Method getSpace

bolt/common/caching/SsdFile.cpp:285–320

std::optional<std::pair<uint64_t, int32_t>> SsdFile::getSpace(
    const std::vector<CachePin>& pins,
    int32_t begin) {
  int32_t next = begin;
  std::lock_guard<std::shared_mutex> l(mutex_);
  for (;;) {
    if (writableRegions_.empty()) {
      if (!growOrEvictLocked()) {
        return std::nullopt;
      }
    }
    assert(!writableRegions_.empty());
    const auto region = writableRegions_[0];
    const auto offset = regionSizes_[region];
    auto available = kRegionSize - offset;
    int64_t toWrite = 0;
    for (; next < pins.size(); ++next) {
      auto* entry = pins[next].checkedEntry();
      if (entry->size() > available) {
        break;
      }
      available -= entry->size();
      toWrite += entry->size();
    }
    if (toWrite > 0) {
      // At least some pins got space from this region. If the region is full
      // the next call will get space from another region.
      regionSizes_[region] += toWrite;
      return std::make_pair<uint64_t, int32_t>(
          region * kRegionSize + offset, toWrite);
    }

    tracker_.regionFilled(region);
    writableRegions_.erase(writableRegions_.begin());
  }
}

bool SsdFile::growOrEvictLocked() {
  if (numRegions_ < maxRegions_) {
    const auto newSize = (numRegions_ + 1) * kRegionSize;
    const auto rc = ::ftruncate(fd_, newSize);