Not deleted mentions of the cleared path

Hi, hackers!

I have already written about the problem of InvalidPath [0] appearing. I
investigated this and found an error in the add_path() function, when we
reject a path, we free up the memory of the path, but do not delete
various mentions of it (for example, in the ancestor of relation, as in
the example below).

Thus, we may face the problem of accessing the freed memory.

I demonstrated this below using gdb when I call a query after running a
test in test/regression/sql/create_misc.sql:

*Query:*

:-- That ALTER TABLE should have added TOAST tables.

SELECT relname, reltoastrelid <> 0 AS has_toast_table
   FROM pg_class
   WHERE oid::regclass IN ('a_star', 'c_star')
   ORDER BY 1;

--UPDATE b_star*
--   SET a = text 'gazpacho'
--   WHERE aa > 4;

SELECT class, aa, a FROM a_star*;

*gdb:
*

0x00007ff3f7325fda in epoll_wait (epfd=5, events=0x55bf9ee75c38,
maxevents=1, timeout=-1) at ../sysdeps/unix/sysv/linux/epoll_wait.c:30
30  ../sysdeps/unix/sysv/linux/epoll_wait.c: No such file or directory.
(gdb) b /home/alena/postgrespro_3/src/backend/optimizer/util/pathnode.c:620
Breakpoint 1 at 0x55bf9cfe4c65: file pathnode.c, line 621.
(gdb) c
Continuing.

Breakpoint 1, add_path (parent_rel=0x55bf9ef7f5c0,
new_path=0x55bf9ef7f4e0) at pathnode.c:621
621     if (!IsA(new_path, IndexPath))
(gdb) n
622       pfree(new_path);
(gdb) n
624 }
(gdb) p *new_path
$1 = {type = T_Invalid, pathtype = T_Invalid, parent =
0x7f7f7f7f7f7f7f7f, pathtarget = 0x7f7f7f7f7f7f7f7f,
  param_info = 0x7f7f7f7f7f7f7f7f, parallel_aware = 127, parallel_safe
= 127, parallel_workers = 2139062143,
  rows = 1.3824172084878715e+306, startup_cost =
1.3824172084878715e+306, total_cost = 1.3824172084878715e+306,
  pathkeys = 0x7f7f7f7f7f7f7f7f}
*(gdb) p new_path
$2 = (Path *) 0x55bf9ef7f4e0*

(gdb) p ((ProjectionPath *)((SortPath*)parent_rel->pathlist->elements
[0].ptr_value)->subpath)->path->parent->cheapest_startup_path
*$20 = (struct Path *) 0x55bf9ef7f4e0*

(gdb) p *((ProjectionPath *)((SortPath*)parent_rel->pathlist->elements
[0].ptr_value)->subpath)->path->parent->cheapest_startup_path
$17 = {type = T_Invalid, pathtype = T_Invalid, parent =
0x7f7f7f7f7f7f7f7f, pathtarget = 0x7f7f7f7f7f7f7f7f,
  param_info = 0x7f7f7f7f7f7f7f7f, parallel_aware = 127, parallel_safe
= 127, parallel_workers = 2139062143,
  rows = 1.3824172084878715e+306, startup_cost =
1.3824172084878715e+306, total_cost = 1.3824172084878715e+306,
  pathkeys = 0x7f7f7f7f7f7f7f7f}

(gdb) p (Path*)(((ProjectionPath
*)((SortPath*)parent_rel->pathlist->elements
[0].ptr_value)->subpath)->path->parent->pathlist->elements[1].ptr_value)
*$21 = (Path *) 0x55bf9ef7f4e0*

(gdb) p *(Path*)(((ProjectionPath
*)((SortPath*)parent_rel->pathlist->elements
[0].ptr_value)->subpath)->path->parent->pathlist->elements[1].ptr_value)
$19 = {type = T_Invalid, pathtype = T_Invalid, parent =
0x7f7f7f7f7f7f7f7f, pathtarget = 0x7f7f7f7f7f7f7f7f,
  param_info = 0x7f7f7f7f7f7f7f7f, parallel_aware = 127, parallel_safe
= 127, parallel_workers = 2139062143,
  rows = 1.3824172084878715e+306, startup_cost =
1.3824172084878715e+306, total_cost = 1.3824172084878715e+306,
  pathkeys = 0x7f7f7f7f7f7f7f7f}
(gdb)

The same problem may be in the add_partial_path() function.

Unfortunately, I have not yet been able to find a problematic query with
the described case, but I have prepared a patch to fix this problem.

What do you think?

0.
https://www.postgresql.org/message-id/flat/01aa76cc-93e1-4de9-ab34-b3a890bf76b3(at)postgrespro(dot)ru

--
Regards,
Alena Rybakina
Postgres Professional