Preserve pg_index.indisclustered across REINDEX CONCURRENTLY

If the flag value is lost, a CLUSTER query following REINDEX
CONCURRENTLY could fail.  Non-concurrent REINDEX is already handling
this case consistently.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20200229024202.GH29456@telsasoft.com
Backpatch-through: 12

Author: michaelpq

src/backend/catalog/index.c

@@ -1527,7 +1527,13 @@ index_concurrently_swap(Oid newIndexId, Oid oldIndexId, const char *oldName)
 	newIndexForm->indimmediate = oldIndexForm->indimmediate;
 	oldIndexForm->indimmediate = true;
 
-	/* Mark old index as valid and new as invalid as index_set_state_flags */
+	/* Preserve indisclustered in the new index */
+	newIndexForm->indisclustered = oldIndexForm->indisclustered;
+
+	/*
+	 * Mark the old index as valid, and the new index as invalid similarly
+	 * to what index_set_state_flags() does.
+	 */
 	newIndexForm->indisvalid = true;
 	oldIndexForm->indisvalid = false;
 	oldIndexForm->indisclustered = false;

src/test/regress/expected/create_index.out

@@ -2128,6 +2128,19 @@ SELECT obj_description('testcomment_idx1'::regclass, 'pg_class');
 (1 row)
 
 DROP TABLE testcomment;
+-- Check that indisclustered updates are preserved
+CREATE TABLE concur_clustered(i int);
+CREATE INDEX concur_clustered_i_idx ON concur_clustered(i);
+ALTER TABLE concur_clustered CLUSTER ON concur_clustered_i_idx;
+REINDEX TABLE CONCURRENTLY concur_clustered;
+SELECT indexrelid::regclass, indisclustered FROM pg_index
+  WHERE indrelid = 'concur_clustered'::regclass;
+       indexrelid       | indisclustered 
+------------------------+----------------
+ concur_clustered_i_idx | t
+(1 row)
+
+DROP TABLE concur_clustered;
 -- Partitions
 -- Create some partitioned tables
 CREATE TABLE concur_reindex_part (c1 int, c2 int) PARTITION BY RANGE (c1);

src/test/regress/sql/create_index.sql

@@ -858,6 +858,15 @@ SELECT obj_description('testcomment_idx1'::regclass, 'pg_class');
 REINDEX TABLE CONCURRENTLY testcomment ;
 SELECT obj_description('testcomment_idx1'::regclass, 'pg_class');
 DROP TABLE testcomment;
+-- Check that indisclustered updates are preserved
+CREATE TABLE concur_clustered(i int);
+CREATE INDEX concur_clustered_i_idx ON concur_clustered(i);
+ALTER TABLE concur_clustered CLUSTER ON concur_clustered_i_idx;
+REINDEX TABLE CONCURRENTLY concur_clustered;
+SELECT indexrelid::regclass, indisclustered FROM pg_index
+  WHERE indrelid = 'concur_clustered'::regclass;
+DROP TABLE concur_clustered;
+
 -- Partitions
 -- Create some partitioned tables
 CREATE TABLE concur_reindex_part (c1 int, c2 int) PARTITION BY RANGE (c1);