fix(dashboard): reap orphaned embedded-chat sessions to stop slash_worker leak

Since #38591 made the dashboard's embedded chat unconditional, every
browser refresh of /chat spins up a fresh session.create (new sid + a
fresh _SlashWorker via _deferred_build) over /api/ws, but the old tab's
WS disconnect only DETACHES the transport (ws.py) — it never closes the
old session or its slash_worker. The dashboard's in-process gateway is
long-lived, so the detached _SlashWorker subprocess's stdin pipe stays
open forever and the worker never reaches EOF: one leaked python process
per refresh.

Fix at the session-lifecycle layer (not PTY signal timing — verified that
a process whose owning gateway dies is always reaped via stdin-EOF; the
leak is specifically the long-lived dashboard process keeping detached
sessions parked). On WS disconnect, schedule a grace-delayed reap of any
session left orphaned (transport detached to stdio, not mid-turn). A quick
reconnect / session.resume / prompt.submit rebinds a live transport and
cancels the reap, preserving the intentional detach-for-reconnect window.

- server.py: extract _teardown_session() (shared with session.close),
  add _ws_session_is_orphaned() + _schedule_ws_orphan_reap(), gated by
  HERMES_TUI_WS_ORPHAN_REAP_GRACE_S (default 20s, 0 disables).
- ws.py: schedule the reap for each detached session on disconnect.
- tests: reap-closes-worker, spares-reattached/mid-turn/finalized,
  disabled-when-grace-zero.

tests/test_tui_gateway_server.py

@@ -876,6 +876,73 @@ def test_session_close_commits_memory_and_fires_finalize_hook(monkeypatch):
         server._sessions.pop("sid", None)
 
 
+def test_ws_orphan_reap_closes_worker_when_session_stays_detached(monkeypatch):
+    """A detached WS session past its grace window has its slash_worker closed.
+
+    Regression for #38591 fallout: every dashboard refresh spawned a fresh
+    session + _SlashWorker but never reaped the previous one, leaking one
+    python subprocess per refresh.
+    """
+    closed = {"worker": False}
+
+    class _FakeWorker:
+        def close(self):
+            closed["worker"] = True
+
+    server._sessions["orphan-sid"] = _session(
+        transport=server._stdio_transport,
+        slash_worker=_FakeWorker(),
+        running=False,
+    )
+    # Run the reap body synchronously (no real timer/grace) to assert behaviour.
+    monkeypatch.setattr(server, "_WS_ORPHAN_REAP_GRACE_S", 0.01)
+    try:
+        # Directly invoke the orphaned-check + teardown the timer would run.
+        assert server._ws_session_is_orphaned(server._sessions["orphan-sid"]) is True
+        session = server._sessions.pop("orphan-sid")
+        server._teardown_session(session)
+        assert closed["worker"] is True
+    finally:
+        server._sessions.pop("orphan-sid", None)
+
+
+def test_ws_orphan_reap_spares_reattached_session(monkeypatch):
+    """A session that rebinds a live transport is NOT considered orphaned."""
+
+    class _LiveTransport:
+        def write(self, *a, **k):
+            return True
+
+    # Reattached: transport is a live (non-stdio) transport.
+    reattached = _session(transport=_LiveTransport(), running=False)
+    assert server._ws_session_is_orphaned(reattached) is False
+
+    # Mid-turn sessions are also spared even if detached.
+    mid_turn = _session(transport=server._stdio_transport, running=True)
+    assert server._ws_session_is_orphaned(mid_turn) is False
+
+    # Already finalized sessions are spared (idempotency).
+    done = _session(transport=server._stdio_transport, running=False, _finalized=True)
+    assert server._ws_session_is_orphaned(done) is False
+
+
+def test_ws_orphan_reap_disabled_when_grace_zero(monkeypatch):
+    """Grace=0 disables the reaper entirely (pre-fix park-forever behaviour)."""
+    fired = {"timer": False}
+
+    class _Timer:
+        def __init__(self, *a, **k):
+            fired["timer"] = True
+
+        def start(self):
+            pass
+
+    monkeypatch.setattr(server, "_WS_ORPHAN_REAP_GRACE_S", 0.0)
+    monkeypatch.setattr(server.threading, "Timer", _Timer)
+    server._schedule_ws_orphan_reap("any-sid")
+    assert fired["timer"] is False
+
+
 def test_init_session_fires_reset_hook(monkeypatch):
     hooks = []
 

tui_gateway/server.py

@@ -136,6 +136,24 @@ try:
 except (ValueError, TypeError):
     _slash_timeout = 45.0
 _SLASH_WORKER_TIMEOUT_S = max(5.0, _slash_timeout)
+
+# When a WebSocket client (the dashboard's embedded-chat tab / desktop app)
+# disconnects, ``tui_gateway.ws`` detaches the transport but intentionally
+# leaves the session parked so a quick reconnect can reattach it (see ws.py).
+# That park is unbounded, though: a browser refresh spins up a brand-new
+# ``session.create`` (new sid + a fresh _SlashWorker via _deferred_build) and
+# never reattaches the OLD sid, so the old session's slash-worker subprocess
+# lingers forever — one leaked python process per refresh (#38591 fallout).
+# After this grace window, an orphaned (transport-detached, not-running) WS
+# session is reaped: its _SlashWorker is closed and the session finalized.
+# Set to 0 to disable (park forever, pre-fix behaviour).
+try:
+    _ws_orphan_reap_grace = float(
+        os.environ.get("HERMES_TUI_WS_ORPHAN_REAP_GRACE_S") or "20"
+    )
+except (ValueError, TypeError):
+    _ws_orphan_reap_grace = 20.0
+_WS_ORPHAN_REAP_GRACE_S = max(0.0, _ws_orphan_reap_grace)
 _DETAIL_SECTION_NAMES = ("thinking", "tools", "subagents", "activity")
 _DETAIL_MODES = frozenset({"hidden", "collapsed", "expanded"})
 
@@ -326,6 +344,78 @@ def _finalize_session(session: dict | None, end_reason: str = "tui_close") -> No
             pass
 
 
+def _teardown_session(session: dict | None) -> None:
+    """Fully tear down a session: finalize, unregister, close agent + worker.
+
+    Shared by ``session.close`` and the orphaned-WS-session reaper so the
+    slash-worker subprocess is always closed exactly once via the same path.
+    Idempotent: the ``_finalized`` guard in ``_finalize_session`` and the
+    ``poll()`` guard in ``_SlashWorker.close`` make repeat calls harmless.
+    """
+    if not session:
+        return
+    _finalize_session(session)
+    try:
+        from tools.approval import unregister_gateway_notify
+
+        unregister_gateway_notify(session["session_key"])
+    except Exception:
+        pass
+    try:
+        agent = session.get("agent")
+        if agent and hasattr(agent, "close"):
+            agent.close()
+    except Exception:
+        pass
+    try:
+        worker = session.get("slash_worker")
+        if worker:
+            worker.close()
+    except Exception:
+        pass
+
+
+def _ws_session_is_orphaned(session: dict | None) -> bool:
+    """True if a WS session has no live transport and no in-flight turn.
+
+    After ``handle_ws`` detaches a disconnected client it points the session
+    at ``_stdio_transport``. In the dashboard's in-process gateway there is no
+    real stdio peer reading those frames, so a session left on the stdio
+    transport (and not mid-turn) is genuinely orphaned and safe to reap.
+    """
+    if not session or session.get("_finalized"):
+        return False
+    if session.get("running"):
+        return False
+    return session.get("transport") is _stdio_transport
+
+
+def _schedule_ws_orphan_reap(sid: str) -> None:
+    """After a grace window, reap session ``sid`` iff it's still orphaned.
+
+    Called from the WS-disconnect path. The grace window lets a transient
+    reconnect (or a ``session.resume`` that reattaches the transport) cancel
+    the reap by re-binding a live transport. Disabled when the grace is 0.
+    """
+    if _WS_ORPHAN_REAP_GRACE_S <= 0:
+        return
+
+    def _reap() -> None:
+        with _session_resume_lock:
+            session = _sessions.get(sid)
+            if not _ws_session_is_orphaned(session):
+                return
+            _sessions.pop(sid, None)
+        try:
+            _teardown_session(session)
+        except Exception:
+            pass
+
+    timer = threading.Timer(_WS_ORPHAN_REAP_GRACE_S, _reap)
+    timer.daemon = True
+    timer.start()
+
+
 def _shutdown_sessions() -> None:
     with _sessions_lock:
         snapshot = list(_sessions.values())
@@ -3675,25 +3765,7 @@ def _(rid, params: dict) -> dict:
             session = _sessions.pop(sid, None)
         if not session:
             return _ok(rid, {"closed": False})
-        _finalize_session(session)
-        try:
-            from tools.approval import unregister_gateway_notify
-
-            unregister_gateway_notify(session["session_key"])
-        except Exception:
-            pass
-        try:
-            agent = session.get("agent")
-            if agent and hasattr(agent, "close"):
-                agent.close()
-        except Exception:
-            pass
-        try:
-            worker = session.get("slash_worker")
-            if worker:
-                worker.close()
-        except Exception:
-            pass
+        _teardown_session(session)
     return _ok(rid, {"closed": True})
 
 

tui_gateway/ws.py

@@ -262,15 +262,32 @@ async def handle_ws(ws: Any) -> None:
                 break
     finally:
         detached_sessions = 0
+        reaped_scheduled = 0
         if transport is not None:
             transport.close()
 
             # Detach the transport from any sessions it owned so later emits
             # fall back to stdio instead of crashing into a closed socket.
-            for _, sess in list(server._sessions.items()):
+            #
+            # In the dashboard's in-process gateway that stdio fallback has no
+            # real reader, so a detached session would otherwise sit forever
+            # holding its _SlashWorker subprocess open (one leaked python proc
+            # per browser refresh — #38591 fallout). Schedule a grace-delayed
+            # reap; a quick reconnect / session.resume re-binds a live
+            # transport and cancels it (see _ws_session_is_orphaned).
+            for _sid, sess in list(server._sessions.items()):
                 if sess.get("transport") is transport:
                     sess["transport"] = server._stdio_transport
                     detached_sessions += 1
+                    try:
+                        server._schedule_ws_orphan_reap(_sid)
+                        reaped_scheduled += 1
+                    except Exception:
+                        _log.exception(
+                            "ws orphan-reap schedule failed peer=%s sid=%s",
+                            peer,
+                            _sid,
+                        )
         try:
             await ws.close()
         except Exception as exc: