Provide notification everytime read is successful

Author: tiif

src/shims/unix/socket.rs

@@ -141,14 +141,19 @@ impl FileDescription for SocketPair {
         // Conveniently, `read` exists on `VecDeque` and has exactly the desired behavior.
         let actual_read_size = readbuf.buf.read(bytes).unwrap();
 
-        // Notification should be provided for peer fd when read emptied the buffer.
+        // The readbuf needs to be explicitly dropped because it will cause panic when
+        // check_and_update_readiness borrows it again.
+        drop(readbuf);
+
+        // A notification should be provided for the peer file description even when it can
+        // only write 1 byte. This implementation is not compliant with the actual Linux kernel
+        // implementation. For optimization reasons, the kernel will only mark the file description
+        // as "writable" when it can write more than a certain number of bytes. Since we
+        // don't know what that *certain number* is, we will provide a notification every time
+        // a read is successful. This might result in our epoll emulation providing more
+        // notifications than the real system.
         if let Some(peer_fd) = self.peer_fd.upgrade() {
-            if readbuf.buf.is_empty() {
-                // The readbuf needs to be explicitly dropped because it will cause panic when
-                // check_and_update_readiness borrows it again.
-                drop(readbuf);
-                peer_fd.check_and_update_readiness(ecx)?;
-            }
+            peer_fd.check_and_update_readiness(ecx)?;
         }
 
         return Ok(Ok(actual_read_size));

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

@@ -485,7 +485,8 @@ fn test_event_overwrite() {
     assert!(check_epoll_wait::<8>(epfd, vec![(expected_event, expected_value)]));
 }
 
-// read in socketpair will only provide epoll notification if it emptied the buffer.
+// An epoll notification will be provided for every succesful read in a socketpair.
+// This behaviour differs from the real system.
 fn test_socketpair_read() {
     // Create an epoll instance.
     let epfd = unsafe { libc::epoll_create1(0) };
@@ -529,8 +530,11 @@ fn test_socketpair_read() {
     assert_eq!(res, 3);
     assert_eq!(buf, "abc".as_bytes());
 
-    // No notification should be provided.
-    assert!(check_epoll_wait::<8>(epfd, vec![]));
+    // Notification will be provided.
+    // But in real system, no notification will be provided here.
+    let expected_event = u32::try_from(libc::EPOLLOUT).unwrap();
+    let expected_value = fds[1] as u64;
+    assert!(check_epoll_wait::<8>(epfd, vec![(expected_event, expected_value)]));
 
     // Read until the buffer is empty.
     let mut buf: [u8; 2] = [0; 2];
@@ -540,7 +544,8 @@ fn test_socketpair_read() {
     assert_eq!(res, 2);
     assert_eq!(buf, "de".as_bytes());
 
-    // Notification should be provided.
+    // Notification will be provided.
+    // In real system, notification will be provided too.
     let expected_event = u32::try_from(libc::EPOLLOUT).unwrap();
     let expected_value = fds[1] as u64;
     assert!(check_epoll_wait::<8>(epfd, vec![(expected_event, expected_value)]));