Initialize merge operator explicitly in PrepareOptionsForRestoredDB() (#12033)

Summary:
We are seeing the following stress test failure: `Failure in DB::Get in backup/restore with: Invalid argument: merge_operator is not properly initialized. Verification failed: Backup/restore failed: Invalid argument: merge_operator is not properly initialized.`. The reason is likely that `GetColumnFamilyOptionsFromString()` does not set merge operator if it's a customized merge operator. Fixing it by initializing merge operator explicitly.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12033

Test Plan:
this repro gives the error consistently before this PR
```
./db_stress --acquire_snapshot_one_in=10000 --adaptive_readahead=0 --allow_concurrent_memtable_write=1 --allow_data_in_errors=True --async_io=0 --atomic_flush=1 --auto_readahead_size=1 --avoid_flush_during_recovery=0 --avoid_unnecessary_blocking_io=1 --backup_max_size=1048576000000 --backup_one_in=50 --batch_protection_bytes_per_key=8 --block_protection_bytes_per_key=2 --block_size=16384 --bloom_before_level=2147483646 --bloom_bits=31.014388066505518 --bottommost_compression_type=lz4hc --bottommost_file_compaction_delay=0 --bytes_per_sync=0 --cache_index_and_filter_blocks=0 --cache_size=33554432 --cache_type=fixed_hyper_clock_cache --charge_compression_dictionary_building_buffer=1 --charge_file_metadata=1 --charge_filter_construction=0 --charge_table_reader=1 --checkpoint_one_in=1000000 --checksum_type=kxxHash --clear_column_family_one_in=0 --column_families=1 --compact_files_one_in=1000000 --compact_range_one_in=1000000 --compaction_pri=3 --compaction_readahead_size=0 --compaction_ttl=10 --compressed_secondary_cache_ratio=0.0 --compressed_secondary_cache_size=0 --compression_checksum=1 --compression_max_dict_buffer_bytes=4095 --compression_max_dict_bytes=16384 --compression_parallel_threads=1 --compression_type=none --compression_use_zstd_dict_trainer=1 --compression_zstd_max_train_bytes=0 --continuous_verification_interval=0 --data_block_index_type=0 --db=/dev/shm/rocksdb_test/rocksdb_crashtest_blackbox --db_write_buffer_size=0 --delpercent=4 --delrangepercent=1 --destroy_db_initially=1 --detect_filter_construct_corruption=0 --disable_wal=1 --enable_compaction_filter=0 --enable_pipelined_write=0 --enable_thread_tracking=0 --expected_values_dir=/dev/shm/rocksdb_test/rocksdb_crashtest_expected --fail_if_options_file_error=1 --fifo_allow_compaction=0 --file_checksum_impl=xxh64 --flush_one_in=1000000 --format_version=2 --get_current_wal_file_one_in=0 --get_live_files_one_in=1000000 --get_property_one_in=1000000 --get_sorted_wal_files_one_in=0 --index_block_restart_interval=10 --index_type=2 --ingest_external_file_one_in=0 --initial_auto_readahead_size=16384 --iterpercent=10 --key_len_percent_dist=1,30,69 --level_compaction_dynamic_level_bytes=1 --lock_wal_one_in=1000000 --long_running_snapshots=1 --manual_wal_flush_one_in=0 --mark_for_compaction_one_file_in=10 --max_auto_readahead_size=524288 --max_background_compactions=1 --max_bytes_for_level_base=67108864 --max_key=100 --max_key_len=3 --max_manifest_file_size=1073741824 --max_write_batch_group_size_bytes=1048576 --max_write_buffer_number=3 --max_write_buffer_size_to_maintain=8388608 --memtable_max_range_deletions=1000 --memtable_prefix_bloom_size_ratio=0 --memtable_protection_bytes_per_key=2 --memtable_whole_key_filtering=0 --memtablerep=skip_list --min_write_buffer_number_to_merge=2 --mmap_read=1 --mock_direct_io=False --nooverwritepercent=1 --num_file_reads_for_auto_readahead=0 --open_files=-1 --open_metadata_write_fault_one_in=0 --open_read_fault_one_in=0 --open_write_fault_one_in=16 --ops_per_thread=100000000 --optimize_filters_for_memory=1 --paranoid_file_checks=0 --partition_filters=0 --partition_pinning=0 --pause_background_one_in=1000000 --periodic_compaction_seconds=0 --prefix_size=-1 --prefixpercent=0 --prepopulate_block_cache=1 --preserve_internal_time_seconds=0 --progress_reports=0 --read_fault_one_in=0 --readahead_size=0 --readpercent=50 --recycle_log_file_num=0 --reopen=0 --secondary_cache_fault_one_in=0 --set_options_one_in=0 --snapshot_hold_ops=100000 --sst_file_manager_bytes_per_sec=0 --sst_file_manager_bytes_per_truncate=0 --stats_dump_period_sec=0 --subcompactions=1 --sync=0 --sync_fault_injection=1 --target_file_size_base=16777216 --target_file_size_multiplier=1 --test_batches_snapshots=0 --top_level_index_pinning=0 --unpartitioned_pinning=1 --use_direct_io_for_flush_and_compaction=0 --use_direct_reads=0 --use_full_merge_v1=0 --use_get_entity=0 --use_merge=1 --use_multi_get_entity=0 --use_multiget=1 --use_put_entity_one_in=10 --use_write_buffer_manager=0 --user_timestamp_size=0 --value_size_mult=32 --verification_only=0 --verify_checksum=1 --verify_checksum_one_in=1000000 --verify_db_one_in=100000 --verify_file_checksums_one_in=1000000 --verify_iterator_with_expected_state_one_in=5 --verify_sst_unique_id_in_manifest=1 --wal_bytes_per_sync=524288 --wal_compression=zstd --write_buffer_size=33554432 --write_dbid_to_manifest=0 --write_fault_one_in=0 --writepercent=35
```

Reviewed By: hx235

Differential Revision: D50825558

Pulled By: cbi42

fbshipit-source-id: 8468dc0444c112415a515af8291ef3abec8a42de

db_stress_tool/db_stress_test_base.cc

@@ -1894,6 +1894,18 @@ Status StressTest::TestBackupRestore(
   return s;
 }
 
+void InitializeMergeOperator(Options& options) {
+  if (FLAGS_use_full_merge_v1) {
+    options.merge_operator = MergeOperators::CreateDeprecatedPutOperator();
+  } else {
+    if (FLAGS_use_put_entity_one_in > 0) {
+      options.merge_operator = std::make_shared<DBStressWideMergeOperator>();
+    } else {
+      options.merge_operator = MergeOperators::CreatePutOperator();
+    }
+  }
+}
+
 Status StressTest::PrepareOptionsForRestoredDB(Options* options) {
   assert(options);
   // To avoid race with other threads' operations (e.g, SetOptions())
@@ -1933,6 +1945,8 @@ Status StressTest::PrepareOptionsForRestoredDB(Options* options) {
   options->listeners.clear();
   // Avoid dangling/shared file descriptors, for reliable destroy
   options->sst_file_manager = nullptr;
+  // GetColumnFamilyOptionsFromString does not create customized merge operator
+  InitializeMergeOperator(*options);
 
   return Status::OK();
 }
@@ -3418,15 +3432,8 @@ void InitializeOptionsFromFlags(
       options.memtable_factory.reset(new VectorRepFactory());
       break;
   }
-  if (FLAGS_use_full_merge_v1) {
-    options.merge_operator = MergeOperators::CreateDeprecatedPutOperator();
-  } else {
-    if (FLAGS_use_put_entity_one_in > 0) {
-      options.merge_operator = std::make_shared<DBStressWideMergeOperator>();
-    } else {
-      options.merge_operator = MergeOperators::CreatePutOperator();
-    }
-  }
+
+  InitializeMergeOperator(options);
 
   if (FLAGS_enable_compaction_filter) {
     options.compaction_filter_factory =