GH-5229: fix left bind join in FedX for single binding input

This change fixes a situation that can incorrectly cause empty results.
It happens when the input of the left argument is a single binding set
and for special source selection situations (e.g. the right argument is
marked as ExclusiveStatement while the endpoint does not provide data)

To avoid the issue we also use the regular left join logic also for a
single binding set input, which can handle the situation properly.

Issue is covered with a unit test.

Author: aschwarte10

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

@@ -953,10 +953,6 @@ public abstract CloseableIteration<BindingSet> evaluateGroupedCheck(
 	 */
 	public CloseableIteration<BindingSet> evaluateLeftBoundJoinStatementPattern(
 			StatementTupleExpr stmt, final List<BindingSet> bindings) throws QueryEvaluationException {
-		// we can omit the bound join handling
-		if (bindings.size() == 1) {
-			return evaluate(stmt, bindings.get(0));
-		}
 
 		FilterValueExpr filterExpr = null;
 		if (stmt instanceof FilterTuple) {

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

@@ -14,6 +14,8 @@
 import java.util.Set;
 
 import org.eclipse.rdf4j.common.iteration.Iterations;
+import org.eclipse.rdf4j.federated.endpoint.Endpoint;
+import org.eclipse.rdf4j.federated.structures.SubQuery;
 import org.eclipse.rdf4j.model.util.Values;
 import org.eclipse.rdf4j.model.vocabulary.FOAF;
 import org.eclipse.rdf4j.model.vocabulary.OWL;
@@ -339,4 +341,81 @@ public void test_leftBindJoin_emptyOptional(boolean bindLeftJoinOptimizationEnab
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(booleans = { true, false })
+	public void test_leftBindJoin_emptyLeftArgumentAsExclusiveGroup(boolean bindLeftJoinOptimizationEnabled)
+			throws Exception {
+
+		var endpoints = prepareTest(
+				Arrays.asList("/tests/basic/data_emptyStore.ttl", "/tests/basic/data_emptyStore.ttl"));
+
+		Repository repo1 = getRepository(1);
+		Repository repo2 = getRepository(2);
+
+		Repository fedxRepo = fedxRule.getRepository();
+
+		fedxRule.setConfig(config -> {
+			config.withBoundJoinBlockSize(10);
+			config.withEnableOptionalAsBindJoin(bindLeftJoinOptimizationEnabled);
+		});
+
+		// add a person
+		try (RepositoryConnection conn = repo1.getConnection()) {
+			var p = Values.iri("http://ex.com/p1");
+			var otherP = Values.iri("http://other.com/p1");
+			conn.add(p, OWL.SAMEAS, otherP);
+		}
+
+		// add name for person 1
+		try (RepositoryConnection conn = repo2.getConnection()) {
+			var otherP = Values.iri("http://other.com/p1");
+			conn.add(otherP, FOAF.NAME, Values.literal("Person 1"));
+		}
+
+		// mark that repo2 for some reason has foaf:age statements (e.g. old cache entry)
+		Endpoint repo2Endpoint = endpoints.get(1);
+		federationContext().getSourceSelectionCache()
+				.updateInformation(new SubQuery(null, FOAF.AGE, null), repo2Endpoint, true);
+
+		fedxRule.enableDebug();
+
+		try {
+			// run query which joins results from multiple repos
+			// the age does not exist for any person
+			try (RepositoryConnection conn = fedxRepo.getConnection()) {
+				String query = "PREFIX foaf: <http://xmlns.com/foaf/0.1/> " +
+						"SELECT * WHERE { "
+						+ " ?person owl:sameAs ?otherPerson . "
+						+ " OPTIONAL { ?otherPerson foaf:age ?age .  } " // age does not exist, however is marked as
+																			// ExclusiveStatement
+						+ "}";
+
+				TupleQuery tupleQuery = conn.prepareTupleQuery(query);
+				try (TupleQueryResult tqr = tupleQuery.evaluate()) {
+					var bindings = Iterations.asList(tqr);
+
+					Assertions.assertEquals(1, bindings.size());
+
+					for (int i = 1; i <= 1; i++) {
+						var p = Values.iri("http://ex.com/p" + i);
+						var otherP = Values.iri("http://other.com/p" + i);
+
+						// find the bindingset for the person in the unordered result
+						BindingSet bs = bindings.stream()
+								.filter(b -> b.getValue("person").equals(p))
+								.findFirst()
+								.orElseThrow();
+
+						Assertions.assertEquals(otherP, bs.getValue("otherPerson"));
+
+						Assertions.assertEquals(otherP, bs.getValue("otherPerson"));
+						Assertions.assertFalse(bs.hasBinding("age"));
+					}
+				}
+			}
+
+		} finally {
+			fedxRepo.shutDown();
+		}
+	}
 }