Re: Attempt to consolidate reading of XLOG page

On 2019-Nov-25, Antonin Houska wrote:

> Alvaro Herrera <alvherre@2ndquadrant.com> 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.
> > 
> > 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.

Excellent, I pushed it with this change included and some other cosmetic
changes.

Now there's only XLogPageRead() ...

> > 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.

Right ... in this interface, we only report an error if pg_pread()
returns negative, which is documented to always set errno.

> I'd appreciate more background about the "partial read" that
> Michael mentions here:
> 
> https://www.postgresql.org/message-id/20191125033048.GG37821%40paquier.xyz

In the current implementation, if pg_pread() does a partial read, we
just loop one more time.

I considered changing the "if (readbytes <= 0)" with "if (readbytes <
segbytes)", but that seemed pointless.

However, writing this now makes me think that we should add a
CHECK_FOR_INTERRUPTS in this loop.  (I also wonder if we shouldn't limit
the number of times we retry if pg_pread returns zero (i.e. no error,
but no bytes read either).  I don't know if this is a real-world
consideration.)

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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