try_to_checkout_new_connection

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 886
        def try_to_checkout_new_connection
          # first in synchronized section check if establishing new conns is allowed
          # and increment @now_connecting, to prevent overstepping this pool's @size
          # constraint
          do_checkout = synchronize do
            if @threads_blocking_new_connections.zero? && (@connections.size + @now_connecting) < @size
              @now_connecting += 1
            end
          end
          if do_checkout
            begin
              # if successfully incremented @now_connecting establish new connection
              # outside of synchronized section
              conn = checkout_new_connection
            ensure
              synchronize do
                if conn
                  adopt_connection(conn)
                  # returned conn needs to be already leased
                  conn.lease
                end
                @now_connecting -= 1
              end
            end
          end
        end