Set FD_CLOEXEC for duped epoll_fd

The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the duplicate descriptor is off
and we should set it manually.

Fixes: tokio-rs/tokio#3809

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/sys/unix/selector/epoll.rs

@@ -41,13 +41,17 @@ impl Selector {
     }
 
     pub fn try_clone(&self) -> io::Result<Selector> {
-        syscall!(dup(self.ep)).map(|ep| Selector {
-            // It's the same selector, so we use the same id.
-            #[cfg(debug_assertions)]
-            id: self.id,
-            ep,
-            #[cfg(debug_assertions)]
-            has_waker: AtomicBool::new(self.has_waker.load(Ordering::Acquire)),
+        syscall!(dup(self.ep)).and_then(|ep| {
+            syscall!(fcntl(ep, libc::F_SETFD, libc::FD_CLOEXEC))?;
+
+            Ok(Selector {
+                // It's the same selector, so we use the same id.
+                #[cfg(debug_assertions)]
+                id: self.id,
+                ep,
+                #[cfg(debug_assertions)]
+                has_waker: AtomicBool::new(self.has_waker.load(Ordering::Acquire)),
+            })
         })
     }