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 +1,69 @@
+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;
+
+/**
+ * Helper type to do optional locking for registration. Allows for
+ * per-bind-point locks and registration, instead of per-type or per-instance
+ * locks which may be misguiding, since for example one instance can be bound to
+ * many MBeanServers etc.
+ * 
+ * Also allows for polled checks, i.e. try-lock and either wait or skip. Wait,
+ * because we probably should not repeat things hidden by this type too often,
+ * and skip because for example a periodic task checking can just skip if a
+ * user-initiated registration check is being done.
+ * 
+ * @author calle
+ *
+ */
+@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()) {
+            if (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,
+}