[Rd] Objects not gc'ed due to caching (?) in R's S3 dispatch mechanism

[Iñaki Úcar]

2018-03-27 6:02 GMT+02:00  <luke-tierney at uiowa.edu>:
> This has nothing to do with printing or dispatch per se. It is the
> result of an internal register (R_ReturnedValue) being protected. It
> gets rewritten whenever there is a jump, e.g. by an explicit return
> call. So a simplified example is
>
> new_foo <- function() {
>   e <- new.env()
>     reg.finalizer(e, function(e) message("Finalizer called"))
>       e
>       }
>
> bar <- function(x) return(x)
>
> bar(new_foo())
> gc() # still in .Last.value
> gc() # nothing
>
> UseMethod essentially does a return call so you see the effect there.

Understood. Thanks for the explanation, Luke.

> The R_ReturnedValue register could probably be safely cleared in more
> places but it isn't clear exactly where. As things stand it will be
> cleared on the next use of a non-local transfer of control, and those
> happen frequently enough that I'm not convinced this is worth
> addressing, at least not at this point in the release cycle.

I barely know the R internals, and I'm sure there's a good reason
behind this change (R 3.2.3 does not show this behaviour), but IMHO
it's, at the very least, confusing. When .Last.value is cleared, that
object loses the last reference, and I'd expect it to be eligible for
gc.

In my case, I was using an object that internally generates a bunch of
data. I discovered this because I was benchmarking the execution, and
I was running out of memory because the memory wasn't been freed as it
was supposed to. So I spent half of the day on this because I thought
I had a memory leak. :-\ (Not blaming anyone here, of course; just
making a case to show that this may be worth addressing at some
point). :-)

Regards,
Iñaki

>
> Best,
>
> luke
>