Add a regression test for output queued to exiting control clients.

Checks that once a control client is marked for exit the server stops
queueing notifications for it (PR 5357). The client's output is drained only
slowly so a backlog holds the exit handshake open; the client is then
detached and the window renamed, and the %window-renamed notifications must
not appear in the client's output stream.

regress/control-client-exit.sh

@@ -0,0 +1,111 @@
+#!/bin/sh
+
+# Once a control client is marked for exit (by detach-client here, or by the
+# "too far behind" eviction in control_check_age), the server must stop
+# queueing new output and notifications for it. The exit handshake only waits
+# for already-queued blocks to drain (control_all_done), so anything queued
+# after that point is written to a client that will never read it, and can hold
+# the handshake - and the pane buffer it pins - open. This checks that a
+# notification generated while the client is exiting never reaches its output.
+#
+# To see this in seconds rather than waiting out CONTROL_MAXIMUM_AGE (300s) the
+# control client's output goes down a fifo that a helper drains only slowly.
+# The slow drain keeps a backlog stuck in the server so the exit handshake
+# cannot complete; while it is stuck we detach the client (which sets the exit
+# flag) and rename the window to a marker. Draining the fifo must then not turn
+# up the marker as a %window-renamed line.
+
+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
+OUT=$DIR/out
+MARKER=exitmarker
+READER=
+
+mkfifo "$FIFO" || exit 1
+: >"$OUT"
+
+cleanup() {
+	[ -n "$READER" ] && kill "$READER" 2>/dev/null
+	$TMUX kill-server 2>/dev/null
+	exec 8<&- 2>/dev/null
+	rm -rf "$DIR"
+}
+trap cleanup 0 1 15
+
+# A detached session whose pane floods printable output forever.
+$TMUX -f/dev/null new -d -x 80 -y 24 -s rt 'cat /dev/zero | tr "\000" x' || exit 1
+$TMUX setw -g automatic-rename off || exit 1
+
+# Attach a control client: output to the fifo, stdin held open by sleep so it
+# stays attached while we drive the session from outside. Open the read end
+# after the client has opened the write end.
+( sleep 30 ) | $TMUX -f/dev/null -C attach -t rt >"$FIFO" 2>&1 &
+exec 8<"$FIFO"
+
+n=0
+while [ $n -lt 50 ]; do
+	$TMUX lsc -F '#{client_name}' 2>/dev/null | grep -q . && break
+	sleep 0.1
+	n=$((n + 1))
+done
+$TMUX lsc -F '#{client_name}' 2>/dev/null | grep -q . ||
+	{ echo "control client did not attach"; exit 1; }
+
+# Drain the control stream slowly (~2.5KB/s). The pane floods far faster, so a
+# backlog stays stuck in the server and the fifo stays full, which is what
+# keeps the exit handshake from completing below.
+( while :; do
+	dd bs=256 count=1 <&8 >>"$OUT" 2>/dev/null || exit 0
+	sleep 0.1
+done ) &
+READER=$!
+
+# Wait until the client is demonstrably backed up: the output only keeps
+# growing at the slow-drain rate while the flood is outrunning the reader.
+n=0
+while [ $n -lt 100 ]; do
+	[ "$(wc -c <"$OUT")" -ge 8000 ] && break
+	sleep 0.1
+	n=$((n + 1))
+done
+[ "$(wc -c <"$OUT")" -ge 8000 ] || { echo "client output did not back up"; exit 1; }
+
+# Detach the control client: this sets CLIENT_EXIT immediately. The stuck
+# backlog holds the window between CLIENT_EXIT and the exit handshake open.
+$TMUX detach-client -s rt
+
+# While the client is exiting, rename the window to the marker. Vanilla queues
+# these %window-renamed notifications for the exiting client; the fix drops them.
+i=0
+while [ $i -lt 5 ]; do
+	$TMUX rename-window -t rt:0 "$MARKER$i"
+	i=$((i + 1))
+done
+sleep 0.5
+
+# Drain fast (bounded) so the backlog, and any notification queued behind it,
+# reaches OUT. A watchdog keeps this from blocking.
+kill "$READER" 2>/dev/null
+READER=
+( dd bs=4096 count=128 <&8 >>"$OUT" 2>/dev/null ) &
+DRAIN=$!
+( sleep 3; kill "$DRAIN" 2>/dev/null ) &
+WATCHDOG=$!
+wait "$DRAIN" 2>/dev/null
+kill "$WATCHDOG" 2>/dev/null
+
+# The marker must never appear as a notification to the exiting client.
+if grep -q "%window-renamed .*$MARKER" "$OUT"; then
+	echo "notification queued to exiting control client:"
+	grep "%window-renamed .*$MARKER" "$OUT"
+	exit 1
+fi
+
+exit 0