R devel - May 2025 - Is it possible to gracefully interrupt a child R process on MS Windows?

In help("pskill", package = "tools") is says:

  Only SIGINT and SIGTERM will be defined on Windows, and pskill will
always use the Windows system call TerminateProcess.

As far as I understand it, TerminateProcess [1] terminates the process
"quite abruptly". Specifically, it is not possible for the process to
intercept the termination and gracefully shutdown. In R terms, we
cannot rely on:

tryCatch({
  ...
}, interrupt = function(int) {
  ## cleanup
})

Similarly, it does not look like R itself can exit gracefully. For
example, when signalling pskill(pid, signal = SIGINT) to another R
process, that R process leaves behind its tempdir(). In contrast, if
the user interrupts the process interactively (Ctrl-C), there is an
'interrupt' condition that can be caught, and R cleans up after itself
before exiting.

QUESTION:

Is it possible to gracefully interrupt a child R process on MS
Windows, e.g. a PSOCK cluster node? (I don't think so, but I figure
it's worth asking)


SUGGESTIONS:

Also, if my understanding that TerminateProcess is abrupt is correct,
and there is no way to exit gracefully, would it make sense to clarify
this fact in help("pskill", package = "tools")? Right now
you either
have to know how 'TerminateProcess' works, or run various tests on MS
Windows to figure out the current behavior.

Also, would a better signal mapping be:

  Only SIGKILL will be defined on Windows, and pskill will always use
the Windows system call TerminateProcess. Signals SIGINT and SIGTERM
are supported for backward compatible reasons, but are effectively
identical to SIGKILL.

? That would change the expectations on what will happen for people
coming from the POSIX world.

[1]
https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-terminateprocess

/Henrik

On Sun, 11 May 2025 10:58:18 -0700
Henrik Bengtsson <henrik.bengtsson at gmail.com> wrote:
> Is it possible to gracefully interrupt a child R process on MS
> Windows, e.g. a PSOCK cluster node?
Not in the general case (I think, based on the code paths leading to
Rf_onintr() on Windows), but PSOCK cluster nodes are instances of
Rscript.exe running the terminal front-end, and the terminal front-end
interrupts R upon receipt of Ctrl+C and Ctrl+Break console events:
https://learn.microsoft.com/en-us/windows/console/setconsolectrlhandler

This makes it possible for ps::ps_interrupt() to spawn a child process
to attach to the console where Rscript.exe is running and generate this
event:
https://github.com/r-lib/ps/blob/042d4836ac584c95a59985171fdfa3b6baf2fa6c/src/interrupt.c#L33-L35

This probably needs specially written code in the children that expects
to be interrupted in order to work reliably, but interrupting the
children and then interrupting the parent and submitting another job to
the PSOCK cluster seems to have worked for me on R-4.5.0.

-- 
Best regards,
Ivan

I think for reliability and portability of termination, one needs to 
implement an application-specific termination protocol on both ends. 
Only within specific application constraints, one can also define what 
graceful termination means. Typically, one also has other expectations 
from the termination process - such that the processes will terminate in 
some finite time/soon. In some cases one also may require certain 
behavior of the cleanup code (such as that wouldn't take long, wouldn't 
do some things, etc), to meet the specific termination requirements. And 
it may require some behavior of the non-cleanup code as well (such as 
polling in some intervals).

Using signals to terminate a process even on Unix may not be seen as 
graceful enough, either. It is not just a Windows problem.

Yes, TerminateProcess() on Windows will not allow the target process to 
run any cleanup. The documentation of "pskill" names 
"TerminateProcess()" explicitly so that the readers interested in the 
details can follow Microsoft documentation. But I think one should avoid 
using pskill()/signals for termination and instead use an 
application-level termination protocol. The parallel package, PSOCK, has 
one, based on socket connections, so perhaps one can take some 
inspiration from there.

Best
Tomas

On 5/11/25 19:58, Henrik Bengtsson wrote:
> In help("pskill", package = "tools") is says:
>
>    Only SIGINT and SIGTERM will be defined on Windows, and pskill will
> always use the Windows system call TerminateProcess.
>
> As far as I understand it, TerminateProcess [1] terminates the process
> "quite abruptly". Specifically, it is not possible for the
process to
> intercept the termination and gracefully shutdown. In R terms, we
> cannot rely on:
>
> tryCatch({
>    ...
> }, interrupt = function(int) {
>    ## cleanup
> })
>
> Similarly, it does not look like R itself can exit gracefully. For
> example, when signalling pskill(pid, signal = SIGINT) to another R
> process, that R process leaves behind its tempdir(). In contrast, if
> the user interrupts the process interactively (Ctrl-C), there is an
> 'interrupt' condition that can be caught, and R cleans up after
itself
> before exiting.
>
> QUESTION:
>
> Is it possible to gracefully interrupt a child R process on MS
> Windows, e.g. a PSOCK cluster node? (I don't think so, but I figure
> it's worth asking)
>
>
> SUGGESTIONS:
>
> Also, if my understanding that TerminateProcess is abrupt is correct,
> and there is no way to exit gracefully, would it make sense to clarify
> this fact in help("pskill", package = "tools")? Right
now you either
> have to know how 'TerminateProcess' works, or run various tests on
MS
> Windows to figure out the current behavior.
>
> Also, would a better signal mapping be:
>
>    Only SIGKILL will be defined on Windows, and pskill will always use
> the Windows system call TerminateProcess. Signals SIGINT and SIGTERM
> are supported for backward compatible reasons, but are effectively
> identical to SIGKILL.
>
> ? That would change the expectations on what will happen for people
> coming from the POSIX world.
>
> [1]
https://learn.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-terminateprocess
>
> /Henrik
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel