I wrote:
> At this point I'm inclined to push both of those patches so we can
> see what the buildfarm makes of them.

So I did that, and while not all of the buildfarm has reported in,
enough of it has that I think we can draw conclusions.  The only member
that's showing any new warnings, AFAICT, is jacana (gcc 4.9 on Windows).
It had no format-related warnings yesterday, but now it has a boatload
of 'em, and it appears that every single one traces to not believing
that printf and friends understand 'l' and 'z' length modifiers.

The reason for this seems to be that we unconditionally replace the
printf function family with snprintf.c on Windows, and port.h causes
those functions to be marked with pg_attribute_printf, which this
patch caused to mean just "printf" not "gnu_printf".  So this gcc
evidently thinks the platform printf doesn't know 'l' and 'z'
(which may or may not be true in reality, but it's irrelevant)
and it complains.

There are also interesting warnings showing up in elog.c, such as

Aug 11 14:26:32 
 warning: function might be possible candidate for 'ms_printf' format attribute 

I think what is happening here is that gcc notices that those functions
call appendStringInfoVA, which is now annotated with the printf
archetype not gnu_printf, so it decides that maybe we marked the elog.c
functions with the wrong archetype.  I have no idea why it's suggesting
"ms_printf" though --- I can find nothing on the web that even admits
that gcc knows such an archetype.

So this is all pretty much of a mess.  If we annotate the elog functions
differently from printf's annotation then we risk getting these complaints
in elog.c, but if we don't do that then we can't really describe their
semantics correctly.  We could possibly mark the replacement snprintf
functions with gnu_printf, but that's a lie with respect to the very
point at issue about %m.  Unless we were to teach snprintf.c about %m
... which probably wouldn't be hard, but I'm not sure I want to go there.
That line of thought leads to deciding that we should treat "printf
doesn't know %m" as a reason to use snprintf.c over the native printf;
and I think we probably do not want to do that, if only because the
native printf is probably more efficient than snprintf.c.  (There are
other reasons to question that too: we probably can't tell without a
run-time test in configure, and even if we detect it correctly, gcc
might be misconfigured to believe the opposite thing about %m support
and hence warn, or fail to warn, anyway.  clang at least seems to get
this wrong frequently.)  But if we do not do such replacement then we
still end up wondering how to mark printf wrapper functions such as

At this point I'm inclined to give up and revert 3a60c8ff8.  It's not
clear that we can really make the warning situation better, as opposed
to just moving the warnings from one platform to another.

                        regards, tom lane

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