usleep hangs indefinitely when using -threaded

[Eelis]

Consider the following two-line program.

  import System.Posix.Unistd
  main = usleep 1000000

When compiled without -threaded, the program nicely sleeps for one second, as expected. When compiled with -threaded, however, the program simply hangs.

[igloo]

We're getting EINTR mid-sleep and throwErrnoIfMinus1Retry_ is ultimately causing us to sleep (for the full length) again.

[Eelis]

This problem is actually one of the listed symptoms of the problem described in ticket #850 (which I personally feel deserves a higher priority than "low", considering that it renders perfectly ordinary system functions utterly unusable in the threaded RTS).