fix leaked sock states, issue 1146 (#2)

Fixed leaked sock states, issue tokio-rs#1146. A reference of the sock state
Arc is blocked by a pending AFD poll request, this reference will leak
if there's no explicit poll done before the program terminates. That's
why doing an explicit "select" during iner selector drop ensures all 
sock state references are freed.

Signed-off-by: Daniel Tacalau <dst4096@gmail.com>

src/sys/windows/selector.rs

@@ -391,6 +391,41 @@ pub struct SelectorInner {
 // We have ensured thread safety by introducing lock manually.
 unsafe impl Sync for SelectorInner {}
 
+impl Drop for SelectorInner {
+    fn drop(&mut self) {
+        loop {
+            let events_num: usize;
+            let mut statuses: [CompletionStatus; 1024] = [CompletionStatus::zero(); 1024];
+
+            let result = self
+                .cp
+                .get_many(&mut statuses, Some(std::time::Duration::from_millis(0)));
+            match result {
+                Ok(iocp_events) => {
+                    events_num = iocp_events.iter().len();
+                    for iocp_event in iocp_events.iter() {
+                        if iocp_event.overlapped().is_null() == false {
+                            // drain sock state to release memory of Arc reference
+                            let _sock_state = from_overlapped(iocp_event.overlapped());
+                        }
+                    }
+                }
+
+                Err(_) => {
+                    break;
+                }
+            }
+
+            if events_num < 1024 {
+                // continue looping until all completion statuses have been drained
+                break;
+            }
+        }
+
+        self.afd_group.release_unused_afd();
+    }
+}
+
 impl SelectorInner {
     pub fn new() -> io::Result<SelectorInner> {
         CompletionPort::new(0).map(|cp| {