illumos epoll may not wake up unless pipe is empty

At present, mio makes use of the emulated epoll(5) available on illumos
systems.  On some versions of illumos the interaction between pipes and
EPOLLET is not quite right.  Specifically, a wakeup may only be
triggered when the buffer pipe transitions from empty to non-empty.  To
work around this, flush the buffer for every call to wake().

src/sys/unix/waker.rs

@@ -137,6 +137,12 @@ mod pipe {
         }
 
         pub fn wake(&self) -> io::Result<()> {
+            // The epoll emulation on some illumos systems currently requires
+            // the pipe buffer to be completely empty for an edge-triggered
+            // wakeup on the pipe read side.
+            #[cfg(target_os = "illumos")]
+            self.empty();
+
             match (&self.sender).write(&[1]) {
                 Ok(_) => Ok(()),
                 Err(ref err) if err.kind() == io::ErrorKind::WouldBlock => {