R devel - Oct 2014 - proper use of reg.finalizer to close connections

Hi all, I have a question about finalizers...
I have a package that manages state for a few connections, and I'd
like to ensure that these connections are 'cleanly' closed upon either
(i) R quitting or (ii) an unloading of the package.
So, in a pared-down example package with a single R file, it looks
something like:

##### BEGIN PACKAGE CODE #####
.CONNS <- new.env(parent = emptyenv())
.CONNS$resource1 <- NULL
.CONNS$resource2 <- NULL
## some more .CONNS resources...

reg.finalizer(.CONNS, function(x) sapply(names(x), disconnect), onexit = TRUE)

connect <- function(x) {
  ## here lies code to connect and update .CONNS[[x]]
}
disconnect <- function(x) {
  print(sprintf("disconnect(%s)", x))
  ## here lies code to disconnect and update .CONNS[[x]]
}
##### END PACKAGE CODE #####

The print(...) statement in disconnect(...) is there as a trace, as I
hoped that I'd see disconnect(...) being called when I quit (or
detach(..., unload = TRUE)).
But, it doesn't appear that disconnect(...) is ever called when the
package (and .CONNS) falls out of memory/scope (and I ran gc() after
detach(...), just to be sure).

In a second 'shot-in-the-dark' attempt, I placed the reg.finalizer
call inside an .onLoad function, but that didn't seem to work, either.

I'm guessing my use of reg.finalizer is way off-base here... but I
cannot infer from the reg.finalizer man page what I might be doing
wrong.
Is there a way to see, at the R-system level, what functions have been
registered as finalizers?

Thanks for any pointers!

-Murat

Hmmm, I guess you will want to put the actual objects that represent
the connections into the environment, at least this seems to be the
easiest to me. Btw. you need ls() to list the contents of an
environment, instead of names(). E.g.

e <- new.env()
e$foo <- 10
e$bar <- "aaa"
names(e)
#> NULL
ls(e)
#> [1] "bar" "foo"
reg.finalizer(e, function(x) { print(ls(x)) })
#> NULL
rm(e)
gc()
#> [1] "bar" "foo"
#>           used (Mb) gc trigger  (Mb) max used  (Mb)
#> Ncells 1528877 81.7    2564037 137.0  2564037 137.0
#> Vcells 3752538 28.7    7930384  60.6  7930356  60.6

More precisely, you probably want to represent each connection as a
separate environment, with its own finalizer. Hope this helps,
Gabor

On Sun, Oct 26, 2014 at 9:49 PM, Murat Tasan <mmuurr at gmail.com>
wrote:
> Hi all, I have a question about finalizers...
> I have a package that manages state for a few connections, and I'd
> like to ensure that these connections are 'cleanly' closed upon
either
> (i) R quitting or (ii) an unloading of the package.
> So, in a pared-down example package with a single R file, it looks
> something like:
>
> ##### BEGIN PACKAGE CODE #####
> .CONNS <- new.env(parent = emptyenv())
> .CONNS$resource1 <- NULL
> .CONNS$resource2 <- NULL
> ## some more .CONNS resources...
>
> reg.finalizer(.CONNS, function(x) sapply(names(x), disconnect), onexit =
TRUE)
>
> connect <- function(x) {
>   ## here lies code to connect and update .CONNS[[x]]
> }
> disconnect <- function(x) {
>   print(sprintf("disconnect(%s)", x))
>   ## here lies code to disconnect and update .CONNS[[x]]
> }
> ##### END PACKAGE CODE #####
>
> The print(...) statement in disconnect(...) is there as a trace, as I
> hoped that I'd see disconnect(...) being called when I quit (or
> detach(..., unload = TRUE)).
> But, it doesn't appear that disconnect(...) is ever called when the
> package (and .CONNS) falls out of memory/scope (and I ran gc() after
> detach(...), just to be sure).
>
> In a second 'shot-in-the-dark' attempt, I placed the reg.finalizer
> call inside an .onLoad function, but that didn't seem to work, either.
>
> I'm guessing my use of reg.finalizer is way off-base here... but I
> cannot infer from the reg.finalizer man page what I might be doing
> wrong.
> Is there a way to see, at the R-system level, what functions have been
> registered as finalizers?
>
> Thanks for any pointers!
>
> -Murat
>
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