pgsql: Skip foreign tablespaces when running pg_checksums/pg_verify_che

Skip foreign tablespaces when running pg_checksums/pg_verify_checksums

Attempting to use pg_checksums (pg_verify_checksums in 11) on a data
folder which includes tablespace paths used across multiple major
versions would cause pg_checksums to scan all directories present in
pg_tblspc, and not only marked with TABLESPACE_VERSION_DIRECTORY. This
could lead to failures when for example running sanity checks on an
upgraded instance with --check. Even worse, it was possible to rewrite
on-disk pages with --enable for a cluster potentially online.

This commit makes pg_checksums skip any directories not named
TABLESPACE_VERSION_DIRECTORY, similarly to what is done for base
backups.

Reported-by: Michael Banck
Author: Michael Banck, Bernd Helmle
Discussion: https://postgr.es/m/62031974fd8e941dd8351fbc8c7eff60d59c5338.camel@credativ.de
backpatch-through: 11

Branch
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REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/8f9aba1874eccae860d5a6ced9b2899b2f7895b4

Modified Files
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src/bin/pg_verify_checksums/pg_verify_checksums.c | 47 ++++++++++++++++++++++-
1 file changed, 46 insertions(+), 1 deletion(-)