Ideas for a relcache test mode about missing invalidations

Hi,

The issue at [1] is caused by missing invalidations, and [2] seems like
a likely candidate too. I wonder if it'd be good to have a relcache test
mode akin to CLOBBER_CACHE_ALWAYS and RELCACHE_FORCE_RELEASE, that tries
to ensure that we've done sufficiently to ensure the right invalidations
are sent.

I think what we'd kind of want is to ensure that relcache entries are
rebuilt at the earliest possible time, but *not* later. That'd mean
they're out of date if there's missing invalidations. Unfortunately I'm
not clear on how that'd be achievable? Ideas?

The best I can come up with is to code some additional dependencies into
CacheInvalidateHeapTuple(), and add tracking ensuring we've sent the
right messages. But that seems somewhat painful and filled with holes.

[1] http://archives.postgresql.org/message-id/CAKoxK%2B5fVodiCtMsXKV_1YAKXbzwSfp7DgDqUmcUAzeAhf%3DHEQ%40mail.gmail.com
[2] https://www.postgresql.org/message-id/12259.1532117714@sss.pgh.pa.us

Greetings,

Andres Freund