Don't do memtable lookup in db_impl_readonly if memtables are empty while opening db.

Summary: In DBImpl::Recover method, while loading memtables, also check if memtables are empty. Use this in DBImplReadonly to determine whether to lookup memtable or not.

Test Plan:
db_test
make check all

Reviewers: sdong, yhchiang, ljin, igor

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22281

Makefile

@@ -117,7 +117,6 @@ TESTS = \
 	thread_local_test \
 	geodb_test \
 	rate_limiter_test \
-	cuckoo_table_builder_test \
 	options_test \
 	cuckoo_table_builder_test \
 	cuckoo_table_reader_test \

db/db_impl_readonly.cc

@@ -16,7 +16,6 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <vector>
-#include <algorithm>
 #include "db/db_iter.h"
 #include "db/dbformat.h"
 #include "db/filename.h"

db/db_test.cc

@@ -1195,6 +1195,16 @@ TEST(DBTest, ReadOnlyDB) {
   }
   ASSERT_EQ(count, 2);
   delete iter;
+  Close();
+
+  // Reopen and flush memtable.
+  Reopen();
+  Flush();
+  Close();
+  // Now check keys in read only mode.
+  ASSERT_OK(ReadOnlyReopen(&options));
+  ASSERT_EQ("v3", Get("foo"));
+  ASSERT_EQ("v2", Get("bar"));
 }
 
 // Make sure that when options.block_cache is set, after a new table is

db/memtable.cc

@@ -417,6 +417,11 @@ static bool SaveValue(void* arg, const char* entry) {
 
 bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
                    MergeContext& merge_context, const Options& options) {
+  // The sequence number is updated synchronously in version_set.h
+  if (first_seqno_ == 0) {
+    // Avoiding recording stats for speed.
+    return false;
+  }
   PERF_TIMER_AUTO(get_from_memtable_time);
 
   Slice user_key = key.user_key();