pgsql: pg_amcheck: avoid unhelpful verification attempts.

pg_amcheck: avoid unhelpful verification attempts.

Avoid calling contrib/amcheck functions with relations that are
unsuitable for checking.  Specifically, don't attempt verification of
temporary relations, or indexes whose pg_index entry indicates that the
index is invalid, or not ready.

These relations are not supported by any of the contrib/amcheck
functions, for reasons that are pretty fundamental.  For example, the
implementation of REINDEX CONCURRENTLY can add its own "transient"
pg_index entries, which has rather unclear implications for the B-Tree
verification functions, at least in the general case -- so they just
treat it as an error.  It falls to the amcheck caller (in this case
pg_amcheck) to deal with the situation at a higher level.

pg_amcheck now simply treats these conditions as additional "visibility
concerns" when it queries system catalogs.  This is a little arbitrary.
It seems to have the least problems among any of the available
alternatives.

Author: Mark Dilger <mark.dilger@enterprisedb.com>
Reported-By: Alexander Lakhin <exclusion@gmail.com>
Reviewed-By: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Bug: #17212
Discussion: https://postgr.es/m/17212-34dd4a1d6bba98bf@postgresql.org
Backpatch: 14-, where pg_amcheck was introduced.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d2bf06db377967b0d671ae372d513806e2a28052

Modified Files
--------------
doc/src/sgml/ref/pg_amcheck.sgml        |  12 ++
src/bin/pg_amcheck/pg_amcheck.c         |  83 ++++++++----
src/bin/pg_amcheck/t/006_bad_targets.pl | 233 ++++++++++++++++++++++++++++++++
3 files changed, 299 insertions(+), 29 deletions(-)