tair源码分析——leveldb新增的CompactRangeSelfLevel过程

// compact filenumber 小于limit的key 在 [begin, end)范围内的 sstable
// compact的时候只有level 0 SSTable会输出到 level 1, 其他的level都是输入输出在同一个level
Status DBImpl::CompactRangeSelfLevel(
  uint64_t limit_filenumber,
  const Slice* begin,
  const Slice* end) {
  // 初始化一个MannualCompaction对象
  manual.limit_filenumber = limit_filenumber;
  manual.bg_compaction_func = &DBImpl::BackgroundCompactionSelfLevel;
  // use TimedCond() 防止丢失唤醒信号
  // 每个层级逐次schedule manualcompaction
  for (int level = 0; level < config::kNumLevels && manual.compaction_status.ok(); ++level) {
    ManualCompaction each_manual = manual;
    each_manual.level = level;
    while (each_manual.compaction_status.ok() && !each_manual.done) {
      // still have compaction running 就等待
      while (bg_compaction_scheduled_) {
        bg_cv_.TimedWait(timed_us);
      }
      manual_compaction_ = &each_manual;
      MaybeScheduleCompaction();
      while (manual_compaction_ == &each_manual) {
        bg_cv_.TimedWait(timed_us);
      }
    }
    manual.compaction_status = each_manual.compaction_status;
  }
  return manual.compaction_status;
}

void DBImpl::BackgroundCompactionSelfLevel() {
  do {
    // level-0 不对 filenumber 进行限制
   /* CompactRangeOneLevel故名思议将该level中的所有符合filenumber 小于limit
        key 在 [begin, end)范围内的 sstable找出来 */
    c = versions_-> CompactRangeOneLevel(m->level,  
            m->level > 0 ? m->limit_filenumber : ~(static_cast<uint64_t>(0)),
            m->begin, m->end);  

    if (NULL == c) { // no compact for this level
      m->done = true; // done all. 
      break;
    }
    // 记录下manual Compaction的结束key
    manual_end = c->input(0, c->num_input_files(0) - 1)->largest;

    CompactionState* compact = new CompactionState(c);
    status = DoCompactionWorkSelfLevel(compact);     // 真正进行Compaction的函数
    CleanupCompaction(compact);
    c->ReleaseInputs();
    DeleteObsoleteFiles();
    delete c;
    if (shutting_down_.Acquire_Load()) {
      // Ignore compaction errors found during shutting down
    } else if (!status.ok()) {
      m->compaction_status = status; // save error
      if (bg_error_.ok()) { // no matter paranoid_checks
        bg_error_ = status;
      }
      break; // exit once fail.
    }
  } while (false);
  if (!m->done) {
    // We only compacted part of the requested range. Update *m
    // to the range that is left to be compacted.
    m->tmp_storage = manual_end;
    m->begin = &m->tmp_storage;
  }
  // Mark it as done
  manual_compaction_ = NULL;
}

Slice key = input->key();
    // if (compact->compaction->ShouldStopBefore(key) &&
    //     compact->builder != NULL) {
    //   status = FinishCompactionOutputFile(compact, input);
    //   if (!status.ok()) {
    //     break;
    //   }
    // }
    // 注释掉的地方即是CompactionWorkSelfLevel减少的部分

} else if (ikey.sequence <= compact->smallest_snapshot &&
                 (ikey.type == kTypeDeletion || // deleted or ..
                  user_comparator()->ShouldDropMaybe(ikey.user_key.data(),
                                                     ikey.sequence, expired_end_time)) &&
                 // .. user-defined should drop(maybe),
                 // based on some condition(eg. this key only has this update.).
                 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;
      }

status = InstallCompactionResults(compact);
// 修改为：
status = InstallCompactionResults(compact, false); // output files is in current level, not level + 1

// 判断这个key是否在需要回收的bucket中，如果是就返回true，那么Compaction的时候直接删除（即回收掉）
  virtual bool ShouldDrop(const char* key, int64_t sequence, uint32_t now = 0) const { return false;}
  // 根据expire_time判断这个key是否已经过期，如过期则返回true
  virtual bool ShouldDropMaybe(const char* key, int64_t sequence, uint32_t now = 0) const { return false;}
  // start_key和key是否依旧属于同一个bucket，是的放回false，否则返回true
  virtual bool ShouldStopBefore(const Slice& start_key, const Slice& key) const { return false;}