fix: add opt-in child-group process detection (#2052)

* fix: add opt-in child-group process detection

refs #1982

* fix: preserve lifecycle probes without foreground groups

refs #1982

2 files changed, 245 insertions, 21 deletions

diff --git a/src/pane.rs b/src/pane.rs
index 3bf94ec0..be1a23da 100644
--- a/src/pane.rs
+++ b/src/pane.rs
@@ -381,11 +381,22 @@ fn foreground_group_changed(
         && (foreground_pgid.is_some() || last_foreground_pgid.is_some())
 }
 
+// Only kernel-observed foreground groups drive change detection. Remembering an
+// inferred group would look like a change on every tick while the kernel stays silent.
+fn process_group_for_change_tracking(
+    observed_foreground_pgid: Option<u32>,
+    probed_process_group_id: Option<u32>,
+) -> Option<u32> {
+    observed_foreground_pgid?;
+    probed_process_group_id.or(observed_foreground_pgid)
+}
+
 fn should_skip_process_probe_for_lifecycle_authority(
     full_lifecycle_authority_active: bool,
     input: ProcessProbeInput,
 ) -> bool {
     full_lifecycle_authority_active
+        && input.foreground_pgid.is_some()
         && !input.pending_foreground_shell_clear
         && input.suppressed_agent.is_none()
         && input.has_process_probe
@@ -713,6 +724,8 @@ fn spawn_basic_detection_task(
                 has_process_probe = true;
                 let probe = probe_foreground_process(pid, foreground_pgid);
                 let process_group_id = probe.process_group_id;
+                let tracked_process_group_id =
+                    process_group_for_change_tracking(foreground_pgid, process_group_id);
                 let foreground_is_pane_shell = probe.foreground_is_pane_shell;
                 let mut new_agent = probe.agent;
                 if let Some(suppressed_agent) = suppressed_agent {
@@ -737,17 +750,15 @@ fn spawn_basic_detection_task(
                     &mut pending_foreground_shell_clear,
                     &mut foreground_shell_exit_reported,
                 );
+                last_foreground_pgid = tracked_process_group_id;
                 if new_agent.is_some() {
-                    last_foreground_pgid = process_group_id.or(foreground_pgid);
                     acquisition_started_at = None;
                     last_content_change_at = None;
-                } else if agent_presence.current_agent().is_none() {
-                    last_foreground_pgid = process_group_id.or(foreground_pgid);
-                    if had_process_probe && process_group_changed {
-                        acquisition_started_at = Some(now);
-                    }
-                } else {
-                    last_foreground_pgid = process_group_id.or(foreground_pgid);
+                } else if agent_presence.current_agent().is_none()
+                    && had_process_probe
+                    && process_group_changed
+                {
+                    acquisition_started_at = Some(now);
                 }
                 if changed {
                     agent = agent_presence.current_agent();
@@ -2151,6 +2162,10 @@ impl PaneRuntime {
                             let probe = probe_foreground_process(pid, foreground_pgid);
                             let process_name = probe.process_name;
                             let process_group_id = probe.process_group_id;
+                            let tracked_process_group_id = process_group_for_change_tracking(
+                                foreground_pgid,
+                                process_group_id,
+                            );
                             let foreground_is_pane_shell = probe.foreground_is_pane_shell;
                             let mut new_agent = probe.agent;
 
@@ -2179,20 +2194,17 @@ impl PaneRuntime {
                                 &mut pending_foreground_shell_clear,
                                 &mut foreground_shell_exit_reported,
                             );
+                            last_foreground_pgid = tracked_process_group_id;
                             if new_agent.is_some() {
-                                last_foreground_pgid = process_group_id;
                                 acquisition_started_at = None;
                                 last_content_change_at = None;
-                                pending_restore_probe = false;
-                            } else if agent_presence.current_agent().is_none() {
-                                last_foreground_pgid = process_group_id.or(foreground_pgid);
-                                if had_process_probe && process_group_changed {
-                                    acquisition_started_at = Some(now);
-                                }
-                                pending_restore_probe = false;
-                            } else {
-                                last_foreground_pgid = process_group_id.or(foreground_pgid);
+                            } else if agent_presence.current_agent().is_none()
+                                && had_process_probe
+                                && process_group_changed
+                            {
+                                acquisition_started_at = Some(now);
                             }
+                            pending_restore_probe = false;
                             if changed {
                                 agent = agent_presence.current_agent();
                                 if agent != previous_agent
@@ -3672,6 +3684,19 @@ mod tests {
     }
 
     #[test]
+    fn inferred_group_does_not_trigger_a_probe_on_every_tick() {
+        let tracked = process_group_for_change_tracking(None, Some(300));
+        assert_eq!(tracked, None);
+        assert!(!should_probe_foreground_job(ProcessProbeInput {
+            current_agent: Some(Agent::Claude),
+            foreground_pgid: None,
+            last_foreground_pgid: tracked,
+            elapsed_since_process_check: std::time::Duration::from_millis(300),
+            ..process_probe_input()
+        }));
+    }
+
+    #[test]
     fn pending_shell_clear_and_restore_force_process_probes() {
         assert!(should_probe_foreground_job(ProcessProbeInput {
             current_agent: Some(Agent::Codex),
@@ -3706,6 +3731,21 @@ mod tests {
     }
 
     #[test]
+    fn lifecycle_authority_keeps_periodic_probes_without_an_observed_group() {
+        let input = ProcessProbeInput {
+            current_agent: Some(Agent::Pi),
+            foreground_pgid: None,
+            last_foreground_pgid: None,
+            elapsed_since_process_check: PROCESS_RECHECK_IDENTIFIED,
+            ..process_probe_input()
+        };
+        assert!(!should_skip_process_probe_for_lifecycle_authority(
+            true, input
+        ));
+        assert!(should_probe_foreground_job(input));
+    }
+
+    #[test]
     fn lifecycle_authority_preserves_process_exit_and_release_probes() {
         assert!(!should_skip_process_probe_for_lifecycle_authority(
             true,
diff --git a/src/platform/linux.rs b/src/platform/linux.rs
index 6557b41b..76568ad0 100644
--- a/src/platform/linux.rs
+++ b/src/platform/linux.rs
@@ -4,6 +4,7 @@ use std::{
     os::fd::RawFd,
     path::PathBuf,
     process::{Command, Stdio},
+    sync::OnceLock,
 };
 
 use super::{
@@ -12,6 +13,14 @@ use super::{
 };
 
 const WSL_MARKER_ENV_VARS: &[&str] = &["WSL_DISTRO_NAME", "WSL_INTEROP"];
+const PROCESS_DETECTION_ENV_VAR: &str = "HERDR_PROCESS_DETECTION";
+const CHILD_GROUPS_SCAN_LIMIT: usize = 64;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum ProcessDetectionMode {
+    Native,
+    ChildGroups,
+}
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 struct ProcGroupMember {
@@ -45,6 +54,29 @@ fn text_indicates_wsl(text: &str) -> bool {
     text.contains("microsoft") || text.contains("wsl")
 }
 
+fn parse_process_detection_mode(value: Option<&str>) -> Result<ProcessDetectionMode, &str> {
+    match value {
+        None | Some("") | Some("native") => Ok(ProcessDetectionMode::Native),
+        Some("child-groups") => Ok(ProcessDetectionMode::ChildGroups),
+        Some(value) => Err(value),
+    }
+}
+
+fn process_detection_mode() -> ProcessDetectionMode {
+    static MODE: OnceLock<ProcessDetectionMode> = OnceLock::new();
+    *MODE.get_or_init(|| {
+        let value = std::env::var(PROCESS_DETECTION_ENV_VAR).ok();
+        parse_process_detection_mode(value.as_deref()).unwrap_or_else(|value| {
+            tracing::warn!(
+                variable = PROCESS_DETECTION_ENV_VAR,
+                %value,
+                "unknown process detection mode; using native detection"
+            );
+            ProcessDetectionMode::Native
+        })
+    })
+}
+
 fn raw_command_argv(command: &str, flag: &str) -> Vec<std::ffi::OsString> {
     vec!["/bin/sh".into(), flag.into(), command.into()]
 }
@@ -96,8 +128,19 @@ pub(crate) fn available_pane_shell(child_pid: u32) -> Option<String> {
 }
 
 pub fn foreground_job(child_pid: u32) -> Option<ForegroundJob> {
-    let tpgid = foreground_process_group_id(child_pid)?;
-    let members = foreground_process_group_members(child_pid, tpgid)?;
+    if let Some(tpgid) = foreground_process_group_id(child_pid) {
+        return foreground_job_for_group(child_pid, tpgid);
+    }
+
+    if process_detection_mode() != ProcessDetectionMode::ChildGroups {
+        return None;
+    }
+
+    foreground_job_for_group(child_pid, child_groups_foreground_process_group(child_pid)?)
+}
+
+fn foreground_job_for_group(child_pid: u32, process_group_id: u32) -> Option<ForegroundJob> {
+    let members = foreground_process_group_members(child_pid, process_group_id)?;
     let processes = members
         .into_iter()
         .map(|member| {
@@ -117,11 +160,60 @@ pub fn foreground_job(child_pid: u32) -> Option<ForegroundJob> {
     }
 
     Some(ForegroundJob {
-        process_group_id: tpgid,
+        process_group_id,
         processes,
     })
 }
 
+/// Best-effort foreground group for environments that do not expose terminal
+/// foreground groups. This mode is explicit because background jobs cannot be
+/// distinguished from foreground jobs without the native terminal signal.
+fn child_groups_foreground_process_group(child_pid: u32) -> Option<u32> {
+    let shell_group_id = process_pgrp_and_comm(child_pid)
+        .map(|(pgrp, _)| pgrp)
+        .filter(|pgrp| *pgrp > 0)? as u32;
+
+    child_groups_foreground_process_group_with(
+        child_pid,
+        shell_group_id,
+        process_task_ids,
+        process_task_children,
+        |pid| process_pgrp_and_comm(pid).map(|(pgrp, _)| pgrp),
+    )
+}
+
+fn child_groups_foreground_process_group_with(
+    child_pid: u32,
+    shell_group_id: u32,
+    mut task_ids: impl FnMut(u32) -> Vec<u32>,
+    mut task_children: impl FnMut(u32, u32) -> Vec<u32>,
+    mut process_group_id: impl FnMut(u32) -> Option<i32>,
+) -> Option<u32> {
+    let mut newest = None;
+    let mut scanned = 0usize;
+    for tid in task_ids(child_pid) {
+        for child in task_children(child_pid, tid) {
+            if scanned >= CHILD_GROUPS_SCAN_LIMIT {
+                return None;
+            }
+            scanned += 1;
+
+            let Some(pgrp) = process_group_id(child) else {
+                continue;
+            };
+            if pgrp <= 0 {
+                continue;
+            }
+            let pgrp = pgrp as u32;
+            if pgrp == shell_group_id {
+                continue;
+            }
+            newest = Some(newest.map_or(pgrp, |current: u32| current.max(pgrp)));
+        }
+    }
+    newest.or(Some(shell_group_id))
+}
+
 fn foreground_process_group_members(
     child_pid: u32,
     process_group_id: u32,
@@ -666,6 +758,98 @@ mod tests {
     }
 
     #[test]
+    fn process_detection_mode_requires_explicit_child_groups_value() {
+        assert_eq!(
+            parse_process_detection_mode(None),
+            Ok(ProcessDetectionMode::Native)
+        );
+        assert_eq!(
+            parse_process_detection_mode(Some("")),
+            Ok(ProcessDetectionMode::Native)
+        );
+        assert_eq!(
+            parse_process_detection_mode(Some("native")),
+            Ok(ProcessDetectionMode::Native)
+        );
+        assert_eq!(
+            parse_process_detection_mode(Some("child-groups")),
+            Ok(ProcessDetectionMode::ChildGroups)
+        );
+        assert_eq!(parse_process_detection_mode(Some("gvisor")), Err("gvisor"));
+    }
+
+    #[test]
+    fn child_groups_foreground_group_picks_the_newest_job() {
+        let tasks = HashMap::from([(100, vec![100])]);
+        let children = HashMap::from([((100, 100), vec![200, 300])]);
+        let groups = HashMap::from([(200, 200), (300, 300)]);
+
+        let group = child_groups_foreground_process_group_with(
+            100,
+            100,
+            |pid| tasks.get(&pid).cloned().unwrap_or_default(),
+            |pid, tid| children.get(&(pid, tid)).cloned().unwrap_or_default(),
+            |pid| groups.get(&pid).copied(),
+        );
+
+        assert_eq!(group, Some(300));
+    }
+
+    #[test]
+    fn child_groups_foreground_group_returns_to_the_shell_group() {
+        let tasks = HashMap::from([(100, vec![100])]);
+        let children = HashMap::from([((100, 100), vec![150, 160])]);
+        let groups = HashMap::from([(150, 90), (160, 90)]);
+
+        let group = child_groups_foreground_process_group_with(
+            100,
+            90,
+            |pid| tasks.get(&pid).cloned().unwrap_or_default(),
+            |pid, tid| children.get(&(pid, tid)).cloned().unwrap_or_default(),
+            |pid| groups.get(&pid).copied(),
+        );
+
+        assert_eq!(group, Some(90));
+    }
+
+    #[test]
+    fn child_groups_foreground_group_skips_the_shell_group() {
+        let tasks = HashMap::from([(100, vec![100])]);
+        let children = HashMap::from([((100, 100), vec![150, 160, 300])]);
+        let groups = HashMap::from([(150, 90), (160, 90), (300, 300)]);
+
+        let group = child_groups_foreground_process_group_with(
+            100,
+            90,
+            |pid| tasks.get(&pid).cloned().unwrap_or_default(),
+            |pid, tid| children.get(&(pid, tid)).cloned().unwrap_or_default(),
+            |pid| groups.get(&pid).copied(),
+        );
+
+        assert_eq!(group, Some(300));
+    }
+
+    #[test]
+    fn child_groups_foreground_group_fails_closed_at_the_scan_limit() {
+        let children: Vec<u32> = (1..=(CHILD_GROUPS_SCAN_LIMIT as u32 + 10)).collect();
+        let mut inspected = 0usize;
+
+        let group = child_groups_foreground_process_group_with(
+            100,
+            100,
+            |_| vec![100],
+            |_, _| children.clone(),
+            |pid| {
+                inspected += 1;
+                Some(pid as i32)
+            },
+        );
+
+        assert_eq!(inspected, CHILD_GROUPS_SCAN_LIMIT);
+        assert_eq!(group, None);
+    }
+
+    #[test]
     fn foreground_members_follow_the_pane_tree_and_filter_by_process_group() {
         let tasks = HashMap::from([
             (100, vec![100, 101]),