Transport request handlers should be registered safely

[jaymode]

Similar to #38560 and #51622, the HandledTransportAction registers a non-static inner class with the transport service as the handler for the requests of a given action as seen below.

elasticsearch/server/src/main/java/org/elasticsearch/action/support/HandledTransportAction.java

Lines 52 to 66 in 8980120

	protected HandledTransportAction(String actionName, boolean canTripCircuitBreaker,
	TransportService transportService, ActionFilters actionFilters,
	Writeable.Reader<Request> requestReader, String executor) {
	super(actionName, actionFilters, transportService.getTaskManager());
	transportService.registerRequestHandler(actionName, executor, false, canTripCircuitBreaker, requestReader,
	new TransportHandler());
	}
	class TransportHandler implements TransportRequestHandler<Request> {
	@Override
	public final void messageReceived(final Request request, final TransportChannel channel, Task task) {
	// We already got the task created on the network layer - no need to create it again on the transport layer
	execute(task, request, new ChannelActionListener<>(channel, actionName, request));
	}
	}

In the other issues, a reference to the instance being constructed, this, was explicitly passed to another service. In this case, the reference is implicit since the inner class TransportHandler is not static and has a reference to the outer class. Since the inner class is published and calls methods within the outer class, there is a chance that this could happen before the outer class is fully constructed, which violates the JLS.

For correctness, this code should be changed so that the handler registration no longer occurs within a constructor. However, given the use of guice to construct transport actions and eventual refactoring to remove guice it may be best to consider this as part of that effort.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)

[jmbessa]

Hi! I would like to know if this issue is still open. If yes, can I work on it? This would be my first issue