Add d8 functionality for switching between realms (a.k.a. contexts), 2nd try

test/mjsunit/bugs/bug-proto.js

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 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#ifndef V8_MARKING_THREAD_H_
-#define V8_MARKING_THREAD_H_
+var realmA = Realm.current();
+var realmB = Realm.create();
+assertEquals(0, realmA);
+assertEquals(1, realmB);
 
-#include "atomicops.h"
-#include "flags.h"
-#include "platform.h"
-#include "v8utils.h"
+// The global objects match the realms' this binding.
+assertSame(this, Realm.global(realmA));
+assertSame(Realm.eval(realmB, "this"), Realm.global(realmB));
+assertFalse(this === Realm.global(realmB));
 
-#include "spaces.h"
+// The global object is not accessible cross-realm.
+var x = 3;
+Realm.shared = this;
+assertThrows("Realm.eval(realmB, 'x')");
+assertSame(undefined, Realm.eval(realmB, "this.x"));
+assertSame(undefined, Realm.eval(realmB, "Realm.shared.x"));
 
-#include "heap.h"
+Realm.eval(realmB, "Realm.global(0).y = 1");
+assertThrows("y");
+assertSame(undefined, this.y);
 
-namespace v8 {
-namespace internal {
+// Can get or set other objects' properties cross-realm.
+var p = {a: 1};
+var o = {__proto__: p, b: 2};
+Realm.shared = o;
+assertSame(1, Realm.eval(realmB, "Realm.shared.a"));
+assertSame(2, Realm.eval(realmB, "Realm.shared.b"));
 
-class MarkingThread : public Thread {
- public:
-  explicit MarkingThread(Isolate* isolate);
+// Cannot get or set a prototype cross-realm.
+assertSame(undefined, Realm.eval(realmB, "Realm.shared.__proto__"));
 
-  void Run();
-  void Stop();
-  void StartMarking();
-  void WaitForMarkingThread();
+Realm.eval(realmB, "Realm.shared.__proto__ = {c: 3}");
+assertSame(1, o.a);
+assertSame(undefined, o.c);
 
-  ~MarkingThread() {
-    delete start_marking_semaphore_;
-    delete end_marking_semaphore_;
-    delete stop_semaphore_;
-  }
-
- private:
-  Isolate* isolate_;
-  Heap* heap_;
-  Semaphore* start_marking_semaphore_;
-  Semaphore* end_marking_semaphore_;
-  Semaphore* stop_semaphore_;
-  volatile AtomicWord stop_thread_;
-  int id_;
-  static Atomic32 id_counter_;
-};
-
-} }  // namespace v8::internal
-
-#endif  // V8_MARKING_THREAD_H_