Re: snapshot too old, configured by time

On Fri, Jan 8, 2016 at 5:22 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

>> People have said that issuing SQL commands directly from a TAP test
>> via DBD::Pg is not acceptable for a core feature, and (despite
>> assertions to the contrary) I see no way to test this feature with
>> existing testing mechanisms. The bigger set of work here, if we
>> don't want this feature to go in without any testing scripts (which
>> is not acceptable IMO), is to enhance the isolation tester or
>> hybridize TAP testing with the isolation tester.
>
> Is it possible to use the PostgresNode stuff to test this? If not,
> perhaps if you restate what additional capabilities you need we could
> look into adding them there. I suspect that what you need is the
> ability to keep more than one session open and feed them commands;
> perhaps we could have the framework have a function that opens a psql
> process and returns a FD to which the test program can write, using the
> IPC::Run stuff (start / pump / finish).

Resubmitting for the March CF.

The main thing that changed is that I can now run all the
regression and isolation tests using installcheck with
old_snapshot_threshold = 0 and get a clean run. That probably gets
better overall coverage than specific tests to demonstrate the
"snapshot too old" error, but of course we need those, too. While
I can do that with hand-run psql sessions or through connectors
from different languages, I have not been able to wrangle the
testing tools we support through the build system into working for
this purpose. (I had been hoping that the recent improvements to
the TAP testing libraries would give me the traction to get there,
but either it's still not there or my perl-fu is just too weak to
figure out how to use those features -- suggestions welcome.)

Basically, a connection needs to remain open and interleave
commands with other connections, which the isolation tester does
just fine; but it needs to do that using a custom postgresql.conf
file, which TAP does just fine. I haven't been able to see the
right way to get a TAP test to set up a customized installation to
run isolation tests against. If I can get that working, I have
additional tests I can drop into that.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company