not split file in compaciton on level 0

Summary: we should not split file on level 0 in compaction because it will fail the following verification of seqno order on level 0

Test Plan: check with filldeterministic in db_bench

Reviewers: yhchiang, andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D65193

db/compaction_job.cc

@@ -854,8 +854,9 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
     // during subcompactions (i.e. if output size, estimated by input size, is
     // going to be 1.2MB and max_output_file_size = 1MB, prefer to have 0.6MB
     // and 0.6MB instead of 1MB and 0.2MB)
-    if (sub_compact->current_output_file_size >=
-        sub_compact->compaction->max_output_file_size()) {
+    if (sub_compact->compaction->output_level() != 0 &&
+        sub_compact->current_output_file_size >=
+            sub_compact->compaction->max_output_file_size()) {
       const Slice* next_key = nullptr;
       if (c_iter->Valid()) {
         next_key = &c_iter->key();