parallel::makeCluster(type="FORK") fails with later attached

[jzhangc]

Whenever later is attached, the FORK type makeCluster will fail. PSOCK works.

library(later)
library(parallel)
cl <- makeCluster(2, type = "FORK")

Error message:

Error in makeForkCluster(nnodes = spec, ...) : Cluster setup failed.
In addition: Warning message:
In socketAccept(socket = socket, blocking = TRUE, open = "a+b", :
problem in accepting connections on this socket

Session Info:

R version 4.0.5 (2021-03-31)
Platform: x86_64-apple-darwin17.0 (64-bit)
Running under: macOS Big Sur 10.16
Matrix products: default
LAPACK: /Library/Frameworks/R.framework/Versions/4.0/Resources/lib/libRlapack.dylib
locale:
[1] en_CA.UTF-8/en_CA.UTF-8/en_CA.UTF-8/C/en_CA.UTF-8/en_CA.UTF-8
attached base packages:
[1] parallel stats graphics grDevices utils datasets methods base
other attached packages:
[1] later_1.2.0
loaded via a namespace (and not attached):
[1] compiler_4.0.5 cli_2.5.0 tools_4.0.5 Rcpp_1.0.6 rlang_0.4.11

[wch]

Using git bisect, it looks like this was introduced by b7ec34c, which was part of #141.