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

[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.

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.

Best,

luke

On Mon, 26 Mar 2018, Iñaki Úcar wrote:

> Hi,
>
> I initially opened an issue in the R6 repo because my issue was with
> an R6 object. But Winston (thanks!) further simplified my example, and
> it turns out that the issue (whether a feature or a bug is yet to be
> seen) had to do with S3 dispatching.
>
> The following example, by Winston, depicts the issue:
>
> print.foo <- function(x, ...) {
>  cat("print.foo called\n")
>  invisible(x)
> }
>
> new_foo <- function() {
>  e <- new.env()
>  reg.finalizer(e, function(e) message("Finalizer called"))
>  class(e) <- "foo"
>  e
> }
>
> new_foo()
> gc() # still in .Last.value
> gc() # nothing
>
> I would expect that the second call to gc() should free 'e', but it's
> not. However, if we call now *any* S3 method, then the object can be
> finally gc'ed:
>
> print(1)
> gc() # Finalizer called
>
> So the hypothesis is that there is some kind of caching (?) mechanism
> going on. Intended behaviour or not, this is something that was
> introduced between R 3.2.3 and 3.3.2 (the first succeeds; from the
> second on, the example fails as described above).
>
> Regards,
> Iñaki
>
> PS: Further discussion and examples in https://github.com/r-lib/R6/issues/140
>
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>

-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney at uiowa.edu
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