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

On Wednesday, October 21, 2020, Bharath Rupireddy <
bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:

> Thanks for the feedback.
>
> On Wed, Oct 21, 2020 at 8:01 PM David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> >
> > On Wed, Oct 21, 2020 at 6:13 AM Magnus Hagander <magnus(at)hagander(dot)net>
> wrote:
> >>
> >> I think it would be nicer to have a pg_terminate_backend(pid,
> wait=false), so a function with a second parameter which defaults to the
> current behaviour of not waiting. And it might be a good idea to also give
> it a timeout parameter?
> >
> > Agreed on the overload, and the timeouts make sense too - with the
> caller deciding whether a timeout results in a failure or a false return
> value.
> >
>
> If the backend is terminated within the user specified timeout then
> the function returns true, otherwise false.

I’m suggesting an option for the second case to fail instead of returning
false.

> >
> >>> 2. pg_wait_backend() -- which waits for a given backend process. Note
> that this function has to be used carefully after pg_terminate_backend(),
> if used on a backend that's not ternmited it simply keeps waiting in a loop.
> >>
> >> It seems this one also very much would need a timeout value.
> >
> > Is there a requirement for waiting to be superuser only? You are not
> affecting any session but your own during the waiting period.
> >
>
> IIUC, in the same patch instead of returning an error in case of
> non-superusers, do we need to wait for user provided timeout
> milliseconds until the current user becomes superuser and then throw
> error if still non-superuser, and proceed further if superuser?
>
> Do we need to have a new function that waits until a current
> non-superuser in a session becomes superuser?
>
> Something else?

Not sure how that would even be possible mid-statement. I was suggesting
removing the superuser check altogether and letting any user execute “wait”.

> >
> > I could imagine, in theory at least, wanting to wait for a backend to go
> idle as well as for it disappearing. Scope creep in terms of this patch's
> goal but worth at least considering now.
> >
>
> 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?
>
> If my understanding is wrong, could you please explain more?
>

Yes, this describes what i was thinking.

David J.