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

On Apr 30, 2019, at 7:03 PM, G?bor Cs?rdi <csardi.gabor at gmail.com> wrote:

Unfortunately --interactive also makes the session interactive(),
which is bad for me, as it is a background session.

In general, I don't want the interactive behavior, but was wondering
if I could send as SIGINT to try to interrupt the computation of the
background process, and if that does not work, then I would send a
SIGKILL and start up another process. It all works nicely, except for
this glitch, but I think I can work around it.

Thanks,
Gabor

On Tue, Apr 30, 2019 at 10:55 PM Tierney, Luke <luke-tierney at uiowa.edu> wrote:

A Simon pointed out the interrupt is recorded but not processed until
a safe point.

When reading from a fifo or pipe R runs non-interactive, which means
is sits in a read() system call and the interrupt isn't seen until
sometime during evaluation when a safe checkpoint is reached.

When reading from a terminal R will use select() to wait for input and
periodically wake and check for interrupts. In that case the interrupt
will probably be seen sooner.

If the interactive behavior is what you want you can add --interactive
to the arguments used to start R.

Best,

luke

On Tue, 30 Apr 2019, G?bor Cs?rdi wrote:

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

--
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
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I think you're talking about two independent things. You can interrupt the computation, no question about that. It's just that if you send an interrupt while you're *not* doing any computations, it will be signaled but not raised until the interrupts are checked since there is no one to check it.

This goes back to my original response - the interactive REPL calls R_CheckUserInterrupt(), but the straight stdin-prcessing doesn't (since it's expected to be a script, not interactive prompt). If you just want to clear interrupts before next processing you can either just run R_CheckUserInterrupt() explicitly, or on R side do anything that does that, e.g. to take your example "tryCatch(Sys.sleep(0), interrupt = function(e) e)" will clear it.