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

[Tierney, Luke]

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, Gábor Csárdi 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 using 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, Gábor Csárdi <csardi.gabor using 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 using 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, Gábor Csárdi <csardi.gabor using 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 using r-project.org mailing list
>>>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>>>
>>>>
>>>
>>
>
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-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
    Actuarial Science
241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu