Delayed write: needs_flush_speedup doesn't actually work

[isaac-io]

needs_flush_speedup is considered in a branch of the code that's effectively dead code.

DBImpl::MaybeScheduleFlushOrCompaction() {
  ...
  bool is_flush_pool_empty =
        env_->GetBackgroundThreads(Env::Priority::HIGH) == 0;
    if (!is_flush_pool_empty) {
      ...
    } else {
  //    // this code is never reached  //     //
      while (unscheduled_flushes_ > 0 &&
            (bg_flush_scheduled_ + bg_compaction_scheduled_ <
                  bg_job_limits.max_flushes ||
              (needs_flush_speedup_ &&
              bg_flush_scheduled_ <= bg_job_limits.max_flushes))) {
        bg_flush_scheduled_++;
        FlushThreadArg* fta = new FlushThreadArg;
        fta->db_ = this;
        fta->thread_pri_ = Env::Priority::LOW;
        env_->Schedule(&DBImpl::BGWorkFlush, fta, Env::Priority::LOW, this,
                      &DBImpl::UnscheduleFlushCallback);
        --unscheduled_flushes_;
    }
}

The reason is that env_->GetBackgroundThreads() returns the pool capacity, not the amount of available slots, so the flush pool will never actually be empty (the intention in RocksDB here was to support a case where flushes are supposed to have the same priority as compactions and thus max_background_flushes is set to 0, but our use of needs_flush_speedup needs to know if there are high priority jobs available, regardless of capacity allocation).

Expected behavior

Available capacity is considered for scheduling based on flush urgency.

Actual behavior

Allocated capacity is considered.

Steps to reproduce the behavior

N/A