Block creation of partitions with open references to its parent

When a partition is created as part of a trigger processing, it is
possible that the partition which just gets created changes the
properties of the table the executor of the ongoing command relies on,
causing a subsequent crash.  This has been found possible when for
example using a BEFORE INSERT which creates a new partition for a
partitioned table being inserted to.

Any attempt to do so is blocked when working on a partition, with
regression tests added for both CREATE TABLE PARTITION OF and ALTER
TABLE ATTACH PARTITION.

Reported-by: Dmitry Shalashov
Author: Amit Langote
Reviewed-by: Michael Paquier, Tom Lane
Discussion: https://postgr.es/m/15437-3fe01ee66bd1bae1@postgresql.org
Backpatch-through: 10

src/backend/commands/tablecmds.c

@@ -1993,6 +1993,14 @@ MergeAttributes(List *schema, List *supers, char relpersistence,
 		else
 			relation = heap_openrv(parent, AccessExclusiveLock);
 
+		/*
+		 * Check for active uses of the parent partitioned table in the
+		 * current transaction, such as being used in some manner by an
+		 * enclosing command.
+		 */
+		if (is_partition)
+			CheckTableNotInUse(relation, "CREATE TABLE .. PARTITION OF");
+
 		/*
 		 * We do not allow partitioned tables and partitions to participate in
 		 * regular inheritance.

src/test/regress/expected/alter_table.out

@@ -3954,6 +3954,24 @@ ERROR:  cannot attach a temporary relation as partition of permanent relation "p
 alter table temp_part_parent attach partition temp_part_child default; -- ok
 drop table perm_part_parent cascade;
 drop table temp_part_parent cascade;
+-- check that attaching partitions to a table while it is being used is
+-- prevented
+create table tab_part_attach (a int) partition by list (a);
+create or replace function func_part_attach() returns trigger
+  language plpgsql as $$
+  begin
+    execute 'create table tab_part_attach_1 (a int)';
+    execute 'alter table tab_part_attach attach partition tab_part_attach_1 for values in (1)';
+    return null;
+  end $$;
+create trigger trig_part_attach before insert on tab_part_attach
+  for each statement execute procedure func_part_attach();
+insert into tab_part_attach values (1);
+ERROR:  cannot ALTER TABLE "tab_part_attach" because it is being used by active queries in this session
+CONTEXT:  SQL statement "alter table tab_part_attach attach partition tab_part_attach_1 for values in (1)"
+PL/pgSQL function func_part_attach() line 4 at EXECUTE
+drop table tab_part_attach;
+drop function func_part_attach();
 -- test case where the partitioning operator is a SQL function whose
 -- evaluation results in the table's relcache being rebuilt partway through
 -- the execution of an ATTACH PARTITION command

src/test/regress/expected/create_table.out

@@ -910,3 +910,19 @@ ERROR:  cannot create a temporary relation as partition of permanent relation "p
 create temp table temp_part partition of temp_parted default; -- ok
 drop table perm_parted cascade;
 drop table temp_parted cascade;
+-- check that adding partitions to a table while it is being used is prevented
+create table tab_part_create (a int) partition by list (a);
+create or replace function func_part_create() returns trigger
+  language plpgsql as $$
+  begin
+    execute 'create table tab_part_create_1 partition of tab_part_create for values in (1)';
+    return null;
+  end $$;
+create trigger trig_part_create before insert on tab_part_create
+  for each statement execute procedure func_part_create();
+insert into tab_part_create values (1);
+ERROR:  cannot CREATE TABLE .. PARTITION OF "tab_part_create" because it is being used by active queries in this session
+CONTEXT:  SQL statement "create table tab_part_create_1 partition of tab_part_create for values in (1)"
+PL/pgSQL function func_part_create() line 3 at EXECUTE
+drop table tab_part_create;
+drop function func_part_create();

src/test/regress/sql/alter_table.sql

@@ -2607,6 +2607,22 @@ alter table temp_part_parent attach partition temp_part_child default; -- ok
 drop table perm_part_parent cascade;
 drop table temp_part_parent cascade;
 
+-- check that attaching partitions to a table while it is being used is
+-- prevented
+create table tab_part_attach (a int) partition by list (a);
+create or replace function func_part_attach() returns trigger
+  language plpgsql as $$
+  begin
+    execute 'create table tab_part_attach_1 (a int)';
+    execute 'alter table tab_part_attach attach partition tab_part_attach_1 for values in (1)';
+    return null;
+  end $$;
+create trigger trig_part_attach before insert on tab_part_attach
+  for each statement execute procedure func_part_attach();
+insert into tab_part_attach values (1);
+drop table tab_part_attach;
+drop function func_part_attach();
+
 -- test case where the partitioning operator is a SQL function whose
 -- evaluation results in the table's relcache being rebuilt partway through
 -- the execution of an ATTACH PARTITION command

src/test/regress/sql/create_table.sql

@@ -730,3 +730,17 @@ create temp table temp_part partition of perm_parted default; -- error
 create temp table temp_part partition of temp_parted default; -- ok
 drop table perm_parted cascade;
 drop table temp_parted cascade;
+
+-- check that adding partitions to a table while it is being used is prevented
+create table tab_part_create (a int) partition by list (a);
+create or replace function func_part_create() returns trigger
+  language plpgsql as $$
+  begin
+    execute 'create table tab_part_create_1 partition of tab_part_create for values in (1)';
+    return null;
+  end $$;
+create trigger trig_part_create before insert on tab_part_create
+  for each statement execute procedure func_part_create();
+insert into tab_part_create values (1);
+drop table tab_part_create;
+drop function func_part_create();