Re: datachecksums: handle invalid and dropped databases during enable

Hi,

I think there may be another case to consider here. DROP DATABASE durably
marks the database invalid before it removes the catalog row and files, so
a crash or ERROR after the marker is written leaves an invalid row whose
files are still present. If checksum enabling is running at the same
time, the worker will fail to connect to that database, and since
1df361e3d82 DatabaseExists() returns false for an invalid row. AFAICT
the launcher then classifies the failure as a concurrent drop and skips
the database, potentially letting data_checksums reach "on" while those
files were never processed.

AFAICS nothing else rules this out, so I am attaching a patch along with
a TAP test that uses a new injection point in dropdb() to exercise the
race directly. The patch makes DatabaseExists() return true for any
present row; ISTM that AccessShareLock already waits out an in-flight
drop, so an invalid-but-present row can only be from an interrupted one
whose files still need checksums. I am not sure if the patch needs a
dedicated TAP test, however I've added one.

Apologies if this is expected behaviour, and I missed on some thread
mentioning it.

Regards,
Ayush