Re: Add LSN along with offset to error messages reported for WAL file read/write/validate header failures

On Mon, Dec 19, 2022 at 5:22 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> On Mon, Dec 19, 2022 at 1:37 PM Michael Paquier <michael@paquier.xyz> wrote:
> >
> > On Tue, Dec 13, 2022 at 09:32:19PM +0530, Bharath Rupireddy wrote:
> > > Okay, here's the v5 patch that I could come up with. It basically adds
> > > functions for dissecting WAL file names and computing offset from lsn.
> > > Thoughts?
> >
> > I had a second look at that, and I still have mixed feelings about the
> > addition of the SQL function, no real objection about
> > pg_dissect_walfile_name().
> >
> > I don't really think that we need a specific handling with a new
> > macro from xlog_internal.h that does its own parsing of the segment
> > number while XLogFromFileName() can do that based on the user input,
> > so I have simplified that.
> >
> > A second thing is the TLI that had better be returned as int8 and not
> > int4 so as we don't have a negative number for a TLI higher than 2B in
> > a WAL segment name.
>
> Thanks. The v6 patch LGTM.

A nitpick - can we also specify a use case for the function
pg_dissect_walfile_name(), that is, computing LSN from offset and WAL
file name, something like [1]?

[1]
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 2f05b06f14..c36fcb83c8 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -26110,7 +26110,17 @@ LOG:  Grand total: 1651920 bytes in 201
blocks; 622360 free (88 chunks); 1029560
        </para>
        <para>
         Extract the file sequence number and timeline ID from a WAL file
-        name.
+        name. This function is useful to compute LSN from a given offset
+        and WAL file name, for example:
+<screen>
+postgres=# \set file_name '000000010000000100C000AB'
+postgres=# \set offset 256
+postgres=# SELECT '0/0'::pg_lsn + pd.segno * ps.setting::int +
:offset AS lsn FROM pg_dissect_walfile_name(:'file_name') pd,
pg_show_all_settings() ps WHERE ps.name = 'wal_segment_size';
+      lsn
+---------------
+ C001/AB000100
+(1 row)
+</screen>
        </para></entry>
       </row>

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com