Prevent pgstats from getting confused when relkind of a relation changes

When the relkind of a relache entry changes, because a table is converted into
a view, pgstats can get confused in 15+, leading to crashes or assertion
failures.

For HEAD, Tom fixed this in b23cd185fd, by removing support for converting a
table to a view, removing the source of the inconsistency. This commit just
adds an assertion that a relcache entry's relkind does not change, just in
case we end up with another case of that in the future. As there's no cases of
changing relkind anymore, we can't add a test that that's handled correctly.

For 15, fix the problem by not maintaining the association with the old pgstat
entry when the relkind changes during a relcache invalidation processing. In
that case the pgstat entry needs to be unlinked first, to avoid
PgStat_TableStatus->relation getting out of sync. Also add a test reproducing
the issues.

No known problem exists in 11-14, so just add the test there.

Reported-by: vignesh C <vignesh21@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CALDaNm2yXz+zOtv7y5zBd5WKT8O0Ld3YxikuU3dcyCvxF7gypA@mail.gmail.com
Discussion: https://postgr.es/m/CALDaNm3oZA-8Wbps2Jd1g5_Gjrr-x3YWrJPek-mF5Asrrvz2Dg@mail.gmail.com
Backpatch: 15-

src/test/regress/expected/create_view.out

@@ -1898,6 +1898,29 @@ select pg_get_viewdef('tt26v', true);
     FROM ( VALUES (1,2,3)) v(x, y, z);
 (1 row)
 
+-- Test that changing the relkind of a relcache entry doesn't cause
+-- trouble. Prior instances of where it did:
+-- CALDaNm2yXz+zOtv7y5zBd5WKT8O0Ld3YxikuU3dcyCvxF7gypA@mail.gmail.com
+-- CALDaNm3oZA-8Wbps2Jd1g5_Gjrr-x3YWrJPek-mF5Asrrvz2Dg@mail.gmail.com
+CREATE TABLE tt26(c int);
+BEGIN;
+CREATE TABLE tt27(c int);
+SAVEPOINT q;
+CREATE RULE "_RETURN" AS ON SELECT TO tt27 DO INSTEAD SELECT * FROM tt26;
+SELECT * FROM tt27;
+ c 
+---
+(0 rows)
+
+ROLLBACK TO q;
+CREATE RULE "_RETURN" AS ON SELECT TO tt27 DO INSTEAD SELECT * FROM tt26;
+ROLLBACK;
+BEGIN;
+CREATE TABLE tt28(c int);
+CREATE RULE "_RETURN" AS ON SELECT TO tt28 DO INSTEAD SELECT * FROM tt26;
+CREATE RULE "_RETURN" AS ON SELECT TO tt28 DO INSTEAD SELECT * FROM tt26;
+ERROR:  "tt28" is already a view
+ROLLBACK;
 -- clean up all the random objects we made above
 DROP SCHEMA temp_view_test CASCADE;
 NOTICE:  drop cascades to 27 other objects
@@ -1929,7 +1952,7 @@ drop cascades to view aliased_view_2
 drop cascades to view aliased_view_3
 drop cascades to view aliased_view_4
 DROP SCHEMA testviewschm2 CASCADE;
-NOTICE:  drop cascades to 68 other objects
+NOTICE:  drop cascades to 69 other objects
 DETAIL:  drop cascades to table t1
 drop cascades to view temporal1
 drop cascades to view temporal2
@@ -1998,3 +2021,4 @@ drop cascades to view tt23v
 drop cascades to view tt24v
 drop cascades to view tt25v
 drop cascades to view tt26v
+drop cascades to table tt26

src/test/regress/sql/create_view.sql

@@ -649,6 +649,29 @@ select x + y + z as c1,
 from (values(1,2,3)) v(x,y,z);
 select pg_get_viewdef('tt26v', true);
 
+
+-- Test that changing the relkind of a relcache entry doesn't cause
+-- trouble. Prior instances of where it did:
+-- CALDaNm2yXz+zOtv7y5zBd5WKT8O0Ld3YxikuU3dcyCvxF7gypA@mail.gmail.com
+-- CALDaNm3oZA-8Wbps2Jd1g5_Gjrr-x3YWrJPek-mF5Asrrvz2Dg@mail.gmail.com
+CREATE TABLE tt26(c int);
+
+BEGIN;
+CREATE TABLE tt27(c int);
+SAVEPOINT q;
+CREATE RULE "_RETURN" AS ON SELECT TO tt27 DO INSTEAD SELECT * FROM tt26;
+SELECT * FROM tt27;
+ROLLBACK TO q;
+CREATE RULE "_RETURN" AS ON SELECT TO tt27 DO INSTEAD SELECT * FROM tt26;
+ROLLBACK;
+
+BEGIN;
+CREATE TABLE tt28(c int);
+CREATE RULE "_RETURN" AS ON SELECT TO tt28 DO INSTEAD SELECT * FROM tt26;
+CREATE RULE "_RETURN" AS ON SELECT TO tt28 DO INSTEAD SELECT * FROM tt26;
+ROLLBACK;
+
+
 -- clean up all the random objects we made above
 DROP SCHEMA temp_view_test CASCADE;
 DROP SCHEMA testviewschm2 CASCADE;