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diff --git a/dom/script/ScriptSettings.h b/dom/script/ScriptSettings.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/. */
+
+/* Utilities for managing the script settings object stack defined in webapps */
+
+#ifndef mozilla_dom_ScriptSettings_h
+#define mozilla_dom_ScriptSettings_h
+
+#include "xpcpublic.h"
+
+#include "mozilla/dom/JSExecutionManager.h"
+#include "mozilla/Maybe.h"
+
+#include "jsapi.h"
+#include "js/Exception.h"
+#include "js/Warnings.h"  // JS::WarningReporter
+
+class JSObject;
+class nsIGlobalObject;
+class nsIPrincipal;
+class nsPIDOMWindowInner;
+class nsGlobalWindowInner;
+class nsIScriptContext;
+struct JSContext;
+
+namespace JS {
+class ExceptionStack;
+class Value;
+}  // namespace JS
+
+namespace mozilla {
+namespace dom {
+
+class Document;
+
+/*
+ * Per thread setup/teardown routines. Init and Destroy should be invoked
+ * once each, at startup and shutdown of the script runtime (respectively).
+ */
+void InitScriptSettings();
+void DestroyScriptSettings();
+
+// To implement a web-compatible browser, it is often necessary to obtain the
+// global object that is "associated" with the currently-running code. This
+// process is made more complicated by the fact that, historically, different
+// algorithms have operated with different definitions of the "associated"
+// global.
+//
+// HTML5 formalizes this into two concepts: the "incumbent global" and the
+// "entry global". The incumbent global corresponds to the global of the
+// current script being executed, whereas the entry global corresponds to the
+// global of the script where the current JS execution began.
+//
+// There is also a potentially-distinct third global that is determined by the
+// current compartment. This roughly corresponds with the notion of Realms in
+// ECMAScript.
+//
+// Suppose some event triggers an event listener in window |A|, which invokes a
+// scripted function in window |B|, which invokes the |window.location.href|
+// setter in window |C|. The entry global would be |A|, the incumbent global
+// would be |B|, and the current compartment would be that of |C|.
+//
+// In general, it's best to use to use the most-closely-associated global
+// unless the spec says to do otherwise. In 95% of the cases, the global of
+// the current compartment (GetCurrentGlobal()) is the right thing. For
+// example, WebIDL constructors (new C.XMLHttpRequest()) are initialized with
+// the global of the current compartment (i.e. |C|).
+//
+// The incumbent global is very similar, but differs in a few edge cases. For
+// example, if window |B| does |C.location.href = "..."|, the incumbent global
+// used for the navigation algorithm is B, because no script from |C| was ever
+// run.
+//
+// The entry global is used for various things like computing base URIs, mostly
+// for historical reasons.
+//
+// Note that all of these functions return bonafide global objects. This means
+// that, for Windows, they always return the inner.
+
+// Returns the global associated with the top-most Candidate Entry Point on
+// the Script Settings Stack. See the HTML spec. This may be null.
+nsIGlobalObject* GetEntryGlobal();
+
+// If the entry global is a window, returns its extant document. Otherwise,
+// returns null.
+Document* GetEntryDocument();
+
+// Returns the global associated with the top-most entry of the the Script
+// Settings Stack. See the HTML spec. This may be null.
+nsIGlobalObject* GetIncumbentGlobal();
+
+// Returns the global associated with the current compartment. This may be null.
+nsIGlobalObject* GetCurrentGlobal();
+
+// JS-implemented WebIDL presents an interesting situation with respect to the
+// subject principal. A regular C++-implemented API can simply examine the
+// compartment of the most-recently-executed script, and use that to infer the
+// responsible party. However, JS-implemented APIs are run with system
+// principal, and thus clobber the subject principal of the script that
+// invoked the API. So we have to do some extra work to keep track of this
+// information.
+//
+// We therefore implement the following behavior:
+// * Each Script Settings Object has an optional WebIDL Caller Principal field.
+//   This defaults to null.
+// * When we push an Entry Point in preparation to run a JS-implemented WebIDL
+//   callback, we grab the subject principal at the time of invocation, and
+//   store that as the WebIDL Caller Principal.
+// * When non-null, callers can query this principal from script via an API on
+//   Components.utils.
+nsIPrincipal* GetWebIDLCallerPrincipal();
+
+// Returns whether JSAPI is active right now.  If it is not, working with a
+// JSContext you grab from somewhere random is not OK and you should be doing
+// AutoJSAPI or AutoEntryScript to get yourself a properly set up JSContext.
+bool IsJSAPIActive();
+
+namespace danger {
+
+// Get the JSContext for this thread.  This is in the "danger" namespace because
+// we generally want people using AutoJSAPI instead, unless they really know
+// what they're doing.
+JSContext* GetJSContext();
+
+}  // namespace danger
+
+JS::RootingContext* RootingCx();
+
+class ScriptSettingsStack;
+class ScriptSettingsStackEntry {
+  friend class ScriptSettingsStack;
+
+ public:
+  ~ScriptSettingsStackEntry();
+
+  bool NoJSAPI() const { return mType == eNoJSAPI; }
+  bool IsEntryCandidate() const {
+    return mType == eEntryScript || mType == eNoJSAPI;
+  }
+  bool IsIncumbentCandidate() { return mType != eJSAPI; }
+  bool IsIncumbentScript() { return mType == eIncumbentScript; }
+
+ protected:
+  enum Type { eEntryScript, eIncumbentScript, eJSAPI, eNoJSAPI };
+
+  ScriptSettingsStackEntry(nsIGlobalObject* aGlobal, Type aEntryType);
+
+  nsCOMPtr<nsIGlobalObject> mGlobalObject;
+  Type mType;
+
+ private:
+  ScriptSettingsStackEntry* mOlder;
+};
+
+/*
+ * For any interaction with JSAPI, an AutoJSAPI (or one of its subclasses)
+ * must be on the stack.
+ *
+ * This base class should be instantiated as-is when the caller wants to use
+ * JSAPI but doesn't expect to run script. The caller must then call one of its
+ * Init functions before being able to access the JSContext through cx().
+ * Its current duties are as-follows (see individual Init comments for details):
+ *
+ * * Grabbing an appropriate JSContext, and, on the main thread, pushing it onto
+ *   the JSContext stack.
+ * * Entering an initial (possibly null) compartment, to ensure that the
+ *   previously entered compartment for that JSContext is not used by mistake.
+ * * Reporting any exceptions left on the JSRuntime, unless the caller steals
+ *   or silences them.
+ *
+ * Additionally, the following duties are planned, but not yet implemented:
+ *
+ * * De-poisoning the JSRuntime to allow manipulation of JSAPI. This requires
+ *   implementing the poisoning first.  For now, this de-poisoning
+ *   effectively corresponds to having a non-null cx on the stack.
+ *
+ * In situations where the consumer expects to run script, AutoEntryScript
+ * should be used, which does additional manipulation of the script settings
+ * stack. In bug 991758, we'll add hard invariants to SpiderMonkey, such that
+ * any attempt to run script without an AutoEntryScript on the stack will
+ * fail. This prevents system code from accidentally triggering script
+ * execution at inopportune moments via surreptitious getters and proxies.
+ */
+class MOZ_STACK_CLASS AutoJSAPI : protected ScriptSettingsStackEntry {
+ public:
+  // Trivial constructor. One of the Init functions must be called before
+  // accessing the JSContext through cx().
+  AutoJSAPI();
+
+  ~AutoJSAPI();
+
+  // This uses the SafeJSContext (or worker equivalent), and enters a null
+  // compartment, so that the consumer is forced to select a compartment to
+  // enter before manipulating objects.
+  //
+  // This variant will ensure that any errors reported by this AutoJSAPI as it
+  // comes off the stack will not fire error events or be associated with any
+  // particular web-visible global.
+  void Init();
+
+  // This uses the SafeJSContext (or worker equivalent), and enters the
+  // compartment of aGlobalObject.
+  // If aGlobalObject or its associated JS global are null then it returns
+  // false and use of cx() will cause an assertion.
+  //
+  // If aGlobalObject represents a web-visible global, errors reported by this
+  // AutoJSAPI as it comes off the stack will fire the relevant error events and
+  // show up in the corresponding web console.
+  //
+  // Successfully initializing the AutoJSAPI will ensure that it enters the
+  // Realm of aGlobalObject's JSObject and exposes that JSObject to active JS.
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject);
+
+  // This is a helper that grabs the native global associated with aObject and
+  // invokes the above Init() with that. aObject must not be a cross-compartment
+  // wrapper: CCWs are not associated with a single global.
+  [[nodiscard]] bool Init(JSObject* aObject);
+
+  // Unsurprisingly, this uses aCx and enters the compartment of aGlobalObject.
+  // If aGlobalObject or its associated JS global are null then it returns
+  // false and use of cx() will cause an assertion.
+  // If aCx is null it will cause an assertion.
+  //
+  // If aGlobalObject represents a web-visible global, errors reported by this
+  // AutoJSAPI as it comes off the stack will fire the relevant error events and
+  // show up in the corresponding web console.
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
+
+  // Convenience functions to take an nsPIDOMWindowInner or nsGlobalWindowInner,
+  // when it is more easily available than an nsIGlobalObject.
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
+
+  [[nodiscard]] bool Init(nsGlobalWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsGlobalWindowInner* aWindow, JSContext* aCx);
+
+  JSContext* cx() const {
+    MOZ_ASSERT(mCx, "Must call Init before using an AutoJSAPI");
+    MOZ_ASSERT(IsStackTop());
+    return mCx;
+  }
+
+#ifdef DEBUG
+  bool IsStackTop() const;
+#endif
+
+  // If HasException, report it.  Otherwise, a no-op.
+  void ReportException();
+
+  bool HasException() const {
+    MOZ_ASSERT(IsStackTop());
+    return JS_IsExceptionPending(cx());
+  };
+
+  // Transfers ownership of the current exception from the JS engine to the
+  // caller. Callers must ensure that HasException() is true, and that cx()
+  // is in a non-null compartment.
+  //
+  // Note that this fails if and only if we OOM while wrapping the exception
+  // into the current compartment.
+  [[nodiscard]] bool StealException(JS::MutableHandle<JS::Value> aVal);
+
+  // As for StealException(), but uses the JS::ExceptionStack class to also
+  // include the exception's stack, represented by SavedFrames.
+  [[nodiscard]] bool StealExceptionAndStack(JS::ExceptionStack* aExnStack);
+
+  // Peek the current exception from the JS engine, without stealing it.
+  // Callers must ensure that HasException() is true, and that cx() is in a
+  // non-null compartment.
+  //
+  // Note that this fails if and only if we OOM while wrapping the exception
+  // into the current compartment.
+  [[nodiscard]] bool PeekException(JS::MutableHandle<JS::Value> aVal);
+
+  void ClearException() {
+    MOZ_ASSERT(IsStackTop());
+    JS_ClearPendingException(cx());
+  }
+
+ protected:
+  // Protected constructor for subclasses.  This constructor initialises the
+  // AutoJSAPI, so Init must NOT be called on subclasses that use this.
+  AutoJSAPI(nsIGlobalObject* aGlobalObject, bool aIsMainThread, Type aType);
+
+  mozilla::Maybe<JSAutoNullableRealm> mAutoNullableRealm;
+  JSContext* mCx;
+
+  // Whether we're mainthread or not; set when we're initialized.
+  bool mIsMainThread;
+  Maybe<JS::WarningReporter> mOldWarningReporter;
+
+ private:
+  void InitInternal(nsIGlobalObject* aGlobalObject, JSObject* aGlobal,
+                    JSContext* aCx, bool aIsMainThread);
+
+  AutoJSAPI(const AutoJSAPI&) = delete;
+  AutoJSAPI& operator=(const AutoJSAPI&) = delete;
+};
+
+/*
+ * A class that can be used to force a particular incumbent script on the stack.
+ */
+class AutoIncumbentScript : protected ScriptSettingsStackEntry {
+ public:
+  explicit AutoIncumbentScript(nsIGlobalObject* aGlobalObject);
+  ~AutoIncumbentScript();
+
+ private:
+  JS::AutoHideScriptedCaller mCallerOverride;
+};
+
+/*
+ * A class to put the JS engine in an unusable state. The subject principal
+ * will become System, the information on the script settings stack is
+ * rendered inaccessible, and JSAPI may not be manipulated until the class is
+ * either popped or an AutoJSAPI instance is subsequently pushed.
+ *
+ * This class may not be instantiated if an exception is pending.
+ */
+class AutoNoJSAPI : protected ScriptSettingsStackEntry,
+                    protected JSAutoNullableRealm {
+ public:
+  AutoNoJSAPI() : AutoNoJSAPI(danger::GetJSContext()) {}
+  ~AutoNoJSAPI();
+
+ private:
+  // Helper constructor to avoid doing GetJSContext() multiple times
+  // during construction.
+  explicit AutoNoJSAPI(JSContext* aCx);
+
+  // Stashed JSContext* so we don't need to GetJSContext in our destructor.
+  // It's probably safe to hold on to this, in the sense that the world should
+  // not get torn down while we're on the stack, and if it's not, we'd need to
+  // fix JSAutoNullableRealm to not hold on to a JSContext either, or
+  // something.
+  JSContext* mCx;
+
+  AutoYieldJSThreadExecution mExecutionYield;
+};
+
+}  // namespace dom
+
+/**
+ * Use AutoJSContext when you need a JS context on the stack but don't have one
+ * passed as a parameter. AutoJSContext will take care of finding the most
+ * appropriate JS context and release it when leaving the stack.
+ */
+class MOZ_RAII AutoJSContext {
+ public:
+  explicit AutoJSContext();
+  operator JSContext*() const;
+
+ protected:
+  JSContext* mCx;
+  dom::AutoJSAPI mJSAPI;
+};
+
+/**
+ * AutoSafeJSContext is similar to AutoJSContext but will only return the safe
+ * JS context. That means it will never call
+ * nsContentUtils::GetCurrentJSContext().
+ *
+ * Note - This is deprecated. Please use AutoJSAPI instead.
+ */
+class MOZ_RAII AutoSafeJSContext : public dom::AutoJSAPI {
+ public:
+  explicit AutoSafeJSContext();
+  operator JSContext*() const { return cx(); }
+
+ private:
+};
+
+/**
+ * Use AutoSlowOperation when native side calls many JS callbacks in a row
+ * and slow script dialog should be activated if too much time is spent going
+ * through those callbacks.
+ * AutoSlowOperation puts an AutoScriptActivity on the stack so that we don't
+ * continue to reset the watchdog. CheckForInterrupt can then be used to check
+ * whether JS execution should be interrupted.
+ * This class (including CheckForInterrupt) is a no-op when used off the main
+ * thread.
+ */
+class MOZ_RAII AutoSlowOperation {
+ public:
+  explicit AutoSlowOperation();
+  void CheckForInterrupt();
+
+ private:
+  bool mIsMainThread;
+  Maybe<xpc::AutoScriptActivity> mScriptActivity;
+};
+
+/**
+ * A class to disable interrupt callback temporary.
+ */
+class MOZ_RAII AutoDisableJSInterruptCallback {
+ public:
+  explicit AutoDisableJSInterruptCallback(JSContext* aCx)
+      : mCx(aCx), mOld(JS_DisableInterruptCallback(aCx)) {}
+
+  ~AutoDisableJSInterruptCallback() { JS_ResetInterruptCallback(mCx, mOld); }
+
+ private:
+  JSContext* mCx;
+  bool mOld;
+};
+
+/**
+ * A helper class which allows to allow-list legacy callers executing script
+ * in the AutoEntryScript constructor. The goal is to remove these exceptions
+ * one by one. Do not add a new one without review from a DOM peer.
+ */
+class MOZ_RAII AutoAllowLegacyScriptExecution {
+ public:
+  AutoAllowLegacyScriptExecution();
+  ~AutoAllowLegacyScriptExecution();
+
+  static bool IsAllowed();
+
+ private:
+  static int sAutoAllowLegacyScriptExecution;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_dom_ScriptSettings_h