Create debug only SkSingleOwner

include/private/SkMutex.h

Index: include/private/SkMutex.h
diff --git a/include/private/SkMutex.h b/include/private/SkMutex.h
index 8c78e1205c8796261be4b19468338a00af8afdc5..d2a2d8230deb2503f211c190380badd351267d31 100644
--- a/include/private/SkMutex.h
+++ b/include/private/SkMutex.h
@@ -122,4 +122,40 @@ typedef SkAutoTAcquire<SkBaseMutex> SkAutoMutexAcquire;
 typedef SkAutoTExclusive<SkBaseMutex> SkAutoMutexExclusive;
 #define SkAutoMutexExclusive(...) SK_REQUIRE_LOCAL_VAR(SkAutoMutexExclusive)
 
+#ifdef SK_DEBUG
+// This is a debug tool to verify an object is only being used from one thread at a time
+class SkSingleOwner {
+public:
+     SkSingleOwner() : fOwner(kIllegalThreadID), fReentranceCount(0) {}
+
+     struct AutoEnforce {
+         AutoEnforce(SkSingleOwner* so) : fSO(so) { fSO->enter(); }
+         ~AutoEnforce() { fSO->exit(); }
+
+         SkSingleOwner* 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//SkMutex_DEFINED