Make |Heap::findGCInfo()| threadsafe.

Source/platform/heap/Heap.cpp

Index: Source/platform/heap/Heap.cpp
diff --git a/Source/platform/heap/Heap.cpp b/Source/platform/heap/Heap.cpp
index f055df916d27a153d4ccc00a1fa90cbafe0dbb5b..17c6ddab553b3e43b5e8fa6fabad5b0c708ee837 100644
--- a/Source/platform/heap/Heap.cpp
+++ b/Source/platform/heap/Heap.cpp
@@ -1600,12 +1600,20 @@ Address Heap::checkAndMarkPointer(Visitor* visitor, Address address)
 #if ENABLE(GC_TRACING)
 const GCInfo* Heap::findGCInfo(Address address)
 {
+    bool needLockForIteration = !ThreadState::isAnyThreadInGC();
+    if (needLockForIteration)
+        ThreadState::threadAttachMutex().lock();
+
     ThreadState::AttachedThreadStateSet& threads = ThreadState::attachedThreads();
     for (ThreadState::AttachedThreadStateSet::iterator it = threads.begin(), end = threads.end(); it != end; ++it) {
         if (const GCInfo* gcInfo = (*it)->findGCInfo(address)) {
+            if (needLockForIteration)
+                ThreadState::threadAttachMutex().unlock();

[Mads Ager (chromium), 2014/06/06 06:54:01]

Do we have a scoped lock object that we could use instead of having to
explicitly unlock at all returns? MutexLocker?

[kouhei (in TOK), 2014/06/06 06:58:26]

I tried to use MutexLocker, but it requires lock at the c-tor, so it can't be
used for optional locks. I'll try to modify it to support use case like this.

[Mads Ager (chromium), 2014/06/06 07:03:30]

Nah, don't bother for this change. This code is never going to be on by default
and it is correct as it is. Was just wondering if we had it already. :)

             return gcInfo;
         }
     }
+    if (needLockForIteration)
+        ThreadState::threadAttachMutex().unlock();
     return 0;
 }