Re: Optional message to user when terminating/cancelling backend

Hi,

On 2017-06-19 20:24:43 +0200, Daniel Gustafsson wrote:
> 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.

For extensions it'd also be useful if it'd be possible to overwrite the
error code. E.g. for citus there's a distributed deadlock detector,
running out of process because there's no way to interrupt lock waits
locally, and we've to do some ugly hacking to generate proper error
messages and code from another session.

- Andres