fix: generate VALUES block for federated queries with variables only (#2084)

Fixed the generation of VALUES block for federated queries.

The values block was including non-variable values like BNodes which resulted
in invalid queries.

Closes #2079.

3 files changed, 55 insertions, 1 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d006172c..bede2f68 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -267,6 +267,24 @@ and will be removed for release.
 <!-- -->
 <!-- -->
 
+
+<!-- -->
+<!-- -->
+<!-- CHANGE BARRIER: START PR #2079 -->
+<!-- -->
+<!-- -->
+
+- Fixed the generation of VALUES block for federated queries.
+  The values block was including non-variable values like BNodes which resulted
+  in invalid queries.
+  [PR #2079](https://github.com/RDFLib/rdflib/pull/2079).
+
+<!-- -->
+<!-- -->
+<!-- CHANGE BARRIER: END PR #2079 -->
+<!-- -->
+<!-- -->
+
 <!-- -->
 <!-- -->
 <!-- CHANGE BARRIER: START -->
diff --git a/rdflib/plugins/sparql/evaluate.py b/rdflib/plugins/sparql/evaluate.py
index 7b0f6e50..706b7673 100644
--- a/rdflib/plugins/sparql/evaluate.py
+++ b/rdflib/plugins/sparql/evaluate.py
@@ -386,7 +386,7 @@ def _buildQueryStringForServiceCall(ctx: QueryContext, match: re.Match) -> str:
         base = ctx.prologue.base  # type: ignore[union-attr]
         if base is not None and len(base) > 0:
             service_query = "BASE <" + base + "> " + service_query
-    sol = ctx.solution()
+    sol = [v for v in ctx.solution() if isinstance(v, Variable)]
     if len(sol) > 0:
         variables = " ".join([v.n3() for v in sol])
         variables_bound = " ".join([ctx.get(v).n3() for v in sol])
diff --git a/test/test_sparql/test_service.py b/test/test_sparql/test_service.py
index 8ed09cc8..53019771 100644
--- a/test/test_sparql/test_service.py
+++ b/test/test_sparql/test_service.py
@@ -70,6 +70,41 @@ def test_service_with_bind():
         assert len(r) == 3
 
 
+def test_service_with_bound_solutions():
+    g = Graph()
+    g.update(
+        """
+        INSERT DATA {
+          []
+            <http://www.w3.org/2002/07/owl#sameAs> <http://de.dbpedia.org/resource/John_Lilburne> ;
+            <http://purl.org/dc/terms/subject> <http://dbpedia.org/resource/Category:1614_births> .
+        }
+        """
+    )
+    q = """
+        SELECT ?sameAs ?dbpComment ?subject WHERE {
+          []
+            <http://www.w3.org/2002/07/owl#sameAs> ?sameAs ;
+            <http://purl.org/dc/terms/subject> ?subject .
+
+          SERVICE <http://DBpedia.org/sparql> {
+            SELECT ?sameAs ?dbpComment ?subject WHERE {
+              <http://dbpedia.org/resource/John_Lilburne>
+                <http://www.w3.org/2002/07/owl#sameAs> ?sameAs ;
+                <http://www.w3.org/2000/01/rdf-schema#comment> ?dbpComment ;
+                <http://purl.org/dc/terms/subject> ?subject .
+            }
+          }
+        }
+        LIMIT 2
+        """
+    results = helper.query_with_retry(g, q)
+    assert len(results) == 2
+
+    for r in results:
+        assert len(r) == 3
+
+
 def test_service_with_values():
     g = Graph()
     q = """select ?sameAs ?dbpComment ?subject
@@ -326,6 +361,7 @@ def test_with_mock(
 if __name__ == "__main__":
     test_service()
     test_service_with_bind()
+    test_service_with_bound_solutions()
     test_service_with_values()
     test_service_with_implicit_select()
     test_service_with_implicit_select_and_prefix()