GH-4990: adjust cloning for context var + unit test coverage

Mage sure to have null-safe clone also for the context variable. Covered
the finding with a unit test

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

@@ -333,7 +333,7 @@ protected Set<Endpoint> performSourceSelection(FedXArbitraryLengthPath pathExpr,
 			identifiedMembers = new HashSet<>(members);
 		} else {
 			StatementPattern checkStmt = new StatementPattern(stmt.getScope(), new Var("subject"),
-					stmt.getPredicateVar().clone(), new Var("object"), stmt.getContextVar());
+					clone(stmt.getPredicateVar()), new Var("object"), clone(stmt.getContextVar()));
 			@SuppressWarnings("unused") // only used as artificial parent
 			HolderNode holderParent = new HolderNode(checkStmt);
 
@@ -364,6 +364,13 @@ protected Set<Endpoint> performSourceSelection(FedXArbitraryLengthPath pathExpr,
 		return identifiedMembers;
 	}
 
+	private Var clone(Var var) {
+		if (var == null) {
+			return null;
+		}
+		return var.clone();
+	}
+
 	protected void optimizeJoinOrder(TupleExpr query, QueryInfo queryInfo, GenericInfoOptimizer info) {
 		// optimize statement groups and join order
 		new StatementGroupAndJoinOptimizer(queryInfo, DefaultFedXCostModel.INSTANCE).optimize(query);

tools/federation/src/test/java/org/eclipse/rdf4j/federated/PropertyPathTests.java

@@ -301,4 +301,52 @@ public void testZeroLengthPath_length2_crossRepository() throws Exception {
 
 	}
 
+	@Test
+	public void testPropertyPath_sourceSelection_crossRepository() throws Exception {
+
+		prepareTest(Arrays.asList("/tests/basic/data_emptyStore.ttl", "/tests/basic/data_emptyStore.ttl"));
+
+		Repository repo1 = getRepository(1);
+		Repository repo2 = getRepository(2);
+
+		try (RepositoryConnection con = repo1.getConnection()) {
+			con.add(Values.iri("http://example.org/A"), RDFS.SUBCLASSOF, Values.iri("http://example.org/B"),
+					Values.iri("http://example.org/graph1"));
+		}
+
+		try (RepositoryConnection con = repo2.getConnection()) {
+			con.add(Values.iri("http://example.org/B"), RDFS.SUBCLASSOF, Values.iri("http://example.org/C"),
+					Values.iri("http://example.org/graph2"));
+		}
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		// 1a: bound (matching) object
+		try (RepositoryConnection con = fedxRepo.getConnection()) {
+			TupleQuery tupleQuery = con.prepareTupleQuery(
+					"PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> "
+							+ "SELECT * WHERE { "
+							+ "   ?subClass (rdfs:subClassOf+) <http://example.org/C> . "
+							+ " } "
+			);
+
+			Assertions.assertEquals(2, QueryResults.asSet(tupleQuery.evaluate()).size());
+		}
+
+		// 1b: with named graph
+		try (RepositoryConnection con = fedxRepo.getConnection()) {
+			TupleQuery tupleQuery = con.prepareTupleQuery(
+					"PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> "
+							+ "SELECT * WHERE { "
+							+ " GRAPH <http://example.org/graph2> {"
+							+ "   ?subClass (rdfs:subClassOf+) <http://example.org/C> . "
+							+ " }"
+							+ "} "
+			);
+
+			Assertions.assertEquals(1, QueryResults.asSet(tupleQuery.evaluate()).size());
+		}
+
+	}
+
 }