Re: pg_xlogdump follow into the future

This patch does not have a reviewer, so I've decided to try myself on.

Disclaimer: although I review quite a lot of code daily, this is my first
review for PostgreSQL. I don't know code very well, and frankly I don't
really know C very well.
Hope my effort are not vain and will be helpful to somebody.
I'll be happy for review on review and any tips on process.

Summary
=======

I favour this patch. Current behaviour is indeed confusing. If we keep
current behaviour we need to update docs and maybe also print a warning
when using -f with a file name.

Thank you for submission, but i'm afraid there is a bit more work here:

- There is a bug, making it hard to test. Please fix.
- Please add some tests.

Submission review
==============

Patch applies cleanly on HEAD. Tests succeed.
There are no new or affected by this patch tests. Given that I've found a
trivial bug (see below), a test should be created.

Usability review
============
I believe I've immediately hit a corner case:

1) I've created a new instance, started it and run `./bin/pg_xlogdump -f
db/pg_wal/000000010000000000000001`.
This spewed quite a lot of stuff, as expected.

2) I've connected to the same instance and ran following:
# SELECT pg_switch_xlog();
pg_switch_xlog
----------------
0/14FA3D8
(1 row)

xlogdump immediately crashed with following:

pg_xlogdump: FATAL: could not find file "000000010000000000000002": No
such file or directory

Problem is that Postgres does not create file until it's actually needed.
So 000000010000000000000002 indeed was not there until after I've run some
transactions. I believe same would happen after pg_start_backup(), etc.

Feature review
===========
See above. Can't do more testing.

Performance review
===============
n/a

Coding review
===========
LGTM

Architecture review
==============
n/a

--
Vladimir Rusinov
Bigtable SRE

Google Ireland Ltd.,Gordon House, Barrow Street, Dublin 4, Ireland
Registered in Dublin, Ireland
Registration Number: 368047

On Mon, Oct 3, 2016 at 5:44 AM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:

> On Mon, Jul 18, 2016 at 8:10 PM, Magnus Hagander <magnus(at)hagander(dot)net>
> wrote:
> >
> >
> > On Thu, Jul 14, 2016 at 8:27 PM, Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> >>
> >> On 2016-07-14 13:46:23 +0200, Magnus Hagander wrote:
> >> > Currently, if you run pg_xlogdump with -f, you have to specify an end
> >> > position in an existing file, or if you don't it will only follow
> until
> >> > the
> >> > end of the current file.
> >>
> >> That's because specifying a file explicitly says that you only want to
> >> look at that file, specifying two files that you want the range
> >> inclusively between the two files. -f works if you just use -s.
> >
> >
> > Hmm. It does now. I'm *sure* it didn't when I was testing it. It must've
> > been something else that was broken at that point :)
>
> Same as Andres here, my understanding is that one file means that you
> just want to look at this file, and two files permits to look at a
> range of changes. But I have no argument against changing the current
> behavior either.
>
> >> > I'd appreciate a review of that by someone who's done more work on the
> >> > xlog
> >> > stuff, but it seems trivial to me. Not sure I can argue it's a bugfix
> >> > though, since the usecase simply did not work...
> >>
> >> I'd say it's working as intended, and you want to change that
> >> intent. That's fair, but I'd not call it a bug, and I'd say it's not
> >> really 9.6 material.
> >
> > Based on that, I agree that it's working as intended.
> >
> > And definitely that it's not 9.6 material.
> >
> > I'll stick it on the CF page so I don't forget about it.
>
> Moved to next CF. Magnus, you may want to finish wrapping that if you
> still intend to change the current behavior.
> --
> Michael
>
>
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