Experimentally relax the constraints of the exchange executor.

Don't fail, when a null executor is passed in.
Still not supported and may fail later, including hard to find race
conditions.

Signed-off-by: Achim Kraus <achim.kraus@cloudcoap.net>

californium-core/src/main/java/org/eclipse/californium/core/network/Exchange.java

@@ -81,6 +81,7 @@
 import org.eclipse.californium.elements.EndpointContext;
 import org.eclipse.californium.elements.EndpointIdentityResolver;
 import org.eclipse.californium.elements.UdpMulticastConnector;
+import org.eclipse.californium.elements.util.CheckedExecutor;
 import org.eclipse.californium.elements.util.ClockUtil;
 import org.eclipse.californium.elements.util.SerialExecutor;
 import org.eclipse.californium.elements.util.StringUtil;
@@ -132,7 +133,31 @@
 public class Exchange {
 
 	private static final Logger LOGGER = LoggerFactory.getLogger(Exchange.class);
-
+
+	/**
+	 * Dummy executor when {@code null} is provided. Experimental, not supported
+	 * and may fail.
+	 * 
+	 * @since 3.9
+	 */
+	private static final CheckedExecutor DUMMY_EXECUTOR = new CheckedExecutor() {
+
+		@Override
+		public void execute(Runnable command) {
+			command.run();
+		}
+
+		@Override
+		public boolean checkOwner() {
+			return true;
+		}
+
+		@Override
+		public void assertOwner() {
+			// no check applied
+		}
+	};
+
 	static final boolean DEBUG = LOGGER.isTraceEnabled();
 
 	private static final int MAX_OBSERVE_NO = (1 << 24) - 1;
@@ -170,7 +195,7 @@ public enum Origin {
 	 * 
 	 * @since 3.0 (changed from optional (unit tests) to mandatory)
 	 */
-	private final SerialExecutor executor;
+	private final CheckedExecutor executor;
 	/** The nano timestamp when this exchange has been created */
 	private final long nanoTimestamp;
 	/**
@@ -207,7 +232,10 @@ public enum Origin {
 	 */
 	private volatile Endpoint endpoint;
 
-	/** An remove handler to be called when a exchange must be removed from the exchange store */
+	/**
+	 * An remove handler to be called when a exchange must be removed from the
+	 * exchange store
+	 */
 	private volatile RemoveHandler removeHandler;
 
 	/** Indicates if the exchange is complete */
@@ -286,7 +314,8 @@ public enum Origin {
 	// true if the exchange has failed due to a timeout
 	private volatile boolean timedOut;
 
-	// the timeout scale factor, exponential back-off between retransmissions, if larger than 1.0F.
+	// the timeout scale factor, exponential back-off between retransmissions,
+	// if larger than 1.0F.
 	private float timeoutScale;
 
 	// the timeout of the current request or response set by reliability layer
@@ -327,19 +356,24 @@ public enum Origin {
 
 	private volatile EndpointContextOperator endpointContextPreOperator;
 
-	//If object security option is used, the Cryptographic context identifier is stored here
+	// If object security option is used, the Cryptographic context identifier
+	// is stored here
 	// for request/response mapping of contexts
 	private byte[] cryptoContextId;
 
 	/**
 	 * Creates a new exchange with the specified request and origin.
 	 * 
+	 * Note: since 3.9 {@code null} as executor doesn't longer fail with a
+	 * {@link NullPointerException}. Using {@code null} is still not supported
+	 * and comes with risks, that especially requires your own responsibility.
+	 * 
 	 * @param request the request that starts the exchange
 	 * @param peersIdentity peer's identity. Usually that's the peer's
 	 *            {@link InetSocketAddress}.
 	 * @param origin the origin of the request (LOCAL or REMOTE)
 	 * @param executor executor to be used for exchanges.
-	 * @throws NullPointerException if request or executor is {@code null}
+	 * @throws NullPointerException if request is {@code null}
 	 * @see EndpointIdentityResolver
 	 * @since 3.0 (added peersIdentity, executor adapted to mandatory)
 	 */
@@ -351,6 +385,10 @@ public Exchange(Request request, Object peersIdentity, Origin origin, Executor e
 	 * Creates a new exchange with the specified request, origin, context, and
 	 * notification marker.
 	 * 
+	 * Note: since 3.9 {@code null} as executor doesn't longer fail with a
+	 * {@link NullPointerException}. Using {@code null} is still not supported
+	 * and comes with risks, that especially requires your own responsibility.
+	 * 
 	 * @param request the request that starts the exchange
 	 * @param peersIdentity peer's identity. Usually that's the peer's
 	 *            {@link InetSocketAddress}.
@@ -359,19 +397,21 @@ public Exchange(Request request, Object peersIdentity, Origin origin, Executor e
 	 * @param ctx the endpoint context of this exchange
 	 * @param notification {@code true} for notification exchange, {@code false}
 	 *            otherwise
-	 * @throws NullPointerException if request or executor is {@code null}
+	 * @throws NullPointerException if request is {@code null}
 	 * @see EndpointIdentityResolver
 	 * @since 3.0 (added peersIdentity, executor adapted to mandatory)
 	 */
-	public Exchange(Request request, Object peersIdentity, Origin origin, Executor executor, EndpointContext ctx, boolean notification) {
+	public Exchange(Request request, Object peersIdentity, Origin origin, Executor executor, EndpointContext ctx,
+			boolean notification) {
 		// might only be the first block of the whole request
 		if (request == null) {
 			throw new NullPointerException("request must not be null!");
 		} else if (executor == null) {
-			throw new NullPointerException("executor must not be null");
+			// Dummy executor.
+			executor = DUMMY_EXECUTOR;
 		}
 		this.id = INSTANCE_COUNTER.incrementAndGet();
-		this.executor = new SerialExecutor(executor);
+		this.executor = executor instanceof CheckedExecutor ? (CheckedExecutor) executor : new SerialExecutor(executor);
 		this.currentRequest = request;
 		this.request = request;
 		this.origin = origin;
@@ -628,12 +668,12 @@ public void setCurrentRequest(Request newCurrentRequest) {
 
 	/**
 	 * Returns the response to the request or {@code null}, if no response has
-	 * arrived yet. 
+	 * arrived yet.
 	 * 
-	 * If there is an observe relation, the last received
-	 * notification is the response on the client side. On the server side, that
-	 * is the last notification to be sent, but may differ from the current
-	 * response, if that is in transit.
+	 * If there is an observe relation, the last received notification is the
+	 * response on the client side. On the server side, that is the last
+	 * notification to be sent, but may differ from the current response, if
+	 * that is in transit.
 	 * 
 	 * @return the response. or {@code null},
 	 */
@@ -878,7 +918,8 @@ public int incrementFailedTransmissionCount() {
 	}
 
 	/**
-	 * Get timeout scale factor for exponential back-off between retransmissions.
+	 * Get timeout scale factor for exponential back-off between
+	 * retransmissions.
 	 * 
 	 * @return timeout scale factor for exponential back-off.
 	 * @since 3.0
@@ -1049,8 +1090,9 @@ public Throwable getCaller() {
 	 * <p>
 	 * This means that both request and response have been sent/received.
 	 * <p>
-	 * This method invokes the {@linkplain RemoveHandler#remove(Exchange, KeyToken, KeyMID)
-	 * remove} method on the observer registered on this exchange (if any).
+	 * This method invokes the
+	 * {@linkplain RemoveHandler#remove(Exchange, KeyToken, KeyMID) remove}
+	 * method on the observer registered on this exchange (if any).
 	 * <p>
 	 * Call this method to trigger a clean-up in the Matcher through its
 	 * ExchangeObserverImpl. Usually, it is called automatically when reaching
@@ -1310,8 +1352,9 @@ public void removeNotifications() {
 	 * exchange to increase security when matching an incoming response to this
 	 * exchange's request.
 	 * </p>
-	 * If a {@link #setEndpointContextPreOperator(EndpointContextOperator)} is used,
-	 * this pre-operator is called before the endpoint context is set and forwarded.
+	 * If a {@link #setEndpointContextPreOperator(EndpointContextOperator)} is
+	 * used, this pre-operator is called before the endpoint context is set and
+	 * forwarded.
 	 * 
 	 * @param ctx the endpoint context information
 	 */

element-connector/src/main/java/org/eclipse/californium/elements/util/CheckedExecutor.java

@@ -0,0 +1,43 @@
+/********************************************************************************
+ * Copyright (c) 2023 Contributors to the Eclipse Foundation
+ * 
+ * See the NOTICE file(s) distributed with this work for additional
+ * information regarding copyright ownership.
+ * 
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * https://www.eclipse.org/legal/epl-2.0, or the Eclipse Distribution License
+ * v1.0 which is available at
+ * https://www.eclipse.org/org/documents/edl-v10.php.
+ * 
+ * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+ ********************************************************************************/
+package org.eclipse.californium.elements.util;
+
+import java.util.ConcurrentModificationException;
+import java.util.concurrent.Executor;
+
+/**
+ * Checked owner executor.
+ * 
+ * @since 3.9
+ */
+public interface CheckedExecutor extends Executor {
+
+	/**
+	 * Assert, that the current thread executes the current job.
+	 * 
+	 * @throws ConcurrentModificationException if current thread doesn't execute
+	 *             the current job
+	 */
+	void assertOwner();
+
+	/**
+	 * Check, if current thread executes the current job.
+	 * 
+	 * @return {@code true}, if current thread executes the current job,
+	 *         {@code false}, otherwise.
+	 */
+	boolean checkOwner();
+
+}

element-connector/src/main/java/org/eclipse/californium/elements/util/SerialExecutor.java

@@ -37,7 +37,7 @@
  * 
  * Serialize job execution before passing the jobs to a provided executor.
  */
-public class SerialExecutor extends AbstractExecutorService {
+public class SerialExecutor extends AbstractExecutorService implements CheckedExecutor {
 
 	private static final Logger LOGGER = LoggerFactory.getLogger(SerialExecutor.class);
 
@@ -119,12 +119,11 @@ public void execute(final Runnable command) {
 	}
 
 	/**
-	 * Assert, that the current thread executes the
-	 * {@link #currentlyExecutedJob}.
+	 * {@inheritDoc}
 	 * 
-	 * @throws ConcurrentModificationException if current thread doesn't execute
-	 *             the {@link #currentlyExecutedJob}.
-	 */
+	 * {@link #currentlyExecutedJob} is used for the current job.
+		 */
+	@Override
 	public void assertOwner() {
 		final Thread me = Thread.currentThread();
 		if (owner.get() != me) {
@@ -138,11 +137,11 @@ public void assertOwner() {
 	}
 
 	/**
-	 * Check, if current thread executes the {@link #currentlyExecutedJob}.
+	 * {@inheritDoc}
 	 * 
-	 * @return {@code true}, if current thread executes the
-	 *         {@link #currentlyExecutedJob}, {@code false}, otherwise.
-	 */
+	 * {@link #currentlyExecutedJob} is used for the current job.
+		 */
+	@Override
 	public boolean checkOwner() {
 		return owner.get() == Thread.currentThread();
 	}