Обсуждение: Wrong command name in writeable-CTE related error messages

Hi!

I noticed that errors due to writable CTEs in read-only or non-volatile context say the offensive command is SELECT.

For example a writeable CTE in a IMMUTABLE function:

 CREATE TABLE t (x INTEGER);

 CREATE FUNCTION immutable_func()
  RETURNS INTEGER
  LANGUAGE SQL
  IMMUTABLE
  AS $$
  WITH x AS (
    INSERT INTO t (x) VALUES (1) RETURNING x
  ) SELECT * FROM x;
  $$;

 SELECT immutable_func();

 ERROR:  SELECT is not allowed in a non-volatile function

Or a writeable CTE in read-only transaction:

 START TRANSACTION READ ONLY;
 WITH x AS (
   INSERT INTO t (x) VALUES (1) RETURNING x
 )
 SELECT * FROM x;

 ERROR:  cannot execute SELECT in a read-only transaction

My first thought was that these error messages should mention INSERT, but after looking into the source I’m not sure
anymore.The name of the command is obtained from CreateCommandName(). After briefly looking around it doesn’t seem to
betrivial to introduce something along the line of CreateModifyingCommandName(). 

So I started by using a different error message at those places where I think it should. I’ve attached a patch for
reference,but I’m not happy with it. In particular I’m unsure about the SPI stuff (how to test?) and if there are more
casesas those covered by the patch. Ultimately getting hold of the command name might also be beneficial for a new
errormessage. 

  A WITH clause containing a data-modifying statement is not allowed in a read-only transaction

It wouldn’t make me sad if somebody who touches the code more often than once every few years can take care of it.

-markus

Markus Winand <markus.winand@winand.at> writes:
> I noticed that errors due to writable CTEs in read-only or non-volatile context say the offensive command is SELECT.

Good point.

> My first thought was that these error messages should mention INSERT, but after looking into the source I’m not sure
anymore.The name of the command is obtained from CreateCommandName(). After briefly looking around it doesn’t seem to
betrivial to introduce something along the line of CreateModifyingCommandName(). 

Yeah, you would have to inspect the plan tree pretty carefully to
determine that.

Given the way the test is written, maybe it'd make sense to forget about
mentioning the command name, and instead identify the table we are
complaining about:

ERROR: table "foo" cannot be modified in a read-only transaction

I don't see any huge point in using PreventCommandIfReadOnly if we
go that way, so no refactoring is needed: just test XactReadOnly
directly.

            regards, tom lane

> On 23.05.2023, at 19:40, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Markus Winand <markus.winand@winand.at> writes:
>> I noticed that errors due to writable CTEs in read-only or non-volatile context say the offensive command is SELECT.
>
> Good point.
>
>> My first thought was that these error messages should mention INSERT, but after looking into the source I’m not sure
anymore.The name of the command is obtained from CreateCommandName(). After briefly looking around it doesn’t seem to
betrivial to introduce something along the line of CreateModifyingCommandName(). 
>
> Yeah, you would have to inspect the plan tree pretty carefully to
> determine that.
>
> Given the way the test is written, maybe it'd make sense to forget about
> mentioning the command name, and instead identify the table we are
> complaining about:
>
> ERROR: table "foo" cannot be modified in a read-only transaction

Attached patch takes the active form:

    cannot modify table ”foo" in a read-only transaction

It obtains the table name by searching rtable for an RTE_RELATION with rellockmode == RowExclusiveLock. Not sure if
thereare any cases where that falls apart. 

> I don't see any huge point in using PreventCommandIfReadOnly if we
> go that way, so no refactoring is needed: just test XactReadOnly
> directly.

As there are several places where this is needed, the patch introduces some utility functions.

More interestingly, I found that BEGIN ATOMIC bodies of non-volatile functions happily accept data-modifying statements
andFOR UPDATE. While they fail at runtime it was my expectation that this would be caught at CREATE time. The attached
patchalso takes care of this by walking the Query tree and looking for resultRelation and hasForUpdate — assuming that
non-volatilefunctions should have neither. Let me know if this is desired behavior or not. 

-markus