Detecting whether a process exists or not by its PID?

[...]
kill(sig=0) is specified by POSIX but indeed as you say there is a race
condition due to PID-reuse.  In principle, detecting that a worker
process is still alive cannot be done correctly outside base R.
I am not sure why you think so.
At user-level I would probably consider some watchdog, e.g. the parallel
tasks would be repeatedly touching a file.
I am pretty sure that there are simpler and better solutions. E.g. one
would be to
ask the worker process for its startup time (with as much precision as possible)
and then use the (pid, startup_time) pair as a unique id.

With this you can check if the process is still running, by checking
that the pid exists,
and that its startup time matches.

This is all very simple with the ps package, on Linux, macOS and Windows.

Gabor
In base R, one can do this correctly for forked processes via
mcparallel/mccollect, not for PSOCK cluster workers which are based on
system() (and I understand it would be a useful feature)

 > j <- mcparallel(Sys.sleep(1000))
 > mccollect(j, wait=FALSE)
NULL

# kill the child process

 > mccollect(j, wait=FALSE)
$`1542`
NULL

More details indeed in ?mcparallel. The key part is that the job must be
started as non-detached and as soon as mccollect() collects is,
mccollect() must never be called on it again.

Tomas

I can the PID of each cluster nodes by querying them for their
Sys.getpid(), e.g.

     pids <- parallel::clusterEvalQ(cl, Sys.getpid())

Is there a function in core R for testing whether a process with a
given PID exists or not? From trial'n'error, I found that on Linux:

   pid_exists <- function(pid) as.logical(tools::pskill(pid, signal = 0L))

returns TRUE for existing processes and FALSE otherwise, but I'm not
sure if I can trust this.  It's not a documented feature in
?tools::pskill, which also warns about 'signal' not being standardized
across OSes.

The other Linux alternative I can imagine is:

   pid_exists <- function(pid) system2("ps", args = c("--pid", pid),
stdout = FALSE) == 0L

Can I expect this to work on macOS as well?  What about other *nix systems?

And, finally, what can be done on Windows?

I'm sure there are packages on CRAN that provides this, but I'd like
to keep dependencies at a minimum.

I appreciate any feedback. Thxs,

Henrik

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Detecting whether a process exists or not by its PID?

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