[Performance Branch] Fix memory leak in HashLinkListRep.GetIterator()

Summary: Full list constructed for full iterator can be leaked. This was a bug introduced when I copy the full iterator codes from hash skip list to hash link list. This patch fixes it.

Test Plan: Run valgrind test against db_test and make sure the memory leak is fixed

Reviewers: kailiu, haobo

Reviewed By: kailiu

CC: igor, leveldb

Differential Revision: https://reviews.facebook.net/D15093

Author: siying

db/prefix_test.cc

@@ -108,7 +108,6 @@ class PrefixTest {
     options.min_write_buffer_number_to_merge =
       FLAGS_min_write_buffer_number_to_merge;
 
-    options.comparator = new TestKeyComparator();
     options.memtable_prefix_bloom_bits = FLAGS_memtable_prefix_bloom_bits;
     options.memtable_prefix_bloom_probes = FLAGS_memtable_prefix_bloom_probes;
 
@@ -142,7 +141,9 @@ class PrefixTest {
     return false;
   }
 
-  PrefixTest() : option_config_(kBegin) { }
+  PrefixTest() : option_config_(kBegin) {
+    options.comparator = new TestKeyComparator();
+  }
   ~PrefixTest() {
     delete options.comparator;
   }

util/hash_linklist_rep.cc

@@ -125,7 +125,7 @@ class HashLinkListRep : public MemTableRep {
   class FullListIterator : public MemTableRep::Iterator {
    public:
     explicit FullListIterator(FullList* list)
-      : iter_(list) {}
+      : iter_(list), full_list_(list) {}
 
     virtual ~FullListIterator() {
     }
@@ -177,6 +177,8 @@ class HashLinkListRep : public MemTableRep {
     }
    private:
     FullList::Iterator iter_;
+    // To destruct with the iterator.
+    std::unique_ptr<FullList> full_list_;
     std::string tmp_;       // For passing to EncodeKey
   };