Events: Allow same event type to be re-registered

When an event type is registered, an exception is raised if the given
event type is already registered.

This has the unfortunate side-effect that if a plugin registers any
event types, that plugin cannot be reloaded. The reason is that when
a plugin is unloaded, its registered events are not unregistered.

As a workaround for this, simply remove the check for duplicate event
types. Long term we will need to find a way to make plugins remove
their events from the list on unload.

Bug: Issue 3301
Change-Id: I85e24f79ea4c901d5141dc9530dff0dc51ff4976

gerrit-server/src/main/java/com/google/gerrit/server/events/EventTypes.java

@@ -40,15 +40,10 @@ public class EventTypes {
   /** Register an event.
    *
    *  @param event The event to register.
-   *  @throws IllegalArgumentException if the event's type is already
    *  registered.
    **/
   public static void registerClass(Event event) {
     String type = event.getType();
-    if (typesByString.containsKey(type)) {
-      throw new IllegalArgumentException(
-          "Event type already registered: " + type);
-    }
     typesByString.put(type, event.getClass());
   }