GH-4990: support for zero-length path expression in FedX

This change adds support for zero-length path expressions in the FedX
engine and also fixes source selection for other path expressions.

Note that the implementation reuses the implementation from
PathIteration and ZeroLengthPathIteration. Unfortunately the existing
implementations cannot be reused without larger refactoring right now.

Functionality is covered with unit tests

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/FedXArbitraryLengthPath.java

@@ -0,0 +1,54 @@
+/*******************************************************************************
+ * Copyright (c) 2024 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.federated.algebra;
+
+import java.util.List;
+
+import org.eclipse.rdf4j.federated.structures.QueryInfo;
+import org.eclipse.rdf4j.federated.util.QueryAlgebraUtil;
+import org.eclipse.rdf4j.query.algebra.ArbitraryLengthPath;
+import org.eclipse.rdf4j.query.algebra.Var;
+
+/**
+ * A specialization of {@link ArbitraryLengthPath} to maintain the {@link QueryInfo}
+ *
+ * @author Andreas Schwarte
+ */
+public class FedXArbitraryLengthPath extends ArbitraryLengthPath implements FedXTupleExpr {
+
+	private static final long serialVersionUID = -7512248084095130084L;
+
+	private final QueryInfo queryInfo;
+
+	public FedXArbitraryLengthPath(ArbitraryLengthPath path, QueryInfo queryInfo) {
+		super(path.getScope(), clone(path.getSubjectVar()), path.getPathExpression(), clone(path.getObjectVar()),
+				clone(path.getContextVar()), path.getMinLength());
+		this.queryInfo = queryInfo;
+	}
+
+	private static Var clone(Var var) {
+		if (var == null) {
+			return null;
+		}
+		return var.clone();
+	}
+
+	@Override
+	public List<String> getFreeVars() {
+		return List.copyOf(QueryAlgebraUtil.getFreeVars(getPathExpression()));
+	}
+
+	@Override
+	public QueryInfo getQueryInfo() {
+		return queryInfo;
+	}
+
+}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/FedXZeroLengthPath.java

@@ -0,0 +1,49 @@
+/*******************************************************************************
+ * Copyright (c) 2024 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.federated.algebra;
+
+import java.util.List;
+
+import org.eclipse.rdf4j.federated.structures.QueryInfo;
+import org.eclipse.rdf4j.query.algebra.StatementPattern.Scope;
+import org.eclipse.rdf4j.query.algebra.Var;
+import org.eclipse.rdf4j.query.algebra.ZeroLengthPath;
+
+/**
+ * A specialization of {@link ZeroLengthPath} that keeps track of {@link QueryInfo} and statement sources.
+ *
+ * @author Andreas Schwarte
+ */
+public class FedXZeroLengthPath extends ZeroLengthPath implements QueryRef {
+
+	private static final long serialVersionUID = 2241037911187178861L;
+
+	private final QueryInfo queryInfo;
+
+	private final List<StatementSource> statementSources;
+
+	public FedXZeroLengthPath(Scope scope, Var subjVar, Var objVar, Var conVar, QueryInfo queryInfo,
+			List<StatementSource> statementSources) {
+		super(scope, subjVar, objVar, conVar);
+		this.queryInfo = queryInfo;
+		this.statementSources = statementSources;
+	}
+
+	public List<StatementSource> getStatementSources() {
+		return statementSources;
+	}
+
+	@Override
+	public QueryInfo getQueryInfo() {
+		return queryInfo;
+	}
+
+}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/algebra/HolderNode.java

@@ -0,0 +1,52 @@
+/*******************************************************************************
+ * Copyright (c) 2024 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.federated.algebra;
+
+import org.eclipse.rdf4j.query.algebra.AbstractQueryModelNode;
+import org.eclipse.rdf4j.query.algebra.QueryModelNode;
+import org.eclipse.rdf4j.query.algebra.QueryModelVisitor;
+
+/**
+ * An artificial holder node serving as parent holder to allow replacement.
+ */
+public class HolderNode extends AbstractQueryModelNode {
+
+	private static final long serialVersionUID = -4689963986499825771L;
+	private QueryModelNode child;
+
+	public HolderNode(QueryModelNode child) {
+		super();
+		setChild(child);
+	}
+
+	public void setChild(QueryModelNode child) {
+		this.child = child;
+		this.child.setParentNode(this);
+	}
+
+	@Override
+	public <X extends Exception> void visitChildren(QueryModelVisitor<X> visitor) throws X {
+		child.visit(visitor);
+	}
+
+	@Override
+	public <X extends Exception> void visit(QueryModelVisitor<X> visitor) throws X {
+		visitor.meetOther(this);
+	}
+
+	@Override
+	public void replaceChildNode(QueryModelNode current, QueryModelNode replacement) {
+		if (child.equals(current)) {
+			setChild(replacement);
+
+		}
+	}
+}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/FedXZeroLengthPathEvaluationStep.java

@@ -0,0 +1,91 @@
+/*******************************************************************************
+ * Copyright (c) 2024 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.federated.evaluation;
+
+import java.util.List;
+import java.util.function.Supplier;
+
+import org.eclipse.rdf4j.common.iteration.CloseableIteration;
+import org.eclipse.rdf4j.federated.algebra.FedXZeroLengthPath;
+import org.eclipse.rdf4j.federated.algebra.StatementSource;
+import org.eclipse.rdf4j.federated.evaluation.iterator.FedXZeroLengthPathIteration;
+import org.eclipse.rdf4j.federated.structures.QueryInfo;
+import org.eclipse.rdf4j.model.Value;
+import org.eclipse.rdf4j.query.BindingSet;
+import org.eclipse.rdf4j.query.QueryEvaluationException;
+import org.eclipse.rdf4j.query.algebra.Var;
+import org.eclipse.rdf4j.query.algebra.evaluation.EvaluationStrategy;
+import org.eclipse.rdf4j.query.algebra.evaluation.QueryEvaluationStep;
+import org.eclipse.rdf4j.query.algebra.evaluation.QueryValueEvaluationStep;
+import org.eclipse.rdf4j.query.algebra.evaluation.impl.QueryEvaluationContext;
+import org.eclipse.rdf4j.query.algebra.evaluation.impl.evaluationsteps.ZeroLengthPathEvaluationStep;
+
+/**
+ * An evaluation step used for {@link FedXZeroLengthPath}.
+ *
+ * @see ZeroLengthPathEvaluationStep
+ */
+public final class FedXZeroLengthPathEvaluationStep implements QueryEvaluationStep {
+	private final Var subjectVar;
+	private final Var objVar;
+	private final Var contextVar;
+	private final QueryValueEvaluationStep subPrep;
+	private final QueryValueEvaluationStep objPrep;
+	private final EvaluationStrategy strategy;
+	private final QueryEvaluationContext context;
+
+	private final Supplier<List<StatementSource>> statementSources;
+	private final Supplier<QueryInfo> queryInfo;
+
+	public FedXZeroLengthPathEvaluationStep(Var subjectVar, Var objVar, Var contextVar,
+			QueryValueEvaluationStep subPrep,
+			QueryValueEvaluationStep objPrep, EvaluationStrategy strategy, QueryEvaluationContext context,
+			Supplier<List<StatementSource>> statementSources, Supplier<QueryInfo> queryInfo) {
+		this.subjectVar = subjectVar;
+		this.objVar = objVar;
+		this.contextVar = contextVar;
+		this.subPrep = subPrep;
+		this.objPrep = objPrep;
+		this.strategy = strategy;
+		this.context = context;
+
+		this.statementSources = statementSources;
+		this.queryInfo = queryInfo;
+	}
+
+	@Override
+	public CloseableIteration<BindingSet, QueryEvaluationException> evaluate(BindingSet bindings) {
+		Value subj = null;
+		try {
+			subj = subPrep.evaluate(bindings);
+		} catch (QueryEvaluationException ignored) {
+		}
+
+		Value obj = null;
+		try {
+			obj = objPrep.evaluate(bindings);
+		} catch (QueryEvaluationException ignored) {
+		}
+
+		if (subj != null && obj != null) {
+			if (!subj.equals(obj)) {
+				return EMPTY_ITERATION;
+			}
+		}
+		return getZeroLengthPathIterator(bindings, subjectVar, objVar, contextVar, subj, obj, context);
+	}
+
+	protected FedXZeroLengthPathIteration getZeroLengthPathIterator(final BindingSet bindings, final Var subjectVar,
+			final Var objVar, final Var contextVar, Value subj, Value obj, QueryEvaluationContext context) {
+		return new FedXZeroLengthPathIteration(strategy, subjectVar, objVar, subj, obj, contextVar, bindings, context,
+				queryInfo.get(), statementSources.get());
+	}
+}