skip generating empty sst

Summary:
When a compaction job output nothing, there is no necessary to generate a empty sst file which will cause `VersionEdit::EncodeTo` failed.
ref https://github.com/facebook/rocksdb/issues/2478
Closes https://github.com/facebook/rocksdb/pull/2503

Differential Revision: D5350799

Pulled By: ajkr

fbshipit-source-id: df0b4fcf3507fe1c3c435208b762e75478e00143

db/compaction_job.cc

@@ -1080,6 +1080,23 @@ Status CompactionJob::FinishCompactionOutputFile(
   }
   sub_compact->outfile.reset();
 
+  if (s.ok() && current_entries == 0) {
+    // If there is nothing to output, no necessary to generate a sst file.
+    // This happens when the output level is bottom level, at the same time
+    // the sub_compact output nothing.
+    std::string fname = TableFileName(
+        db_options_.db_paths, meta->fd.GetNumber(), meta->fd.GetPathId());
+    env_->DeleteFile(fname);
+
+    // Also need to remove the file from outputs, or it will be added to the
+    // VersionEdit.
+    assert(!sub_compact->outputs.empty());
+    sub_compact->outputs.pop_back();
+    sub_compact->builder.reset();
+    sub_compact->current_output_file_size = 0;
+    return s;
+  }
+
   ColumnFamilyData* cfd = sub_compact->compaction->column_family_data();
   TableProperties tp;
   if (s.ok() && current_entries > 0) {

db/compaction_job_test.cc

@@ -340,6 +340,24 @@ TEST_F(CompactionJobTest, SimpleDeletion) {
   RunCompaction({files}, expected_results);
 }
 
+TEST_F(CompactionJobTest, OutputNothing) {
+  NewDB();
+
+  auto file1 = mock::MakeMockFile({{KeyStr("a", 1U, kTypeValue), "val"}});
+
+  AddMockFile(file1);
+
+  auto file2 = mock::MakeMockFile({{KeyStr("a", 2U, kTypeDeletion), ""}});
+
+  AddMockFile(file2);
+
+  auto expected_results = mock::MakeMockFile();
+
+  SetLastSequence(4U);
+  auto files = cfd_->current()->storage_info()->LevelFiles(0);
+  RunCompaction({files}, expected_results);
+}
+
 TEST_F(CompactionJobTest, SimpleOverwrite) {
   NewDB();