Re: memory leak in trigger handling (since PG12)

On 5/24/23 20:55, Andres Freund wrote:
> 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.
> 

Yeah, we could do that. I was thinking about backpatching, and modifying
 ExecGetAllUpdatedCols signature would be ABI break, but adding a
variant should be fine.

> 
>> 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?
> 

No, I simply tried to do the context switch and then gave up when it
crashed on the ExecGetRootToChildMap info. I haven't looked much
further, but you may be right it's the only bit.

It didn't occur to me it might be a bug - I think the code simply
assumes it gets called with suitable memory context, just like we do in
various other places. Maybe it should document the assumption.

> 
>> 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.
> 

WFM


regards

-- 
Tomas Vondra
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