Fix more issues with dependency handling at swap phase of REINDEX CONCURRENTLY

When canceling a REINDEX CONCURRENTLY operation after swapping is done,
a drop of the parent table would leave behind old indexes.  This is a
consequence of 68ac9cf, which fixed the case of pg_depend bloat when
repeating REINDEX CONCURRENTLY on the same relation.

In order to take care of the problem without breaking the previous fix,
this uses a different strategy, possible even with the exiting set of
routines to handle dependency changes.  The dependencies of/on the
new index are additionally switched to the old one, allowing an old
invalid index remaining around because of a cancellation or a failure to
use the dependency links of the concurrently-created index.  This
ensures that dropping any objects the old invalid index depends on also
drops the old index automatically.

Reported-by: Julien Rouhaud
Author: Michael Paquier
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/20200227080735.l32fqcauy73lon7o@nol
Backpatch-through: 12

src/backend/catalog/index.c

@@ -1675,12 +1675,13 @@ index_concurrently_swap(Oid newIndexId, Oid oldIndexId, const char *oldName)
 	}
 
 	/*
-	 * Move all dependencies of and on the old index to the new one.  First
-	 * remove any dependencies that the new index may have to provide an
-	 * initial clean state for the dependency switch, and then move all the
-	 * dependencies from the old index to the new one.
+	 * Swap all dependencies of and on the old index to the new one, and
+	 * vice-versa.  Note that a call to CommandCounterIncrement() would cause
+	 * duplicate entries in pg_depend, so this should not be done.
 	 */
-	deleteDependencyRecordsFor(RelationRelationId, newIndexId, false);
+	changeDependenciesOf(RelationRelationId, newIndexId, oldIndexId);
+	changeDependenciesOn(RelationRelationId, newIndexId, oldIndexId);
+
 	changeDependenciesOf(RelationRelationId, oldIndexId, newIndexId);
 	changeDependenciesOn(RelationRelationId, oldIndexId, newIndexId);