Fix some defects reported by Coverity Scan (#6933)

Summary:
Confusing checks for null that are never null
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6933

Test Plan: make check

Reviewed By: cheng-chang

Differential Revision: D21885466

Pulled By: pdillinger

fbshipit-source-id: 4b48e03c2a33727f2702b0d12292f9fda5a3c475

db/memtable_list.cc

@@ -43,24 +43,22 @@ void MemTableListVersion::UnrefMemTable(autovector<MemTable*>* to_delete,
 }
 
 MemTableListVersion::MemTableListVersion(
-    size_t* parent_memtable_list_memory_usage, MemTableListVersion* old)
+    size_t* parent_memtable_list_memory_usage, const MemTableListVersion& old)
     : max_write_buffer_number_to_maintain_(
-          old->max_write_buffer_number_to_maintain_),
+          old.max_write_buffer_number_to_maintain_),
       max_write_buffer_size_to_maintain_(
-          old->max_write_buffer_size_to_maintain_),
+          old.max_write_buffer_size_to_maintain_),
       parent_memtable_list_memory_usage_(parent_memtable_list_memory_usage) {
-  if (old != nullptr) {
-    memlist_ = old->memlist_;
+  memlist_ = old.memlist_;
   for (auto& m : memlist_) {
     m->Ref();
   }
 
-    memlist_history_ = old->memlist_history_;
+  memlist_history_ = old.memlist_history_;
   for (auto& m : memlist_history_) {
     m->Ref();
   }
 }
-}
 
 MemTableListVersion::MemTableListVersion(
     size_t* parent_memtable_list_memory_usage,
@@ -604,7 +602,7 @@ void MemTableList::InstallNewVersion() {
   } else {
     // somebody else holds the current version, we need to create new one
     MemTableListVersion* version = current_;
-    current_ = new MemTableListVersion(&current_memory_usage_, current_);
+    current_ = new MemTableListVersion(&current_memory_usage_, *version);
     current_->Ref();
     version->Unref();
   }

db/memtable_list.h

@@ -44,7 +44,7 @@ struct FlushJobInfo;
 class MemTableListVersion {
  public:
   explicit MemTableListVersion(size_t* parent_memtable_list_memory_usage,
-                               MemTableListVersion* old = nullptr);
+                               const MemTableListVersion& old);
   explicit MemTableListVersion(size_t* parent_memtable_list_memory_usage,
                                int max_write_buffer_number_to_maintain,
                                int64_t max_write_buffer_size_to_maintain);

utilities/transactions/transaction_base.cc

@@ -312,8 +312,7 @@ std::vector<Status> TransactionBaseImpl::MultiGet(
 
   std::vector<Status> stat_list(num_keys);
   for (size_t i = 0; i < num_keys; ++i) {
-    std::string* value = values ? &(*values)[i] : nullptr;
-    stat_list[i] = Get(read_options, column_family[i], keys[i], value);
+    stat_list[i] = Get(read_options, column_family[i], keys[i], &(*values)[i]);
   }
 
   return stat_list;
@@ -350,8 +349,7 @@ std::vector<Status> TransactionBaseImpl::MultiGetForUpdate(
   // TODO(agiardullo): optimize multiget?
   std::vector<Status> stat_list(num_keys);
   for (size_t i = 0; i < num_keys; ++i) {
-    std::string* value = values ? &(*values)[i] : nullptr;
-    stat_list[i] = Get(read_options, column_family[i], keys[i], value);
+    stat_list[i] = Get(read_options, column_family[i], keys[i], &(*values)[i]);
   }
 
   return stat_list;

utilities/transactions/write_prepared_txn_db.cc

@@ -294,8 +294,7 @@ std::vector<Status> WritePreparedTxnDB::MultiGet(
 
   std::vector<Status> stat_list(num_keys);
   for (size_t i = 0; i < num_keys; ++i) {
-    std::string* value = values ? &(*values)[i] : nullptr;
-    stat_list[i] = this->Get(options, column_family[i], keys[i], value);
+    stat_list[i] = this->Get(options, column_family[i], keys[i], &(*values)[i]);
   }
   return stat_list;
 }