feat(serve): burst-gated body stall eviction

The body budget from the prior commit is a generous absolute cap; alone it
just hands a body-phase slowloris a bigger window. Add a stall gate under
that cap that distinguishes a slowloris trickle from a slow-but-legit
client by requiring BURSTS, not a mere average rate:

- BodyStallGate: each body read is bounded by min(body cap, mark + stall).
  The stall mark advances only when body_burst_bytes accumulate since the
  last advance, so a steady sub-burst trickle never moves it and is evicted
  at ~body_stall_timeout, while a bursty slow client keeps resetting it.
- Two words of state; one add + one compare per read. Raw socket bytes are
  counted, so chunked framing counts and an MSS-fragmented burst still
  accumulates. Reuses the existing read_some deadline plumbing.
- Wired into read_body (fixed CL) and read_chunked_body (via fill_to, the
  single choke point all chunked reads pass through).
- New knobs body_burst_bytes (4 KiB) + body_stall_timeout (20s); effective
  floor ~205 B/s enforced in bursts.

Tests: body_smooth_trickle_evicted_at_stall_timeout (chunked; active
sub-burst trickle evicted at ~stall while the cap is far away) and
bursty_slow_body_survives_stall_gate (fixed CL; real bursts with sub-stall
gaps complete intact). 79 lib + 45 integration green; clippy --lib clean.

src/conn_actor.rs

@@ -61,6 +61,19 @@ pub struct ConnLimits {
     /// covered (that's the handler's business); the write phase has its own
     /// per-write budget (`write_timeout`).
     pub body_timeout:       Duration,
+    /// Burst-gated body stall eviction: the bytes that must accumulate
+    /// since the last advance to count as a "burst" and reset the stall
+    /// clock. A body that dribbles fewer than this per `body_stall_timeout`
+    /// window is evicted — the discriminator between a slowloris trickle
+    /// (near-zero, smooth) and a slow-but-legit client (delivers real
+    /// bursts). The pair implies an effective floor of
+    /// body_burst_bytes / body_stall_timeout, enforced in bursts.
+    pub body_burst_bytes:   usize,
+    /// Max time since the last qualifying burst (`body_burst_bytes`)
+    /// before a stalled body read is evicted. Must comfortably exceed a
+    /// legit client's worst quiet gap (e.g. cellular RRC/handover/DRX
+    /// stalls). The absolute `body_timeout` always backstops it.
+    pub body_stall_timeout: Duration,
     /// Per-write budget for response bytes: every `write_all` (the fixed
     /// head+body, and each streamed chunk) must complete within this.
     /// A client that stops reading mid-response is dropped when its
@@ -85,6 +98,8 @@ impl Default for ConnLimits {
             keep_alive_timeout: Duration::from_secs(60),
             head_timeout:       Duration::from_secs(30),
             body_timeout:       Duration::from_secs(300),
+            body_burst_bytes:   4 * 1024,
+            body_stall_timeout: Duration::from_secs(20),
             write_timeout:      Duration::from_secs(30),
             max_frame_payload:  1024 * 1024,
             max_message_bytes:  4 * 1024 * 1024,
@@ -204,7 +219,7 @@ pub fn run_connection(
                 Err(ChunkedBodyErr::Io(_)) => return,
             }
         } else {
-            match read_body(raw, &mut buf, parsed.head_len, body_len, body_deadline) {
+            match read_body(raw, &mut buf, parsed.head_len, body_len, &limits, body_deadline) {
                 Ok(b) => (b, body_len),
                 // Timeout mid-body (and any other body io error) -> just
                 // close; there's no point talking HTTP to a client this far
@@ -421,6 +436,58 @@ fn read_head(
     }
 }
 
+// ---------------------------------------------------------------------------
+// BodyStallGate — burst-gated stall eviction for body reads
+// ---------------------------------------------------------------------------
+//
+// Each body read is bounded by the SOONER of two deadlines: the absolute
+// body cap (`body_timeout`, passed in as `cap`) and a sliding stall window
+// (`mark + body_stall_timeout`). The stall mark only advances when the
+// client delivers a full burst (`body_burst_bytes` accumulated since the
+// last advance) — so a steady sub-burst trickle never moves the mark and
+// is evicted at ~body_stall_timeout, while a bursty slow-but-legit client
+// keeps resetting it and survives up to the absolute cap.
+//
+// State is two words (`mark`, `since_mark`); the per-read cost is one add
+// and one compare. Bytes counted are RAW socket bytes (progress = the
+// client is sending *something*), so chunked framing counts too, and a
+// burst that the kernel fragments into several reads still accumulates.
+struct BodyStallGate {
+    cap:           Instant,
+    stall_timeout: Duration,
+    burst_bytes:   usize,
+    mark:          Instant,
+    since_mark:    usize,
+}
+
+impl BodyStallGate {
+    fn new(cap: Instant, limits: &ConnLimits, now: Instant) -> Self {
+        Self {
+            cap,
+            stall_timeout: limits.body_stall_timeout,
+            burst_bytes:   limits.body_burst_bytes,
+            mark:          now,
+            since_mark:    0,
+        }
+    }
+
+    /// Deadline for the next read: the sooner of the absolute cap and the
+    /// current stall window.
+    fn deadline(&self) -> Instant {
+        (self.mark + self.stall_timeout).min(self.cap)
+    }
+
+    /// Record `n` freshly-read raw body bytes; advance the stall mark if a
+    /// full burst has accumulated since the last advance.
+    fn record(&mut self, n: usize, now: Instant) {
+        self.since_mark += n;
+        if self.since_mark >= self.burst_bytes {
+            self.mark = now;
+            self.since_mark = 0;
+        }
+    }
+}
+
 // ---------------------------------------------------------------------------
 // read_body
 // ---------------------------------------------------------------------------
@@ -430,7 +497,8 @@ fn read_body(
     buf: &mut Vec<u8>,
     head_len: usize,
     body_len: usize,
-    deadline: Instant,
+    limits: &ConnLimits,
+    cap: Instant,
 ) -> io::Result<Vec<u8>> {
     // Bytes already in `buf` past the head belong to the body.
     let already = buf.len().saturating_sub(head_len);
@@ -442,13 +510,17 @@ fn read_body(
         return Ok(buf[head_len..head_len + body_len].to_vec());
     }
 
-    // Read until we have the rest, bounded by the body budget (anchored
+    // Read until we have the rest, bounded by the body cap AND the
-    // by the caller when the head finished parsing).
+    // burst-gated stall window (whichever is sooner).
+    let mut gate = BodyStallGate::new(cap, limits, Instant::now());
     let mut total_read = already;
     while total_read < body_len {
-        match read_some(fd, buf, 8 * 1024, deadline) {
+        match read_some(fd, buf, 8 * 1024, gate.deadline()) {
             Ok(0)  => return Err(io::Error::new(ErrorKind::UnexpectedEof, "client closed during body")),
-            Ok(n)  => total_read += n,
+            Ok(n)  => {
+                total_read += n;
+                gate.record(n, Instant::now());
+            }
             Err(e) => return Err(e),
         }
     }
@@ -489,23 +561,25 @@ fn read_chunked_body(
     limits: &ConnLimits,
     deadline: Instant,
 ) -> Result<(Vec<u8>, usize), ChunkedBodyErr> {
-    // Ensure `buf` holds at least `until` bytes, reading on the request
+    // Ensure `buf` holds at least `until` bytes, reading under the body
-    // deadline. Io(TimedOut) on expiry, UnexpectedEof on early close.
+    // stall gate (absolute cap AND burst-gated stall window). Io(TimedOut)
+    // on expiry, UnexpectedEof on early close. All chunked socket reads
+    // funnel through here, so recording bytes here covers the whole path.
     fn fill_to(
         fd: RawFd,
         buf: &mut Vec<u8>,
         until: usize,
-        deadline: Instant,
+        gate: &mut BodyStallGate,
     ) -> Result<(), ChunkedBodyErr> {
         while buf.len() < until {
-            match read_some(fd, buf, 8 * 1024, deadline) {
+            match read_some(fd, buf, 8 * 1024, gate.deadline()) {
                 Ok(0) => {
                     return Err(ChunkedBodyErr::Io(io::Error::new(
                         ErrorKind::UnexpectedEof,
                         "client closed during chunked body",
                     )))
                 }
-                Ok(_) => {}
+                Ok(n) => gate.record(n, Instant::now()),
                 Err(e) => return Err(ChunkedBodyErr::Io(e)),
             }
         }
@@ -520,7 +594,7 @@ fn read_chunked_body(
         buf: &mut Vec<u8>,
         from: usize,
         max_line: usize,
-        deadline: Instant,
+        gate: &mut BodyStallGate,
     ) -> Result<usize, ChunkedBodyErr> {
         let mut scan = from;
         loop {
@@ -533,16 +607,19 @@ fn read_chunked_body(
                     return Err(ChunkedBodyErr::Malformed);
                 }
             }
-            fill_to(fd, buf, buf.len() + 1, deadline)?;
+            fill_to(fd, buf, buf.len() + 1, gate)?;
         }
     }
 
+    // `deadline` is the absolute body cap; the gate layers the burst-gated
+    // stall window under it. All reads below go through find_crlf/fill_to.
+    let mut gate = BodyStallGate::new(deadline, limits, Instant::now());
     let mut pos = head_len;
     let mut decoded: Vec<u8> = Vec::new();
 
     loop {
         // ----- size line: HEX[;extensions]\r\n -----
-        let line_end = find_crlf(fd, buf, pos, MAX_SIZE_LINE, deadline)?;
+        let line_end = find_crlf(fd, buf, pos, MAX_SIZE_LINE, &mut gate)?;
         let line = &buf[pos..line_end];
         let size_str = match line.iter().position(|&b| b == b';') {
             Some(i) => &line[..i], // chunk extensions: ignored
@@ -559,7 +636,7 @@ fn read_chunked_body(
             // ----- trailer section: zero or more header lines, then CRLF -----
             let trailer_start = pos;
             loop {
-                let t_end = find_crlf(fd, buf, pos, MAX_SIZE_LINE.max(1024), deadline)?;
+                let t_end = find_crlf(fd, buf, pos, MAX_SIZE_LINE.max(1024), &mut gate)?;
                 let empty = t_end == pos;
                 pos = t_end + 2;
                 if empty {
@@ -576,7 +653,7 @@ fn read_chunked_body(
         }
 
         // ----- chunk payload + trailing CRLF -----
-        fill_to(fd, buf, pos + size + 2, deadline)?;
+        fill_to(fd, buf, pos + size + 2, &mut gate)?;
         decoded.extend_from_slice(&buf[pos..pos + size]);
         if &buf[pos + size..pos + size + 2] != b"\r\n" {
             return Err(ChunkedBodyErr::Malformed);

src/serve.rs

@@ -45,6 +45,13 @@ pub struct Config {
     /// to full body). Sized for slow links, so much larger than
     /// `head_timeout`. See `ConnLimits::body_timeout`.
     pub body_timeout:       Duration,
+    /// Burst size that resets the body stall clock. A body dribbling fewer
+    /// than this per `body_stall_timeout` window is evicted — the slowloris
+    /// / slow-legit discriminator. See `ConnLimits::body_burst_bytes`.
+    pub body_burst_bytes:   usize,
+    /// Max time since the last qualifying body burst before eviction;
+    /// backstopped by `body_timeout`. See `ConnLimits::body_stall_timeout`.
+    pub body_stall_timeout: Duration,
     /// Per-write budget for response bytes (the fixed head+body write, and
     /// each streamed chunk). See `ConnLimits::write_timeout`.
     pub write_timeout:      Duration,
@@ -97,6 +104,8 @@ impl Config {
             read_buf_size:      8 * 1024,
             head_timeout:       Duration::from_secs(30),
             body_timeout:       Duration::from_secs(300),
+            body_burst_bytes:   4 * 1024,
+            body_stall_timeout: Duration::from_secs(20),
             write_timeout:      Duration::from_secs(30),
             max_body_bytes:     16 * 1024 * 1024,
             drain_timeout:      Duration::from_secs(30),
@@ -117,6 +126,8 @@ impl Config {
             keep_alive_timeout: self.keep_alive_timeout,
             head_timeout:       self.head_timeout,
             body_timeout:       self.body_timeout,
+            body_burst_bytes:   self.body_burst_bytes,
+            body_stall_timeout: self.body_stall_timeout,
             write_timeout:      self.write_timeout,
             max_frame_payload:  self.max_frame_payload,
             max_message_bytes:  self.max_message_bytes,

tests/integration.rs

@@ -462,6 +462,39 @@ fn spawn_server_with_timeouts(
     panic!("server didn't come up on {addr}");
 }
 
+/// Spawn a server with the body stall-gate knobs under test; keep-alive
+/// and head budgets are set out of the way so only the body path matters.
+fn spawn_server_with_body_gate(
+    pipeline: Pipeline,
+    head: Duration,
+    body: Duration,
+    burst_bytes: usize,
+    stall: Duration,
+) -> u16 {
+    let port = free_port();
+    let addr: SocketAddr = format!("127.0.0.1:{port}").parse().unwrap();
+    std::thread::spawn(move || {
+        let cfg = Config {
+            listener_pool:      2,
+            scheduler_threads:  Some(2),
+            keep_alive_timeout: Duration::from_secs(30),
+            head_timeout:       head,
+            body_timeout:       body,
+            body_burst_bytes:   burst_bytes,
+            body_stall_timeout: stall,
+            ..Config::new(addr)
+        };
+        serve_with(cfg, pipeline).unwrap();
+    });
+    for _ in 0..50 {
+        if TcpStream::connect(addr).is_ok() {
+            return port;
+        }
+        std::thread::sleep(Duration::from_millis(50));
+    }
+    panic!("server didn't come up on {addr}");
+}
+
 /// Read from `s` until the response head is complete (double CRLF). Only
 /// suitable for responses with an empty body.
 fn read_response_head(s: &mut TcpStream) -> Vec<u8> {
@@ -1003,6 +1036,78 @@ fn fixed_body_stall_killed_at_body_timeout() {
     assert!(start.elapsed() < Duration::from_secs(3), "close took too long");
 }
 
+/// Burst gate, NEGATIVE (chunked path): a client that ACTIVELY but SMOOTHLY
+/// trickles sub-burst bytes is evicted at ~body_stall_timeout — even though
+/// the absolute body_timeout is far away and the client never goes fully
+/// silent. This is the slowloris-body case the gate exists to catch, and
+/// exercises the fill_to gate in read_chunked_body.
+#[test]
+fn body_smooth_trickle_evicted_at_stall_timeout() {
+    let port = spawn_server_with_body_gate(
+        echo_pipeline(),
+        Duration::from_secs(30),     // head_timeout out of the way
+        Duration::from_secs(30),     // body_timeout out of the way (prove it's the STALL gate)
+        4096,                        // body_burst_bytes
+        Duration::from_millis(800),  // body_stall_timeout under test
+    );
+    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
+    s.set_read_timeout(Some(Duration::from_secs(5))).unwrap();
+    // Head + a chunk-size line announcing a 4096-byte chunk, then trickle
+    // its payload one byte at a time: never a full burst, so the stall mark
+    // never advances.
+    s.write_all(b"POST /echo HTTP/1.1\r\nHost: x\r\nTransfer-Encoding: chunked\r\n\r\n1000\r\n")
+        .unwrap();
+    let start = std::time::Instant::now();
+    let mut evicted = false;
+    for _ in 0..200 { // up to ~20s; eviction expected at ~800ms
+        if s.write_all(&[b'x']).is_err() {
+            evicted = true; // server closed on us -> write failed
+            break;
+        }
+        std::thread::sleep(Duration::from_millis(100));
+    }
+    assert!(evicted, "server never evicted the smooth sub-burst trickle");
+    assert!(
+        start.elapsed() < Duration::from_secs(3),
+        "eviction took {:?}, expected ~800ms (stall gate, not the 30s cap)", start.elapsed()
+    );
+}
+
+/// Burst gate, POSITIVE (fixed-CL path): a slow-but-legit client that
+/// delivers real bursts with gaps SHORTER than body_stall_timeout keeps
+/// resetting the stall mark and completes intact. This is the slow IoT
+/// upload the gate must NOT punish; exercises the read_body gate.
+#[test]
+fn bursty_slow_body_survives_stall_gate() {
+    let port = spawn_server_with_body_gate(
+        echo_pipeline(),
+        Duration::from_secs(30),    // head_timeout out of the way
+        Duration::from_secs(30),    // body_timeout out of the way
+        4096,                       // body_burst_bytes
+        Duration::from_secs(2),     // body_stall_timeout: gaps stay under this
+    );
+    let mut s = TcpStream::connect(("127.0.0.1", port)).unwrap();
+    s.set_read_timeout(Some(Duration::from_secs(10))).unwrap();
+    // Promise 3 * 4096 bytes, Connection: close so read_to_end lands the
+    // full echo.
+    let burst = vec![b'x'; 4096];
+    s.write_all(b"POST /echo HTTP/1.1\r\nHost: x\r\nContent-Length: 12288\r\nConnection: close\r\n\r\n")
+        .unwrap();
+    for i in 0..3 {
+        s.write_all(&burst).unwrap();
+        if i < 2 {
+            std::thread::sleep(Duration::from_millis(500)); // < 2s stall window
+        }
+    }
+    let mut resp = Vec::new();
+    s.read_to_end(&mut resp).expect("expected full response");
+    assert_eq!(http_status(&resp), 200, "resp head: {:?}", String::from_utf8_lossy(&resp[..resp.len().min(120)]));
+    let body_at = resp.windows(4).position(|w| w == b"\r\n\r\n").expect("no head terminator") + 4;
+    let body = &resp[body_at..];
+    assert_eq!(body.len(), 12288, "echoed body truncated: {} bytes", body.len());
+    assert!(body.iter().all(|&b| b == b'x'), "echoed body corrupted");
+}
+
 // ---------------------------------------------------------------------------
 // SSE (v0.3 chunk 3)
 // ---------------------------------------------------------------------------