fix: reduce detach latency and stabilize detach path

libshpool/src/common.rs

@@ -14,10 +14,65 @@
 
 //! The common module is a grab bag of shared utility functions.
 
-use std::env;
+use std::{env, thread, time};
 
 use anyhow::anyhow;
 
+/// Controls how often `sleep_unless` re-checks its stop predicate.
+#[derive(Clone, Copy, Debug)]
+pub enum PollStrategy {
+    /// Poll at a fixed interval.
+    Uniform { interval: time::Duration },
+    /// Poll with exponential backoff up to a maximum interval.
+    ///
+    /// Values <= 1 disable growth and behave like uniform polling.
+    Backoff { initial_interval: time::Duration, factor: f32, max_interval: time::Duration },
+}
+
+/// Sleeps for up to `total_sleep`, but returns early if `stop` becomes true.
+///
+/// Returns `true` when `stop` triggered before timeout and `false` when the
+/// full sleep window elapsed.
+pub fn sleep_unless<F>(
+    total_sleep: time::Duration,
+    mut stop: F,
+    poll_strategy: PollStrategy,
+) -> bool
+where
+    F: FnMut() -> bool,
+{
+    let deadline = time::Instant::now() + total_sleep;
+    let mut next_interval = match poll_strategy {
+        PollStrategy::Uniform { interval } => interval,
+        PollStrategy::Backoff { initial_interval, .. } => initial_interval,
+    };
+
+    if next_interval.is_zero() {
+        // Avoid a tight spin-loop if a zero interval is accidentally configured.
+        next_interval = time::Duration::from_millis(1);
+    }
+
+    loop {
+        if stop() {
+            return true;
+        }
+
+        let now = time::Instant::now();
+        if now >= deadline {
+            return false;
+        }
+
+        let remaining = deadline.saturating_duration_since(now);
+        thread::sleep(remaining.min(next_interval));
+
+        if let PollStrategy::Backoff { factor, max_interval, .. } = poll_strategy {
+            if factor > 1.0 {
+                next_interval = std::cmp::min(next_interval.mul_f32(factor), max_interval);
+            }
+        }
+    }
+}
+
 pub fn resolve_sessions(sessions: &mut Vec<String>, action: &str) -> anyhow::Result<()> {
     if sessions.is_empty() {
         if let Ok(current_session) = env::var("SHPOOL_SESSION_NAME") {
@@ -32,3 +87,54 @@ pub fn resolve_sessions(sessions: &mut Vec<String>, action: &str) -> anyhow::Res
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use std::cell::Cell;
+    use std::time::Duration;
+
+    use super::{sleep_unless, PollStrategy};
+
+    #[test]
+    fn sleep_unless_returns_immediately_when_stop_is_true() {
+        let stopped = sleep_unless(
+            Duration::from_millis(10),
+            || true,
+            PollStrategy::Uniform { interval: Duration::from_millis(1) },
+        );
+
+        assert!(stopped);
+    }
+
+    #[test]
+    fn sleep_unless_times_out_when_stop_is_false() {
+        let stopped = sleep_unless(
+            Duration::from_millis(3),
+            || false,
+            PollStrategy::Uniform { interval: Duration::from_millis(1) },
+        );
+
+        assert!(!stopped);
+    }
+
+    #[test]
+    fn sleep_unless_rechecks_stop_with_backoff() {
+        let checks = Cell::new(0usize);
+        let stopped = sleep_unless(
+            Duration::from_millis(20),
+            || {
+                let n = checks.get() + 1;
+                checks.set(n);
+                n >= 3
+            },
+            PollStrategy::Backoff {
+                initial_interval: Duration::from_millis(1),
+                factor: 2.0,
+                max_interval: Duration::from_millis(4),
+            },
+        );
+
+        assert!(stopped);
+        assert!(checks.get() >= 3);
+    }
+}

libshpool/src/consts.rs

@@ -15,7 +15,7 @@
 use std::time;
 
 pub const SOCK_STREAM_TIMEOUT: time::Duration = time::Duration::from_millis(200);
-pub const JOIN_POLL_DURATION: time::Duration = time::Duration::from_millis(100);
+pub const JOIN_POLL_DURATION: time::Duration = time::Duration::from_millis(50);
 
 pub const BUF_SIZE: usize = 1024 * 16;
 

libshpool/src/daemon/shell.rs

@@ -32,7 +32,7 @@ use shpool_protocol::{Chunk, ChunkKind, TtySize};
 use tracing::{debug, error, info, instrument, span, trace, warn, Level};
 
 use crate::{
-    consts,
+    common, consts,
     daemon::{config, exit_notify::ExitNotifier, keybindings, pager::PagerCtl, prompt, show_motd},
     protocol::ChunkExt as _,
     session_restore, test_hooks,
@@ -51,10 +51,9 @@ const SUPERVISOR_POLL_DUR: time::Duration = time::Duration::from_millis(300);
 // size.
 const REATTACH_RESIZE_DELAY: time::Duration = time::Duration::from_millis(50);
 
-// The shell->client thread should wake up relatively frequently so it can
-// detect reattach, but we don't need to go crazy since reattach is not part of
-// the inner loop.
-const SHELL_TO_CLIENT_POLL_MS: u16 = 100;
+// The shell->client thread should poll frequently so detach/reattach control
+// messages are noticed quickly without spinning the CPU.
+const SHELL_TO_CLIENT_POLL_MS: u16 = 50;
 
 // How long to wait before giving up while trying to talk to the
 // shell->client thread.
@@ -835,7 +834,11 @@ impl SessionInner {
 
                                 use keybindings::Action::*;
                                 match action {
-                                    Detach => self.action_detach()?,
+                                    Detach => {
+                                        self.action_detach()?;
+                                        debug!("exiting client->shell thread after detach");
+                                        return Ok(());
+                                    }
                                     NoOp => {}
                                 }
                             }
@@ -883,12 +886,15 @@ impl SessionInner {
 
                 loop {
                     trace!("checking stop_rx");
-                    if stop.load(Ordering::Relaxed) {
+                    let stop_early = common::sleep_unless(
+                        consts::HEARTBEAT_DURATION,
+                        || stop.load(Ordering::Relaxed),
+                        common::PollStrategy::Uniform { interval: consts::JOIN_POLL_DURATION },
+                    );
+                    if stop_early {
                         info!("recvd stop msg");
                         return Ok(());
                     }
-
-                    thread::sleep(consts::HEARTBEAT_DURATION);
                     {
                         let shell_to_client_ctl = self.shell_to_client_ctl.lock().unwrap();
                         match shell_to_client_ctl

libshpool/src/protocol.rs

@@ -27,10 +27,14 @@ use serde::{Deserialize, Serialize};
 use shpool_protocol::{Chunk, ChunkKind, ConnectHeader, VersionHeader};
 use tracing::{debug, error, info, instrument, span, trace, warn, Level};
 
-use super::{consts, tty};
+use super::{common, consts, tty};
 
-const JOIN_POLL_DUR: time::Duration = time::Duration::from_millis(100);
-const JOIN_HANGUP_DUR: time::Duration = time::Duration::from_millis(300);
+const DETACH_DISCONNECT_FAST_WAIT_DUR: time::Duration = time::Duration::from_millis(10);
+const MAX_DETACH_WAIT_DUR: time::Duration = time::Duration::from_millis(300);
+const DETACH_BACKOFF_INITIAL_DUR: time::Duration = time::Duration::from_millis(1);
+// Cap backoff steps so slow-path stays responsive while still avoiding busy
+// waits.
+const DETACH_BACKOFF_MAX_STEP_DUR: time::Duration = time::Duration::from_millis(25);
 
 /// The centralized encoding function that should be used for all protocol
 /// serialization.
@@ -50,7 +54,7 @@ where
     Ok(())
 }
 
-/// The centralized decoding focuntion that should be used for all protocol
+/// The centralized decoding function that should be used for all protocol
 /// deserialization.
 pub fn decode_from<T, R>(r: R) -> anyhow::Result<T>
 where
@@ -333,18 +337,49 @@ impl Client {
                 if sock_to_stdout_h.is_finished() {
                     nfinished_threads += 1;
                 }
+
                 if nfinished_threads > 0 {
                     if nfinished_threads < 2 {
-                        thread::sleep(JOIN_HANGUP_DUR);
-                        nfinished_threads = 0;
-                        if stdin_to_sock_h.is_finished() {
-                            info!("recheck: stdin->sock thread done");
-                            nfinished_threads += 1;
-                        }
-                        if sock_to_stdout_h.is_finished() {
-                            info!("recheck: sock->stdout thread done");
-                            nfinished_threads += 1;
+                        // Fast-path: when sock->stdout already ended (detach/disconnect),
+                        // stdin->sock can stay blocked on stdin. In that case, do a very
+                        // short grace wait and then exit quickly. This is independent
+                        // of stdin being a TTY or a pipe.
+                        // Slow-path: for other shutdown orders, keep compatibility by
+                        // waiting up to 300ms with backoff.
+                        let mut stdin_done = stdin_to_sock_h.is_finished();
+                        let mut stdout_done = sock_to_stdout_h.is_finished();
+
+                        // Keep max_wait fixed for this detach sequence. Recomputing it inside
+                        // the loop could accidentally switch paths mid-cleanup.
+                        let max_wait = if stdout_done && !stdin_done {
+                            DETACH_DISCONNECT_FAST_WAIT_DUR
+                        } else {
+                            MAX_DETACH_WAIT_DUR
+                        };
+
+                        let finished_waiting = common::sleep_unless(
+                            max_wait,
+                            || {
+                                stdin_done = stdin_to_sock_h.is_finished();
+                                stdout_done = sock_to_stdout_h.is_finished();
+                                nfinished_threads = (stdin_done as usize) + (stdout_done as usize);
+                                nfinished_threads >= 2
+                            },
+                            common::PollStrategy::Backoff {
+                                initial_interval: DETACH_BACKOFF_INITIAL_DUR,
+                                factor: 2.0,
+                                max_interval: DETACH_BACKOFF_MAX_STEP_DUR,
+                            },
+                        );
+
+                        if !finished_waiting {
+                            // Re-probe after timeout because thread state can change
+                            // during the final sleep inside sleep_unless.
+                            stdin_done = stdin_to_sock_h.is_finished();
+                            stdout_done = sock_to_stdout_h.is_finished();
+                            nfinished_threads = (stdin_done as usize) + (stdout_done as usize);
                         }
+
                         if nfinished_threads < 2 {
                             // If one of the worker threads is done and the
                             // other is not exiting, we are likely blocked on
@@ -355,8 +390,8 @@ impl Client {
                             // us to use simple blocking IO.
                             warn!(
                                 "exiting due to a stuck IO thread stdin_to_sock_finished={} sock_to_stdout_finished={}",
-                                stdin_to_sock_h.is_finished(),
-                                sock_to_stdout_h.is_finished()
+                                stdin_done,
+                                stdout_done
                             );
                             // make sure that we restore the tty flags on the input
                             // tty before exiting the process.
@@ -367,7 +402,8 @@ impl Client {
                     }
                     break;
                 }
-                thread::sleep(JOIN_POLL_DUR);
+
+                thread::sleep(consts::JOIN_POLL_DURATION);
             }
 
             match stdin_to_sock_h.join() {