amcheck: Fix snapshot usage in bt_index_parent_check

We were using SnapshotAny to do some index checks, but that's wrong and
causes spurious errors when used on indexes created by CREATE INDEX
CONCURRENTLY.  Fix it to use an MVCC snapshot, and add a test for it.

Backpatch of 6bd469d26a to branches 14-16.  I previously misidentified
the bug's origin: it came in with commit 7f563c09f8 (pg11-era, not
5ae2087202 as claimed previously), so all live branches are affected.

Also take the opportunity to fix some comments that we failed to update
in the original commits and apply pgperltidy.  In branch 14, remove the
unnecessary test plan specification (which would have need to have been
changed anyway; c.f. commit 549ec201d613.)

Diagnosed-by: Donghang Lin <donghanglin@gmail.com>
Author: Mihail Nikalayeu <mihailnikalayeu@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Backpatch-through: 17
Discussion: https://postgr.es/m/CANtu0ojmVd27fEhfpST7RG2KZvwkX=dMyKUqg0KM87FkOSdz8Q@mail.gmail.com

contrib/amcheck/t/002_cic.pl

@@ -74,7 +74,8 @@ $in_progress_h->query_safe(q(BEGIN; SELECT pg_current_xact_id();));
 # delete one row from table, while background transaction is in progress
 $node->safe_psql('postgres', q(DELETE FROM quebec WHERE i = 1;));
 # create index concurrently, which will skip the deleted row
-$node->safe_psql('postgres', q(CREATE INDEX CONCURRENTLY oscar ON quebec(i);));
+$node->safe_psql('postgres',
+	q(CREATE INDEX CONCURRENTLY oscar ON quebec(i);));
 
 # check index using bt_index_parent_check
 $result = $node->psql('postgres',

contrib/amcheck/verify_nbtree.c

@@ -90,9 +90,11 @@ typedef struct BtreeCheckState
 	BufferAccessStrategy checkstrategy;
 
 	/*
-	 * Info for uniqueness checking. Fill these fields once per index check.
+	 * Info for uniqueness checking. Fill this field and the one below once
+	 * per index check.
 	 */
 	IndexInfo  *indexinfo;
+	/* Table scan snapshot for heapallindexed and checkunique */
 	Snapshot	snapshot;
 
 	/*