Fix potential NPE in RemoteEndpoint.handleRequest

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/Endpoint.java

@@ -18,8 +18,22 @@
  */
 public interface Endpoint {
 
+	/**
+	 * Accepts the given request for further processing.
+	 *
+	 * @param method a request method, may not be <code>null</code>
+	 * @param parameter a request parameter
+	 * @return an instance of CompletableFuture representing the result computed in response to the request.
+	 *  May not be <code>null</code>.
+	 */
 	CompletableFuture<?> request(String method, Object parameter);
 
+	/**
+	 * Accepts the given notification for further processing.
+	 *
+	 * @param method a notification method, may not be <code>null</code>
+	 * @param parameter a notification parameter
+	 */
 	void notify(String method, Object parameter);
 
 }

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/RemoteEndpoint.java

@@ -269,6 +269,9 @@ protected void handleRequest(RequestMessage requestMessage) {
 		try {
 			// Forward the request to the local endpoint
 			future = localEndpoint.request(requestMessage.getMethod(), requestMessage.getParams());
+			if (future == null) {
+				throw new IllegalStateException("Local endpoint returned null from its request method, whereas an instance of CompletableFuture is expected");
+			}
 		} catch (Throwable throwable) {
 			// The local endpoint has failed handling the request - reply with an error response
 			ResponseError errorObject = exceptionHandler.apply(throwable);

org.eclipse.lsp4j.jsonrpc/src/main/java/org/eclipse/lsp4j/jsonrpc/services/JsonRequest.java

@@ -21,11 +21,13 @@
  * Annotation to mark a request method on an interface or class.
  * <p>
  * A request method must have the return type {@link CompletableFuture} with an
- * object parameter type or Void and have zero or one argument.
+ * object parameter type or Void, and have zero or one argument.
  * <p>
- * According to jsonrpc an argument must be an 'object' (a java bean, not e,g.
+ * According to jsonrpc an argument must be an 'object' (a java bean, not e.g. a
  * String).
  * <p>
+ * A request method is not allowed to return <code>null</code>.
+ * <p>
  * The name of the jsonrpc request will be the optional segment, followed by the
  * name of the Java method that is annotated with JsonRequest. The name of the
  * jsonrpc request can be customized by using the {@link #value()} field of this

org.eclipse.lsp4j.jsonrpc/src/test/java/org/eclipse/lsp4j/jsonrpc/test/RemoteEndpointTest.java

@@ -464,4 +464,35 @@ public CompletableFuture<Object> request(String method, Object parameter) {
 			logMessages.unregister();
 		}
 	}
-}
+
+	@Test
+	public void testEndpointReturningNull() {
+		final var logMessages = new LogMessageAccumulator();
+		try {
+			// Don't show the exception in the test execution log
+			logMessages.registerTo(RemoteEndpoint.class);
+
+			final var endp = new TestEndpoint() {
+				@Override
+				public CompletableFuture<Object> request(String method, Object parameter) {
+					return null;
+				}
+			};
+			final var consumer = new TestMessageConsumer();
+			final var endpoint = new RemoteEndpoint(consumer, endp);
+
+			endpoint.consume(init(new RequestMessage(), it -> {
+				it.setId("1");
+				it.setMethod("foo");
+				it.setParams("myparam");
+			}));
+
+			assertEquals("Check some response received", 1, consumer.messages.size());
+			final var response = (ResponseMessage) consumer.messages.get(0);
+			assertNotNull("Check response has error", response.getError());
+			assertEquals(ResponseErrorCode.InternalError.getValue(), response.getError().getCode());
+		} finally {
+			logMessages.unregister();
+		}
+	}
+}