Re: pgsql: dshash: Add sequential scan support.

Hi,

On 2022-03-10 20:09:56 -0500, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > dshash: Add sequential scan support.
> > Add ability to scan all entries sequentially to dshash. The interface is
> > similar but a bit different both from that of dynahash and simple dshash
> > search functions. The most significant differences is that dshash's interfac
> > always needs a call to dshash_seq_term when scan ends.
>
> Umm ... what about error recovery? Or have you just cemented the
> proposition that long-lived dshashes are unsafe?

I don't think this commit made it worse. dshash_seq_term() releases an lwlock
(which will be released in case of an error) and unsets
hash_table->find_[exclusively_]locked. The latter weren't introduced by this
patch, and are also set by dshash_find().

I agree that ->find_[exclusively_]locked are problematic from an error
recovery perspective.

It's per-backend state at least and just used for assertions. We could remove
it. Or stop checking it in places where it could be set wrongly: dshash_find()
and dshash_detach() couldn't check anymore, but the rest of the assertions
would still be valid afaics?

Greetings,

Andres Freund