Remove the possibility of disabling fsync completely

Author: Ostrzyciel

core/sail/lucene-api/src/main/java/org/eclipse/rdf4j/sail/lucene/LuceneSail.java

@@ -308,9 +308,8 @@ public class LuceneSail extends NotifyingSailWrapper {
 	/**
 	 * Set the key "fsyncInterval=<t>" as sail parameter to configure the interval in milliseconds in which fsync
 	 * is called on the Lucene index, default is defined in {@link #DEFAULT_FSYNC_INTERVAL}. Changes in the index will
-	 * become visible to readers at most after the interval is elapsed. If set to 0 or a negative value, fsync will be
-	 * disabled completely and there are no guarantees as to when your data will be persisted. This setting is only used
-	 * when {@link #TRANSACTIONAL_KEY} is set to false (rollbacks disabled).
+	 * become visible to readers at most after the interval is elapsed. This must be set to a value > 0. This setting
+	 * is only used when {@link #TRANSACTIONAL_KEY} is set to false (rollbacks disabled).
 	 */
 	public static final String FSYNC_INTERVAL_KEY = "fsyncInterval";
 

core/sail/lucene/src/main/java/org/eclipse/rdf4j/sail/lucene/impl/LuceneIndex.java

@@ -229,6 +229,9 @@ public synchronized void initialize(Properties parameters) throws Exception {
 			this.fsyncIntervalMillis = NumberUtils.toLong(
 					parameters.getProperty(LuceneSail.FSYNC_INTERVAL_KEY),
 					LuceneSail.DEFAULT_FSYNC_INTERVAL);
+			if (this.fsyncIntervalMillis <= 0) {
+				throw new IllegalArgumentException(LuceneSail.FSYNC_INTERVAL_KEY + " must be > 0");
+			}
 		}
 
 		postInit();
@@ -294,9 +297,11 @@ private void postInit() throws IOException {
 
 	private void setUpFsyncScheduler() {
 		// If transactions are disabled, launch a background thread to fsync periodically.
-		if (this.transactionsEnabled || this.fsyncIntervalMillis <= 0) {
+		if (this.transactionsEnabled) {
 			return;
 		}
+		// Checked in initialize(), assertion here for clarity.
+		assert this.fsyncIntervalMillis > 0;
 
 		// Use a daemon thread so the scheduler does not prevent JVM shutdown.
 		this.fsyncScheduler = new ScheduledThreadPoolExecutor(1, r -> {

core/sail/lucene/src/test/java/org/eclipse/rdf4j/sail/lucene/impl/LuceneNonTransactionalTest.java

@@ -1,5 +1,5 @@
 /*******************************************************************************
- * Copyright (c) 2025 Eclipse RDF4J contributors.
+ * Copyright (c) 2026 Eclipse RDF4J contributors.
  *
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Distribution License v1.0

core/sail/lucene/src/test/java/org/eclipse/rdf4j/sail/lucene/impl/LuceneParametersTest.java

@@ -0,0 +1,53 @@
+/*******************************************************************************
+ * Copyright (c) 2026 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.sail.lucene.impl;
+
+import static org.junit.Assert.assertThrows;
+
+import java.util.Properties;
+
+import org.eclipse.rdf4j.sail.lucene.LuceneSail;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test to verify that LuceneSail parameters are validated correctly.
+ *
+ * @author Piotr Sowiński
+ */
+public class LuceneParametersTest {
+
+	@Test
+	public void testZeroFsyncInterval() {
+		var index = new LuceneIndex();
+		var params = new Properties();
+		params.setProperty(LuceneSail.FSYNC_INTERVAL_KEY, "0");
+		params.setProperty(LuceneSail.LUCENE_RAMDIR_KEY, "true");
+		var e = assertThrows(IllegalArgumentException.class, () -> index.initialize(params));
+		Assertions.assertTrue(
+				e.getMessage().contains(LuceneSail.FSYNC_INTERVAL_KEY),
+				"Message should mention fsync interval"
+		);
+	}
+
+	@Test
+	public void testNegativeFsyncInterval() {
+		var index = new LuceneIndex();
+		var params = new Properties();
+		params.setProperty(LuceneSail.FSYNC_INTERVAL_KEY, "-10");
+		params.setProperty(LuceneSail.LUCENE_RAMDIR_KEY, "true");
+		var e = assertThrows(IllegalArgumentException.class, () -> index.initialize(params));
+		Assertions.assertTrue(
+				e.getMessage().contains(LuceneSail.FSYNC_INTERVAL_KEY),
+				"Message should mention fsync interval"
+		);
+	}
+}