extend comments on HUP vs RDHUP

Author: RalfJung

src/tools/miri/src/shims/unix/linux/epoll.rs

@@ -71,6 +71,9 @@ pub struct EpollReadyEvents {
     pub epollrdhup: bool,
     /// For stream socket, this event merely indicates that the peer
     /// closed its end of the channel.
+    /// Unlike epollrdhup, this should only be set when the stream is fully closed.
+    /// epollrdhup also gets set when only the write half is closed, which is possible
+    /// via `shutdown(_, SHUT_WR)`.
     pub epollhup: bool,
 }
 

src/tools/miri/src/shims/unix/socket.rs

@@ -69,7 +69,8 @@ impl FileDescription for SocketPair {
                 epoll_ready_events.epollout = true;
             }
         } else {
-            // Peer FD has been closed.
+            // Peer FD has been closed. This always sets both the RDHUP and HUP flags
+            // as we do not support `shutdown` that could be used to partially close the stream.
             epoll_ready_events.epollrdhup = true;
             epoll_ready_events.epollhup = true;
             // Since the peer is closed, even if no data is available reads will return EOF and