Re: tuplesort test coverage

Hi,

On 2019-10-24 11:10:34 -0700, Andres Freund wrote:
> On 2019-10-15 13:05:32 +0100, Peter Geoghegan wrote:
> > > - aborted abbreviated keys (lines 1522, 1608, 1774, 3620, 3739, 3867, 4266)
> > 
> > Also hard to test -- there was a bug here when abbreviated keys first
> > went in -- that was detected by amcheck.
> > 
> > All of the places where we abort are essentially the same, though.
> 
> Why is it that hard?  Seems fairly easy to create cases that reliably
> abort.
> 
> I really don't think it's ok to have as many abbrev abort related paths
> without any coverage - the relevant code isn't that trivial. And
> something like amcheck really doesn't strike me as sufficient. For one,
> it doesn't provide any coverage either. For another, plenty sorts don't
> end up in a form that amcheck sees.
> 
> Tests aren't just there to verify that the current behaviour isn't
> broken, they're also there to allow to develop with some confidence. And
> I don't think tuplesort as is really allows that, and e.g. abbreviated
> keys made that substantially worse. That happens, but I think it'd be
> good if you could help improving the situation.
> 
> E.g.
> SELECT * FROM (SELECT ('00000000-0000-0000-0000-'||to_char(g.i, '000000000000FM'))::uuid uuid FROM generate_series(15000, 0, -1) g(i)) d ORDER BY uuid
> reliably triggers abbreviated keys, and it looks to me like that should
> be portable.  With a few tweaks it'd be fairly easy to use that to
> provide some OK coverage for most the abbrev key cases.

Here's a first stab at getting the coverage of tuplesort.c to a
satisfying level.  There's still bits uncovered, but that's largely
either a) trace_sort related b) hopefully unreachable stuff c) explain
related. The largest actually missing thing is a disk-based
mark/restore, which probably ought be covered.

I think the the test time of this would still be OK, but if not we could
also work a bit more on that angle.

I'm pretty sure there's some minor copy & paste mistakes in the test,
but I want to get this out there and get some reactions before investing
further time.

Peter, Tom?

- Andres