attempt_to_checkout_all_existing_connections

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 753
        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      = Concurrent.monotonic_time + (@checkout_timeout * 2)

          @available.with_a_bias_for(Thread.current) do
            loop do
              synchronize do
                return if collected_conns.size == @connections.size && @now_connecting == 0
                remaining_timeout = timeout_time - Concurrent.monotonic_time
                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