Fix leaked sock states (#1154)

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 inner selector drop ensures all sock state references are freed.

Fixes: #1146

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() {
+                            // 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| {