Re: IndexTupleDSize macro seems redundant

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Ildar Musin <i(dot)musin(at)postgrespro(dot)ru>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: IndexTupleDSize macro seems redundant
Date: 2018-01-11 18:17:13
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* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Stephen Frost <sfrost(at)snowman(dot)net> writes:
> > I'll leave the patch status in 'Needs review' since there's more
> > changes, but hopefully someone can take a look and we can move this
> > along, seems like a pretty small and reasonable improvement.
> I'm on board with Stephen's changes, except in _bt_restore_page.
> The issue there is that the "from" pointer isn't necessarily adequately
> aligned to be considered an IndexTuple pointer; that's why we're doing
> the memcpy dance to get a length out of it. "Item" doesn't connote
> anything about alignment (it's the same as Pointer, ie char*). Even
> though we don't do anything with items[i] except pass it as an Item
> to PageAddItem, the proposed change could result in breakage, because
> the compiler could take it as license to assume that "from" is aligned,
> and perhaps change what it generates for the memcpy.

I certainly hadn't been thinking about that. I didn't see any
issues in my testing (where I created a table with a btree index and
insert'd a bunch of records into and then killed the server, forcing WAL
replay and then checked that the index appeared to be valid using order
by queries; perhaps I should have tried amcheck, but doesn't sound like
this is something that would have been guaranteed to break anyway).

That said, since it's not aligned, regardless of the what craziness the
compiler might try to pull, we probably shouldn't go casting it
to something that later hackers might think will be aligned, but we
should add a comment to clarify that it's not aligned and that we can't
act like it is.

> I think that in the other places where Stephen wants to change Item
> to something else, the alignment expectation actually does hold,
> so we're OK if we want to do it in those places.

Yes, everywhere else it's a pointer returned from PageGetItem() or
XLogRecGetBlockData() (which always returns a MAXALIGNed pointer).



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