Re: [Patch] ALTER SYSTEM READ ONLY

On Wed, Sep 15, 2021 at 4:34 AM Mark Dilger
<mark.dilger@enterprisedb.com> wrote:
>
>
>
> > On Jun 16, 2020, at 6:55 AM, amul sul <sulamul@gmail.com> wrote:
> >
> > (2) if the session is idle, we also need the top-level abort
> > record to be written immediately, but can't send an error to the client until the next
> > command is issued without losing wire protocol synchronization. For now, we just use
> > FATAL to kill the session; maybe this can be improved in the future.
>
> Andres,
>
> I'd like to have a patch that tests the impact of a vacuum running for xid wraparound purposes, blocked on a pinned
pageheld by the cursor, when another session disables WAL.  It would be very interesting to test how the vacuum handles
thatspecific change.  I have not figured out the cleanest way to do this, though, as we don't as a project yet have a
standardway of setting up xid exhaustion in a regression test, do we?  The closest I saw to it was your work in [1],
butthat doesn't seem to have made much headway recently, and is designed for the TAP testing infrastructure, which
isn'tuseable from inside an isolation test.  Do you have a suggestion how best to continue developing out the test
infrastructure?
>
>
> Amul,
>
> The most obvious way to test how your ALTER SYSTEM READ ONLY feature interacts with concurrent sessions is using the
isolationtester in src/test/isolation/, but as it stands now, the first permutation that gets a FATAL causes the test
toabort and all subsequent permutations to not run.  Attached patch v34-0009 fixes that. 
>

Interesting.

> Attached patch v34-0010 adds a test of cursors opened FOR UPDATE interacting with a system that is set read-only by a
differentsession.  The expected output is worth reviewing to see how this plays out.  I don't see anything in there
whichis obviously wrong, but some of it is a bit clunky.  For example, by the time the client sees an error "FATAL:
WALis now prohibited", the system may already have switched back to read-write.  Also, it is a bit strange to get one
ofthese errors on an attempted ROLLBACK.  Once again, not wrong as such, but clunky. 
>

Can't we do the same in the TAP test? If the intention is only to test
session termination when the system changes to WAL are prohibited then
that I have added in the latest version, but that test does not
reinitiate the same connection again, I think that is not possible
there too.


Regards,
Amul