attempt_to_checkout_all_existing_connections

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 611
      def attempt_to_checkout_all_existing_connections(raise_on_acquisition_timeout = true)
        collected_conns = synchronize do
          # account for our own connections
          @connections.select {|conn| conn.owner == Thread.current}
        end

        newly_checked_out = []
        timeout_time      = Time.now + (@checkout_timeout * 2)

        @available.with_a_bias_for(Thread.current) do
          while true
            synchronize do
              return if collected_conns.size == @connections.size && @now_connecting == 0
              remaining_timeout = timeout_time - Time.now
              remaining_timeout = 0 if remaining_timeout < 0
              conn = checkout_for_exclusive_access(remaining_timeout)
              collected_conns   << conn
              newly_checked_out << conn
            end
          end
        end
      rescue ExclusiveConnectionTimeoutError
        # <tt>raise_on_acquisition_timeout == false</tt> means we are directed to ignore any
        # timeouts and are expected to just give up: we've obtained as many connections
        # as possible, note that in a case like that we don't return any of the
        # +newly_checked_out+ connections.

        if raise_on_acquisition_timeout
          release_newly_checked_out = true
          raise
        end
      rescue Exception # if something else went wrong
        # this can't be a "naked" rescue, because we have should return conns
        # even for non-StandardErrors
        release_newly_checked_out = true
        raise
      ensure
        if release_newly_checked_out && newly_checked_out
          # releasing only those conns that were checked out in this method, conns
          # checked outside this method (before it was called) are not for us to release
          newly_checked_out.each {|conn| checkin(conn)}
        end
      end