From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: pg_preadv() and pg_pwritev()
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Comments: In-reply-to Thomas Munro <thomas.munro@gmail.com>
	message dated "Sun, 20 Dec 2020 11:38:51 +1300"
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Date: Sat, 19 Dec 2020 18:34:17 -0500
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Thomas Munro <thomas.munro@gmail.com> writes:
> I want to be able to do synchronous vectored file I/O, so I made
> wrapper macros for preadv() and pwritev() with fallbacks for systems
> that don't have them.  Following the precedent of the pg_pread() and
> pg_pwrite() macros, the "pg_" prefix reflects a subtle contract
> change: the fallback paths might have the side effect of changing the
> file position.

In a quick look, seems OK with some nits:

1. port.h cannot assume that <limits.h> has already been included;
nor do I want to fix that by including <limits.h> there.  Do we
really need to define a fallback value of IOV_MAX?  If so,
maybe the answer is to put the replacement struct iovec and
IOV_MAX in some new header.

2. I'm not really that happy about loading <sys/uio.h> into
every compilation we do, which would be another reason for a
new specialized header that either includes <sys/uio.h> or
provides fallback definitions.

3. The patch as given won't prove anything except that the code
compiles.  Is it worth fixing at least one code path to make
use of pg_preadv and pg_pwritev, so we can make sure this code
is tested before there's layers of other new code on top?

			regards, tom lane