GH-5433 Use AtomicReferenceArray for value cache to ensure inter-thread visibility

Author: kenwenzel

core/sail/lmdb/src/main/java/org/eclipse/rdf4j/sail/lmdb/ValueStore.java

@@ -57,6 +57,7 @@
 import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicReferenceArray;
 import java.util.concurrent.locks.ReadWriteLock;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.concurrent.locks.StampedLock;
@@ -125,7 +126,7 @@ class ValueStore extends AbstractValueFactory {
 	/**
 	 * A simple cache containing the [VALUE_CACHE_SIZE] most-recently used values stored by their ID.
 	 */
-	private final LmdbValue[] valueCache;
+	private final AtomicReferenceArray<LmdbValue> valueCache;
 	/**
 	 * A simple cache containing the [ID_CACHE_SIZE] most-recently used value-IDs stored by their value.
 	 */
@@ -193,7 +194,7 @@ class ValueStore extends AbstractValueFactory {
 		this.mapSize = config.getValueDBSize();
 		open();
 
-		valueCache = new LmdbValue[config.getValueCacheSize()];
+		valueCache = new AtomicReferenceArray<>(config.getValueCacheSize());
 		valueIDCache = new ConcurrentCache<>(config.getValueIDCacheSize());
 		namespaceCache = new ConcurrentCache<>(config.getNamespaceCacheSize());
 		namespaceIDCache = new ConcurrentCache<>(config.getNamespaceIDCacheSize());
@@ -442,7 +443,7 @@ protected byte[] getData(long id) throws IOException {
 	 * @return the value object or <code>null</code> if not found
 	 */
 	LmdbValue cachedValue(long id) {
-		LmdbValue value = valueCache[(int) (id % valueCache.length)];
+		LmdbValue value = valueCache.get((int) (id % valueCache.length()));
 		if (value != null && value.getInternalID() == id) {
 			return value;
 		}
@@ -459,7 +460,7 @@ LmdbValue cachedValue(long id) {
 	 * @return the value object or <code>null</code> if not found
 	 */
 	void cacheValue(long id, LmdbValue value) {
-		valueCache[(int) (id % valueCache.length)] = value;
+		valueCache.lazySet((int) (id % valueCache.length()), value);
 	}
 
 	/**
@@ -1236,7 +1237,9 @@ public void clear() throws IOException {
 	}
 
 	protected void clearCaches() {
-		Arrays.fill(valueCache, null);
+		for (int i = 0; i < valueCache.length(); i++) {
+			valueCache.set(i, null);
+		}
 		valueIDCache.clear();
 		namespaceCache.clear();
 		namespaceIDCache.clear();