Create debug only SkSingleOwner

include/private/GrSingleOwner.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef GrSingleOwner_DEFINED
+#define GrSingleOwner_DEFINED
+
+#include "SkTypes.h"
+
+#ifdef SK_DEBUG
+#include "SkMutex.h"
+#include "SkThreadID.h"
+
+// This is a debug tool to verify an object is only being used from one thread at a time.
+class GrSingleOwner {
+public:
+     GrSingleOwner() : fOwner(kIllegalThreadID), fReentranceCount(0) {}
+
+     struct AutoEnforce {
+         AutoEnforce(GrSingleOwner* so) : fSO(so) { fSO->enter(); }
+         ~AutoEnforce() { fSO->exit(); }
+
+         GrSingleOwner* fSO;
+     };
+
+private:
+     void enter() {
+         SkAutoMutexAcquire lock(fMutex);
+         SkThreadID self = SkGetThreadID();
+         SkASSERT(fOwner == self || fOwner == kIllegalThreadID);
+         fReentranceCount++;
+         fOwner = self;
+     }
+
+     void exit() {
+         SkAutoMutexAcquire lock(fMutex);
+         fReentranceCount--;
+         if (fReentranceCount == 0) {
+             fOwner = kIllegalThreadID;
+         }
+     }
+
+     SkMutex fMutex;
+     SkThreadID fOwner;    // guarded by fMutex
+     int fReentranceCount; // guarded by fMutex
+};
+#endif
+
+#endif