Re: pread() and pwrite()

Hi,

On 11/5/18 9:10 PM, Thomas Munro wrote:
> On Tue, Nov 6, 2018 at 5:07 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
>> Please remove Tell from line 18 in fd.h.  To Küssnacht with him!
> 
> Thanks, done.  But what is this arrow sticking through my Mac laptop's
> screen...?
> 
> On Tue, Nov 6, 2018 at 6:23 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.
> 
> OK.  But since we're using this from both fd.c and xlog.c, I put that
> into src/include/port.h.
> 
>> FWIW, I tested the given patches on HPUX 10.20; they compiled cleanly
>> and pass the core regression tests.
> 

Passes check-world, and includes the feedback on this thread.

New status: Ready for Committer

Best regards,
  Jesper