code: func (c *RegionCache) getRegionByIDFromCache(regionID uint64) *Region {
  	for v, r := range c.mu.regions {
  		if v.id == regionID {
  			return r.region
  		}
  	}
  	return nil
  }
docs: // getRegionByIDFromCache tries to get region by regionID from cache. Like
  // `getCachedRegion`, it should be called with c.mu.RLock(), and the returned
  // Region should not be used after c.mu is RUnlock().