Re: memory leak in trigger handling (since PG12)

Hi,

On 2023-05-24 15:38:41 +0200, Tomas Vondra wrote:
> I looked at this again, and I think GetPerTupleMemoryContext(estate)
> might do the trick, see the 0002 part.

Yea, that seems like the right thing here.

> Unfortunately it's not much
> smaller/simpler than just freeing the chunks, because we end up doing
>
> oldcxt = MemoryContextSwitchTo(GetPerTupleMemoryContext(estate));
> updatedCols = ExecGetAllUpdatedCols(relinfo, estate);
> MemoryContextSwitchTo(oldcxt);

We could add a variant of ExecGetAllUpdatedCols that switches the context.

> and then have to pass updatedCols elsewhere. It's tricky to just switch
> to the context (e.g. in ExecASUpdateTriggers/ExecARUpdateTriggers), as
> AfterTriggerSaveEvent allocates other bits of memory too (in a longer
> lived context).

Hm - on a quick look the allocations in trigger.c itself are done with
MemoryContextAlloc().

I did find a problematic path, namely that ExecGetChildToRootMap() ends up
building resultRelInfo->ri_ChildToRootMap in CurrentMemoryContext.

That seems like a flat out bug to me - we can't just store data in a
ResultRelInfo without ensuring the memory is lives long enough. Nearby places
like ExecGetRootToChildMap() do make sure to switch to es_query_cxt.

Did you see other bits of memory getting allocated in CurrentMemoryContext?

> So we'd have to do another switch again. Not sure how
> backpatch-friendly would that be.

Yea, that's a valid concern. I think it might be reasonable to use something
like ExecGetAllUpdatedColsCtx() in the backbranches, and switch to a
short-lived context for the trigger invocations in >= 16.

Greetings,

Andres Freund