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From: Thomas Munro <thomas.munro@gmail.com>
Date: Mon, 19 Sep 2022 10:16:24 +1200
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Subject: Re: Tree-walker callbacks vs -Wdeprecated-non-prototype
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
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On Mon, Sep 19, 2022 at 8:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I think we ought to seriously consider the alternative of changing
> nodeFuncs.c about like I have here, but not touching the walkers/mutators,
> and silencing the resulting complaints about function type casting by
> doing the equivalent of
>
> -    return expression_tree_walker(node, cost_qual_eval_walker,
> -                                  (void *) context);
> +    return expression_tree_walker(node,
> +                                  (tree_walker_callback) cost_qual_eval_walker,
> +                                  (void *) context);
>
> We could avoid touching all the call sites by turning
> expression_tree_walker and friends into macro wrappers that incorporate
> these casts.  This is fairly annoying, in that it gives up the function
> type safety the C committee wants to impose on us; but I really think
> the data type safety that we're giving up in this version of the patch
> is a worse hazard.

But is it defined behaviour?

https://stackoverflow.com/questions/559581/casting-a-function-pointer-to-another-type

> BTW, I was distressed to discover that someone decided they could
> use ExecShutdownNode as a planstate_tree_walker() walker even though
> its argument list is not even the right length.  I'm a bit flabbergasted
> that we seem to have gotten away with that so far, because I'd have
> thought for sure that it'd break some platform's convention for which
> argument gets passed where.  I think we need to fix that, independently
> of what we do about the larger scope of these problems.  To avoid an
> API break, I propose making ExecShutdownNode just be a one-liner that
> calls an internal ExecShutdownNode_walker() function.  (I've not done
> it that way in the attached, though.)

Huh... wouldn't systems that pass arguments right-to-left on the stack
receive NULL for node?  That'd include the SysV i386 convention used
on Linux, *BSD etc.  But that can't be right or we'd know about it...

But certainly +1 for fixing that regardless.