ApplyToAllCacheEntries

Summary: Added a method that executes a callback on every cache entry.

Test Plan: added a unit test

Reviewers: haobo

Reviewed By: haobo

CC: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D18441

include/rocksdb/cache.h

@@ -116,6 +116,12 @@ class Cache {
     // default implementation is noop
   };
 
+  // Apply callback to all entries in the cache
+  // If thread_safe is true, it will also lock the accesses. Otherwise, it will
+  // access the cache without the lock held
+  virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t),
+                                      bool thread_safe) = 0;
+
  private:
   void LRU_Remove(Handle* e);
   void LRU_Append(Handle* e);