From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Thomas Munro <thomas.munro@enterprisedb.com>,
        Jesper Pedersen <jesper.pedersen@redhat.com>,
        Magnus Hagander <magnus@hagander.net>, Oskari Saarenmaa <os@ohmu.fi>,
        Pg Hackers <pgsql-hackers@postgresql.org>,
        Tobias Oberstein <tobias.oberstein@gmail.com>
Subject: Re: pread() and pwrite()
In-reply-to: <20181105155918.jg36uj4rtnxwqtzt@alvherre.pgsql>
References: <20181105155918.jg36uj4rtnxwqtzt@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Mon, 05 Nov 2018 12:59:18 -0300"
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Date: Mon, 05 Nov 2018 12:23:52 -0500
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2018-Nov-04, Thomas Munro wrote:
>> Here's a patch to add Windows support by supplying
>> src/backend/port/win32/pread.c.  Thoughts?

> Hmm, so how easy is to detect that somebody runs read/write on fds where
> pread/pwrite have occurred?  I guess for data files it's easy to detect
> since you'd quickly end up with corrupted files, but what about other
> kinds of files?  I wonder if we should be worrying about using this
> interface somewhere other than fd.c and forgetting about the limitation.

Yeah.  I think the patch as presented is OK; it uses pread/pwrite only
inside fd.c, which is a reasonably non-leaky abstraction.  But there's
definitely a hazard of somebody submitting a patch that depends on
using pread/pwrite elsewhere, and then that maybe not working.

What I suggest is that we *not* try to make this a completely transparent
substitute.  Instead, make the functions exported by src/port/ be
"pg_pread" and "pg_pwrite", and inside fd.c we'd write something like

#ifdef HAVE_PREAD
#define pg_pread pread
#endif

and then refer to pg_pread/pg_pwrite in the body of that file.  That
way, if someone refers to pread and expects standard functionality
from it, they'll get a failure on platforms not supporting it.

FWIW, I tested the given patches on HPUX 10.20; they compiled cleanly
and pass the core regression tests.

			regards, tom lane