[Rd] Background R session on Unix and SIGINT

Gábor Csárdi
Tue, 30 Apr 2019 21:09:51 -0000

OK, I managed to create an example without callr, but it is still
somewhat cumbersome. Anyway, here it is.

Terminal 1:
mkfifo fif
R --no-readline --slave --no-save --no-restore < fif

Terminal 2:
cat > fif
Sys.getpid()

This will make Terminal 1 print the pid of the R process, so we can
send a SIGINT:

Terminal 3:
kill -INT pid

The R process is of course still running happily.

Terminal 2 again:
tryCatch(Sys.sleep(10), interrupt = function(e) e)

and then Terminal 1 prints the interrupt condition:
<interrupt: >

This is macOS and 3.5.3, although I don't think it matters much.

Thanks much!
G.

On Tue, Apr 30, 2019 at 9:50 PM Simon Urbanek
<simon.urbanek at r-project.org> wrote:
>
> Can you give an example without callr? The key is how is the process stated
and what it is doing which is entirely opaque in callr.
>
> Windows doesn't have signals, so the process there is entirely
different. Most of the WIN32 processing is event-based.
>
> Cheers,
> Simon
>
>
> > On Apr 30, 2019, at 4:17 PM, G?bor Cs?rdi <csardi.gabor at
gmail.com> wrote:
> >
> > Yeah, I get that they are async.
> >
> > What happens is that the background process is not doing anything when
> > the process gets a SIGINT. I.e. the background process is just
> > listening on its standard input.
> >
> > AFAICT for an interactive process such a SIGINT is just swallowed,
> > with a newline outputted to the terminal.
> >
> > But apparently, for this background process, it is not swallowed, and
> > it is triggered later. FWIW it does not happen on Windows, not very
> > surprisingly.
> >
> > Gabor
> >
> > On Tue, Apr 30, 2019 at 9:13 PM Simon Urbanek
> > <simon.urbanek at r-project.org> wrote:
> >>
> >> Interrupts are not synchronous in R - the signal only flags the
request for interruption. Nothing actually happens until R_CheckUserInterrupt()
is called at an interruptible point. In you case your code is apparently not
calling R_CheckUserInterrupt() until later as a side-effect of the next
evaluation.
> >>
> >> Cheers,
> >> Simon
> >>
> >>
> >>> On Apr 30, 2019, at 3:44 PM, G?bor Cs?rdi <csardi.gabor at
gmail.com> wrote:
> >>>
> >>> Hi All,
> >>>
> >>> I realize that this is not a really nice reprex, but anyone
has an
> >>> idea why a background R session would "remember" an
interrupt (SIGINT)
> >>> on Unix?
> >>>
> >>> rs <- callr::r_session$new()
> >>> rs$interrupt()     # just sends a SIGINT
> >>> #> [1] TRUE
> >>>
> >>> rs$run(function() 1+1)
> >>> #> Error: interrupt
> >>>
> >>> rs$run(function() 1+1)
> >>> #> [1] 2
> >>>
> >>> It seems that the main loop somehow stores the SIGINT it
receives
> >>> while it is waiting on stdin, and then it triggers it when
some input
> >>> comes in.... Maybe. Just speculating....
> >>>
> >>> Thanks,
> >>> Gabor
> >>>
> >>> ______________________________________________
> >>> R-devel at r-project.org mailing list
> >>> https://stat.ethz.ch/mailman/listinfo/r-devel
> >>>
> >>
> >
>