db_impl.cc

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "db/db_impl.h"

#include <algorithm>
#include <set>
#include <string>
#include <stdint.h>
#include <stdio.h>
#include <vector>
#include "db/builder.h"
#include "db/db_iter.h"
#include "db/dbformat.h"
#include "db/filename.h"
#include "db/log_reader.h"
#include "db/log_writer.h"
#include "db/memtable.h"
#include "db/table_cache.h"
#include "db/version_set.h"
#include "db/write_batch_internal.h"
#include "leveldb/db.h"
#include "leveldb/env.h"
#include "leveldb/status.h"
#include "leveldb/table.h"
#include "leveldb/table_builder.h"
#include "port/port.h"
#include "table/block.h"
#include "table/merger.h"
#include "table/two_level_iterator.h"
#include "util/coding.h"
#include "util/logging.h"
#include "util/mutexlock.h"

namespace leveldb {

struct DBImpl::CompactionState {
  Compaction* const compaction;

  // Sequence numbers < smallest_snapshot are not significant since we
  // will never have to service a snapshot below smallest_snapshot.
  // Therefore if we have seen a sequence number S <= smallest_snapshot,
  // we can drop all entries for the same key with sequence numbers < S.
  SequenceNumber smallest_snapshot;

  // Files produced by compaction
  struct Output {
    uint64_t number;
    uint64_t file_size;
    InternalKey smallest, largest;
  };
  std::vector<Output> outputs;

  // State kept for output being generated
  WritableFile* outfile;
  TableBuilder* builder;

  uint64_t total_bytes;

  Output* current_output() { return &outputs[outputs.size()-1]; }

  explicit CompactionState(Compaction* c)
      : compaction(c),
        outfile(NULL),
        builder(NULL),
        total_bytes(0) {
  }
};

namespace {
class NullWritableFile : public WritableFile {
 public:
  virtual Status Append(const Slice& data) { return Status::OK(); }
  virtual Status Close() { return Status::OK(); }
  virtual Status Flush() { return Status::OK(); }
  virtual Status Sync() { return Status::OK(); }
};
}

// Fix user-supplied options to be reasonable
template <class T,class V>
static void ClipToRange(T* ptr, V minvalue, V maxvalue) {
  if (static_cast<V>(*ptr) > maxvalue) *ptr = maxvalue;
  if (static_cast<V>(*ptr) < minvalue) *ptr = minvalue;
}
Options SanitizeOptions(const std::string& dbname,
                        const InternalKeyComparator* icmp,
                        const Options& src) {
  Options result = src;
  result.comparator = icmp;
  ClipToRange(&result.max_open_files,           20,     50000);
  ClipToRange(&result.write_buffer_size,        64<<10, 1<<30);
  ClipToRange(&result.block_size,               1<<10,  4<<20);
  if (result.info_log == NULL) {
    // Open a log file in the same directory as the db
    src.env->CreateDir(dbname);  // In case it does not exist
    src.env->RenameFile(InfoLogFileName(dbname), OldInfoLogFileName(dbname));
    Status s = src.env->NewWritableFile(InfoLogFileName(dbname),
                                        &result.info_log);
    if (!s.ok()) {
      // No place suitable for logging
      result.info_log = new NullWritableFile;
    }
  }
  if (result.block_cache == NULL) {
    result.block_cache = NewLRUCache(8 << 20);
  }
  return result;
}

DBImpl::DBImpl(const Options& options, const std::string& dbname)
    : env_(options.env),
      internal_comparator_(options.comparator),
      options_(SanitizeOptions(dbname, &internal_comparator_, options)),
      owns_info_log_(options_.info_log != options.info_log),
      owns_cache_(options_.block_cache != options.block_cache),
      dbname_(dbname),
      db_lock_(NULL),
      shutting_down_(NULL),
      bg_cv_(&mutex_),
      compacting_cv_(&mutex_),
      mem_(new MemTable(internal_comparator_)),
      imm_(NULL),
      logfile_(NULL),
      log_(NULL),
      bg_compaction_scheduled_(false),
      compacting_(false) {
  mem_->Ref();
  has_imm_.Release_Store(NULL);

  // Reserve ten files or so for other uses and give the rest to TableCache.
  const int table_cache_size = options.max_open_files - 10;
  table_cache_ = new TableCache(dbname_, &options_, table_cache_size);

  versions_ = new VersionSet(dbname_, &options_, table_cache_,
                             &internal_comparator_);
}

DBImpl::~DBImpl() {
  // Wait for background work to finish
  mutex_.Lock();
  shutting_down_.Release_Store(this);  // Any non-NULL value is ok
  if (bg_compaction_scheduled_) {
    while (bg_compaction_scheduled_) {
      bg_cv_.Wait();
    }
  }
  mutex_.Unlock();

  if (db_lock_ != NULL) {
    env_->UnlockFile(db_lock_);
  }

  delete versions_;
  if (mem_ != NULL) mem_->Unref();
  if (imm_ != NULL) imm_->Unref();
  delete log_;
  delete logfile_;
  delete table_cache_;

  if (owns_info_log_) {
    delete options_.info_log;
  }
  if (owns_cache_) {
    delete options_.block_cache;
  }
}

Status DBImpl::NewDB() {
  VersionEdit new_db;
  new_db.SetComparatorName(user_comparator()->Name());
  new_db.SetLogNumber(0);
  new_db.SetNextFile(2);
  new_db.SetLastSequence(0);

  const std::string manifest = DescriptorFileName(dbname_, 1);
  WritableFile* file;
  Status s = env_->NewWritableFile(manifest, &file);
  if (!s.ok()) {
    return s;
  }
  {
    log::Writer log(file);
    std::string record;
    new_db.EncodeTo(&record);
    s = log.AddRecord(record);
    if (s.ok()) {
      s = file->Close();
    }
  }
  delete file;
  if (s.ok()) {
    // Make "CURRENT" file that points to the new manifest file.
    s = SetCurrentFile(env_, dbname_, 1);
  } else {
    env_->DeleteFile(manifest);
  }
  return s;
}

void DBImpl::MaybeIgnoreError(Status* s) const {
  if (s->ok() || options_.paranoid_checks) {
    // No change needed
  } else {
    Log(env_, options_.info_log, "Ignoring error %s", s->ToString().c_str());
    *s = Status::OK();
  }
}

void DBImpl::DeleteObsoleteFiles() {
  // Make a set of all of the live files
  std::set<uint64_t> live = pending_outputs_;
  versions_->AddLiveFiles(&live);

  std::vector<std::string> filenames;
  env_->GetChildren(dbname_, &filenames); // Ignoring errors on purpose
  uint64_t number;
  FileType type;
  for (size_t i = 0; i < filenames.size(); i++) {
    if (ParseFileName(filenames[i], &number, &type)) {
      bool keep = true;
      switch (type) {
        case kLogFile:
          keep = ((number == versions_->LogNumber()) ||
                  (number == versions_->PrevLogNumber()));
          break;
        case kDescriptorFile:
          // Keep my manifest file, and any newer incarnations'
          // (in case there is a race that allows other incarnations)
          keep = (number >= versions_->ManifestFileNumber());
          break;
        case kTableFile:
          keep = (live.find(number) != live.end());
          break;
        case kTempFile:
          // Any temp files that are currently being written to must
          // be recorded in pending_outputs_, which is inserted into "live"
          keep = (live.find(number) != live.end());
          break;
        case kCurrentFile:
        case kDBLockFile:
        case kInfoLogFile:
          keep = true;
          break;
      }

      if (!keep) {
        if (type == kTableFile) {
          table_cache_->Evict(number);
        }
        Log(env_, options_.info_log, "Delete type=%d #%lld\n",
            int(type),
            static_cast<unsigned long long>(number));
        env_->DeleteFile(dbname_ + "/" + filenames[i]);
      }
    }
  }
}

Status DBImpl::Recover(VersionEdit* edit) {
  mutex_.AssertHeld();

  // Ignore error from CreateDir since the creation of the DB is
  // committed only when the descriptor is created, and this directory
  // may already exist from a previous failed creation attempt.
  env_->CreateDir(dbname_);
  assert(db_lock_ == NULL);
  Status s = env_->LockFile(LockFileName(dbname_), &db_lock_);
  if (!s.ok()) {
    return s;
  }

  if (!env_->FileExists(CurrentFileName(dbname_))) {
    if (options_.create_if_missing) {
      s = NewDB();
      if (!s.ok()) {
        return s;
      }
    } else {
      return Status::InvalidArgument(
          dbname_, "does not exist (create_if_missing is false)");
    }
  } else {
    if (options_.error_if_exists) {
      return Status::InvalidArgument(
          dbname_, "exists (error_if_exists is true)");
    }
  }

  s = versions_->Recover();
  if (s.ok()) {
    // Recover from the log files named in the descriptor
    SequenceNumber max_sequence(0);
    if (versions_->PrevLogNumber() != 0) { // log#==0 means no prev log
      s = RecoverLogFile(versions_->PrevLogNumber(), edit, &max_sequence);
    }
    if (s.ok() && versions_->LogNumber() != 0) { // log#==0 for initial state
      s = RecoverLogFile(versions_->LogNumber(), edit, &max_sequence);
    }
    if (s.ok()) {
      if (versions_->LastSequence() < max_sequence) {
        versions_->SetLastSequence(max_sequence);
      }
    }
  }

  return s;
}

Status DBImpl::RecoverLogFile(uint64_t log_number,
                              VersionEdit* edit,
                              SequenceNumber* max_sequence) {
  struct LogReporter : public log::Reader::Reporter {
    Env* env;
    WritableFile* info_log;
    const char* fname;
    Status* status;  // NULL if options_.paranoid_checks==false
    virtual void Corruption(size_t bytes, const Status& s) {
      Log(env, info_log, "%s%s: dropping %d bytes; %s",
          (this->status == NULL ? "(ignoring error) " : ""),
          fname, static_cast<int>(bytes), s.ToString().c_str());
      if (this->status != NULL && this->status->ok()) *this->status = s;
    }
  };

  mutex_.AssertHeld();

  // Open the log file
  std::string fname = LogFileName(dbname_, log_number);
  SequentialFile* file;
  Status status = env_->NewSequentialFile(fname, &file);
  if (!status.ok()) {
    MaybeIgnoreError(&status);
    return status;
  }

  // Create the log reader.
  LogReporter reporter;
  reporter.env = env_;
  reporter.info_log = options_.info_log;
  reporter.fname = fname.c_str();
  reporter.status = (options_.paranoid_checks ? &status : NULL);
  // We intentially make log::Reader do checksumming even if
  // paranoid_checks==false so that corruptions cause entire commits
  // to be skipped instead of propagating bad information (like overly
  // large sequence numbers).
  log::Reader reader(file, &reporter, true/*checksum*/,
                     0/*initial_offset*/);
  Log(env_, options_.info_log, "Recovering log #%llu",
      (unsigned long long) log_number);

  // Read all the records and add to a memtable
  std::string scratch;
  Slice record;
  WriteBatch batch;
  MemTable* mem = NULL;
  while (reader.ReadRecord(&record, &scratch) &&
         status.ok()) {
    if (record.size() < 12) {
      reporter.Corruption(
          record.size(), Status::Corruption("log record too small"));
      continue;
    }
    WriteBatchInternal::SetContents(&batch, record);

    if (mem == NULL) {
      mem = new MemTable(internal_comparator_);
      mem->Ref();
    }
    status = WriteBatchInternal::InsertInto(&batch, mem);
    MaybeIgnoreError(&status);
    if (!status.ok()) {
      break;
    }
    const SequenceNumber last_seq =
        WriteBatchInternal::Sequence(&batch) +
        WriteBatchInternal::Count(&batch) - 1;
    if (last_seq > *max_sequence) {
      *max_sequence = last_seq;
    }

    if (mem->ApproximateMemoryUsage() > options_.write_buffer_size) {
      status = WriteLevel0Table(mem, edit);
      if (!status.ok()) {
        // Reflect errors immediately so that conditions like full
        // file-systems cause the DB::Open() to fail.
        break;
      }
      mem->Unref();
      mem = NULL;
    }
  }

  if (status.ok() && mem != NULL) {
    status = WriteLevel0Table(mem, edit);
    // Reflect errors immediately so that conditions like full
    // file-systems cause the DB::Open() to fail.
  }

  if (mem != NULL) mem->Unref();
  delete file;
  return status;
}

Status DBImpl::WriteLevel0Table(MemTable* mem, VersionEdit* edit) {
  mutex_.AssertHeld();
  const uint64_t start_micros = env_->NowMicros();
  FileMetaData meta;
  meta.number = versions_->NewFileNumber();
  pending_outputs_.insert(meta.number);
  Iterator* iter = mem->NewIterator();
  Log(env_, options_.info_log, "Level-0 table #%llu: started",
      (unsigned long long) meta.number);

  Status s;
  {
    mutex_.Unlock();
    s = BuildTable(dbname_, env_, options_, table_cache_, iter, &meta, edit);
    mutex_.Lock();
  }

  Log(env_, options_.info_log, "Level-0 table #%llu: %lld bytes %s",
      (unsigned long long) meta.number,
      (unsigned long long) meta.file_size,
      s.ToString().c_str());
  delete iter;
  pending_outputs_.erase(meta.number);

  CompactionStats stats;
  stats.micros = env_->NowMicros() - start_micros;
  stats.bytes_written = meta.file_size;
  stats_[0].Add(stats);
  return s;
}

Status DBImpl::CompactMemTable() {
  mutex_.AssertHeld();
  assert(imm_ != NULL);
  assert(compacting_);

  // Save the contents of the memtable as a new Table
  VersionEdit edit;
  Status s = WriteLevel0Table(imm_, &edit);

  // Replace immutable memtable with the generated Table
  if (s.ok()) {
    edit.SetPrevLogNumber(0);
    s = versions_->LogAndApply(&edit);
  }

  if (s.ok()) {
    // Commit to the new state
    imm_->Unref();
    imm_ = NULL;
    has_imm_.Release_Store(NULL);
    DeleteObsoleteFiles();
  }

  compacting_cv_.SignalAll();  // Wake up waiter even if there was an error
  return s;
}

void DBImpl::TEST_CompactRange(
    int level,
    const std::string& begin,
    const std::string& end) {
  MutexLock l(&mutex_);
  while (compacting_) {
    compacting_cv_.Wait();
  }
  Compaction* c = versions_->CompactRange(
      level,
      InternalKey(begin, kMaxSequenceNumber, kValueTypeForSeek),
      InternalKey(end, 0, static_cast<ValueType>(0)));

  if (c != NULL) {
    CompactionState* compact = new CompactionState(c);
    DoCompactionWork(compact);  // Ignore error in test compaction
    CleanupCompaction(compact);
  }

  // Start any background compaction that may have been delayed by this thread
  MaybeScheduleCompaction();
}

Status DBImpl::TEST_CompactMemTable() {
  MutexLock l(&mutex_);
  Status s = MakeRoomForWrite(true /* force compaction */);
  if (s.ok()) {
    // Wait until the compaction completes
    while (imm_ != NULL && bg_error_.ok()) {
      compacting_cv_.Wait();
    }
    if (imm_ != NULL) {
      s = bg_error_;
    }
  }
  return s;
}

void DBImpl::MaybeScheduleCompaction() {
  mutex_.AssertHeld();
  if (bg_compaction_scheduled_) {
    // Already scheduled
  } else if (compacting_) {
    // Some other thread is running a compaction.  Do not conflict with it.
  } else if (shutting_down_.Acquire_Load()) {
    // DB is being deleted; no more background compactions
  } else if (imm_ == NULL && !versions_->NeedsCompaction()) {
    // No work to be done
  } else {
    bg_compaction_scheduled_ = true;
    env_->Schedule(&DBImpl::BGWork, this);
  }
}

void DBImpl::BGWork(void* db) {
  reinterpret_cast<DBImpl*>(db)->BackgroundCall();
}

void DBImpl::BackgroundCall() {
  MutexLock l(&mutex_);
  assert(bg_compaction_scheduled_);
  if (!shutting_down_.Acquire_Load() &&
      !compacting_) {
    BackgroundCompaction();
  }
  bg_compaction_scheduled_ = false;
  bg_cv_.SignalAll();

  // Previous compaction may have produced too many files in a level,
  // so reschedule another compaction if needed.
  MaybeScheduleCompaction();
}

void DBImpl::BackgroundCompaction() {
  mutex_.AssertHeld();
  assert(!compacting_);

  if (imm_ != NULL) {
    compacting_ = true;
    CompactMemTable();
    compacting_ = false;
    compacting_cv_.SignalAll();
    return;
  }

  Compaction* c = versions_->PickCompaction();
  if (c == NULL) {
    // Nothing to do
    return;
  }

  Status status;
  if (c->IsTrivialMove()) {
    // Move file to next level
    assert(c->num_input_files(0) == 1);
    FileMetaData* f = c->input(0, 0);
    c->edit()->DeleteFile(c->level(), f->number);
    c->edit()->AddFile(c->level() + 1, f->number, f->file_size,
                       f->smallest, f->largest);
    status = versions_->LogAndApply(c->edit());
    Log(env_, options_.info_log, "Moved #%lld to level-%d %lld bytes %s\n",
        static_cast<unsigned long long>(f->number),
        c->level() + 1,
        static_cast<unsigned long long>(f->file_size),
        status.ToString().c_str());
  } else {
    CompactionState* compact = new CompactionState(c);
    status = DoCompactionWork(compact);
    CleanupCompaction(compact);
  }
  delete c;

  if (status.ok()) {
    // Done
  } else if (shutting_down_.Acquire_Load()) {
    // Ignore compaction errors found during shutting down
  } else {
    Log(env_, options_.info_log,
        "Compaction error: %s", status.ToString().c_str());
    if (options_.paranoid_checks && bg_error_.ok()) {
      bg_error_ = status;
    }
  }
}

void DBImpl::CleanupCompaction(CompactionState* compact) {
  mutex_.AssertHeld();
  if (compact->builder != NULL) {
    // May happen if we get a shutdown call in the middle of compaction
    compact->builder->Abandon();
    delete compact->builder;
  } else {
    assert(compact->outfile == NULL);
  }
  delete compact->outfile;
  for (size_t i = 0; i < compact->outputs.size(); i++) {
    const CompactionState::Output& out = compact->outputs[i];
    pending_outputs_.erase(out.number);
  }
  delete compact;
}

Status DBImpl::OpenCompactionOutputFile(CompactionState* compact) {
  assert(compact != NULL);
  assert(compact->builder == NULL);
  uint64_t file_number;
  {
    mutex_.Lock();
    file_number = versions_->NewFileNumber();
    pending_outputs_.insert(file_number);
    CompactionState::Output out;
    out.number = file_number;
    out.smallest.Clear();
    out.largest.Clear();
    compact->outputs.push_back(out);
    mutex_.Unlock();
  }

  // Make the output file
  std::string fname = TableFileName(dbname_, file_number);
  Status s = env_->NewWritableFile(fname, &compact->outfile);
  if (s.ok()) {
    compact->builder = new TableBuilder(options_, compact->outfile);
  }
  return s;
}

Status DBImpl::FinishCompactionOutputFile(CompactionState* compact,
                                          Iterator* input) {
  assert(compact != NULL);
  assert(compact->outfile != NULL);
  assert(compact->builder != NULL);

  const uint64_t output_number = compact->current_output()->number;
  assert(output_number != 0);

  // Check for iterator errors
  Status s = input->status();
  const uint64_t current_entries = compact->builder->NumEntries();
  if (s.ok()) {
    s = compact->builder->Finish();
  } else {
    compact->builder->Abandon();
  }
  const uint64_t current_bytes = compact->builder->FileSize();
  compact->current_output()->file_size = current_bytes;
  compact->total_bytes += current_bytes;
  delete compact->builder;
  compact->builder = NULL;

  // Finish and check for file errors
  if (s.ok()) {
    s = compact->outfile->Sync();
  }
  if (s.ok()) {
    s = compact->outfile->Close();
  }
  delete compact->outfile;
  compact->outfile = NULL;

  if (s.ok() && current_entries > 0) {
    // Verify that the table is usable
    Iterator* iter = table_cache_->NewIterator(ReadOptions(),
                                               output_number,
                                               current_bytes);
    s = iter->status();
    delete iter;
    if (s.ok()) {
      Log(env_, options_.info_log,
          "Generated table #%llu: %lld keys, %lld bytes",
          (unsigned long long) output_number,
          (unsigned long long) current_entries,
          (unsigned long long) current_bytes);
    }
  }
  return s;
}


Status DBImpl::InstallCompactionResults(CompactionState* compact) {
  mutex_.AssertHeld();
  Log(env_, options_.info_log,  "Compacted %d@%d + %d@%d files => %lld bytes",
      compact->compaction->num_input_files(0),
      compact->compaction->level(),
      compact->compaction->num_input_files(1),
      compact->compaction->level() + 1,
      static_cast<long long>(compact->total_bytes));

  // Add compaction outputs
  compact->compaction->AddInputDeletions(compact->compaction->edit());
  const int level = compact->compaction->level();
  for (size_t i = 0; i < compact->outputs.size(); i++) {
    const CompactionState::Output& out = compact->outputs[i];
    compact->compaction->edit()->AddFile(
        level + 1,
        out.number, out.file_size, out.smallest, out.largest);
    pending_outputs_.erase(out.number);
  }
  compact->outputs.clear();

  Status s = versions_->LogAndApply(compact->compaction->edit());
  if (s.ok()) {
    compact->compaction->ReleaseInputs();
    DeleteObsoleteFiles();
  } else {
    // Discard any files we may have created during this failed compaction
    for (size_t i = 0; i < compact->outputs.size(); i++) {
      env_->DeleteFile(TableFileName(dbname_, compact->outputs[i].number));
    }
  }
  return s;
}

Status DBImpl::DoCompactionWork(CompactionState* compact) {
  const uint64_t start_micros = env_->NowMicros();
  int64_t imm_micros = 0;  // Micros spent doing imm_ compactions

  Log(env_, options_.info_log,  "Compacting %d@%d + %d@%d files",
      compact->compaction->num_input_files(0),
      compact->compaction->level(),
      compact->compaction->num_input_files(1),
      compact->compaction->level() + 1);

  assert(versions_->NumLevelFiles(compact->compaction->level()) > 0);
  assert(compact->builder == NULL);
  assert(compact->outfile == NULL);
  if (snapshots_.empty()) {
    compact->smallest_snapshot = versions_->LastSequence();
  } else {
    compact->smallest_snapshot = snapshots_.oldest()->number_;
  }

  // Release mutex while we're actually doing the compaction work
  compacting_ = true;
  mutex_.Unlock();

  Iterator* input = versions_->MakeInputIterator(compact->compaction);
  input->SeekToFirst();
  Status status;
  ParsedInternalKey ikey;
  std::string current_user_key;
  bool has_current_user_key = false;
  SequenceNumber last_sequence_for_key = kMaxSequenceNumber;
  for (; input->Valid() && !shutting_down_.Acquire_Load(); ) {
    // Prioritize immutable compaction work
    if (has_imm_.NoBarrier_Load() != NULL) {
      const uint64_t imm_start = env_->NowMicros();
      mutex_.Lock();
      if (imm_ != NULL) {
        CompactMemTable();
        compacting_cv_.SignalAll();  // Wakeup MakeRoomForWrite() if necessary
      }
      mutex_.Unlock();
      imm_micros += (env_->NowMicros() - imm_start);
    }

    Slice key = input->key();
    if (compact->compaction->ShouldStopBefore(key) &&
        compact->builder != NULL) {
      status = FinishCompactionOutputFile(compact, input);
      if (!status.ok()) {
        break;
      }
    }

    // Handle key/value, add to state, etc.
    bool drop = false;
    if (!ParseInternalKey(key, &ikey)) {
      // Do not hide error keys
      current_user_key.clear();
      has_current_user_key = false;
      last_sequence_for_key = kMaxSequenceNumber;
    } else {
      if (!has_current_user_key ||
          user_comparator()->Compare(ikey.user_key,
                                     Slice(current_user_key)) != 0) {
        // First occurrence of this user key
        current_user_key.assign(ikey.user_key.data(), ikey.user_key.size());
        has_current_user_key = true;
        last_sequence_for_key = kMaxSequenceNumber;
      }

      if (last_sequence_for_key <= compact->smallest_snapshot) {
        // Hidden by an newer entry for same user key
        drop = true;    // (A)
      } else if (ikey.type == kTypeDeletion &&
                 ikey.sequence <= compact->smallest_snapshot &&
                 compact->compaction->IsBaseLevelForKey(ikey.user_key)) {
        // For this user key:
        // (1) there is no data in higher levels
        // (2) data in lower levels will have larger sequence numbers
        // (3) data in layers that are being compacted here and have
        //     smaller sequence numbers will be dropped in the next
        //     few iterations of this loop (by rule (A) above).
        // Therefore this deletion marker is obsolete and can be dropped.
        drop = true;
      }

      last_sequence_for_key = ikey.sequence;
    }
#if 0
    Log(env_, options_.info_log,
        "  Compact: %s, seq %d, type: %d %d, drop: %d, is_base: %d, "
        "%d smallest_snapshot: %d",
        ikey.user_key.ToString().c_str(),
        (int)ikey.sequence, ikey.type, kTypeValue, drop,
        compact->compaction->IsBaseLevelForKey(ikey.user_key),
        (int)last_sequence_for_key, (int)compact->smallest_snapshot);
#endif

    if (!drop) {
      // Open output file if necessary
      if (compact->builder == NULL) {
        status = OpenCompactionOutputFile(compact);
        if (!status.ok()) {
          break;
        }
      }
      if (compact->builder->NumEntries() == 0) {
        compact->current_output()->smallest.DecodeFrom(key);
      }
      compact->current_output()->largest.DecodeFrom(key);
      compact->builder->Add(key, input->value());

      // Close output file if it is big enough
      if (compact->builder->FileSize() >=
          compact->compaction->MaxOutputFileSize()) {
        status = FinishCompactionOutputFile(compact, input);
        if (!status.ok()) {
          break;
        }
      }
    }

    input->Next();
  }

  if (status.ok() && shutting_down_.Acquire_Load()) {
    status = Status::IOError("Deleting DB during compaction");
  }
  if (status.ok() && compact->builder != NULL) {
    status = FinishCompactionOutputFile(compact, input);
  }
  if (status.ok()) {
    status = input->status();
  }
  delete input;
  input = NULL;

  CompactionStats stats;
  stats.micros = env_->NowMicros() - start_micros - imm_micros;
  for (int which = 0; which < 2; which++) {
    for (int i = 0; i < compact->compaction->num_input_files(which); i++) {
      stats.bytes_read += compact->compaction->input(which, i)->file_size;
    }
  }
  for (size_t i = 0; i < compact->outputs.size(); i++) {
    stats.bytes_written += compact->outputs[i].file_size;
  }

  mutex_.Lock();
  stats_[compact->compaction->level() + 1].Add(stats);

  if (status.ok()) {
    status = InstallCompactionResults(compact);
  }
  compacting_ = false;
  compacting_cv_.SignalAll();
  VersionSet::LevelSummaryStorage tmp;
  Log(env_, options_.info_log,
      "compacted to: %s", versions_->LevelSummary(&tmp));
  return status;
}

Iterator* DBImpl::NewInternalIterator(const ReadOptions& options,
                                      SequenceNumber* latest_snapshot) {
  mutex_.Lock();
  *latest_snapshot = versions_->LastSequence();

  // Collect together all needed child iterators
  std::vector<Iterator*> list;
  list.push_back(mem_->NewIterator());
  if (imm_ != NULL) {
    list.push_back(imm_->NewIterator());
  }
  versions_->current()->AddIterators(options, &list);
  Iterator* internal_iter =
      NewMergingIterator(&internal_comparator_, &list[0], list.size());
  versions_->current()->Ref();
  internal_iter->RegisterCleanup(&DBImpl::Unref, this, versions_->current());

  mutex_.Unlock();
  return internal_iter;
}

Iterator* DBImpl::TEST_NewInternalIterator() {
  SequenceNumber ignored;
  return NewInternalIterator(ReadOptions(), &ignored);
}

int64_t DBImpl::TEST_MaxNextLevelOverlappingBytes() {
  MutexLock l(&mutex_);
  return versions_->MaxNextLevelOverlappingBytes();
}

Status DBImpl::Get(const ReadOptions& options,
                   const Slice& key,
                   std::string* value) {
  // TODO(opt): faster implementation
  Iterator* iter = NewIterator(options);
  iter->Seek(key);
  bool found = false;
  if (iter->Valid() && user_comparator()->Compare(key, iter->key()) == 0) {
    Slice v = iter->value();
    value->assign(v.data(), v.size());
    found = true;
  }
  // Non-OK iterator status trumps everything else
  Status result = iter->status();
  if (result.ok() && !found) {
    result = Status::NotFound(Slice());  // Use an empty error message for speed
  }
  delete iter;
  return result;
}

Iterator* DBImpl::NewIterator(const ReadOptions& options) {
  SequenceNumber latest_snapshot;
  Iterator* internal_iter = NewInternalIterator(options, &latest_snapshot);
  return NewDBIterator(
      &dbname_, env_, user_comparator(), internal_iter,
      (options.snapshot != NULL
       ? reinterpret_cast<const SnapshotImpl*>(options.snapshot)->number_
       : latest_snapshot));
}

void DBImpl::Unref(void* arg1, void* arg2) {
  DBImpl* impl = reinterpret_cast<DBImpl*>(arg1);
  Version* v = reinterpret_cast<Version*>(arg2);
  MutexLock l(&impl->mutex_);
  v->Unref();
}

const Snapshot* DBImpl::GetSnapshot() {
  MutexLock l(&mutex_);
  return snapshots_.New(versions_->LastSequence());
}

void DBImpl::ReleaseSnapshot(const Snapshot* s) {
  MutexLock l(&mutex_);
  snapshots_.Delete(reinterpret_cast<const SnapshotImpl*>(s));
}

// Convenience methods
Status DBImpl::Put(const WriteOptions& o, const Slice& key, const Slice& val) {
  return DB::Put(o, key, val);
}

Status DBImpl::Delete(const WriteOptions& options, const Slice& key) {
  return DB::Delete(options, key);
}

Status DBImpl::Write(const WriteOptions& options, WriteBatch* updates) {
  Status status;
  MutexLock l(&mutex_);
  status = MakeRoomForWrite(false);  // May temporarily release lock and wait
  uint64_t last_sequence = versions_->LastSequence();
  if (status.ok()) {
    WriteBatchInternal::SetSequence(updates, last_sequence + 1);
    last_sequence += WriteBatchInternal::Count(updates);
    versions_->SetLastSequence(last_sequence);

    // Add to log and apply to memtable
    status = log_->AddRecord(WriteBatchInternal::Contents(updates));
    if (status.ok() && options.sync) {
      status = logfile_->Sync();
    }
    if (status.ok()) {
      status = WriteBatchInternal::InsertInto(updates, mem_);
    }
  }
  if (options.post_write_snapshot != NULL) {
    *options.post_write_snapshot =
        status.ok() ? snapshots_.New(last_sequence) : NULL;
  }
  return status;
}

Status DBImpl::MakeRoomForWrite(bool force) {
  mutex_.AssertHeld();
  bool allow_delay = !force;
  Status s;
  while (true) {
    if (!bg_error_.ok()) {
      // Yield previous error
      s = bg_error_;
      break;
    } else if (
        allow_delay &&
        versions_->NumLevelFiles(0) >= config::kL0_SlowdownWritesTrigger) {
      // We are getting close to hitting a hard limit on the number of
      // L0 files.  Rather than delaying a single write by several
      // seconds when we hit the hard limit, start delaying each
      // individual write by 1ms to reduce latency variance.  Also,
      // this delay hands over some CPU to the compaction thread in
      // case it is sharing the same core as the writer.
      mutex_.Unlock();
      env_->SleepForMicroseconds(1000);
      allow_delay = false;  // Do not delay a single write more than once
      mutex_.Lock();
    } else if (!force &&
               (mem_->ApproximateMemoryUsage() <= options_.write_buffer_size)) {
      // There is room in current memtable
      break;
    } else if (imm_ != NULL) {
      // We have filled up the current memtable, but the previous
      // one is still being compacted, so we wait.
      compacting_cv_.Wait();
    } else if (versions_->NumLevelFiles(0) >= config::kL0_StopWritesTrigger) {
      // There are too many level-0 files.
      compacting_cv_.Wait();
    } else {
      // Attempt to switch to a new memtable and trigger compaction of old
      assert(versions_->PrevLogNumber() == 0);
      uint64_t new_log_number = versions_->NewFileNumber();
      WritableFile* lfile = NULL;
      s = env_->NewWritableFile(LogFileName(dbname_, new_log_number), &lfile);
      if (!s.ok()) {
        break;
      }
      VersionEdit edit;
      edit.SetPrevLogNumber(versions_->LogNumber());
      edit.SetLogNumber(new_log_number);
      s = versions_->LogAndApply(&edit);
      if (!s.ok()) {
        delete lfile;
        env_->DeleteFile(LogFileName(dbname_, new_log_number));
        break;
      }
      delete log_;
      delete logfile_;
      logfile_ = lfile;
      log_ = new log::Writer(lfile);
      imm_ = mem_;
      has_imm_.Release_Store(imm_);
      mem_ = new MemTable(internal_comparator_);
      mem_->Ref();
      force = false;   // Do not force another compaction if have room
      MaybeScheduleCompaction();
    }
  }
  return s;
}

bool DBImpl::GetProperty(const Slice& property, std::string* value) {
  value->clear();

  MutexLock l(&mutex_);
  Slice in = property;
  Slice prefix("leveldb.");
  if (!in.starts_with(prefix)) return false;
  in.remove_prefix(prefix.size());

  if (in.starts_with("num-files-at-level")) {
    in.remove_prefix(strlen("num-files-at-level"));
    uint64_t level;
    bool ok = ConsumeDecimalNumber(&in, &level) && in.empty();
    if (!ok || level < 0 || level >= config::kNumLevels) {
      return false;
    } else {
      char buf[100];
      snprintf(buf, sizeof(buf), "%d",
               versions_->NumLevelFiles(static_cast<int>(level)));
      *value = buf;
      return true;
    }
  } else if (in == "stats") {
    char buf[200];
    snprintf(buf, sizeof(buf),
             "                               Compactions\n"
             "Level  Files Size(MB) Time(sec) Read(MB) Write(MB)\n"
             "--------------------------------------------------\n"
             );
    value->append(buf);
    for (int level = 0; level < config::kNumLevels; level++) {
      int files = versions_->NumLevelFiles(level);
      if (stats_[level].micros > 0 || files > 0) {
        snprintf(
            buf, sizeof(buf),
            "%3d %8d %8.0f %9.0f %8.0f %9.0f\n",
            level,
            files,
            versions_->NumLevelBytes(level) / 1048576.0,
            stats_[level].micros / 1e6,
            stats_[level].bytes_read / 1048576.0,
            stats_[level].bytes_written / 1048576.0);
        value->append(buf);
      }
    }
    return true;
  }

  return false;
}

void DBImpl::GetApproximateSizes(
    const Range* range, int n,
    uint64_t* sizes) {
  // TODO(opt): better implementation
  Version* v;
  {
    MutexLock l(&mutex_);
    versions_->current()->Ref();
    v = versions_->current();
  }

  for (int i = 0; i < n; i++) {
    // Convert user_key into a corresponding internal key.
    InternalKey k1(range[i].start, kMaxSequenceNumber, kValueTypeForSeek);
    InternalKey k2(range[i].limit, kMaxSequenceNumber, kValueTypeForSeek);
    uint64_t start = versions_->ApproximateOffsetOf(v, k1);
    uint64_t limit = versions_->ApproximateOffsetOf(v, k2);
    sizes[i] = (limit >= start ? limit - start : 0);
  }

  {
    MutexLock l(&mutex_);
    v->Unref();
  }
}

// Default implementations of convenience methods that subclasses of DB
// can call if they wish
Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {
  WriteBatch batch;
  batch.Put(key, value);
  return Write(opt, &batch);
}

Status DB::Delete(const WriteOptions& opt, const Slice& key) {
  WriteBatch batch;
  batch.Delete(key);
  return Write(opt, &batch);
}

DB::~DB() { }

Status DB::Open(const Options& options, const std::string& dbname,
                DB** dbptr) {
  *dbptr = NULL;

  DBImpl* impl = new DBImpl(options, dbname);
  impl->mutex_.Lock();
  VersionEdit edit;
  Status s = impl->Recover(&edit); // Handles create_if_missing, error_if_exists
  if (s.ok()) {
    uint64_t new_log_number = impl->versions_->NewFileNumber();
    WritableFile* lfile;
    s = options.env->NewWritableFile(LogFileName(dbname, new_log_number),
                                     &lfile);
    if (s.ok()) {
      edit.SetLogNumber(new_log_number);
      impl->logfile_ = lfile;
      impl->log_ = new log::Writer(lfile);
      s = impl->versions_->LogAndApply(&edit);
    }
    if (s.ok()) {
      impl->DeleteObsoleteFiles();
      impl->MaybeScheduleCompaction();
    }
  }
  impl->mutex_.Unlock();
  if (s.ok()) {
    *dbptr = impl;
  } else {
    delete impl;
  }
  return s;
}

Snapshot::~Snapshot() {
}

Status DestroyDB(const std::string& dbname, const Options& options) {
  Env* env = options.env;
  std::vector<std::string> filenames;
  // Ignore error in case directory does not exist
  env->GetChildren(dbname, &filenames);
  if (filenames.empty()) {
    return Status::OK();
  }

  FileLock* lock;
  Status result = env->LockFile(LockFileName(dbname), &lock);
  if (result.ok()) {
    uint64_t number;
    FileType type;
    for (size_t i = 0; i < filenames.size(); i++) {
      if (ParseFileName(filenames[i], &number, &type)) {
        Status del = env->DeleteFile(dbname + "/" + filenames[i]);
        if (result.ok() && !del.ok()) {
          result = del;
        }
      }
    }
    env->UnlockFile(lock);  // Ignore error since state is already gone
    env->DeleteFile(LockFileName(dbname));
    env->DeleteDir(dbname);  // Ignore error in case dir contains other files
  }
  return result;
}

}