Re: [HACKERS] parallel.c oblivion of worker-startup failures

On Thu, Dec 21, 2017 at 6:46 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> What if we don't allow to reuse such slots till the backend/session
> that has registered it performs unregister? Currently, we don't seem
> to have an API corresponding to Register*BackgroundWorker() which can
> be used to unregister, but maybe we can provide such an API.

Well, then we could have slots pinned down for a long time, if the
backend never gets around to calling unregister. Furthermore, that's
absolutely not back-patchable, because we can't put a requirement like
that on code running in the back branches. Also, what if the code
path that would have done the unregister eventually errors out? We'd
need TRY/CATCH blocks everywhere that registers the worker. In short,
this seems terrible for multiple reasons.

>> Furthermore, it doesn't help in the case where the worker starts and
>> immediately exits without attaching to the DSM.
>
> Yeah, but can't we detect that case? After the worker exits, we can
> know its exit status as is passed to CleanupBackgroundWorker, we can
> use that to mark the worker state as BGWH_ERROR_STOPPED (or something
> like BGWH_IMMEDIATE_STOPPED).
>
> I think above way sounds invasive, but it seems to me that it can be
> used by other users of background workers as well.

The exit status doesn't tell us whether the worker attached to the DSM.

I'm relatively puzzled as to why you're rejecting a relatively
low-impact way of handling a corner case that was missed in the
original design in favor of major architectural changes.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company