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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Date: Fri, 24 Feb 2017 09:48:30 -0500
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On 02/23/2017 04:41 PM, Tom Lane wrote:
> Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
>> I'm not entirely sure how I should replace those DirectFunctionCall2 calls.
> Basically what we want is for the called function to be able to use the
> fcinfo->flinfo->fn_extra and fcinfo->flinfo->fn_mcxt fields of the
> FmgrInfo struct that GIN has set up for the btree_gin support function.
>
> The fast but somewhat scary way to do it would just be to pass through
> the flinfo pointer as-is.  Imagine that fmgr.c grows a set of functions
> like
>
> Datum
> DontKnowWhatToCallThisFunctionCall2(PGFunction func,
>                                     FmgrInfo *flinfo, Oid collation,
>                                     Datum arg1, Datum arg2)
> {
>     FunctionCallInfoData fcinfo;
>     Datum        result;
>
>     InitFunctionCallInfoData(fcinfo, flinfo, 2, collation, NULL, NULL);
>
>     fcinfo.arg[0] = arg1;
>     fcinfo.arg[1] = arg2;
>     fcinfo.argnull[0] = false;
>     fcinfo.argnull[1] = false;
>
>     result = (*func) (&fcinfo);
>
>     /* Check for null result, since caller is clearly not expecting one */
>     if (fcinfo.isnull)
>         elog(ERROR, "function %p returned NULL", (void *) func);
>
>     return result;
> }
>
> and then you'd just pass through the fcinfo->flinfo you got.
>
> 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.

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.

The technique is somewhat similar to what we do in plperl.c where we
fake up a function call for handling DO blocks.

cheers

andrew

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


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