Automatic notification for top transaction IDs

I came across this thread [1] to disallow canceling a transaction not
yet confirmed by a synchronous replica. I think my proposed patch
might help that case as well, hence adding all involved in that thread
to BCC, for one-time notification.

As mentioned in that thread, when sending a cancellation signal, the
client cannot be sure if the cancel signal was honored, and if the
transaction was cancelled successfully. In the attached patch, the
backend emits a NotificationResponse containing the current full
transaction id. It does so only if the relevant GUC is enabled, and
when the top-transaction is being assigned the ID.

This information can be useful to the client, when:
i) it wants to cancel a transaction _after_ issuing a COMMIT, and
ii) it wants to check the status of its transaction that it sent
COMMIT for, but never received a response (perhaps because the server
crashed).

Additionally, this information can be useful for middleware, like
Transaction Processing Monitors, which can now transparently (without
any change in application code) monitor the status of transactions (by
watching for the transaction status indicator in the ReadyForQuery
protocol message). They can use the transaction ID from the
NotificationResponse to open a watcher, and on seeing either an 'E' or
'I' payload in subsequent ReadyForQuery messages, close the watcher.
On server crash, or other adverse events, they can then use the
transaction IDs still being watched to check status of those
transactions, and take appropriate actions, e.g. retry any aborted
transactions.

We cannot use the elog() mechanism for this notification because it is
sensitive to the value of client_min_messages. Hence I used the NOTIFY
infrastructure for this message. I understand that this usage violates
some expectations as to how NOTIFY messages are supposed to behave
(see [2] below), but I think these are acceptable violations; open to
hearing if/why this might not be acceptable, and any possible
alternatives.

I'm not very familiar with the parallel workers infrastructure, so the
patch is missing any consideration for those.

Reviews welcome.

[1]: subject was: Re: Disallow cancellation of waiting for synchronous
replication
thread: https://www.postgresql.org/message-id/flat/C1F7905E-5DB2-497D-ABCC-E14D4DEE506C%40yandex-team.ru

[2]:
At present, NotificationResponse can only be sent outside a
transaction, and thus it will not occur in the middle of a
command-response series, though it might occur just before ReadyForQuery.
It is unwise to design frontend logic that assumes that, however.
Good practice is to be able to accept NotificationResponse at any
point in the protocol.

Best regards,
--
Gurjeet Singh http://gurjeet.singh.im/