From da641e4e8d2b0547487e8c6f28aca0e5981d5a92 Mon Sep 17 00:00:00 2001
From: Nicholas Marriott <nicholas.marriott@gmail.com>
Date: Tue, 18 Aug 2026 08:45:11 +0100
Subject: [PATCH] Add test for GitHub issue 5498 from Alex Rattray.

---
 regress/respawn-pane-control-lag.sh | 87 +++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 regress/respawn-pane-control-lag.sh

diff --git a/regress/respawn-pane-control-lag.sh b/regress/respawn-pane-control-lag.sh
new file mode 100644
index 000000000..b71ce1946
--- /dev/null
+++ b/regress/respawn-pane-control-lag.sh
@@ -0,0 +1,87 @@
+#!/bin/sh
+
+# respawn-pane frees the pane's buffer and creates an empty one. The pane
+# offsets and each control client's offsets used to keep pointing into the old
+# buffer, so if an attached control client had not caught up on the pane's
+# output, the next read from the new process ran off the end of the new buffer
+# and the server crashed in input_parse.
+#
+# Two ways for a client to be behind are tested: a control client whose output
+# goes down a fifo that is never read (a second, healthy control client keeps
+# the pane being read), and a healthy control client viewing a session that
+# the pane's window has been moved out of.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+TMUX="$TEST_TMUX -Ltest"
+$TMUX kill-server 2>/dev/null
+
+DIR=$(mktemp -d)
+FIFO=$DIR/fifo
+SERVER=
+
+mkfifo "$FIFO" || exit 1
+
+cleanup() {
+	[ -n "$SERVER" ] && kill -9 "$SERVER" 2>/dev/null
+	$TMUX kill-server 2>/dev/null
+	exec 8<&- 2>/dev/null
+	rm -rf "$DIR"
+}
+trap cleanup 0 1 15
+
+alive() {
+	if ! kill -0 "$SERVER" 2>/dev/null; then
+		SERVER=
+		echo "server died after $1"
+		exit 1
+	fi
+	$TMUX has -t rt || exit 1
+}
+
+wait_clients() {
+	n=0
+	while [ $n -lt 50 ]; do
+		[ "$($TMUX lsc 2>/dev/null | wc -l)" -ge $1 ] && return
+		sleep 0.1
+		n=$((n + 1))
+	done
+	echo "control clients did not attach"; exit 1
+}
+
+# A detached session with a pane that writes a line every 10 milliseconds.
+$TMUX -f/dev/null new -d -x 80 -y 24 -s rt \
+	'while :; do echo xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx; sleep 0.01; done' ||
+	exit 1
+SERVER=$($TMUX display -pt rt '#{pid}')
+
+# One control client with output down the unread fifo and one reading
+# normally; stdin of both held open by sleep so they stay attached.
+( sleep 60 ) | $TMUX -f/dev/null -C attach -t rt >"$FIFO" 2>&1 &
+exec 8<"$FIFO"
+( sleep 60 ) | $TMUX -f/dev/null -C attach -t rt >/dev/null 2>&1 &
+wait_clients 2
+
+# Let the first client fall behind, then respawn the pane. The new process
+# writes at once and the server must survive reading it.
+sleep 3
+$TMUX respawn-pane -k -t rt:0 'echo respawned; sleep 60' || exit 1
+sleep 1
+alive "respawn-pane with a lagging control client"
+
+# Now a pane whose window is moved out of the session both clients view: their
+# offsets for it stop advancing, so they fall behind until it is respawned.
+$TMUX neww -d -t rt \
+	'while :; do echo yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy; sleep 0.01; done' ||
+	exit 1
+sleep 1
+$TMUX new -d -s other || exit 1
+$TMUX movew -d -s rt:1 -t other: || exit 1
+sleep 2
+$TMUX respawn-pane -k -t other:1 'echo respawned; sleep 60' || exit 1
+sleep 1
+alive "respawn-pane on a window moved out of the clients' session"
+
+exit 0