epoll: allow controlling write event notifications for bidirectional I/O

For sockets that frequently switch between read/write but can be
readable at any time, add a `notify_writable` method to EventGuard which
re-arms the socket with either EPOLLOUT set or cleared.
When there is writable data in userspace but writing is blocked, the
guard can temporarily enable writable events then disable them when
user code has flushed its buffers, instead of having to manually manage
separate 'writer' sockets.

src/device/epoll.rs

@@ -381,6 +381,16 @@ impl<'a, H> EventGuard<'a, H> {
         unsafe { self.poll.clear_event_by_fd(self.event.fd) };
         std::mem::forget(self); // Don't call the regular drop that would enable the event
     }
+
+    /// Change the event flags to enable or disable notifying when the fd is writable
+    pub fn notify_writable(&mut self, enabled: bool) {
+        let flags = if enabled {
+            EPOLLOUT | EPOLLIN | EPOLLET | EPOLLONESHOT
+        } else {
+            EPOLLIN | EPOLLONESHOT
+        };
+        self.event.event.events = flags as _;
+    }
 }
 
 pub fn block_signal(signal: c_int) -> Result<sigset_t, String> {