1 files changed, 72 insertions, 0 deletions
diff --git a/org.eclipse.jdt.core/search/org/eclipse/jdt/internal/core/nd/DatabaseRef.java b/org.eclipse.jdt.core/search/org/eclipse/jdt/internal/core/nd/DatabaseRef.java
new file mode 100644
index 000000000..044fe241d
--- /dev/null
+++ b/org.eclipse.jdt.core/search/org/eclipse/jdt/internal/core/nd/DatabaseRef.java
@@ -0,0 +1,72 @@
+/*******************************************************************************
+ * Copyright (c) 2015, 2016 Google, Inc and others.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *   Stefan Xenos (Google) - Initial implementation
+ *******************************************************************************/
+package org.eclipse.jdt.internal.core.nd;
+
+import java.util.function.Supplier;
+
+/**
+ * Holds a reference to a database entity that may be retained across read locks. In normal circumstances, it
+ * is unsafe to retain a database address after a read lock is released since the object pointed to at that
+ * address may have been deleted in the meantime. This class addresses this problem by remembering both the
+ * address itself and enough information to determine whether that address is invalid and search for an
+ * equivalent object if the original is lost.
+ */
+public class DatabaseRef<T extends NdNode> implements Supplier<T> {
+	private final Nd nd;
+	private T lastResult;
+	private long writeCounter;
+	private final Supplier<T> searchFunction;
+
+	/**
+	 * Constructs a new {@link DatabaseRef} that will search for its target using the given search function.
+	 */
+	public DatabaseRef(Nd nd, Supplier<T> searchFunction) {
+		this.nd = nd;
+		this.searchFunction = searchFunction;
+		this.writeCounter = -1;
+	}
+
+	/**
+	 * Constructs a new {@link DatabaseRef} that will search for its target using the given search function.
+	 */
+	public DatabaseRef(Nd nd, Supplier<T> searchFunction, T initialResult) {
+		this.nd = nd;
+		this.searchFunction = searchFunction;
+		this.lastResult = initialResult;
+		this.writeCounter = this.nd.getWriteNumber();
+	}
+
+	/**
+	 * Returns the referenced object or null if the object is no longer present in the database.
+	 */
+	public T get() {
+		long ndWriteNumber = this.nd.getWriteNumber();
+		if (this.writeCounter == ndWriteNumber) {
+			return this.lastResult;
+		}
+
+		T result = this.searchFunction.get();
+		this.writeCounter = ndWriteNumber;
+		this.lastResult = result;
+		return result;
+	}
+
+	public Nd getNd() {
+		return this.nd;
+	}
+
+	/**
+	 * Acquires a read lock. Callers must invoke close() on the result when done.
+	 */
+	public IReader lock() {
+		return this.nd.acquireReadLock();
+	}
+}