Add test for CREATE INDEX CONCURRENTLY with not-so-immutable predicate

83158f7 has improved index_set_state_flags() so as it is possible to use
transactional updates when updating pg_index state flags, but there was
not really a test case which stressed directly the possibility it fixed.
This commit adds such a test, using a predicate that looks valid in
appearance but calls a stable function.

Author: Andrey Lepikhov
Discussion: https://postgr.es/m/9b905019-5297-7372-0ad2-e1a4bb66a719@postgrespro.ru
Backpatch-through: 9.6

src/test/regress/expected/create_index.out

@@ -1397,6 +1397,18 @@ BEGIN;
 CREATE INDEX CONCURRENTLY concur_index7 ON concur_heap(f1);
 ERROR:  CREATE INDEX CONCURRENTLY cannot run inside a transaction block
 COMMIT;
+-- test where predicate is able to do a transactional update during
+-- a concurrent build before switching pg_index state flags.
+CREATE FUNCTION predicate_stable() RETURNS bool IMMUTABLE
+LANGUAGE plpgsql AS $$
+BEGIN
+  EXECUTE 'SELECT txid_current()';
+  RETURN true;
+END; $$;
+CREATE INDEX CONCURRENTLY concur_index8 ON concur_heap (f1)
+  WHERE predicate_stable();
+DROP INDEX concur_index8;
+DROP FUNCTION predicate_stable();
 -- But you can do a regular index build in a transaction
 BEGIN;
 CREATE INDEX std_index on concur_heap(f2);

src/test/regress/sql/create_index.sql

@@ -481,11 +481,22 @@ CREATE INDEX CONCURRENTLY concur_index4 on concur_heap(f2) WHERE f1='a';
 CREATE INDEX CONCURRENTLY concur_index5 on concur_heap(f2) WHERE f1='x';
 -- here we also check that you can default the index name
 CREATE INDEX CONCURRENTLY on concur_heap((f2||f1));
-
 -- You can't do a concurrent index build in a transaction
 BEGIN;
 CREATE INDEX CONCURRENTLY concur_index7 ON concur_heap(f1);
 COMMIT;
+-- test where predicate is able to do a transactional update during
+-- a concurrent build before switching pg_index state flags.
+CREATE FUNCTION predicate_stable() RETURNS bool IMMUTABLE
+LANGUAGE plpgsql AS $$
+BEGIN
+  EXECUTE 'SELECT txid_current()';
+  RETURN true;
+END; $$;
+CREATE INDEX CONCURRENTLY concur_index8 ON concur_heap (f1)
+  WHERE predicate_stable();
+DROP INDEX concur_index8;
+DROP FUNCTION predicate_stable();
 
 -- But you can do a regular index build in a transaction
 BEGIN;