Add a regress test for the server's session keyring on Linux

Install keyutils in CI so the test runs rather than being skipped.

.github/workflows/regress.yml

@@ -51,6 +51,7 @@ jobs:
             automake \
             bison \
             build-essential \
+            keyutils \
             libevent-dev \
             libncurses-dev \
             libutf8proc-dev \

regress/session-keyring.sh

@@ -0,0 +1,89 @@
+#!/bin/sh
+
+# On Linux the server should join a session keyring of its own unless the
+# session-keyring option is off, so it keeps working after the login session's
+# keyring is revoked, and revoke it when it exits.
+
+PATH=/bin:/usr/bin
+TERM=screen
+
+[ "$(uname)" = Linux ] || exit 0
+keyctl session - true >/dev/null 2>&1 || exit 0
+
+[ -z "$TEST_TMUX" ] && TEST_TMUX=$(readlink -f ../tmux)
+OUT=$(mktemp -d)
+TMUX="$TEST_TMUX -LtestA$$ -f/dev/null"
+TMUX2="$TEST_TMUX -LtestB$$ -f$OUT/conf"
+$TMUX kill-server 2>/dev/null
+$TMUX2 kill-server 2>/dev/null
+
+fail()
+{
+	echo "$*"
+	exit 1
+}
+
+cleanup()
+{
+	$TMUX kill-server 2>/dev/null
+	$TMUX2 kill-server 2>/dev/null
+	[ -s $OUT/holder ] && kill $(cat $OUT/holder) 2>/dev/null
+	rm -rf "$OUT"
+}
+trap cleanup 0 1 15
+
+# Set KEYRING to the session keyring of a new pane on server $1.
+pane_keyring()
+{
+	$1 new-window -d "keyctl id @s >$OUT/pane 2>&1; $1 wait-for -S pane" ||
+		fail "could not create pane"
+	$1 wait-for pane
+	KEYRING=$(cat $OUT/pane)
+	case "$KEYRING" in
+	''|*[!0-9]*)
+		fail "pane keyring not usable: $KEYRING"
+		;;
+	esac
+}
+
+# With the option off, the server should keep the keyring it was started with.
+echo 'set -g session-keyring off' >$OUT/conf
+OFF=$(keyctl session - sh -c "
+	$TMUX2 new -d 'sleep 1000' && keyctl id @s
+" 2>/dev/null) || fail "could not start server"
+pane_keyring "$TMUX2"
+[ "$KEYRING" = "$OFF" ] || fail "pane has a new keyring with option off"
+$TMUX2 kill-server
+
+# Start the server from a login session with its own keyring, then revoke that
+# keyring as pam_keyinit(8) does when the login session ends.
+LOGIN=$(keyctl session - sh -c "
+	$TMUX new -d 'sleep 1000' && keyctl id @s && keyctl revoke @s
+" 2>/dev/null) || fail "could not start server"
+
+# A new pane should still have a usable keyring, with the user keyring linked.
+pane_keyring "$TMUX"
+[ "$KEYRING" != "$LOGIN" ] || fail "pane has the login keyring"
+keyctl rlist $KEYRING | grep -qw "$(keyctl id @u)" ||
+	fail "user keyring not linked"
+
+# The server should revoke its keyring when it exits, even while a process it
+# started (which ignores the SIGHUP from its pane closing) still holds it.
+$TMUX new-window -d "
+	trap '' HUP
+	echo \$\$ >$OUT/holder
+	$TMUX wait-for -S held
+	exec sleep 100
+" || fail "could not create pane"
+$TMUX wait-for held
+SERVER=$($TMUX display -p '#{pid}')
+$TMUX kill-server
+n=0
+while kill -0 $SERVER 2>/dev/null && [ $n -lt 50 ]; do
+	sleep 0.1
+	n=$((n + 1))
+done
+keyctl rdescribe $KEYRING >$OUT/exit 2>&1
+grep -q "revoked" $OUT/exit || fail "keyring not revoked: $(cat $OUT/exit)"
+
+exit 0