Add scope guard

Summary: Small change: replace mutex_.Lock/mutex_.Unlock() with scope guard

Test Plan: make all check

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21609

db/db_impl.cc

@@ -1865,9 +1865,11 @@ Status DBImpl::FlushMemTable(ColumnFamilyData* cfd,
   w.done = false;
   w.timeout_hint_us = kNoTimeOut;
 
+  Status s;
+  {
     WriteContext context;
-  mutex_.Lock();
-  Status s = BeginWrite(&w, 0);
+    MutexLock guard_lock(&mutex_);
+    s = BeginWrite(&w, 0);
     assert(s.ok() && !w.done);  // No timeout and nobody should do our job
 
     // SetNewMemtableAndNewLogFile() will release and reacquire mutex
@@ -1879,7 +1881,7 @@ Status DBImpl::FlushMemTable(ColumnFamilyData* cfd,
     assert(!writers_.empty());
     assert(writers_.front() == &w);
     EndWrite(&w, &w, s);
-  mutex_.Unlock();
+  }
 
 
   if (s.ok() && options.wait) {