Fix assertion failure in pg_prewarm() on objects without storage.

An assertion test added in commit 049ef33 could fail when pg_prewarm()
was called on objects without storage, such as partitioned tables.
This resulted in the following failure in assert-enabled builds:

    Failed Assert("RelFileNumberIsValid(rlocator.relNumber)")

Note that, in non-assert builds, pg_prewarm() just failed with an error
in that case, so there was no ill effect in practice.

This commit fixes the issue by having pg_prewarm() raise an error early
if the specified object has no storage. This approach is similar to
the fix in commit 4623d7144 for pg_freespacemap.

Back-patched to v17, where the issue was introduced.

Author: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/e082e6027610fd0a4091ae6d033aa117@oss.nttdata.com
Backpatch-through: 17

contrib/pg_prewarm/Makefile

@@ -10,6 +10,8 @@ EXTENSION = pg_prewarm
 DATA = pg_prewarm--1.1--1.2.sql pg_prewarm--1.1.sql pg_prewarm--1.0--1.1.sql
 PGFILEDESC = "pg_prewarm - preload relation data into system buffer cache"
 
+REGRESS = pg_prewarm
+
 TAP_TESTS = 1
 
 ifdef USE_PGXS

contrib/pg_prewarm/expected/pg_prewarm.out

@@ -0,0 +1,10 @@
+-- Test pg_prewarm extension
+CREATE EXTENSION pg_prewarm;
+-- pg_prewarm() should fail if the target relation has no storage.
+CREATE TABLE test (c1 int) PARTITION BY RANGE (c1);
+SELECT pg_prewarm('test', 'buffer');
+ERROR:  relation "test" does not have storage
+DETAIL:  This operation is not supported for partitioned tables.
+-- Cleanup
+DROP TABLE test;
+DROP EXTENSION pg_prewarm;

contrib/pg_prewarm/meson.build

@@ -29,6 +29,11 @@ tests += {
   'name': 'pg_prewarm',
   'sd': meson.current_source_dir(),
   'bd': meson.current_build_dir(),
+  'regress': {
+    'sql': [
+      'pg_prewarm',
+    ],
+  },
   'tap': {
     'tests': [
       't/001_basic.pl',

contrib/pg_prewarm/pg_prewarm.c

@@ -128,6 +128,14 @@ pg_prewarm(PG_FUNCTION_ARGS)
 	if (aclresult != ACLCHECK_OK)
 		aclcheck_error(aclresult, get_relkind_objtype(rel->rd_rel->relkind), get_rel_name(relOid));
 
+	/* Check that the relation has storage. */
+	if (!RELKIND_HAS_STORAGE(rel->rd_rel->relkind))
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("relation \"%s\" does not have storage",
+						RelationGetRelationName(rel)),
+				 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
+
 	/* Check that the fork exists. */
 	if (!smgrexists(RelationGetSmgr(rel), forkNumber))
 		ereport(ERROR,

contrib/pg_prewarm/sql/pg_prewarm.sql

@@ -0,0 +1,10 @@
+-- Test pg_prewarm extension
+CREATE EXTENSION pg_prewarm;
+
+-- pg_prewarm() should fail if the target relation has no storage.
+CREATE TABLE test (c1 int) PARTITION BY RANGE (c1);
+SELECT pg_prewarm('test', 'buffer');
+
+-- Cleanup
+DROP TABLE test;
+DROP EXTENSION pg_prewarm;