There's an obvious bug, plus another bug that I hit trying to reproduce
the first bug, plus another... it's a long story.
Basically: If you get nothing back from executeDistinctShardBSI on a
Timestamp field, the request for a large enough pool of strings to hold
timestamp conversions of the nothing segfaults because r.Columns() on
a nil row segfaults.
To try to test this better, I added a filter to the executor test that
we use for this case, which got me a different result complaining about
a DistinctTimestamp result not being a SignedRow.
So, there's a couple of issues. One is that, in the case where a filter
is present, if the filter comes up with nothing, we can bail early
and return a result of the SignedRow type, which then breaks the reduce
part of our map/reduce when we try to reduce DistinctTimestamp values
into a SignedRow. To fix this, we make sure that we return the expected
type even in the case where we're bailing early.
A simpler way to see the actual original bug is, rather than having
a filter, just have a shard that has a value in *some other field*
but not in the timestamp field. So we add that to the test, too.
But also, really, since this is a problem that's happened more than
once, I propose that we also just make nil rows allow you to request
their columns and get back nil, so things like this don't bite us as
much. This wouldn't be a sufficient fix for the filter case, and I
still have the short-circuit for the nil row case explicitly in this
particular case because relying on the nil behavior bugs me, but I
think it's safer to allow .Columns on nil rows.
empty string doesn't work because gitlab doesn't set the variable at
all. How do I know that "null" is correct? Because Fletcher told
me... apparently it's a ruby-ism
This commit changes `RankCache.BulkAdd()` so that entries are
limited to an upper bound of 2x `maxEntries`. When this bound
is exceeded then the cache is automatically recalculated.
