add some debugging to SkNVRefCnt

include/core/SkRefCnt.h

 
 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkRefCnt_DEFINED
@@ skipping 235 matching lines @@
     SkAutoUnref(SkRefCnt* obj) : SkAutoTUnref<SkRefCnt>(obj) {}
 };
 #define SkAutoUnref(...) SK_REQUIRE_LOCAL_VAR(SkAutoUnref)
 
 // This is a variant of SkRefCnt that's Not Virtual, so weighs 4 bytes instead of 8 or 16.
 // There's only benefit to using this if the deriving class does not otherwise need a vtable.
 template <typename Derived>
 class SkNVRefCnt : SkNoncopyable {
 public:
     SkNVRefCnt() : fRefCnt(1) {}
+#ifdef SK_DEBUG
+    ~SkNVRefCnt() {

[mtklein, 2014/11/24 17:34:05]

Oh wait, I forgot, not lgtm.  Make this protected?

+        SkASSERTF(0 == fRefCnt, "NVRefCnt was %d", fRefCnt);
+    }
+#endif
 
     // Implementation is pretty much the same as SkRefCntBase. All required barriers are the same:
     //   - unique() needs acquire when it returns true, and no barrier if it returns false;
     //   - ref() doesn't need any barrier;
     //   - unref() needs a release barrier, and an acquire if it's going to call delete.
 
     bool unique() const { return 1 == sk_acquire_load(&fRefCnt); }
     void    ref() const { sk_atomic_inc(&fRefCnt); }
-    void  unref() const { if (1 == sk_atomic_dec(&fRefCnt)) { SkDELETE((const Derived*)this); } }
+    void  unref() const {
+        int32_t prevValue = sk_atomic_dec(&fRefCnt);
+        SkASSERT(prevValue >= 1);
+        if (1 == prevValue) {
+            SkDELETE((const Derived*)this);
+        }
+    }
     void  deref() const { this->unref(); }  // Chrome prefers to call deref().
     int32_t getRefCnt() const { return fRefCnt; } // Used by Chrome unit tests.
 
 private:
     mutable int32_t fRefCnt;
 };
 
 #endif