fix(http): advertise keep-alive on HTTP/1.0 responses we keep open

1.0 defaults to close: honoring 'Connection: Keep-Alive' without echoing
it back left spec-following clients waiting for an EOF that never came
(ab -k deadlocked to its poll timeout on response 1).

As agreed:
- serialise_response echoes 'connection: keep-alive' when keep_alive
  is on AND version is 1.0. 1.1 keep-alive stays implicit.
- Error statuses (>=400) on 1.0 force keep_alive off in the conn actor
  (joins the existing 1.0-stream force-close): we don't advertise reuse
  on a 4xx/5xx to a protocol generation where reuse is opt-in.
- HEAD needs nothing: the echo is framing-neutral.
- Parse-error responses already carried 'connection: close'.

Tests: unit pair (1.0 echo / 1.1 implicit) + integration pair (socket
actually reused on 1.0; 404 forces close + EOF). Validated against the
original repro: ab -k -n 5000 -c 50 GET /users -> 5000/5000 keep-alive,
0 failed, ~63k rps (~18.9k without keep-alive).

ROADMAP.md

@@ -229,21 +229,19 @@ PubSub underneath.
 
 ## Known bugs
 
-- **HTTP/1.0 keep-alive: server honors but never advertises** (found
+- ~~**HTTP/1.0 keep-alive: server honors but never advertises**~~ FIXED
-  2026-06-12 via `ab -k`). A 1.0 request with `Connection: Keep-Alive`
+  (2026-06-12, same session it was found). As-landed decisions:
-  IS kept alive (second pipelined request on the same socket gets
+  serialise_response now echoes `connection: keep-alive` on a kept-alive
-  answered), but the response carries no `Connection: Keep-Alive`
+  1.0 response (1.1 keep-alive stays implicit); error statuses (>=400)
-  header. Per 1.0 convention the client may only reuse when the server
+  on 1.0 force close — no keep-alive advertised on a 4xx/5xx where reuse
-  explicitly opts in; absent the header, clients assume the server will
+  is opt-in (conn_actor, alongside the existing 1.0-stream force-close);
-  close and wait for EOF — which never comes. `ab -k` therefore
+  HEAD needs nothing special (header echo only, framing untouched);
-  deadlocks until its poll timeout (`apr_pollset_poll: timeout 70007`)
+  parse-error responses already carried `connection: close`. Validated
-  on the FIRST response; no keep-alive bench numbers obtainable.
+  against the original repro: `ab -k -n 5000 -c 50` on GET /users now
-  Fix direction: when the conn stays alive on a 1.0 exchange, echo
+  reports 5000/5000 keep-alive requests, 0 failed, ~63k req/s
-  `Connection: Keep-Alive` in serialise_response — or don't keep 1.0
+  (vs ~18.9k no-keep-alive baseline, same box). Watch out benching:
-  alive at all. Decide scope (error responses, HEAD) with user first.
+  ab counts a response toward "Non-2xx" silently — a 404'd route still
-  Baseline for reference: GET /users (crud example, release, loopback,
+  shows "Failed requests: 0" with great rps; check the route first.
-  stdout logging silenced), no keep-alive, c=50: ~18.9k req/s, 0
-  failed, p50 3ms / p99 5ms. HTTP/1.1 path responds normally.
 
 ## Later / icebox
 

src/conn_actor.rs

@@ -248,10 +248,13 @@ pub fn run_connection(
         // ----- 4. Write the response. -----
         // A Stream body on HTTP/1.0 has no chunked framing: the body is
         // delimited by EOF, so keep-alive is forced off for this response
-        // (and `connection: close` goes on the wire).
+        // (and `connection: close` goes on the wire). Error statuses on
+        // 1.0 also force close: we don't advertise keep-alive on a
+        // 4xx/5xx to a protocol generation where reuse is opt-in.
         let is_stream = matches!(response_conn.resp_body, RespBody::Stream(_));
+        let is_error  = response_conn.status.unwrap_or(200) >= 400;
         let keep_alive = keep_alive
-            && !(is_stream && version == HttpVersion::Http10);
+            && !(version == HttpVersion::Http10 && (is_stream || is_error));
 
         let head_bytes = parser::serialise_response(&response_conn, keep_alive);
         if write_all(raw, &head_bytes, Instant::now() + limits.write_timeout).is_err() {

src/parser.rs

@@ -252,8 +252,17 @@ pub fn serialise_response(conn: &Conn, keep_alive: bool) -> Vec<u8> {
         out.extend_from_slice(b"\r\n");
     }
 
-    if !wrote_connection && !keep_alive {
+    if !wrote_connection {
+        if !keep_alive {
             out.extend_from_slice(b"connection: close\r\n");
+        } else if conn.version == HttpVersion::Http10 {
+            // HTTP/1.0 defaults to close: a connection we intend to keep
+            // open MUST be advertised back, or a spec-following client
+            // waits for an EOF that never comes (`ab -k` deadlocked on
+            // exactly this). 1.1 keep-alive is the default and stays
+            // implicit.
+            out.extend_from_slice(b"connection: keep-alive\r\n");
+        }
     }
 
     out.extend_from_slice(b"\r\n");
@@ -452,6 +461,23 @@ mod tests {
         assert!(s.contains("connection: close"));
     }
 
+    #[test]
+    fn serialise_http10_keepalive_is_echoed() {
+        let mut conn = Conn::new().put_status(200).put_body("hi");
+        conn.version = HttpVersion::Http10;
+        let bytes = serialise_response(&conn, true);
+        let s = std::str::from_utf8(&bytes).unwrap();
+        assert!(s.contains("connection: keep-alive"), "head: {s}");
+    }
+
+    #[test]
+    fn serialise_http11_keepalive_stays_implicit() {
+        let conn = Conn::new().put_status(200).put_body("hi");
+        let bytes = serialise_response(&conn, true);
+        let s = std::str::from_utf8(&bytes).unwrap();
+        assert!(!s.contains("connection:"), "head: {s}");
+    }
+
     #[test]
     fn serialise_user_content_length_is_respected() {
         let conn = Conn::new().put_status(200)

tests/integration.rs

@@ -660,6 +660,51 @@ fn keep_alive_after_chunked_response() {
     assert_eq!(http_body(&resp), b"hello");
 }
 
+/// HTTP/1.0 keep-alive must be advertised back (`connection: keep-alive`)
+/// when honored — 1.0 defaults to close, so a silent keep-open leaves
+/// spec-following clients (ab -k) waiting for EOF. The socket then serves
+/// a second request.
+#[test]
+fn http10_keepalive_advertised_and_socket_reused() {
+    let pipe = Pipeline::new().plug(
+        Router::new().get("/", |c: Conn, _n: Next| c.put_status(200))
+    );
+    let port = spawn_server(pipe);
+    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
+    s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
+
+    let req = b"GET / HTTP/1.0\r\nHost: x\r\nConnection: keep-alive\r\n\r\n";
+    s.write_all(req).unwrap();
+    let head = String::from_utf8(read_response_head(&mut s)).unwrap();
+    assert!(head.starts_with("HTTP/1.0 200 OK\r\n"), "head: {head}");
+    assert!(head.contains("connection: keep-alive"), "head: {head}");
+
+    // Same socket, second request — the keep-alive was real.
+    s.write_all(req).unwrap();
+    let head2 = String::from_utf8(read_response_head(&mut s)).unwrap();
+    assert!(head2.starts_with("HTTP/1.0 200 OK\r\n"), "head: {head2}");
+}
+
+/// Error statuses on HTTP/1.0 force close even when the client asked for
+/// keep-alive: `connection: close` on the wire, then EOF.
+#[test]
+fn http10_keepalive_forced_off_on_error_status() {
+    let pipe = Pipeline::new().plug(
+        Router::new().get("/", |c: Conn, _n: Next| c.put_status(200))
+    );
+    let port = spawn_server(pipe);
+    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
+    s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
+
+    s.write_all(b"GET /missing HTTP/1.0\r\nHost: x\r\nConnection: keep-alive\r\n\r\n")
+        .unwrap();
+    let mut resp = Vec::new();
+    s.read_to_end(&mut resp).unwrap(); // EOF: server closed
+    let head = String::from_utf8_lossy(&resp).to_string();
+    assert!(head.starts_with("HTTP/1.0 404 Not Found\r\n"), "head: {head}");
+    assert!(head.contains("connection: close"), "head: {head}");
+}
+
 /// HTTP/1.0 has no chunked framing: a stream body goes out raw, the
 /// response carries `connection: close` and no transfer-encoding, and EOF
 /// delimits the body.