From: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
Subject: Re: btree_gin and btree_gist for enums
To: Tom Lane <tgl@sss.pgh.pa.us>
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Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Message-ID: <930cb563-6dc6-c4af-3be9-31f06c60361e@2ndQuadrant.com>
Date: Fri, 24 Feb 2017 14:55:21 -0500
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On 02/24/2017 11:02 AM, Tom Lane wrote:
> Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
>> On 02/23/2017 04:41 PM, Tom Lane wrote:
>>> The reason this is kind of scary is that it's just blithely assuming
>>> that the function won't look at the *other* fields of the FmgrInfo.
>>> If it did, it would likely get very confused, since those fields
>>> would be describing the GIN support function, not the function we're
>>> calling.
>>>
>>> We could alternatively have this trampoline function set up a fresh, local
>>> FmgrInfo struct that it zeroes except for copying fn_extra and fn_mcxt
>>> from the caller's struct, and then it copies fn_extra back again on the
>>> way out.  That's a few more cycles but it would be safer, I think; if the
>>> function tried to look at the other fields such as fn_oid it would see
>>> obviously bogus data.
>> Do we want one or both of these? I'm prepared to code up a patch to
>> fmgr.[ch] to provide them.
> On reflection I'm not sure that the double-copy approach is all that much
> safer than just passing down the caller's flinfo pointer.  Most of the
> time it would be better, but suppose that the callee updates fn_extra
> and then throws elog(ERROR) --- the outcome would be different, probably
> creating a leak in fn_mcxt.  Maybe this would still be okay, because
> perhaps that FmgrInfo is never used again, but I don't think we can assume
> that for the case at hand.
>
> At this point I'd be inclined to just document that the called function
> should only use fn_extra/fn_mcxt.


fair enough. Will do it that way.


>
>> I don't know what to call it either. In my test I used
>> CallerContextFunctionCall2 - not sure if that's quite right, but should
>> be close.
> CallerInfo?  CallerFInfo?  Or we could spell out CallerFmgrInfo but
> that seems a bit verbose.
>
> 			

I'll go with CallerFInfoFunctionCall2 etc.

In the btree_gist system the calls to the routines like enum_cmp are
buried about three levels deep. I'm thinking I'll just pass the flinfo
down the stack and just call these routines at the bottom level.


cheers

andrew

-- 
Andrew Dunstan                https://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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