Check for leaving behind opened connections, sinks, graphical devices

[HenrikBengtsson]

Issue

For example,

f <- future({
  db <- DBI::dbConnect(RSQLite::SQLite(), "db.sqlite", extended_types = TRUE)
  42L
})

will leave behind an open connection on the worker.

Task

We could check for this and produce an informative warning when there's a connection left behind. This can be done by looking at getAllConnections() before and after evaluating the future and report on details from showConnections().

Minimal reproducible example

> ## Before
> cons0 <- getAllConnections()
> cons0
[1] 0 1 2

> f <- future({ con <- textConnection("abc"); as.integer(con) })

> ## During ...
> cons1 <- getAllConnections()
> cons1
[1] 0 1 2 4

> id <- value(f)
> id
[1] 4

> ## After ...
> cons2 <- getAllConnections()
> cons2
[1] 0 1 2 4
> setdiff(cons2, cons1)
[1] 4