Re: Optional message to user when terminating/cancelling backend

+1.

This really helps PostgreSQL Azure service as well. When we are doing the upgrades to the service, instead of abruptly terminating the sessions we can provide this message.

Thanks,
Satya

From: pgsql-hackers-owner(at)postgresql(dot)org [mailto:pgsql-hackers-owner(at)postgresql(dot)org] On Behalf Of Pavel Stehule
Sent: Monday, June 19, 2017 11:41 AM
To: Daniel Gustafsson <daniel(at)yesql(dot)se>
Cc: PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: [HACKERS] Optional message to user when terminating/cancelling backend

2017-06-19 20:24 GMT+02:00 Daniel Gustafsson <daniel(at)yesql(dot)se<mailto:daniel(at)yesql(dot)se>>:
When terminating, or cancelling, a backend it’s currently not possible to let
the signalled session know *why* it was dropped. This has nagged me in the
past and now it happened to come up again, so I took a stab at this. The
attached patch implements the ability to pass an optional text message to the
signalled session which is included in the error message:

SELECT pg_terminate_backend(<pid> [, message]);
SELECT pg_cancel_backend(<pid> [, message]);

Right now the message is simply appended on the error message, not sure if
errdetail or errhint would be better? Calling:

select pg_terminate_backend(<pid>, 'server rebooting');

..leads to:

FATAL: terminating connection due to administrator command: "server rebooting"

Omitting the message invokes the command just like today.

The message is stored in a new shmem area which is checked when the session is
aborted. To keep things simple a small buffer is kept per backend for the
message. If deemed too costly, keeping a central buffer from which slabs are
allocated can be done (but seemed rather complicated for little gain compared
to the quite moderate memory spend.)

cheers ./daniel

+1

very good idea

Pavel

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