bug#29826: nondeterministic Broken pipe

[Mark H Weaver <mhw@netris.org>]

Alex Vong <alexvong1995@gmail.com> writes:

> I get the following error when running ``guix --version | head -n 1''. I
> can get similar after replacing ``--version'' with ``--help''. Also, the
> error is nondeterministic. Any idea?

Attempts to write to a pipe that has already been closed on the other
end results in EPIPE.  From the write(2) man page:

  EPIPE fd is connected to a pipe or socket whose reading end is closed.
        When this happens the writing process will also receive a
        SIGPIPE signal.  (Thus, the write return value is seen only if
        the program catches, blocks or ignores this signal.)

In this case, there's a race condition.  The result depends on whether
"head -n 1" closes its end of the pipe before or after "guix --version"
is finished writing all of its output.  If "head -n 1" closes the pipe
first, then "guix --version" will receive EPIPE while attempting to
write to it.

What normally happens is that the sending process receives SIGPIPE,
which simply causes it to exit prematurely without ever receiving this
error.  However, since Guix arranges to ignore SIGPIPE in
'initialize-guix' in guix/ui.scm, we receive EPIPE.

That's what's happening here.  I'll need to think on how best to fix it.

     Regards,
       Mark