Reimplement restarting of scm-manager

SCM-Manager tries now to figure out which is the best strategy for the restart.
It chooses from one of the following strategies:

* PosixRestartStrategy which uses native LibC
* ExitRestartStrategy uses System.exit and relies on external mechanism to start again
* InjectionContextRestartStrategy destroys and re initializes the injection context

scm-webapp/src/main/java/sonia/scm/lifecycle/ExitRestartStrategy.java

@@ -0,0 +1,52 @@
+package sonia.scm.lifecycle;
+
+import com.google.common.annotations.VisibleForTesting;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.function.IntConsumer;
+
+/**
+ * {@link RestartStrategy} which tears down the scm-manager context and
+ * then exists the java process with {@link System#exit(int)}.
+ * <p>
+ * This is useful if an external mechanism is able to restart the process after it has exited.
+ */
+class ExitRestartStrategy implements RestartStrategy {
+
+  private static final Logger LOG = LoggerFactory.getLogger(ExitRestartStrategy.class);
+
+  static final String NAME = "exit";
+
+  static final String PROPERTY_EXIT_CODE = "sonia.scm.restart.exit-code";
+
+  private IntConsumer exiter = System::exit;
+
+  ExitRestartStrategy() {
+  }
+
+  @VisibleForTesting
+  void setExiter(IntConsumer exiter) {
+    this.exiter = exiter;
+  }
+
+  @Override
+  public void restart(InjectionContext context) {
+    int exitCode = determineExitCode();
+
+    LOG.warn("destroy injection context");
+    context.destroy();
+
+    LOG.warn("exit scm-manager with exit code {}", exitCode);
+    exiter.accept(exitCode);
+  }
+
+  private int determineExitCode() {
+    String exitCodeAsString = System.getProperty(PROPERTY_EXIT_CODE, "0");
+    try {
+      return Integer.parseInt(exitCodeAsString);
+    } catch (NumberFormatException ex) {
+      throw new RestartNotSupportedException("invalid exit code " + exitCodeAsString, ex);
+    }
+  }
+}