pgsql: amcheck: Skip unlogged relations during recovery.

amcheck: Skip unlogged relations during recovery.

contrib/amcheck failed to consider the possibility that unlogged
relations will not have any main relation fork files when running in hot
standby mode. This led to low-level "can't happen" errors that complain
about the absence of a relfilenode file.

To fix, simply skip verification of unlogged index relations during
recovery. In passing, add a direct check for the presence of a main
fork just before verification proper begins, so that we cleanly verify
the presence of the main relation fork file.

Author: Andrey Borodin, Peter Geoghegan
Reported-By: Andrey Borodin
Diagnosed-By: Andrey Borodin
Discussion: https://postgr.es/m/DA9B33AC-53CB-4643-96D4-7A0BBC037FA1@yandex-team.ru
Backpatch: 10-, where amcheck was introduced.

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a05fa2c0e77535470b6ee1029671eb41c612f4ef

Modified Files
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contrib/amcheck/verify_nbtree.c | 45 ++++++++++++++++++++++++++++++++++++-----
1 file changed, 40 insertions(+), 5 deletions(-)