[Rd] Race condition on parallel package's mcexit and rmChild

[Simon Urbanek]

Because that's the communication protocol between the parent and child. There is a difference between unsolicited exit and empty result exit.

Cheers,
Simon


> On May 20, 2019, at 11:22 AM, Sun Yijiang <sunyijiang using gmail.com> wrote:
> 
> Have read the latest code, but I still don't understand why mc_exit
> needs to write zero on exit.  If a child closes its pipe, parent will
> know that on next select.
> 
> Best,
> Yijiang
> 
> Tomas Kalibera <tomas.kalibera using gmail.com> 于2019年5月20日周一 下午10:52写道：
>> 
>> This issue has already been addressed in 76462 (R-devel) and also ported
>> to R-patched. In fact rmChild() is used in mccollect(wait=FALSE).
>> 
>> Best
>> Tomas
>> 
>> On 5/19/19 11:39 AM, Sun Yijiang wrote:
>>> I've been hacking with parallel package for some time and built a
>>> parallel processing framework with it.  However, although very rarely,
>>> I did notice "ignoring SIGPIPE signal" error every now and then.
>>> After a deep dig into the source code, I think I found something worth
>>> noticing.
>>> 
>>> In short, wring to pipe in the C function mc_exit(SEXP sRes) may cause
>>> a SIGPIPE.  Code from src/library/parallel/src/fork.c:
>>> 
>>> SEXP NORET mc_exit(SEXP sRes)
>>> {
>>>     int res = asInteger(sRes);
>>> ... ...
>>>     if (master_fd != -1) { /* send 0 to signify that we're leaving */
>>>         size_t len = 0;
>>>         /* assign result for Fedora security settings */
>>>         ssize_t n = write(master_fd, &len, sizeof(len));
>>> ... ...
>>> }
>>> 
>>> So a pipe write is made in mc_exit, and here's how this function is
>>> used in src/library/parallel/R/unix/mcfork.R:
>>> 
>>> mcexit <- function(exit.code = 0L, send = NULL)
>>> {
>>>     if (!is.null(send)) try(sendMaster(send), silent = TRUE)
>>>     .Call(C_mc_exit, as.integer(exit.code))
>>> }
>>> 
>>> Between sendMaster() and mc_exit() calls, which are made in the child
>>> process, the master process may call readChild() followed by
>>> rmChild().  rmChild closes the pipe on the master side, and if it's
>>> called before child calls mc_exit, a SIGPIPE will be raised when child
>>> tries to write to the pipe in mc_exit.
>>> 
>>> rmChild is defined but not used in parallel package, so this problem
>>> won't surface in most cases.  However, it is a useful API and may be
>>> used by users like me for advanced control over child processes.  I
>>> hope we can discuss a solution on it.
>>> 
>>> In fact, I don't see why we need to write to the pipe on child exit
>>> and how it has anything to do with "Fedora security settings" as
>>> suggested in the comments.  Removing it, IMHO, would be a good and
>>> clean way to solve this problem.
>>> 
>>> Regards,
>>> Yijiang
>>> 
>>> ______________________________________________
>>> R-devel using r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>> 
>> 
> 
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