The worker is not freed if the call is interrupted.

[randy3k]

Describe the bug

The discussion originates from HenrikBengtsson/future.callr#20 where I discovered that
the worker is not freed if the call to future_lapply is interrupted. The issue is not only applied to future.callr, multisession has similar issues.

Reproduce example

future::plan(future.callr::callr, workers = 5)
R.utils::withTimeout(
    future.apply::future_lapply(1:5, function(.) Sys.sleep(1000)), 
    timeout = 2,
    onTimeout = "silent")
#> NULL
for (i in 1:10) gc()
future::nbrOfFreeWorkers()
#> [1] 0

^{Created on 2021-12-06 by the reprex package (v2.0.1)}

Expected behavior

Expect that future::nbrOfFreeWorkers should recover.

Session information

r$> sessionInfo()
R version 4.1.2 (2021-11-01)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Big Sur 10.16

Matrix products: default
BLAS:   /Library/Frameworks/R.framework/Versions/4.1/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.1/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

loaded via a namespace (and not attached):
[1] compiler_4.1.2

[randy3k]

In the context of future.apply, I think that we should install an interrupt handler to cleanup the unresolved futures when the code is interrupted.

As described in HenrikBengtsson/future.callr#20, a more general issue is that we should free up the workers when the future loses its reference.