Question about client_connection_check_interval

Dear Hackers,

While reading source codes about timeouts and GUC and I found that
strange behavior about client_connection_check_interval.

Currently we did not an assign_hook about client_connection_check_interval,
that means a timeout will not turn on immediately if users change the GUC
from zero to arbitrary positive integer.
In my understanding the timeout will fire only when:

* before starting transaction
* after firing the CLIENT_CONNECTION_CHECK_TIMEOUT timeout

Hence I thought following inconvenient scenario:

1. set client_connection_check_interval = 0 in postgresql.conf
2. start a tx
3. SET LOCAL client_connection_check_interval to non-zero value
  in order to checking clients until the end of the tx
4. users expect to firing the timeout, but it does not work
  because enable_timeout_after() will never execute in the tx

Is this an expected behavior?  If so, I think this spec should be documented.
If not, I think an assign_hook is needed for resolving the problem.

How do you think?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED