Re: Attempt to consolidate reading of XLOG page

From: Antonin Houska <ah(at)cybertec(dot)at>
To: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Attempt to consolidate reading of XLOG page
Date: 2019-11-25 09:02:00
Message-ID: 50470.1574672520@antos
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Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

> On 2019-Nov-22, Antonin Houska wrote:
> > As I pointed out in
> >
> >
> >
> > seg.ws_off only replaced readOff in XLogReaderState. So we should only update
> > ws_off where readOff was updated before commit 709d003. This does happen in
> > ReadPageInternal (see HEAD) and I see no reason for the final patch to update
> > ws_off anywhere else.
> Oh you're right.
> I see no reason to leave ws_off. We can move that to XLogReaderState; I
> did that here. We also need the offset in WALReadError, though, so I
> added it there too. Conceptually it seems clearer to me this way.
> What do you think of the attached?

It looks good to me. Attached is just a fix of a minor problem in error
reporting that Michael pointed out earlier.

> BTW I'm not clear what errors can pread()/pg_pread() report that do not
> set errno. I think lines 1083/1084 of WALRead are spurious now.

All I can say is that the existing calls of pg_pread() do not clear errno, so
you may be right. I'd appreciate more background about the "partial read" that
Michael mentions here:

Antonin Houska

Attachment Content-Type Size
wal_read_errno.patch text/x-diff 823 bytes

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