[Patch] Make pg_checksums skip foreign tablespace directories

Hi,

we had a customer who ran pg_checksums on their v12 cluster and got a
confusing error:

|pg_checksums: error: invalid segment number 0 in file name "/PG-
|Data/foo_12_data/pg_tblspc/16402/PG_10_201707211/16390/pg_internal.init
|.10028"

Turns out the customer ran a pg_ugprade in copy mode before and started
up the old cluster again which pg_checksums decided to checked as well -
note the PG_10_201707211 in the error message. The attached patch is a
stab at teaching pg_checksums to only check its own
TABLESPACE_VERSION_DIRECTORY directory. I guess this implies that it
would ignore tablespace directories of outdated catversion instances
during development, which I think should be ok, but others might not
agree?

The other question is whether it is possible to end up with a
pg_internal.init.$PID file in a running cluster. E.g. if an instance
crashes and gets started up again - are those cleaned up during crash
recovery, or should pg_checksums ignore them? Right now pg_checksums
only checks against a list of filenames and only skips on exact matches
not prefixes so that might take a bit of work.

Michael

--
Michael Banck
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