GH-4990: support for zero-length path expression in FedX (#4991)

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());
+	}
+}