Make tailing iterator show new entries in memtable.

Summary:
Reseek mutable_iter if it is invalid in Next and immutable_iter
is invalid.

Test Plan: DBTestTailingIterator.TailingIteratorSeekToNext

Reviewers: tnovak, march, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D44865

db/db_tailing_iter_test.cc

@@ -68,6 +68,7 @@ TEST_F(DBTestTailingIterator, TailingIteratorSeekToNext) {
   read_options.tailing = true;
 
   std::unique_ptr<Iterator> iter(db_->NewIterator(read_options, handles_[1]));
+  std::unique_ptr<Iterator> itern(db_->NewIterator(read_options, handles_[1]));
   std::string value(1024, 'a');
 
   const int num_records = 1000;
@@ -88,6 +89,13 @@ TEST_F(DBTestTailingIterator, TailingIteratorSeekToNext) {
     iter->Seek(target);
     ASSERT_TRUE(iter->Valid());
     ASSERT_EQ(iter->key().compare(key), 0);
+    if (i == 1) {
+      itern->SeekToFirst();
+    } else {
+      itern->Next();
+    }
+    ASSERT_TRUE(itern->Valid());
+    ASSERT_EQ(itern->key().compare(key), 0);
   }
   for (int i = 2 * num_records; i > 0; --i) {
     char buf1[32];

db/forward_iterator.cc

@@ -341,6 +341,7 @@ void ForwardIterator::SeekInternal(const Slice& internal_key,
 
 void ForwardIterator::Next() {
   assert(valid_);
+  bool update_prev_key = false;
 
   if (sv_ == nullptr ||
       sv_->version_number != cfd_->GetSuperVersionNumber()) {
@@ -355,14 +356,16 @@ void ForwardIterator::Next() {
   } else if (current_ != mutable_iter_) {
     // It is going to advance immutable iterator
 
-    bool update_prev_key = true;
     if (is_prev_set_ && prefix_extractor_) {
       // advance prev_key_ to current_ only if they share the same prefix
       update_prev_key =
         prefix_extractor_->Transform(prev_key_.GetKey()).compare(
           prefix_extractor_->Transform(current_->key())) == 0;
+    } else {
+      update_prev_key = true;
     }
 
+
     if (update_prev_key) {
       prev_key_.SetKey(current_->key());
       is_prev_set_ = true;
@@ -376,6 +379,8 @@ void ForwardIterator::Next() {
       immutable_status_ = current_->status();
     } else if (current_->Valid()) {
       immutable_min_heap_.push(current_);
+    } else if ((!mutable_iter_->Valid()) && update_prev_key) {
+        mutable_iter_->Seek(prev_key_.GetKey());
     }
   }