Fix a tsan warning due to reading flags in LRUHandle without holding a mutex (#8433)

Summary:
Tsan complains due to a perceived race condition in accessing LRUHandle flags. One thread calls ```LRUHandle::SetHit()``` from ```LRUCacheShard::Lookup()```, while another thread calls ```LRUHandle::IsPending()``` from ```LRUCacheShard::IsReady()```. The latter call is from ```MultiGet```. It doesn't actually have to call ```IsReady``` since a null value indicates the cache handle is not ready, so its sufficient to check for a null value.

Also modify ```IsReady``` to acquire the LRU shard mutex.

Tests:
1. make check
2. Run tsan_crash

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8433

Reviewed By: zhichao-cao

Differential Revision: D29278030

Pulled By: anand1976

fbshipit-source-id: 0c9fed56d12eda853e72dadebe75038361bd257f

cache/lru_cache.cc

@@ -596,15 +596,12 @@ void LRUCacheShard::Erase(const Slice& key, uint32_t hash) {
 
 bool LRUCacheShard::IsReady(Cache::Handle* handle) {
   LRUHandle* e = reinterpret_cast<LRUHandle*>(handle);
+  MutexLock l(&mutex_);
   bool ready = true;
   if (e->IsPending()) {
     assert(secondary_cache_);
     assert(e->sec_handle);
-    if (e->sec_handle->IsReady()) {
-      Promote(e);
-    } else {
-      ready = false;
-    }
+    ready = e->sec_handle->IsReady();
   }
   return ready;
 }

table/block_based/block_based_table_reader.cc

@@ -2564,24 +2564,21 @@ void BlockBasedTable::MultiGet(const ReadOptions& read_options,
           s = Status::OK();
         }
         if (s.ok() && !results.back().IsEmpty()) {
-          if (results.back().IsReady()) {
+          // Since we have a valid handle, check the value. If its nullptr,
+          // it means the cache is waiting for the final result and we're
+          // supposed to call WaitAll() to wait for the result.
+          if (results.back().GetValue() != nullptr) {
             // Found it in the cache. Add NULL handle to indicate there is
             // nothing to read from disk.
             if (results.back().GetCacheHandle()) {
               results.back().UpdateCachedValue();
-              // Its possible the cache lookup returned a non-null handle,
-              // but the lookup actually failed to produce a valid value
-              if (results.back().GetValue() == nullptr) {
-                block_handles.emplace_back(handle);
-                total_len += block_size(handle);
-              }
-            }
-            if (results.back().GetValue() != nullptr) {
-              block_handles.emplace_back(BlockHandle::NullBlockHandle());
             }
+            block_handles.emplace_back(BlockHandle::NullBlockHandle());
           } else {
-            // We have to wait for the asynchronous cache lookup to finish,
-            // and then we may have to read the block from disk anyway
+            // We have to wait for the cache lookup to finish in the
+            // background, and then we may have to read the block from disk
+            // anyway
+            assert(results.back().GetCacheHandle());
             wait_for_cache_results = true;
             block_handles.emplace_back(handle);
             cache_handles.emplace_back(results.back().GetCacheHandle());