[CF] Flush all memtables on column family drop

Summary: When column family is dropped, we want to delete all WALs that refer to it. To do that, we need to make them obsolete by flushing all the memtables

Test Plan: column_family_test

Reviewers: dhruba, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16557

db/db_impl.cc

@@ -3155,17 +3155,23 @@ Status DBImpl::DropColumnFamily(ColumnFamilyHandle* column_family) {
   edit.DropColumnFamily();
   edit.SetColumnFamily(cfd->GetID());
 
-  MutexLock l(&mutex_);
   Status s;
+  {
+    MutexLock l(&mutex_);
     if (cfd->IsDropped()) {
       s = Status::InvalidArgument("Column family already dropped!\n");
     }
     if (s.ok()) {
       s = versions_->LogAndApply(cfd, &edit, &mutex_);
     }
+  }
 
   if (s.ok()) {
     Log(options_.info_log, "Dropped column family with id %u\n", cfd->GetID());
+    // Flush the memtables. This will make all WAL files referencing dropped
+    // column family to be obsolete. They will be deleted when user deletes
+    // column family handle
+    Write(WriteOptions(), nullptr);  // ignore error
   } else {
     Log(options_.info_log, "Dropping column family with id %u FAILED -- %s\n",
         cfd->GetID(), s.ToString().c_str());