test: add case for the Reloading bus signal

Check that the manager broadcasts Reloading(true/false) on the API bus
for daemon-reload, and the one-shot Reloading(false) after a
daemon-reexec, which requires the subscribers of the previous instance
to be coldplugged before the D-Bus queue is dispatched.

test/units/TEST-07-PID1.reloading-signal.sh

@@ -0,0 +1,73 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -eux
+set -o pipefail
+
+# Verify that the manager broadcasts the Reloading signal on the API bus,
+# both for daemon-reload and, as a one-shot Reloading(false), after a
+# daemon-reexec.
+#
+# The reexec case is a regression test: API bus subscribers from the
+# pre-reexec instance are coldplugged only once the asynchronous GetId
+# reply is processed (see bus_init_api()). The D-Bus queue dispatch must
+# not consume send_reloading_done before that happened, otherwise the
+# Reloading(false) signal is lost and clients waiting for it to detect the
+# end of the reexec time out.
+
+MONITOR_LOG=/run/TEST-07-PID1-reloading-signal.log
+MONITOR_UNIT=
+
+at_exit() {
+    [[ -n "$MONITOR_UNIT" ]] && systemctl stop "$MONITOR_UNIT" 2>/dev/null || :
+    rm -f "$MONITOR_LOG"
+}
+
+trap at_exit EXIT
+
+start_monitor() {
+    # Use a fresh unit name every time, a previous instance might not have
+    # been garbage-collected yet. busctl monitor sends READY=1 once the
+    # monitor is installed, so with Type=notify the monitor is guaranteed
+    # to be running when systemd-run returns.
+    MONITOR_UNIT="busctl-monitor-$RANDOM.service"
+    systemd-run --unit="$MONITOR_UNIT" --service-type=notify --quiet \
+        --property=StandardOutput=file:"$MONITOR_LOG" \
+        busctl monitor \
+            --match="type=signal,path=/org/freedesktop/systemd1,interface=org.freedesktop.systemd1.Manager,member=Reloading"
+}
+
+stop_monitor() {
+    systemctl stop "$MONITOR_UNIT"
+    MONITOR_UNIT=
+    rm -f "$MONITOR_LOG"
+}
+
+wait_for_reloading() {
+    local value=${1:?}
+
+    timeout 30 bash -ec "until grep -F 'BOOLEAN ${value};' $MONITOR_LOG >/dev/null; do sleep .5; done"
+}
+
+# The manager sends the Reloading signal on the API bus only if it has at
+# least one subscriber. logind subscribes, and its subscription is carried
+# over a daemon-reexec via serialization, so make sure it is running.
+systemctl start systemd-logind.service
+
+# logind's Subscribe call is asynchronous, so it may not have been processed
+# by the manager yet when logind announces readiness. Keep reloading until
+# the signal shows up in the monitor output.
+start_monitor
+timeout 30 bash -ec "until grep -F 'BOOLEAN true;' $MONITOR_LOG >/dev/null; do systemctl daemon-reload; sleep 1; done"
+
+# A daemon-reload sends Reloading(true) followed by Reloading(false).
+wait_for_reloading true
+wait_for_reloading false
+stop_monitor
+
+# A daemon-reexec must send the one-shot Reloading(false) once the new
+# manager instance is up, even though the subscribers of the old instance
+# are re-added only after the asynchronous API bus setup finished.
+start_monitor
+systemctl daemon-reexec
+wait_for_reloading false
+stop_monitor