Bug 774633 - Assert against switching origins on a compartment. r=jst

dom/base/nsGlobalWindow.cpp

@@ -1884,7 +1884,15 @@ nsGlobalWindow::SetNewDocument(nsIDocument* aDocument,
     // match the new document.
     // NB: We don't just call currentInner->RefreshCompartmentPrincipals() here
     // because we haven't yet set its mDoc to aDocument.
-    JS_SetCompartmentPrincipals(js::GetObjectCompartment(currentInner->mJSObject),
+    JSCompartment *compartment = js::GetObjectCompartment(currentInner->mJSObject);
+#ifdef DEBUG
+    bool sameOrigin = false;
+    nsIPrincipal *existing =
+      nsJSPrincipals::get(JS_GetCompartmentPrincipals(compartment));
+    aDocument->NodePrincipal()->Equals(existing, &sameOrigin);
+    MOZ_ASSERT(sameOrigin);
+#endif
+    JS_SetCompartmentPrincipals(compartment,
                                 nsJSPrincipals::get(aDocument->NodePrincipal()));
   } else {
     if (aState) {

js/src/jsfriendapi.cpp

@@ -196,10 +196,20 @@ JS_SetCompartmentPrincipals(JSCompartment *compartment, JSPrincipals *principals
     if (principals == compartment->principals)
         return;
 
+    // Any compartment with the trusted principals -- and there can be
+    // multiple -- is a system compartment.
+    JSPrincipals *trusted = compartment->rt->trustedPrincipals();
+    bool isSystem = principals && principals == trusted;
+
     // Clear out the old principals, if any.
     if (compartment->principals) {
         JS_DropPrincipals(compartment->rt, compartment->principals);
         compartment->principals = NULL;
+        // We'd like to assert that our new principals is always same-origin
+        // with the old one, but JSPrincipals doesn't give us a way to do that.
+        // But we can at least assert that we're not switching between system
+        // and non-system.
+        JS_ASSERT(compartment->isSystemCompartment == isSystem);
     }
 
     // Set up the new principals.
@@ -208,10 +218,8 @@ JS_SetCompartmentPrincipals(JSCompartment *compartment, JSPrincipals *principals
         compartment->principals = principals;
     }
 
-    // Any compartment with the trusted principals -- and there can be
-    // multiple -- is a system compartment.
-    JSPrincipals *trusted = compartment->rt->trustedPrincipals();
-    compartment->isSystemCompartment = principals && principals == trusted;
+    // Update the system flag.
+    compartment->isSystemCompartment = isSystem;
 }
 
 JS_FRIEND_API(JSBool)