_PUBLIC_API const RealmBehaviors& RealmBehaviorsRef(Realm* realm);
extern JS_PUBLIC_API const RealmBehaviors& RealmBehaviorsRef(JSContext* cx);
extern JS_PUBLIC_API void SetRealmNonLive(Realm* realm);
// This behaves like RealmBehaviors::setReduceTimerPrecisionCallerType, but
// can be used even after the Realm has already been created.
extern JS_PUBLIC_API void SetRealmReduceTimerPrecisionCallerType(
Realm* realm, RTPCallerTypeToken type);
} // namespace JS
#endif // js_RealmOptions_h