From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Michael Banck <michael.banck@credativ.de>,
        Magnus Hagander <magnus@hagander.net>,
        Fabien COELHO <coelho@cri.ensmp.fr>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: pg_verify_checksums and -fno-strict-aliasing
In-reply-to: <20180830220409.4zhdka6ibs66enbk@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Thu, 30 Aug 2018 15:04:09 -0700"
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Date: Thu, 30 Aug 2018 18:11:40 -0400
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Andres Freund <andres@anarazel.de> writes:
> On 2018-08-30 14:46:06 -0700, Andres Freund wrote:
>> One way to fix it would be to memcpy in/out the modified PageHeader, or
>> just do offset math and memcpy to that offset.

> It took me a bit to reproduce the issue (due to sheer stupidity on my
> part: no, changing the flags passed to gcc to link pg_verify_checksums
> doesn't do the trick), but the above indeed fixes the issue for me.

I suspect people will complain about the added cost of doing that.

I've been AFK all afternoon, but what I was intending to try next was
the union approach, specifically union'ing PageHeaderData with the uint32
array representation needed by pg_checksum_block().  That might also
end up giving us code less unreadable than this:

	uint32		(*dataArr)[N_SUMS] = (uint32 (*)[N_SUMS]) data;


BTW, not to mention the elephant in the room, but: is it *really* OK
that pg_checksum_page scribbles on the page buffer, even temporarily?
It's certainly quite horrid that there aren't large warnings about
that in the function's API comment.

			regards, tom lane