pgsql: nbtree VACUUM: cope with topparent inconsistencies.

nbtree VACUUM: cope with topparent inconsistencies.

Avoid "right sibling %u of block %u is not next child" errors when
vacuuming a corrupt nbtree index. Just LOG the issue and press on.
That way VACUUM will have a decent chance of finishing off all required
processing for the index (and for the table as a whole).

This is similar to recent work from commit 5abff197, as well as work
from commit 5b861baa (later backpatched as commit 43e409ce), which
taught nbtree VACUUM to keep going when its "re-find" check fails. The
hardening added by this commit takes place directly after the "re-find"
check, right before the critical section for the first stage of page
deletion.

Author: Peter Geoghegan <pg(at)bowt(dot)ie>
Discussion: https://postgr.es/m/CAH2-Wz=dayg0vjs4+er84TS9ami=csdzjpuiCGbEw=idhwqhzQ@mail.gmail.com
Backpatch: 11- (all supported versions).

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/642bec1f8dd2e05b769a4ed3b42ef0fe57804647

Modified Files
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src/backend/access/nbtree/nbtpage.c | 23 ++++++++++++++++-------
1 file changed, 16 insertions(+), 7 deletions(-)