Re: [PROPOSAL] Shared Ispell dictionaries

On Wed, May 16, 2018 at 7:36 AM, Arthur Zakirov
<a(dot)zakirov(at)postgrespro(dot)ru> wrote:
>> I don't quite understand the problem you're trying to solve here, but:
>>
>> 1. Unless dsm_pin_segment() is called, a DSM segment will
>> automatically be removed when there are no remaining mappings.
>>
>> 2. Unless dsm_pin_mapping() is called, a DSM segment will be unmapped
>> when the currently-in-scope resource owner is cleaned up, like at the
>> end of the query. If it is called, then the mapping will stick around
>> until the backend exits.
>
> I tried to solve the case when DSM segment remains mapped even a
> dictionary was dropped. It may happen in the following situation:
>
> Backend 1:
>
> =# select ts_lexize('english_shared', 'test');
> -- The dictionary is loaded into DSM, the segment and the mapping is
> pinned
> ...
> -- Call ts_lexize() from backend 2 below
> =# drop text search dictionary english_shared;
> -- The segment and the mapping is unpinned, see ts_dict_shmem_release()
>
> Backend 2:
>
> =# select ts_lexize('english_shared', 'test');
> -- The dictionary got from DSM, the mapping is pinned
> ...
> -- The dictionary was dropped by backend 1, but the mapping still is
> pinned

Yeah, there's really nothing we can do about that (except switch from
processes to threads). There's no way for one process to force
another process to unmap something. As you've observed, you can get
it to be dropped eventually, but not immediately.

> In sum, I think the problem is mostly solved. Backend 2 unpins the
> segment in next ts_lexize() call. But if backend 2 doesn't call
> ts_lexize() (or other TS function) anymore the segment will remain mapped.
> It is the only problem I see for now.

Maybe you could use CacheRegisterSyscacheCallback to get a callback
when the backend notices that a DROP has occurred.

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