[External] Re: Background R session on Unix and SIGINT

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