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

[Sun Yijiang]

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
> >
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>
>