disallow TRUNCATE on user_catalog_table

When TRUNCATE command is executed on user_catalog_table
in synchronous_mode during logial replication, TRUNCATE waits for
the subscriber's synchronization. At the same time, we don't have
a restriction that output plugin cannot take a lock on it. Therefore,
the walsender can hang due to the deadlock to take a new lock
on a user_catalog_table, since the table is locked by TRUNCATE already.
This patch addresses such a deadlock hazards by prohibiting
an operation to issue TRUNCATE on user_catalog_table.

Author: Osumi Takamichi

doc/src/sgml/ref/truncate.sgml

@@ -176,6 +176,10 @@ TRUNCATE [ TABLE ] [ ONLY ] <replaceable class="parameter">name</replaceable> [
    supported by the foreign data wrapper, for instance,
    see <xref linkend="postgres-fdw"/>.
   </para>
+
+  <para>
+    <command>TRUNCATE</command> on user_catalog_table is not supported.
+  </para>
  </refsect1>
 
  <refsect1>

src/backend/commands/tablecmds.c

@@ -2435,6 +2435,7 @@ static void
 truncate_check_rel(Oid relid, Form_pg_class reltuple)
 {
 	char	   *relname = NameStr(reltuple->relname);
+	Relation	relation = NULL;
 
 	/*
 	 * Only allow truncate on regular tables, foreign tables using foreign
@@ -2478,6 +2479,18 @@ truncate_check_rel(Oid relid, Form_pg_class reltuple)
 				 errmsg("permission denied: \"%s\" is a system catalog",
 						relname)));
 
+	/*
+	 * Disallow truncate on user_catalog_table, with attention to the deadlock
+	 * scenario that output plugin takes an lock on it in synchronous mode of
+	 * logical replication.
+	 */
+	relation = RelationIdGetRelation(relid);
+	if (RelationIsUsedAsCatalogTable(relation))
+		ereport(ERROR,
+				errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				errmsg("cannot TRUNCATE an user_catalog_table"));
+	RelationClose(relation);
+
 	InvokeObjectTruncateHook(relid);
 }
 

src/test/regress/expected/truncate.out

@@ -592,3 +592,8 @@ SELECT a as "from table trunc_a" FROM trunc_a ORDER BY a;
 (3 rows)
 
 DROP TABLE trunc_a, ref_c;
+-- truncate an user_catalog_table
+CREATE TABLE my_user_catalog_table (col integer) WITH (user_catalog_table = true);
+TRUNCATE my_user_catalog_table; -- should fail
+ERROR:  cannot TRUNCATE an user_catalog_table
+DROP TABLE my_user_catalog_table;

src/test/regress/sql/truncate.sql

@@ -327,3 +327,8 @@ SELECT a as "from table ref_c" FROM ref_c;
 SELECT a as "from table trunc_a" FROM trunc_a ORDER BY a;
 
 DROP TABLE trunc_a, ref_c;
+
+-- truncate an user_catalog_table
+CREATE TABLE my_user_catalog_table (col integer) WITH (user_catalog_table = true);
+TRUNCATE my_user_catalog_table; -- should fail
+DROP TABLE my_user_catalog_table;