[DAP] Only need to add completion listener to main module

The language-specific debuggers are the ones responsible for copying over execution
listeners when we cross language boundaries. This avoids one risk related to concurrent
modifications and accesses of the execution listeners.

plugins/org.eclipse.epsilon.eol.dap/src/org/eclipse/epsilon/eol/dap/EpsilonDebugAdapter.java

@@ -323,18 +323,6 @@ protected ThreadState attachTo(IEolModule module) {
 			module.getContext().getExecutorFactory().setExecutionController(thread.debugger);
 		}
 
-		// Attach the completion listener to this module, if we haven't done so yet
-		boolean hasListener = false;
-		for (IExecutionListener l : module.getContext().getExecutorFactory().getExecutionListeners()) {
-			if (l == completionListener) {
-				hasListener = true;
-				break;
-			}
-		}
-		if (!hasListener) {
-			module.getContext().getExecutorFactory().addExecutionListener(completionListener);
-		}
-
 		// Attach to the module's output and error streams
 		final PrintStream outStream = createStream(module.getContext(), OutputEventArgumentsCategory.STDOUT);
 		module.getContext().setOutputStream(outStream);
@@ -397,6 +385,7 @@ public void connect(IDebugProtocolClient client) {
 			throw new IllegalStateException("connect(): the module has not been set up yet");
 		}
 		attachTo(mainModule);
+		mainModule.getContext().getExecutorFactory().addExecutionListener(completionListener);
 
 		this.client = client;
 		client.initialized();

plugins/org.eclipse.epsilon.eol.engine/src/org/eclipse/epsilon/eol/execute/ExecutorFactory.java

@@ -77,7 +77,7 @@ public final StackTraceManager getStackTraceManager() {
 
 	public void addExecutionListener(IExecutionListener listener) {
 		if (executionListeners == null) {
-			executionListeners = new CopyOnWriteArrayList<>();
+			executionListeners = new ArrayList<>(1);
 		}
 		executionListeners.add(listener);
 	}