Re: A new function to wait for the backend exit after termination

Thanks for the comments.

On Wed, Oct 28, 2020 at 6:41 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>
> I prefer that false is returned when the timeout happens,
> like pg_promote() does.
>

Earlier it was suggested to error out on timeout. Since users can not
guess on time it takes to terminate or become idle, throwing error
seems to be odd on timeout. And also in case if the given pid is not a
backend pid, we are throwing a warning and returning false but not
error. Similarly we can return false on timeout, if required a
warning. Thoughts?

>
> >> IIUC, do we need a new option, something like pg_wait_backend(pid,
> >> timeout, waituntil) where "waituntil" if specified "idle" waits until
> >> the given backend goes to idle mode, or "termination" waits until
> >> termination?
>
> Isn't this wait-for-idle mode fragile? Because there is no guarantee
> that the backend is still in idle state when pg_wait_backend(idle) returns.
>

Yeah this can happen. By the time pg_wait_backend returns we could
have the idle state of the backend changed. Looks like this is also a
problem with the existing pgstat_get_backend_current_activity()
function. There we have a comment saying below and the function
returns a pointer to the current activity string. Maybe we could have
similar comments about the usage in the document?

 *    It is the caller's responsibility to invoke this only for backends whose
 *    state is expected to remain stable while the result is in use.

Does this problem exist even if we use  pg_stat_activity()?

>
> When the specified timeout is negative, the following error is thrown *after*
> SIGTERM is signaled to the target backend. This seems strange to me.
> The timeout value should be verified at the beginning of the function, instead.
>
>      ERROR:  timeout cannot be negative
>

Okay. I will change that.

>
> pg_terminate_backend(xxx, false) failed with the following error. I think
> it's more helpful if the function can work even without the timeout value.
> That is, what about redefining the function in src/backend/catalog/system_views.sql
> and specifying the DEFAULT values for the arguments "wait" and "timeout"?
> The similar function "pg_promote" would be good reference to you.
>
>      ERROR:  function pg_terminate_backend(integer, boolean) does not exist at character 8
>

Yeah. This seems good. I will have false as default value for the wait
parameter. I have defined the timeout to be in milliseconds, then how
about having a default value of 100 milliseconds?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com