TEST-07-PID1: check reloads while the API bus is being set up

Freeze the bus daemon and reexecute the manager, so that it waits for the
GetId() reply of its new API bus connection, and check that reloads over
D-Bus and Varlink are answered meanwhile and that Reloading(false) still
follows once the bus daemon is thawed.

(cherry picked from commit 09b626bcc0)

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

@@ -13,11 +13,23 @@ set -o pipefail
 # 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.
+#
+# Replies to Reload() calls must not be held back until then: a second
+# Reload() arriving while the first reply was still pending used to trip an
+# assertion in method_reload()/vl_method_reload_manager() and freeze PID 1.
 
 MONITOR_LOG=/run/TEST-07-PID1-reloading-signal.log
 MONITOR_UNIT=
+DBUS_CGROUP=
+
+bus_freeze() {
+    [[ -n "$DBUS_CGROUP" ]] || return 0
+    echo "${1:?}" >"$DBUS_CGROUP/cgroup.freeze"
+}
 
 at_exit() {
+    # Thawing twice is harmless.
+    bus_freeze 0 2>/dev/null || :
     [[ -n "$MONITOR_UNIT" ]] && systemctl stop "$MONITOR_UNIT" 2>/dev/null || :
     rm -f "$MONITOR_LOG"
 }
@@ -71,3 +83,32 @@ start_monitor
 systemctl daemon-reexec
 wait_for_reloading false
 stop_monitor
+
+# Reloads over D-Bus and Varlink must be answered while the reexecuted manager
+# waits for the GetId reply, and Reloading(false) must still follow. Freeze the
+# bus daemon via its cgroup: "systemctl freeze/thaw" go through the bus daemon
+# itself, so they cannot be used on it.
+start_monitor
+DBUS_CGROUP="/sys/fs/cgroup$(systemctl show -P ControlGroup dbus.service)"
+bus_freeze 1
+timeout 10 bash -c "until grep -qFx 'frozen 1' $DBUS_CGROUP/cgroup.events; do sleep .1; done"
+systemctl daemon-reexec
+
+timeout 10 systemctl daemon-reload
+timeout 10 varlinkctl call /run/systemd/io.systemd.Manager io.systemd.Manager.Reload '{}'
+
+pids=()
+for _ in 1 2; do
+    timeout 15 systemctl daemon-reload &
+    pids+=("$!")
+    timeout 15 varlinkctl call /run/systemd/io.systemd.Manager io.systemd.Manager.Reload '{}' &
+    pids+=("$!")
+done
+for pid in "${pids[@]}"; do
+    wait "$pid"
+done
+
+bus_freeze 0
+DBUS_CGROUP=
+wait_for_reloading false
+stop_monitor