Re: Transaction timeout - Mailing list pgsql-hackers

> On 26 Jan 2024, at 19:58, Japin Li <japinli@hotmail.com> wrote:
>
> Thanks for updating the patch.  Here are some comments for v24.
>
> +       <para>
> +        Terminate any session that spans longer than the specified amount of
> +        time in transaction. The limit applies both to explicit transactions
> +        (started with <command>BEGIN</command>) and to implicitly started
> +        transaction corresponding to single statement. But this limit is not
> +        applied to prepared transactions.
> +        If this value is specified without units, it is taken as milliseconds.
> +        A value of zero (the default) disables the timeout.
> +       </para>
> The sentence "But this limit is not applied to prepared transactions" is redundant,
> since we have a paragraph to describe this later.
Fixed.
>
> +
> +       <para>
> +        If <varname>transaction_timeout</varname> is shorter than
> +        <varname>idle_in_transaction_session_timeout</varname> or <varname>statement_timeout</varname>
> +        <varname>transaction_timeout</varname> will invalidate longer timeout.
> +       </para>
> +
>
> Since we are already try to disable the timeouts, should we try to disable
> them even if they are equal.

Well, we disable timeouts on equality. Fixed docs.

>
> +
> +       <para>
> +        Prepared transactions are not subject for this timeout.
> +       </para>
>
> Maybe wrap this with <note> is a good idea.
Done.

>
>> I’ve inspected CI fails and they were caused by two different problems:
>> 1. It’s unsafe for isaoltion tester to await transaction_timeout within a query. Usually it gets
>> FATAL: terminating connection due to transaction timeout
>> But if VM is a bit slow it can get occasional
>> PQconsumeInput failed: server closed the connection unexpectedly
>> So, currently all tests use “passive waiting”, in a session that will not timeout.
>>
>> 2. In some cases pg_sleep(0.1) were sleeping up to 200 ms. That was making s7 and s8 fail, because they rely on this
margin.
>
> I'm curious why this happened.
I think pg_sleep() cannot provide guarantees on when next query will be executed. In our case we need that isolation
testersee that sleep is over and continue in other session... 

>> I’ve separated these tests into different test timeouts-long and increased margin to 300ms. Now tests run horrible
2431ms. Moreover I’m afraid that on buildfarm we can have much randomly-slower machines so this test might be excluded. 
>> This test checks COMMIT AND CHAIN and flow of small queries (Nik’s case).
>>
>> Also I’ve verified that every "enable_timeout_after(TRANSACTION_TIMEOUT)” and “disable_timeout(TRANSACTION_TIMEOUT)”
isnecessary and found that case of aborting "idle in transaction (aborted)” is not covered by tests. I’m not sure we
needa test for this. 
>
> I see there is a test about idle_in_transaction_timeout and transaction_timeout.
>
> Both of them only check the session, but don't check the reason, so we cannot
> distinguish the reason they are terminated.  Right?
Yes.
>
>> Japin, Junwang, what do you think?
>
> However, checking the reason on the timeout session may cause regression test
> failed (as you point in 1), I don't strongly insist on it.

Indeed, if we check a reason of FATAL timeouts - we get flaky tests.


Best regards, Andrey Borodin.