reg.finalizer() in an R package does not execute at the end of an R session

Question

From the documentation ?reg.finalizer in R:

Inter alia, it provides a way to program code to be run at the end of an R session without manipulating .Last. For use in a package, it is often a good idea to set a finalizer on an object in the namespace: then it will be called at the end of the session, or soon after the namespace is unloaded if that is done during the session.

It seems that I can use reg.finalizer() to run certain code when an R session is ended, but it does not work for me. I have prepared a minimal package at https://github.com/yihui/finalizer-test, which basically contains the code below:

e = new.env()

reg.finalizer(e, function(e) {
  message('Bye!')
}, onexit = TRUE)

If I simply run the above code in an interactive R session and quit the session, I can see the message Bye!, but if I install the above package (you can use devtools::install_github('yihui/finalizer-test')), load it in an R session, and quit the R session, I don't see the message. I wonder why the finalizer is not executed in this case.

FWIW, when I install the package, I can see the message Bye!:

$ R CMD INSTALL .

* installing to library ‘/Users/yihui/R’
* installing *source* package ‘finalizer’ ...
** R
** preparing package for lazy loading
No man pages found in package  ‘finalizer’ 
** help
*** installing help indices
Bye!
** building package indices
** testing if installed package can be loaded
* DONE (finalizer)

I don't see the message, either, when I run the command below:

$ R -e "library(finalizer)"
> library(finalizer)
> 
> 
$ 

Improbable

Answer 1

It works if you register the finalizer function to the package name space environment. You can use the .onLoad hook for that:

e = new.env()

reg.finalizer(e, function(e) {
  message('Object Bye!')
}, onexit = TRUE)


finalize <- function(env) {
   print(ls(env))
   message("Bye from Name space Finalizer")
}


.onLoad <- function(libname, pkgname) {
   parent <- parent.env(environment())
   print(str(parent))
   reg.finalizer(parent, finalize, onexit= TRUE)
}

The object finalizer function is not called but since you have the whole namespace environment you probably don't need it.

I created a fork of your test package on https://github.com/mpbastos/finalizer-test:

> devtools::install_git("https://github.com/mpbastos/finalizer-test")
Downloading git repo https://github.com/mpbastos/finalizer-test
Installing finalizer
"C:/PROGRA~1/R/R-34~1.0/bin/x64/R" --no-site-file --no-environ --no-save  \
  --no-restore --quiet CMD INSTALL  \
  "C:/Users/mbastos/AppData/Local/Temp/RtmpOGymjQ/file5cf829e63957"  \
  --library="\\sharedfs/MyDocs6/mbastos/Documents/R/win-library/3.4"  \
  --install-tests

* installing *source* package 'finalizer' ...
** R
** preparing package for lazy loading
** help
No man pages found in package  'finalizer'
*** installing help indices
** building package indices
Object Bye!
** testing if installed package can be loaded
*** arch - i386
<environment: namespace:finalizer>
NULL
[1] "e"        "finalize"
Bye from Name space Finalizer
*** arch - x64
<environment: namespace:finalizer>
NULL
[1] "e"        "finalize"
Bye from Name space Finalizer
* DONE (finalizer)
> library(finalizer)
<environment: namespace:finalizer>
NULL
> q()
Save workspace image? [y/n/c]: n
[1] "e"        "finalize"
Bye from Name space Finalizer

Bobbiebobbin

Answer 2

Not really an answer (the accepted one is fine), but I've also noted that in some cases functions like print and cat produce no visible output when the finalizer is run on exit, at least when using RStudio.

I haven't been able to see a pattern, but I guess there is a racing condition, and that in some cases stdout is closed before the finalizer-function is executed/finished. And I guess the same issue could play up with using message()

So testing the function with message might be misleading you if you want to execute it on exit.

Keldah