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diff --git a/js/public/Realm.h b/js/public/Realm.h
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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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef js_Realm_h
+#define js_Realm_h
+
+#include "js/shadow/Realm.h"  // JS::shadow::Realm
+
+#include "js/GCPolicyAPI.h"
+#include "js/TypeDecls.h"  // forward-declaration of JS::Realm
+
+/************************************************************************/
+
+// [SMDOC] Realms
+//
+// Data associated with a global object. In the browser each frame has its
+// own global/realm.
+
+namespace js {
+namespace gc {
+JS_PUBLIC_API void TraceRealm(JSTracer* trc, JS::Realm* realm,
+                              const char* name);
+}  // namespace gc
+}  // namespace js
+
+namespace JS {
+class JS_PUBLIC_API AutoRequireNoGC;
+
+// Each Realm holds a weak reference to its GlobalObject.
+template <>
+struct GCPolicy<Realm*> : public NonGCPointerPolicy<Realm*> {
+  static void trace(JSTracer* trc, Realm** vp, const char* name) {
+    if (*vp) {
+      ::js::gc::TraceRealm(trc, *vp, name);
+    }
+  }
+};
+
+// Get the current realm, if any. The ECMAScript spec calls this "the current
+// Realm Record".
+extern JS_PUBLIC_API Realm* GetCurrentRealmOrNull(JSContext* cx);
+
+// Return the compartment that contains a given realm.
+inline JS::Compartment* GetCompartmentForRealm(Realm* realm) {
+  return shadow::Realm::get(realm)->compartment();
+}
+
+// Return an object's realm. All objects except cross-compartment wrappers are
+// created in a particular realm, which never changes. Returns null if obj is
+// a cross-compartment wrapper.
+extern JS_PUBLIC_API Realm* GetObjectRealmOrNull(JSObject* obj);
+
+// Get the value of the "private data" internal field of the given Realm.
+// This field is initially null and is set using SetRealmPrivate.
+// It's a pointer to embeddding-specific data that SpiderMonkey never uses.
+extern JS_PUBLIC_API void* GetRealmPrivate(Realm* realm);
+
+// Set the "private data" internal field of the given Realm.
+extern JS_PUBLIC_API void SetRealmPrivate(Realm* realm, void* data);
+
+typedef void (*DestroyRealmCallback)(JS::GCContext* gcx, Realm* realm);
+
+// Set the callback SpiderMonkey calls just before garbage-collecting a realm.
+// Embeddings can use this callback to free private data associated with the
+// realm via SetRealmPrivate.
+//
+// By the time this is called, the global object for the realm has already been
+// collected.
+extern JS_PUBLIC_API void SetDestroyRealmCallback(
+    JSContext* cx, DestroyRealmCallback callback);
+
+using RealmNameCallback = void (*)(JSContext* cx, Realm* realm, char* buf,
+                                   size_t bufsize,
+                                   const JS::AutoRequireNoGC& nogc);
+
+// Set the callback SpiderMonkey calls to get the name of a realm, for
+// diagnostic output.
+extern JS_PUBLIC_API void SetRealmNameCallback(JSContext* cx,
+                                               RealmNameCallback callback);
+
+// Get the global object for the given realm. This only returns nullptr during
+// GC, between collecting the global object and destroying the Realm.
+extern JS_PUBLIC_API JSObject* GetRealmGlobalOrNull(Realm* realm);
+
+// Initialize standard JS class constructors, prototypes, and any top-level
+// functions and constants associated with the standard classes (e.g. isNaN
+// for Number).
+extern JS_PUBLIC_API bool InitRealmStandardClasses(JSContext* cx);
+
+// If the current realm has the non-standard freezeBuiltins option set to true,
+// freeze the constructor object and seal the prototype.
+extern JS_PUBLIC_API bool MaybeFreezeCtorAndPrototype(JSContext* cx,
+                                                      HandleObject ctor,
+                                                      HandleObject maybeProto);
+
+/*
+ * Ways to get various per-Realm objects. All the getters declared below operate
+ * on the JSContext's current Realm.
+ */
+
+extern JS_PUBLIC_API JSObject* GetRealmObjectPrototype(JSContext* cx);
+extern JS_PUBLIC_API JS::Handle<JSObject*> GetRealmObjectPrototypeHandle(
+    JSContext* cx);
+
+extern JS_PUBLIC_API JSObject* GetRealmFunctionPrototype(JSContext* cx);
+
+extern JS_PUBLIC_API JSObject* GetRealmArrayPrototype(JSContext* cx);
+
+extern JS_PUBLIC_API JSObject* GetRealmErrorPrototype(JSContext* cx);
+
+extern JS_PUBLIC_API JSObject* GetRealmIteratorPrototype(JSContext* cx);
+
+extern JS_PUBLIC_API JSObject* GetRealmAsyncIteratorPrototype(JSContext* cx);
+
+// Returns an object that represents the realm, that can be referred from
+// other realm/compartment.
+// See the consumer in `MaybeCrossOriginObjectMixins::EnsureHolder` for details.
+extern JS_PUBLIC_API JSObject* GetRealmKeyObject(JSContext* cx);
+
+// Implements https://tc39.github.io/ecma262/#sec-getfunctionrealm
+// 7.3.22 GetFunctionRealm ( obj )
+//
+// WARNING: may return a realm in a different compartment!
+//
+// Will throw an exception and return nullptr when a security wrapper or revoked
+// proxy is encountered.
+extern JS_PUBLIC_API Realm* GetFunctionRealm(JSContext* cx,
+                                             HandleObject objArg);
+
+/** NB: This API is infallible; a nullptr return value does not indicate error.
+ *
+ * |target| must not be a cross-compartment wrapper because CCWs are not
+ * associated with a single realm.
+ *
+ * Entering a realm roots the realm and its global object until the matching
+ * JS::LeaveRealm() call.
+ */
+extern JS_PUBLIC_API JS::Realm* EnterRealm(JSContext* cx, JSObject* target);
+
+extern JS_PUBLIC_API void LeaveRealm(JSContext* cx, JS::Realm* oldRealm);
+
+}  // namespace JS
+
+/*
+ * At any time, a JSContext has a current (possibly-nullptr) realm. The
+ * preferred way to change the current realm is with JSAutoRealm:
+ *
+ *   void foo(JSContext* cx, JSObject* obj) {
+ *     // in some realm 'r'
+ *     {
+ *       JSAutoRealm ar(cx, obj);  // constructor enters
+ *       // in the realm of 'obj'
+ *     }                           // destructor leaves
+ *     // back in realm 'r'
+ *   }
+ *
+ * The object passed to JSAutoRealm must *not* be a cross-compartment wrapper,
+ * because CCWs are not associated with a single realm.
+ *
+ * For more complicated uses that don't neatly fit in a C++ stack frame, the
+ * realm can be entered and left using separate function calls:
+ *
+ *   void foo(JSContext* cx, JSObject* obj) {
+ *     // in 'oldRealm'
+ *     JS::Realm* oldRealm = JS::EnterRealm(cx, obj);
+ *     // in the realm of 'obj'
+ *     JS::LeaveRealm(cx, oldRealm);
+ *     // back in 'oldRealm'
+ *   }
+ *
+ * Note: these calls must still execute in a LIFO manner w.r.t all other
+ * enter/leave calls on the context. Furthermore, only the return value of a
+ * JS::EnterRealm call may be passed as the 'oldRealm' argument of
+ * the corresponding JS::LeaveRealm call.
+ *
+ * Entering a realm roots the realm and its global object for the lifetime of
+ * the JSAutoRealm.
+ */
+
+class MOZ_RAII JS_PUBLIC_API JSAutoRealm {
+  JSContext* cx_;
+  JS::Realm* oldRealm_;
+
+ public:
+  JSAutoRealm(JSContext* cx, JSObject* target);
+  JSAutoRealm(JSContext* cx, JSScript* target);
+  ~JSAutoRealm();
+};
+
+class MOZ_RAII JS_PUBLIC_API JSAutoNullableRealm {
+  JSContext* cx_;
+  JS::Realm* oldRealm_;
+
+ public:
+  explicit JSAutoNullableRealm(JSContext* cx, JSObject* targetOrNull);
+  ~JSAutoNullableRealm();
+};
+
+#endif  // js_Realm_h