From 21b3da3babae3c9a39c5181a16ea5c662cb44e07 Mon Sep 17 00:00:00 2001
From: nicm <nicm@openbsd.org>
Date: Mon, 21 Sep 2026 12:43:36 +0000
Subject: [PATCH] Do not do anything in session_detach and rely on the caller
 destroying the session, otherwise grouped sessions can reuse a session with
 no windows. From Jeong, Heon in GitHub issue 5620.

---
 server-fn.c |  6 +++---
 session.c   | 13 +++++++++----
 spawn.c     |  6 +++---
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/server-fn.c b/server-fn.c
index 754e0a514..21b8829eb 100644
--- a/server-fn.c
+++ b/server-fn.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: server-fn.c,v 1.153 2026/09/10 11:02:18 nicm Exp $ */
+/* $OpenBSD: server-fn.c,v 1.154 2026/09/21 12:43:36 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -311,8 +311,8 @@ server_link_window(struct session *src, struct winlink *srcwl,
 		}
 		if (killflag) {
 			/*
-			 * Can't use session_detach as it will destroy session
-			 * if this makes it empty.
+			 * Can't use session_detach as it won't detach the only
+			 * window.
 			 */
 			events_fire_winlink("window-unlinked", dstwl);
 			dstwl->flags &= ~WINLINK_ALERTFLAGS;
diff --git a/session.c b/session.c
index c0ea1306a..060e01fac 100644
--- a/session.c
+++ b/session.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: session.c,v 1.107 2026/08/05 07:35:35 nicm Exp $ */
+/* $OpenBSD: session.c,v 1.108 2026/09/21 12:43:36 nicm Exp $ */
 
 /*
  * Copyright (c) 2007 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -338,10 +338,17 @@ session_attach(struct session *s, struct window *w, int idx, char **cause)
 	return (wl);
 }
 
-/* Detach a window from a session. */
+/*
+ * Detach a window from a session. Returns 1 if the window has not been
+ * detached - the caller must destroy the session.
+ */
 int
 session_detach(struct session *s, struct winlink *wl)
 {
+	if (RB_MIN(winlinks, &s->windows) == wl &&
+	    RB_MAX(winlinks, &s->windows) == wl)
+		return (1);
+
 	if (s->curw == wl &&
 	    session_last(s) != 0 &&
 	    session_previous(s, 0) != 0)
@@ -354,8 +361,6 @@ session_detach(struct session *s, struct winlink *wl)
 
 	session_group_synchronize_from(s);
 
-	if (RB_EMPTY(&s->windows))
-		return (1);
 	return (0);
 }
 
diff --git a/spawn.c b/spawn.c
index 290b5da6f..22dbe041d 100644
--- a/spawn.c
+++ b/spawn.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: spawn.c,v 1.52 2026/08/20 09:19:24 nicm Exp $ */
+/* $OpenBSD: spawn.c,v 1.53 2026/09/21 12:43:36 nicm Exp $ */
 
 /*
  * Copyright (c) 2019 Nicholas Marriott <nicholas.marriott@gmail.com>
@@ -168,8 +168,8 @@ spawn_window(struct spawn_context *sc, char **cause)
 		}
 		if (wl != NULL) {
 			/*
-			 * Can't use session_detach as it will destroy session
-			 * if this makes it empty.
+			 * Can't use session_detach as it won't detach the only
+			 * window.
 			 */
 			wl->flags &= ~WINLINK_ALERTFLAGS;
 			events_fire_winlink("window-unlinked", wl);