Bug 1328307. AutoNoJSAPI should make it clearer that script is not running. r=bholley

Differential Revision: https://phabricator.services.mozilla.com/D41792

--HG--
extra : moz-landing-system : lando

dom/script/ScriptSettings.cpp

@@ -677,11 +677,23 @@ AutoIncumbentScript::AutoIncumbentScript(nsIGlobalObject* aGlobalObject)
 
 AutoIncumbentScript::~AutoIncumbentScript() { ScriptSettingsStack::Pop(this); }
 
-AutoNoJSAPI::AutoNoJSAPI() : ScriptSettingsStackEntry(nullptr, eNoJSAPI) {
+AutoNoJSAPI::AutoNoJSAPI(JSContext* aCx)
+    : ScriptSettingsStackEntry(nullptr, eNoJSAPI),
+      JSAutoNullableRealm(aCx, nullptr),
+      mCx(aCx) {
+  // Make sure we don't seem to have an incumbent global due to
+  // whatever script is running right now.
+  JS::HideScriptedCaller(aCx);
+
+  // Make sure the fallback GetIncumbentGlobal() behavior and
+  // GetEntryGlobal() both return null.
   ScriptSettingsStack::Push(this);
 }
 
-AutoNoJSAPI::~AutoNoJSAPI() { ScriptSettingsStack::Pop(this); }
+AutoNoJSAPI::~AutoNoJSAPI() {
+  ScriptSettingsStack::Pop(this);
+  JS::UnhideScriptedCaller(mCx);
+}
 
 }  // namespace dom
 

dom/script/ScriptSettings.h

@@ -407,10 +407,23 @@ class AutoIncumbentScript : protected ScriptSettingsStackEntry {
  *
  * This class may not be instantiated if an exception is pending.
  */
-class AutoNoJSAPI : protected ScriptSettingsStackEntry {
+class AutoNoJSAPI : protected ScriptSettingsStackEntry,
+                    protected JSAutoNullableRealm {
  public:
-  explicit AutoNoJSAPI();
+  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;
 };
 
 }  // namespace dom