Re: On login trigger: take three

On Tue, Jan 23, 2024 at 8:00 PM Alexander Lakhin <exclusion@gmail.com> wrote:
> Please look at the following query, which triggers an assertion failure on
> updating the field dathasloginevt for an entry in pg_database:
> SELECT format('CREATE DATABASE db1 LOCALE_PROVIDER icu ICU_LOCALE en ENCODING utf8
> ICU_RULES ''' || repeat(' ', 200000) || ''' TEMPLATE template0;')
> \gexec
> \c db1 -
>
> CREATE FUNCTION on_login_proc() RETURNS event_trigger AS $$
> BEGIN
>    RAISE NOTICE 'You are welcome!';
> END;
> $$ LANGUAGE plpgsql;
>
> CREATE EVENT TRIGGER on_login_trigger ON login EXECUTE PROCEDURE on_login_proc();
> DROP EVENT TRIGGER on_login_trigger;
>
> \c
>
> \connect: connection to server on socket "/tmp/.s.PGSQL.5432" failed: server closed the connection unexpectedly
>
> The stack trace of the assertion failure is:
> ...
> #5  0x000055c8699b9b8d in ExceptionalCondition (
>      conditionName=conditionName@entry=0x55c869a1f7c0 "HaveRegisteredOrActiveSnapshot()",
>      fileName=fileName@entry=0x55c869a1f4c6 "toast_internals.c", lineNumber=lineNumber@entry=669) at assert.c:66
> #6  0x000055c86945df0a in init_toast_snapshot (...) at toast_internals.c:669
> #7  0x000055c86945dfbe in toast_delete_datum (...) at toast_internals.c:429
> #8  0x000055c8694fd1da in toast_tuple_cleanup (...) at toast_helper.c:309
> #9  0x000055c8694b55a1 in heap_toast_insert_or_update (...) at heaptoast.c:333
> #10 0x000055c8694a8c6c in heap_update (... at heapam.c:3604
> #11 0x000055c8694a96cb in simple_heap_update (...) at heapam.c:4062
> #12 0x000055c869555b7b in CatalogTupleUpdate (...) at indexing.c:322
> #13 0x000055c8695f0725 in EventTriggerOnLogin () at event_trigger.c:957
> ...
>
> Funnily enough, when Tom Lane was wondering, whether pg_database's toast
> table could pose a risk [1], I came to the conclusion that it's impossible,
> but that was before the login triggers introduction...
>
> [1] https://www.postgresql.org/message-id/1284094.1695479962%40sss.pgh.pa.us

Thank you for reporting.  I'm looking into this...
I wonder if there is a way to avoid toast update given that we don't
touch the toasted field here.

------
Regards,
Alexander Korotkov