Fix LRUCache missing null check on destruct

Summary:
Fix LRUCache missing null check on destruct. The check is needed if LRUCache::DisownData is called.
Closes https://github.com/facebook/rocksdb/pull/3920

Differential Revision: D8191631

Pulled By: yiwu-arbug

fbshipit-source-id: d5014f6e49b51692c18a25fb55ece935f5a023c4

cache/lru_cache.cc

@@ -474,11 +474,14 @@ LRUCache::LRUCache(size_t capacity, int num_shard_bits,
 }
 
 LRUCache::~LRUCache() {
+  if (shards_ != nullptr) {
+    assert(num_shards_ > 0);
     for (int i = 0; i < num_shards_; i++) {
       shards_[i].~LRUCacheShard();
     }
     port::cacheline_aligned_free(shards_);
   }
+}
 
 CacheShard* LRUCache::GetShard(int shard) {
   return reinterpret_cast<CacheShard*>(&shards_[shard]);
@@ -504,6 +507,7 @@ void LRUCache::DisownData() {
 // Do not drop data if compile with ASAN to suppress leak warning.
 #ifndef __SANITIZE_ADDRESS__
   shards_ = nullptr;
+  num_shards_ = 0;
 #endif  // !__SANITIZE_ADDRESS__
 }
 

cache/lru_cache.h

@@ -295,7 +295,7 @@ class LRUCache : public ShardedCache {
   double GetHighPriPoolRatio();
 
  private:
-  LRUCacheShard* shards_;
+  LRUCacheShard* shards_ = nullptr;
   int num_shards_ = 0;
 };