fix db_stress assertion failure

Summary: in rocksdb::DBIter::FindValueForCurrentKey(), last_not_merge_type could also be SingleDelete() which is omitted

Test Plan: db_iter_test

Reviewers: yhchiang, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D65187

db/db_iter.cc

@@ -658,7 +658,8 @@ bool DBIter::FindValueForCurrentKey() {
       return false;
     case kTypeMerge:
       current_entry_is_merged_ = true;
-      if (last_not_merge_type == kTypeDeletion) {
+      if (last_not_merge_type == kTypeDeletion ||
+          last_not_merge_type == kTypeSingleDeletion) {
         MergeHelper::TimedFullMerge(merge_operator_, saved_key_.GetKey(),
                                     nullptr, merge_context_.GetOperands(),
                                     &saved_value_, logger_, statistics_, env_,

db/db_iter_test.cc

@@ -1129,6 +1129,26 @@ TEST_F(DBIteratorTest, DBIterator5) {
     db_iter->Prev();
     ASSERT_TRUE(!db_iter->Valid());
   }
+
+  {
+    // put, singledelete, merge
+    TestIterator* internal_iter = new TestIterator(BytewiseComparator());
+    internal_iter->AddPut("a", "val_a");
+    internal_iter->AddSingleDeletion("a");
+    internal_iter->AddMerge("a", "merge_1");
+    internal_iter->AddMerge("a", "merge_2");
+    internal_iter->AddPut("b", "val_b");
+    internal_iter->Finish();
+    std::unique_ptr<Iterator> db_iter(NewDBIterator(
+        env_, ImmutableCFOptions(options), BytewiseComparator(), internal_iter,
+        10, options.max_sequential_skip_in_iterations, 0));
+    db_iter->Seek("b");
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "b");
+    db_iter->Prev();
+    ASSERT_TRUE(db_iter->Valid());
+    ASSERT_EQ(db_iter->key().ToString(), "a");
+  }
 }
 
 TEST_F(DBIteratorTest, DBIterator6) {