Bug 990353 - Flag for discarding where appropriate. r=bent

dom/workers/ScriptLoader.cpp

@@ -699,6 +699,20 @@ ScriptExecutorRunnable::WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate)
   JS::Rooted<JSObject*> global(aCx, JS::CurrentGlobalOrNull(aCx));
   NS_ASSERTION(global, "Must have a global by now!");
 
+  // Determine whether we want to be discarding source on this global to save
+  // memory. It would make more sense to do this when we create the global, but
+  // the information behind UsesSystemPrincipal() et al isn't finalized until
+  // the call to SetPrincipal during the first script load. After that, however,
+  // it never changes. So we can just idempotently set the bits here.
+  //
+  // Note that we read a pref that is cached on the main thread. This is benignly
+  // racey.
+  if (xpc::ShouldDiscardSystemSource()) {
+    bool discard = aWorkerPrivate->UsesSystemPrincipal() ||
+                   aWorkerPrivate->IsInPrivilegedApp();
+    JS::CompartmentOptionsRef(global).setDiscardSource(discard);
+  }
+
   for (uint32_t index = mFirstIndex; index <= mLastIndex; index++) {
     ScriptLoadInfo& loadInfo = loadInfos.ElementAt(index);
 

js/xpconnect/src/XPCJSRuntime.cpp

@@ -3510,6 +3510,7 @@ XPCJSRuntime::GetCompilationScope()
         NS_ENSURE_SUCCESS(rv, nullptr);
 
         mCompilationScope = js::UncheckedUnwrap(&v.toObject());
+        CompartmentOptionsRef(mCompilationScope).setDiscardSource(ShouldDiscardSystemSource());
     }
     return mCompilationScope;
 }

js/xpconnect/src/nsXPConnect.cpp

@@ -414,6 +414,17 @@ InitGlobalObject(JSContext* aJSContext, JS::Handle<JSObject*> aGlobal, uint32_t
         }
     }
 
+    if (ShouldDiscardSystemSource()) {
+        nsIPrincipal *prin = GetObjectPrincipal(aGlobal);
+        bool isSystem = nsContentUtils::IsSystemPrincipal(prin);
+        if (!isSystem) {
+            short status = prin->GetAppStatus();
+            isSystem = status == nsIPrincipal::APP_STATUS_PRIVILEGED ||
+                       status == nsIPrincipal::APP_STATUS_CERTIFIED;
+        }
+        JS::CompartmentOptionsRef(aGlobal).setDiscardSource(isSystem);
+    }
+
     // Stuff coming through this path always ends up as a DOM global.
     MOZ_ASSERT(js::GetObjectClass(aGlobal)->flags & JSCLASS_DOM_GLOBAL);