Make Pollee stateless

kernel/libs/aster-bigtcp/src/socket/event.rs

@@ -3,9 +3,19 @@
 /// A observer that will be invoked whenever events occur on the socket.
 pub trait SocketEventObserver: Send + Sync {
     /// Notifies that events occurred on the socket.
-    fn on_events(&self);
+    fn on_events(&self, events: SocketEvents);
 }
 
 impl SocketEventObserver for () {
-    fn on_events(&self) {}
+    fn on_events(&self, _events: SocketEvents) {}
+}
+
+bitflags::bitflags! {
+    /// Socket events caused by the _network_.
+    pub struct SocketEvents: u8 {
+        const CAN_RECV = 1;
+        const CAN_SEND = 2;
+        const PEER_CLOSED = 4;
+        const CLOSED = 8;
+    }
 }