Re: Attempt to consolidate reading of XLOG page

On Fri, Nov 22, 2019 at 07:56:32PM -0300, Alvaro Herrera wrote:
> 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.

Yeah, that seems cleaner.

> What do you think of the attached?

Looks rather fine to me.

> 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.

Because we have no guarantee that errno will be cleared if you do a
partial read where errno is not set, so you may finish by reporting
the state of a previous failed read instead of the partially-failed
one depending on how WALReadError is treated?  In short, I don't see
any actual reason why it would be good to remove the reset of errno
either before the calls to pread and pwrite().
--
Michael