Add signal_ignore() and signal_default()

Provide safe mechanisms to ignore or reset a signal's action to the
default.

Fixes #587.

src/sys/signal.rs

@@ -964,6 +964,22 @@ pub unsafe fn signal(signal: Signal, handler: SigHandler) -> Result<SigHandler>
     })
 }
 
+/// Reset the specified signal to its default action.
+///
+/// `signal` can be any signal except `SIGKILL` or `SIGSTOP`.
+pub fn signal_default(signal: Signal) -> Result<()> {
+    // SAFETY: restoring a default handler is safe if the old handler isn't called
+    unsafe { self::signal(signal, SigHandler::SigDfl) }.map(drop)
+}
+
+/// Ignore the specified signal.
+///
+/// `signal` can be any signal except `SIGKILL` or `SIGSTOP`.
+pub fn signal_ignore(signal: Signal) -> Result<()> {
+    // SAFETY: ignoring a signal is safe if the old handler isn't called
+    unsafe { self::signal(signal, SigHandler::SigIgn) }.map(drop)
+}
+
 fn do_pthread_sigmask(how: SigmaskHow,
                        set: Option<&SigSet>,
                        oldset: Option<*mut libc::sigset_t>) -> Result<()> {

test/sys/test_signal.rs

@@ -141,3 +141,19 @@ fn test_signal() {
     // Restore default signal handler
     unsafe { signal(Signal::SIGINT, SigHandler::SigDfl) }.unwrap();
 }
+
+#[test]
+fn test_safe_signal() {
+    let _m = crate::SIGNAL_MTX.lock();
+
+    signal_ignore(SIGINT).unwrap();
+    raise(Signal::SIGINT).unwrap();
+
+    signal_default(SIGINT).unwrap();
+
+    // Ensure default was restored
+    assert_eq!(
+        unsafe { signal(Signal::SIGINT, SigHandler::SigDfl) }.unwrap(),
+        SigHandler::SigDfl
+    );
+}