[JENKINS-47043] Introduce exit lifecycle to exit instead of restart

If hudson.lifecycle.ExitLifecycle is selected, jenkins will exit when
restart is requested. Exit code is taken out of
jenkins.model.Jenkins.exitCodeOnRestart, default is 5.

This is usable when there is an external watchdog such as SystemD.

SystemD service example:

   ExecStart=@/usr/bin/java jenkins $JAVA_OPTIONS -DJENKINS_HOME=${JENKINS_HOME} -Dhudson.lifecycle=hudson.lifecycle.ExitLifecycle -Djenkins.model.Jenkins.exitCodeOnRestart=55 -jar $JENKINS_WAR --debug=${JENKINS_DEBUG_LEVEL} --httpPort=${JENKINS_PORT} --httpListenAddress=${JENKINS_LISTEN_ADDRESS} $JENKINS_ARGS

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>

core/src/main/java/hudson/lifecycle/ExitLifecycle.java

@@ -0,0 +1,75 @@
+/*
+ * The MIT License
+ *
+ * Copyright 2018 Alon Bar-Lev <alon.barlev@gmail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson.lifecycle;
+
+import hudson.Extension;
+
+import org.kohsuke.accmod.Restricted;
+import org.kohsuke.accmod.restrictions.NoExternalUse;
+
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+import jenkins.model.Configuration;
+import jenkins.model.Jenkins;
+
+/**
+ * {@link Lifecycle} that delegates the responsibility to restart Jenkins to an external
+ * watchdog such as SystemD or OpenRC.
+ *
+ * <p>
+ * Restart by exit with specific code.
+ *
+ * @author Alon Bar-Lev
+ */
+@Restricted(NoExternalUse.class)
+@Extension
+public class ExitLifecycle extends Lifecycle {
+
+    private static final Logger LOGGER = Logger.getLogger(ExitLifecycle.class.getName());
+
+    private static final String EXIT_CODE_ON_RESTART = "exitCodeOnRestart";
+    private static final String DEFAULT_EXIT_CODE = "5";
+
+    private Integer exitOnRestart;
+
+    public ExitLifecycle() {
+        exitOnRestart = Integer.parseInt(Configuration.getStringConfigParameter(EXIT_CODE_ON_RESTART, DEFAULT_EXIT_CODE));
+    }
+
+    @Override
+    public void restart() {
+        Jenkins jenkins = Jenkins.getInstanceOrNull(); // guard against repeated concurrent calls to restart
+
+        try {
+            if (jenkins != null) {
+                jenkins.cleanUp();
+            }
+        } catch (Exception e) {
+            LOGGER.log(Level.SEVERE, "Failed to clean up. Restart will continue.", e);
+        }
+
+        System.exit(exitOnRestart);
+    }
+}