Fix blocking of multiple signals per pipe

- Also update the example test application to catch a similar error on
  execution.

examples/test_block_specific.rs

@@ -6,9 +6,11 @@ use chan_signal::{Signal, kill_this};
 
 fn main() {
  chan_signal::unblock_signals_by_default();
- let r_usr1 = chan_signal::notify(&[Signal::USR1]);
+ let r_usr1 = chan_signal::notify(&[Signal::USR1, Signal::ALRM]);
  kill_this(Signal::USR1);
+ kill_this(Signal::ALRM);
  assert_eq!(r_usr1.recv(), Some(Signal::USR1));
+ assert_eq!(r_usr1.recv(), Some(Signal::ALRM));
 
  let (s, r_usr2) = chan::sync(1);
  chan_signal::notify_on(&s, Signal::USR2);

src/lib.rs

@@ -203,12 +203,13 @@ pub fn notify_on(chan: &Sender<Signal>, signal: Signal) {
  let mut sigs = BitSet::new();
  sigs.insert(signal.as_sig() as usize);
  subs.insert((*chan).clone(), sigs);
-
- // Make sure that the signal that we want notifications on is blocked
- let mut sig_set = SigSet::empty();
- sig_set.add(signal.as_sig()).unwrap();
- sig_set.thread_block_signals().unwrap();
  }
+
+ // Make sure that the signal that we want notifications on is blocked
+ // It does not matter if we unblock the same signal twice.
+ let mut sig_set = SigSet::empty();
+ sig_set.add(signal.as_sig()).unwrap();
+ sig_set.thread_block_signals().unwrap();
 }
 
 /// Unblock all subscribable signals that are blocked by default.