service: add new RestartMode option

When this option is set to direct, the service restarts without entering a failed
state. Dependent units are not notified of transitory failure.

This is useful for the following use case:

We have a target with Requires=my-service, After=my-service.
my-service.service is a oneshot service and has Restart=on-failure in
its definition.

my-service.service can get stuck for various reasons and time out, in
which case it is restarted. Currently, when it fails the first time, the
target fails, even though my-service is restarted.

The behavior we're looking for is that until my-service is not restarted
anymore, the target stays pending waiting for my-service.service to
start successfully or fail without being restarted anymore.

test/testsuite-03.units/fails-on-restart-restartdirect.service

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Description=Fail on restart
+StartLimitIntervalSec=1m
+StartLimitBurst=3
+
+[Service]
+Type=oneshot
+ExecStart=false
+Restart=on-failure
+RestartMode=direct

test/testsuite-03.units/fails-on-restart-restartdirect.target

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Requires=fails-on-restart-restartdirect.service

test/testsuite-03.units/fails-on-restart.service

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Description=Fail on restart
+StartLimitIntervalSec=1m
+StartLimitBurst=3
+
+[Service]
+Type=oneshot
+ExecStart=false
+Restart=on-failure
+RestartMode=normal

test/testsuite-03.units/fails-on-restart.target

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Requires=fails-on-restart.service

test/testsuite-03.units/succeeds-on-restart-restartdirect.service

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Service]
+Type=oneshot
+ExecStart=/usr/lib/systemd/tests/testdata/testsuite-03.units/succeeds-on-restart.sh
+Restart=on-failure
+RestartMode=direct

test/testsuite-03.units/succeeds-on-restart-restartdirect.target

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Requires=succeeds-on-restart-restartdirect.service

test/testsuite-03.units/succeeds-on-restart.service

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Service]
+Type=oneshot
+ExecStart=/usr/lib/systemd/tests/testdata/testsuite-03.units/succeeds-on-restart.sh
+Restart=on-failure
+RestartMode=normal

test/testsuite-03.units/succeeds-on-restart.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+if [[ ! -f "/succeeds-on-restart.ko" ]]
+then
+    touch "/succeeds-on-restart.ko"
+    exit 1
+else
+    rm "/succeeds-on-restart.ko"
+    exit 0
+fi

test/testsuite-03.units/succeeds-on-restart.target

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Unit]
+Requires=succeeds-on-restart.service

test/units/testsuite-03.sh

@@ -153,4 +153,17 @@ systemctl restart propagatestopto-indirect.target
 assert_rc 3 systemctl --quiet is-active propagatestopto-and-pullin.target
 assert_rc 3 systemctl --quiet is-active sleep-infinity-simple.service
 
+# Test restart mode direct
+systemctl start succeeds-on-restart-restartdirect.target
+assert_rc 0 systemctl --quiet is-active succeeds-on-restart-restartdirect.target
+
+systemctl start fails-on-restart-restartdirect.target || :
+assert_rc 3 systemctl --quiet is-active fails-on-restart-restartdirect.target
+
+systemctl start succeeds-on-restart.target || :
+assert_rc 3 systemctl --quiet is-active succeeds-on-restart.target
+
+systemctl start fails-on-restart.target || :
+assert_rc 3 systemctl --quiet is-active fails-on-restart.target
+
 touch /testok