Message-ID: <4de6a4f5-1c1e-2dbd-a650-38f90baa12e9@postgrespro.ru>
Date: Thu, 15 Jun 2023 13:46:02 +0300
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Subject: Re: When IMMUTABLE is not.
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To: pgsql-hackers@lists.postgresql.org
References: <389c986d-fbb4-c644-9280-db7836af7ca9@postgrespro.ru>
From: Yura Sokolov <y.sokolov@postgrespro.ru>
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Sorry, previous message were smashed for some reason.

I'll try to repeat

I found, than declaration of function as IMMUTABLE/STABLE is not enough 
to be sure
function doesn't manipulate data.

In fact, SPI checks only direct function kind, but fails to check 
indirect call.

Attached immutable_not.sql creates 3 functions:

- `immutable_direct` is IMMUTABLE and tries to insert into table directly.
   PostgreSQL correctly detects and forbids this action.

- `volatile_direct` is VOLATILE and inserts into table directly.
   It is allowed and executed well.

- `immutable_indirect` is IMMUTABLE and calls `volatile_direct`.
   PostgreSQL failed to detect and prevent this DML manipulation.

Output:

select immutable_direct('immutable_direct');
psql:immutable_not.sql:28: ERROR:  INSERT is not allowed in a 
non-volatile function
CONTEXT:  SQL statement "insert into xxx values(j)"
PL/pgSQL function immutable_direct(character varying) line 3 at SQL 
statement

select volatile_direct('volatile_direct');
volatile_direct
-----------------
volatile_direct
(1 row)

select immutable_indirect('immutable_indirect');
immutable_indirect
--------------------
immutable_indirect
(1 row)

select * from xxx;
         i
--------------------
volatile_direct
immutable_indirect
(2 rows)

Attached forbid-non-volatile-mutations.diff add checks readonly function 
didn't made data manipulations.
Output for patched version:

select immutable_indirect('immutable_indirect');
psql:immutable_not.sql:32: ERROR:  Damn2! Update were done in a 
non-volatile function
CONTEXT:  SQL statement "SELECT volatile_direct(j)"
PL/pgSQL function immutable_indirect(character varying) line 3 at PERFORM

I doubt check should be done this way. This check is necessary, but it 
should be
FATAL instead of ERROR. And ERROR should be generated at same place, when
it is generated for `immutable_direct`, but with check of "read_only" 
status through
whole call stack instead of just direct function kind.

-----

regards,
Yura Sokolov
Postgres Professional