Re: [PROPOSAL] Shared Ispell dictionaries

On Mon, Mar 26, 2018 at 11:27:48AM -0400, Tom Lane wrote:
> Arthur Zakirov  writes:
> > I'm not sure that I understood the second case correclty. Can cache
> > invalidation help in this case? I don't have confident knowledge of cache
> > invalidation. It seems to me that InvalidateTSCacheCallBack() should
> > release segment after commit.
> 
> "Release after commit" sounds like a pretty dangerous design to me,
> because a release necessarily implies some kernel calls, which could
> fail.  We can't afford to inject steps that might fail into post-commit
> cleanup (because it's too late to recover by failing the transaction).
> It'd be better to do cleanup while searching for a dictionary to use.
> 
> I assume the DSM infrastructure already has some solution for getting
> rid of DSM segments when the last interested process disconnects,
> so maybe you could piggyback on that somehow.

Yes, there is dsm_pin_mapping() for this. But it is necessary to keep a
segment even if there are no attached processes. From 0003:

+	/* Remain attached until end of postmaster */
+	dsm_pin_segment(seg);
+	/* Remain attached until end of session */
+	dsm_pin_mapping(seg);

-- 
Arthur Zakirov
Postgres Professional: http://www.postgrespro.com
Russian Postgres Company