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
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