tcp: accept FIN on zero window.

src/socket/tcp.rs

@@ -1473,7 +1473,7 @@ impl<'a> Socket<'a> {
         let window_start = self.remote_seq_no + self.rx_buffer.len();
         let window_end = self.remote_seq_no + self.rx_buffer.capacity();
         let segment_start = repr.seq_number;
-        let segment_end = repr.seq_number + repr.segment_len();
+        let segment_end = repr.seq_number + repr.payload.len();
 
         let (payload, payload_offset) = match self.state {
             // In LISTEN and SYN-SENT states, we have not yet synchronized with the remote end.
@@ -1533,9 +1533,8 @@ impl<'a> Socket<'a> {
                 };
 
                 if segment_in_window {
-                    let segment_data_end = repr.seq_number + repr.payload.len();
                     let overlap_start = window_start.max(segment_start);
-                    let overlap_end = window_end.min(segment_data_end);
+                    let overlap_end = window_end.min(segment_end);
 
                     // the checks done above imply this.
                     debug_assert!(overlap_start <= overlap_end);
@@ -1595,6 +1594,7 @@ impl<'a> Socket<'a> {
         // If a FIN is received at the end of the current segment but the start of the segment
         // is not at the start of the receive window, disregard this FIN.
         if control == TcpControl::Fin && window_start != segment_start {
+            tcp_trace!("ignoring FIN because we don't have full data yet. window_start={} segment_start={}", window_start, segment_start);
             control = TcpControl::None;
         }
 
@@ -1868,7 +1868,10 @@ impl<'a> Socket<'a> {
         let assembler_was_empty = self.assembler.is_empty();
 
         // Try adding payload octets to the assembler.
-        let Ok(contig_len) = self.assembler.add_then_remove_front(payload_offset, payload_len) else {
+        let Ok(contig_len) = self
+            .assembler
+            .add_then_remove_front(payload_offset, payload_len)
+        else {
             net_debug!(
                 "assembler: too many holes to add {} octets at offset {}",
                 payload_len,
@@ -6051,6 +6054,57 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_zero_window_fin() {
+        let mut s = socket_established();
+        s.rx_buffer = SocketBuffer::new(vec![0; 6]);
+        s.assembler = Assembler::new();
+        s.ack_delay = None;
+
+        send!(
+            s,
+            TcpRepr {
+                seq_number: REMOTE_SEQ + 1,
+                ack_number: Some(LOCAL_SEQ + 1),
+                payload: &b"abcdef"[..],
+                ..SEND_TEMPL
+            }
+        );
+        recv!(
+            s,
+            [TcpRepr {
+                seq_number: LOCAL_SEQ + 1,
+                ack_number: Some(REMOTE_SEQ + 1 + 6),
+                window_len: 0,
+                ..RECV_TEMPL
+            }]
+        );
+
+        // Even though the sequence space for the FIN itself is outside the window,
+        // it is not data, so FIN must be accepted when window full.
+        send!(
+            s,
+            TcpRepr {
+                seq_number: REMOTE_SEQ + 1 + 6,
+                ack_number: Some(LOCAL_SEQ + 1),
+                payload: &[],
+                control: TcpControl::Fin,
+                ..SEND_TEMPL
+            }
+        );
+        assert_eq!(s.state, State::CloseWait);
+
+        recv!(
+            s,
+            [TcpRepr {
+                seq_number: LOCAL_SEQ + 1,
+                ack_number: Some(REMOTE_SEQ + 1 + 7),
+                window_len: 0,
+                ..RECV_TEMPL
+            }]
+        );
+    }
+
     #[test]
     fn test_zero_window_ack_on_window_growth() {
         let mut s = socket_established();