Re: segfault in HEAD when too many nested functions call

On 15/07/2017 17:22, Tom Lane wrote:
> Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com> writes:
>> Since b8d7f053c5c2bf2a7e8734fe3327f6a8bc711755 (Andres in Cc), if you
>> write queries which result in infinite recursion (or just too many
>> nested function calls), execution ends with segfault instead of intended
>> exhausted max_stack_depth:
>
> Yes. We discussed this before the patch went in [1].

Ah, I totally forgot about it, sorry.

> I wanted to put
> a stack depth check in ExecProcNode(), and still do. Andres demurred,
> claiming that that was too much overhead, but didn't really provide a
> credible alternative. The thread drifted off without any resolution,
> but clearly we need to do something before 10.0 final.
>

I wanted to add an open item but I see you already did, thanks!

>> Please find attached a trivial patch to fix this. I'm not sure
>> ExecMakeTableFunctionResult() is the best or only place that needs to
>> check the stack depth.
>
> I don't think that that's adequate at all.
>
> I experimented with a variant that doesn't go through
> ExecMakeTableFunctionResult:
>
> [...]
> This manages not to crash, but I think the reason it does not is purely
> accidental: "SELECT so()" has a nontrivial targetlist so we end up running
> ExecBuildProjectionInfo on that, meaning that a fresh expression
> compilation happens at every nesting depth, and there are
> check_stack_depth calls in expression compilation. Surely that's
> something we'd try to improve someday. Or it could accidentally get
> broken by unrelated changes in the way plpgsql sets up queries to be
> executed.
>
> I still think that we really need to add a check in ExecProcNode().
> Even if there's an argument to be made that every recursion would
> somewhere go through ExecMakeTableFunctionResult, very large/complex
> queries could result in substantial stack getting chewed up before
> we get to that --- and we don't have an infinite amount of stack slop.

I was pretty sceptical about checking depth in
ExecMakeTableFunctionResult() only vs ExecProcNode(), and this has
finished convincing me so I definitely agree.

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org