Document that Isolate::GetCurrent() must not be called before initialization

Also, add a check for debug mode.

BUG=none
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/758043002

Cr-Commit-Position: refs/heads/master@{#25507}

include/v8.h

@@ -4650,6 +4650,8 @@ class V8_EXPORT Isolate {
   /**
    * Returns the entered isolate for the current thread or NULL in
    * case there is no current isolate.
+   *
+   * This method must not be invoked before V8::Initialize() was invoked.
    */
   static Isolate* GetCurrent();
 

src/isolate.cc

@@ -118,6 +118,9 @@ base::Thread::LocalStorageKey Isolate::per_isolate_thread_data_key_;
 base::LazyMutex Isolate::thread_data_table_mutex_ = LAZY_MUTEX_INITIALIZER;
 Isolate::ThreadDataTable* Isolate::thread_data_table_ = NULL;
 base::Atomic32 Isolate::isolate_counter_ = 0;
+#if DEBUG
+base::Atomic32 Isolate::isolate_key_created_ = 0;
+#endif
 
 Isolate::PerIsolateThreadData*
     Isolate::FindOrAllocatePerThreadDataForThisThread() {
@@ -157,6 +160,9 @@ void Isolate::InitializeOncePerProcess() {
   base::LockGuard<base::Mutex> lock_guard(thread_data_table_mutex_.Pointer());
   CHECK(thread_data_table_ == NULL);
   isolate_key_ = base::Thread::CreateThreadLocalKey();
+#if DEBUG
+  base::NoBarrier_Store(&isolate_key_created_, 1);
+#endif
   thread_id_key_ = base::Thread::CreateThreadLocalKey();
   per_isolate_thread_data_key_ = base::Thread::CreateThreadLocalKey();
   thread_data_table_ = new Isolate::ThreadDataTable();

src/isolate.h

@@ -486,6 +486,7 @@ class Isolate {
 
   // Returns the isolate inside which the current thread is running.
   INLINE(static Isolate* Current()) {
+    DCHECK(base::NoBarrier_Load(&isolate_key_created_) == 1);
     Isolate* isolate = reinterpret_cast<Isolate*>(
         base::Thread::GetExistingThreadLocal(isolate_key_));
     DCHECK(isolate != NULL);
@@ -493,6 +494,7 @@ class Isolate {
   }
 
   INLINE(static Isolate* UncheckedCurrent()) {
+    DCHECK(base::NoBarrier_Load(&isolate_key_created_) == 1);
     return reinterpret_cast<Isolate*>(
         base::Thread::GetThreadLocal(isolate_key_));
   }
@@ -1177,6 +1179,10 @@ class Isolate {
   // A global counter for all generated Isolates, might overflow.
   static base::Atomic32 isolate_counter_;
 
+#if DEBUG
+  static base::Atomic32 isolate_key_created_;
+#endif
+
   void Deinit();
 
   static void SetIsolateThreadLocals(Isolate* isolate,