scylla-jmx: Introduce a registration check object

Allows for shared code for synchronized and optionally
partial update checks.

src/main/java/com/scylladb/jmx/metrics/RegistrationChecker.java

@@ -0,0 +1,53 @@
+package com.scylladb.jmx.metrics;
+
+import static com.scylladb.jmx.metrics.RegistrationMode.Remove;
+import static com.scylladb.jmx.metrics.RegistrationMode.Wait;
+import static java.util.EnumSet.allOf;
+import static java.util.EnumSet.of;
+
+import java.net.UnknownHostException;
+import java.util.EnumSet;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
+import javax.management.OperationsException;
+
+import com.scylladb.jmx.api.APIClient;
+import com.sun.jmx.mbeanserver.JmxMBeanServer;
+
+@SuppressWarnings("restriction")
+public abstract class RegistrationChecker {
+	private final Lock lock = new ReentrantLock();
+
+	public static final EnumSet<RegistrationMode> REMOVE_NO_WAIT = of(Remove);
+	public static final EnumSet<RegistrationMode> ADD_AND_REMOVE = allOf(RegistrationMode.class);
+
+	public final void reap(APIClient client, JmxMBeanServer server) throws OperationsException, UnknownHostException {
+		check(client, server, REMOVE_NO_WAIT);
+	}
+
+	public final void check(APIClient client, JmxMBeanServer server) throws OperationsException, UnknownHostException {
+		check(client, server, ADD_AND_REMOVE);
+	}
+
+	public final void check(APIClient client, JmxMBeanServer server, EnumSet<RegistrationMode> mode)
+			throws OperationsException, UnknownHostException {
+		if (!lock.tryLock() && mode.contains(Wait)) {
+			// someone is doing update.
+			// since this is jmx, and sloppy, we'll just
+			// assume that once he is done, things are
+			// good enough.
+			lock.lock();
+			lock.unlock();
+			return;
+		}
+		try {
+			doCheck(client, server, mode);
+		} finally {
+			lock.unlock();
+		}
+	}
+
+	protected abstract void doCheck(APIClient client, JmxMBeanServer server, EnumSet<RegistrationMode> mode)
+			throws OperationsException, UnknownHostException;
+}

src/main/java/com/scylladb/jmx/metrics/RegistrationMode.java

@@ -0,0 +1,5 @@
+package com.scylladb.jmx.metrics;
+
+public enum RegistrationMode {
+	Wait, Add, Remove,
+}