explain the behavior on closed peers

Author: RalfJung

src/shims/unix/linux/epoll.rs

@@ -433,11 +433,13 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         Ok(Scalar::from_i32(num_of_events))
     }
 
-    /// For a specific file description, get its ready events and update
-    /// the corresponding ready list. This function is called whenever a file description
-    /// is registered with epoll, or the buffer it reads from / writes to changed.
-    /// This *will* report an event if anyone is subscribed to it, without any further
-    /// filtering, so do not call this function when an FD didn't have anything happen to it!
+    /// For a specific file description, get its ready events and update the corresponding ready
+    /// list. This function should be called whenever you change how many bytes can be read from an
+    /// FD or whether reading returns EOF, and whenever you change the amount of bytes that can be
+    /// written to an FD or whether writing will succeed.
+    ///
+    /// This *will* report an event if anyone is subscribed to it, without any further filtering, so
+    /// do not call this function when an FD didn't have anything happen to it!
     fn check_and_update_readiness(&self, fd_ref: &FileDescriptionRef) -> InterpResult<'tcx, ()> {
         let this = self.eval_context_ref();
         let id = fd_ref.get_id();

src/shims/unix/socket.rs

@@ -71,9 +71,9 @@ impl FileDescription for SocketPair {
         } else {
             // Peer FD has been closed.
             epoll_ready_events.epollrdhup = true;
-            // This is an edge case. Whenever epollrdhup is triggered, epollin and epollout will be
-            // added even though there is no data in the buffer.
-            // FIXME: Figure out why. This looks like a bug.
+            // Since the peer is closed, even if no data is available reads will return EOF and
+            // writes will return EPIPE. In other words, they won't block, so we mark this as ready
+            // for read and write.
             epoll_ready_events.epollin = true;
             epoll_ready_events.epollout = true;
         }

tests/pass-dep/libc/libc-epoll.rs

@@ -96,6 +96,7 @@ fn test_epoll_socketpair() {
     assert_eq!(res, 0);
 
     // Check result from epoll_wait.
+    // We expect to get a read, write, HUP notification from the close since closing an FD always unblocks reads and writes on its peer.
     let expected_event = u32::try_from(libc::EPOLLRDHUP | libc::EPOLLIN | libc::EPOLLOUT).unwrap();
     let expected_value = u64::try_from(fds[1]).unwrap();
     assert!(check_epoll_wait::<8>(epfd, &[(expected_event, expected_value)]));