From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Michael Paquier <michael@paquier.xyz>,
    Thomas Munro <thomas.munro@gmail.com>,
    Robert Haas <robertmhaas@gmail.com>,
    "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Attempt to consolidate reading of XLOG page
In-reply-to: <20191122225632.GA9169@alvherre.pgsql>
References: <20191122225632.GA9169@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
   message dated "Fri, 22 Nov 2019 19:56:32 -0300."
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="=-=-="
Date: Mon, 25 Nov 2019 10:02:00 +0100
Message-ID: <50470.1574672520@antos>
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Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

> On 2019-Nov-22, Antonin Houska wrote:
> 
> > As I pointed out in
> > 
> > https://www.postgresql.org/message-id/88183.1574261429%40antos
> > 
> > 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:

https://www.postgresql.org/message-id/20191125033048.GG37821%40paquier.xyz

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com


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Content-Type: text/x-diff
Content-Disposition: attachment; filename=wal_read_errno.patch

diff --git a/src/bin/pg_waldump/pg_waldump.c b/src/bin/pg_waldump/pg_waldump.c
index 04124bc254..eda81c1df1 100644
--- a/src/bin/pg_waldump/pg_waldump.c
+++ b/src/bin/pg_waldump/pg_waldump.c
@@ -354,9 +354,11 @@ WALDumpReadPage(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
 					 state->segcxt.ws_segsize);
 
 		if (errinfo.wre_errno != 0)
-			fatal_error("could not read in file %s, offset %u, length %zu: %s",
-						fname, errinfo.wre_off, (Size) errinfo.wre_req,
-						strerror(errinfo.wre_errno));
+		{
+			errno = errinfo.wre_errno;
+			fatal_error("could not read in file %s, offset %u, length %zu: %m",
+						fname, errinfo.wre_off, (Size) errinfo.wre_req);
+		}
 		else
 			fatal_error("could not read in file %s, offset %u: length: %zu",
 						fname, errinfo.wre_off, (Size) errinfo.wre_req);

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