Re: aggregate crash

Andres Freund <andres@anarazel.de> writes:
> On 2020-01-14 17:01:01 -0500, Tom Lane wrote:
>> But I agree that not checking null-ness
>> explicitly is kind of unsafe.  We've never before had any expectation
>> that the Datum value of a null is anything in particular.

> I'm still not sure I actually fully understand the bug. It's obvious how
> returning the input value again could lead to memory not being freed (so
> that leak seems to go all the way back). And similarly, since the
> introduction of expanded objects, it can also lead to the expanded
> object not being deleted.
> But that's not the problem causing the crash here. What I think must
> instead be the problem is that pergroupstate->transValueIsNull, but
> pergroupstate->transValue is set to something looking like a
> pointer. Which caused us not to datumCopy() a new transition value into
> a long lived context. and then a later transition causes us to free the
> short-lived value?

Yeah, I was kind of wondering that too.  While formally the Datum value
for a null is undefined, I'm not aware offhand of any functions that
wouldn't return zero --- and this would have to be an aggregate transition
function doing so, which reduces the universe of candidates quite a lot.
Plus there's the question of how often a transition function would return
null for non-null input at all.

Could we see a test case that provokes this crash, even if it doesn't
do so reliably?

            regards, tom lane