Re: strong memory leak in plpgsql from handled rollback and lazy cast

Hi,

On 2019-09-22 20:16:15 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2019-09-22 18:43:23 -0400, Tom Lane wrote:
> >> I'm not convinced that it'd be safe to re-use an ExprState after a
> >> previous execution failed (though perhaps Andres has a different
> >> opinion?)
>
> > I don't immediately see why it'd be problematic to reuse at a later
> > time, as long as it's guaranteed that a) there's only one execution
> > happening at a time b) the failure wasn't in the middle of writing a
> > value. a) would be problematic regardless of reuse-after-failure, and
> > b) should be satisfied by only failing at ereport etc.
>
> I think you're considering a much smaller slice of the system than
> what seems to me to be at risk here.

Yea, I was only referencing the expression eval logic itself, as I
understood your question to aim mainly at that...

> As an example, an ExprState tree would also include any
> fn_extra-linked state that individual functions might've set up.
> We've got very little control over what the validity requirements are
> for those or how robust the code that creates them is. I *think* that
> most of the core code that makes such things is written in a way that
> it doesn't leave partially-valid cache state if setup fails partway
> through ... but I wouldn't swear that it all is, and I'd certainly bet
> money on there being third-party code that isn't careful about that.

Hm. I'd be kinda willing to just declare such code broken. But given
that the memory leak situation, as you say, still exists, I don't think
it matters for now.

> > Hm. I interestingly am working on a patch that merges all the memory
> > allocations done for an ExprState into one or two allocations (by
> > basically doing the traversal twice). Then it'd be feasible to just
> > pfree() the memory, if that helps.
>
> Again, that'd do nothing to clean up subsidiary fn_extra state.
> If we want no leaks, we need a separate context for the tree
> to live in.

Well, it'd presumably leak a lot less :/.

Greetings,

Andres Freund