fix(proto): floor negative echo lengths

crates/irtt-proto/tests/directional_codecs.rs

@@ -305,6 +305,32 @@ fn echo_payload_is_copied_zero_filled_and_bounded_in_both_directions() {
     );
 }
 
+#[test]
+fn a_negative_negotiated_length_still_encodes_in_both_directions() {
+    // A server accepts a negative negotiated length during open and returns it
+    // unchanged, so a live session can hold one and must stay executable. There
+    // is no datagram shorter than none, so both directions fall back to the
+    // mandatory field block — header (4) + token (8) + sequence (4) — exactly
+    // as a zero length would.
+    let params = Params {
+        length: -4096,
+        ..Params::default()
+    };
+    assert_eq!(echo_packet_len(false, &params), 16);
+
+    let request = encode_request(echo_request(&params, &[]), None)
+        .expect("a negative negotiated length must still encode a request");
+    assert_eq!(request.len(), 16);
+
+    let reply = encode_echo_reply(&echo_reply(&params, FLAG_REPLY), &params, None)
+        .expect("a negative negotiated length must still encode a reply");
+    assert_eq!(reply.len(), 16);
+    assert_eq!(
+        decode_echo_reply(&reply, &params, None).unwrap().token,
+        TOKEN
+    );
+}
+
 // ---------- Upstream midpoint ECHO-reply compatibility ----------
 //
 // Verified upstream irtt 0.9.1 behavior: for single-clock
@@ -346,7 +372,7 @@ fn midpoint_params(clock: Clock, payload_space: usize) -> Params {
 
 fn midpoint_compat_len(params: &Params, hmac: bool) -> usize {
     let compat_header = PacketLayout::echo(hmac, params).header_len() + 8;
-    compat_header.max(echo_packet_len(hmac, params).unwrap())
+    compat_header.max(echo_packet_len(hmac, params))
 }
 
 /// Hand-builds upstream's dual-midpoint ECHO reply: both midpoint fields on
@@ -399,7 +425,7 @@ fn midpoint_dual_field_reply_is_accepted_wherever_length_identifies_it() {
     for (payload_space, expected_extra) in [(0, 8), (1, 7), (4, 4), (7, 1)] {
         for clock in [Clock::Wall, Clock::Monotonic] {
             let params = midpoint_params(clock, payload_space);
-            let normal_len = echo_packet_len(false, &params).unwrap();
+            let normal_len = echo_packet_len(false, &params);
             let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 3, (111, 222), None);
             assert_eq!(
                 packet.len(),
@@ -438,7 +464,7 @@ fn midpoint_dual_field_reply_of_equal_length_is_accepted_but_not_correctable() {
     for payload_space in [8, 64] {
         for clock in [Clock::Wall, Clock::Monotonic] {
             let params = midpoint_params(clock, payload_space);
-            let normal_len = echo_packet_len(false, &params).unwrap();
+            let normal_len = echo_packet_len(false, &params);
             let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 4, (111, 222), None);
             assert_eq!(
                 packet.len(),
@@ -491,7 +517,7 @@ fn midpoint_dual_field_reply_authenticates_over_full_packet() {
 fn only_the_exact_compat_length_is_accepted_as_an_extension() {
     let params = midpoint_params(Clock::Wall, 0);
     let packet = build_midpoint_dual_field_reply(&params, FLAG_REPLY, 1, (10, 20), None);
-    let expected_len = echo_packet_len(false, &params).unwrap();
+    let expected_len = echo_packet_len(false, &params);
     assert_eq!(packet.len(), expected_len + 8);
 
     // Every other length around the compatibility size stays rejected,

crates/irtt-proto/tests/request_codec.rs

@@ -450,21 +450,34 @@ fn encoded_requests_decode_back_to_their_sender_side_identity() {
 }
 
 #[test]
-fn echo_encoding_rejects_a_negative_negotiated_length() {
+fn echo_encoding_floors_a_negative_negotiated_length_at_the_field_block() {
+    // A negative negotiated length is accepted during open and echoed back
+    // unchanged, so a real session can carry one. It must therefore stay
+    // encodable: it asks for no space beyond the mandatory fields.
     let params = Params {
         length: -1,
         ..Params::default()
     };
+    let packet = encode_request(
+        RequestToEncode::Echo {
+            token: TOKEN,
+            sequence: SEQUENCE,
+            params: &params,
+            payload: &[],
+        },
+        None,
+    )
+    .expect("a negative negotiated length must still encode");
+
+    // header (4) + token (8) + sequence (4).
+    assert_eq!(packet.len(), 16);
+    let request = decode_request(&packet).unwrap();
     assert_eq!(
-        encode_request(
-            RequestToEncode::Echo {
-                token: TOKEN,
-                sequence: SEQUENCE,
-                params: &params,
-                payload: &[],
-            },
-            None,
-        ),
-        Err(ProtoError::NegativePacketLength { length: -1 })
+        request.kind,
+        DecodedRequestKind::Echo {
+            token: TOKEN,
+            sequence: SEQUENCE,
+            tail: &[],
+        }
     );
 }