Fix Selector::try_clone

Calls fcntl F_DUPFD_CLOEXEC expects two arguments; the command
(F_DUPFD_CLOEXEC) and an argument for the command. In this case an lower
bound for the resulting file descriptor. Because we didn't provide a
value it would take whatever value was left in the register from
whatever code used it before the system call.

This caused Waker::new to fail, see issue
tokio-rs#1497.

src/sys/unix/selector/epoll.rs

@@ -41,7 +41,7 @@ impl Selector {
     }
 
     pub fn try_clone(&self) -> io::Result<Selector> {
-        syscall!(fcntl(self.ep, libc::F_DUPFD_CLOEXEC, 0)).map(|ep| Selector {
+        syscall!(fcntl(self.ep, libc::F_DUPFD_CLOEXEC, super::LOWEST_FD)).map(|ep| Selector {
             // It's the same selector, so we use the same id.
             #[cfg(debug_assertions)]
             id: self.id,

src/sys/unix/selector/kqueue.rs

@@ -87,7 +87,7 @@ impl Selector {
     }
 
     pub fn try_clone(&self) -> io::Result<Selector> {
-        syscall!(fcntl(self.kq, libc::F_DUPFD_CLOEXEC)).map(|kq| Selector {
+        syscall!(fcntl(self.kq, libc::F_DUPFD_CLOEXEC, super::LOWEST_FD)).map(|kq| Selector {
             // It's the same selector, so we use the same id.
             #[cfg(debug_assertions)]
             id: self.id,

src/sys/unix/selector/mod.rs

@@ -33,3 +33,13 @@ mod kqueue;
     target_os = "openbsd"
 ))]
 pub(crate) use self::kqueue::{event, Event, Events, Selector};
+
+/// Lowest file descriptor used in `Selector::try_clone`.
+///
+/// # Notes
+///
+/// Usually fds 0, 1 and 2 are standard in, out and error. Some application
+/// blindly assume this to be true, which means using any one of those a select
+/// could result in some interesting and unexpected errors. Avoid that by using
+/// an fd that doesn't have a pre-determined usage.
+const LOWEST_FD: libc::c_int = 3;