Fixes a bug in SPARQL DESCRIBE implementation

Adds tests to ensure it won't happen again

lib/src/sparql/eval.rs

@@ -2731,16 +2731,20 @@ impl<S: ReadableEncodedStore<Error = EvaluationError> + 'static> Iterator for De
                 Ok(tuple) => tuple,
                 Err(error) => return Some(Err(error)),
             };
-            for subject in tuple.iter() {
-                if let Some(subject) = subject {
-                    self.quads = Box::new(self.eval.dataset.encoded_quads_for_pattern(
+            let eval = self.eval.clone();
+            self.quads = Box::new(tuple.into_iter().flatten().flat_map(move |subject| {
+                eval.dataset
+                    .encoded_quads_for_pattern(
                         Some(subject),
                         None,
                         None,
-                        None,
-                    ));
-                }
-            }
+                        Some(EncodedTerm::DefaultGraph),
+                    )
+                    .chain(
+                        eval.dataset
+                            .encoded_quads_for_pattern(Some(subject), None, None, None),
+                    )
+            }));
         }
     }
 }

testsuite/oxigraph-tests/sparql/describe.rq

@@ -0,0 +1 @@
+DESCRIBE <http://example.com/not_exist> <http://example.com/test>

testsuite/oxigraph-tests/sparql/describe_input.ttl

@@ -0,0 +1,9 @@
+PREFIX schema: <http://schema.org/>
+
+<http://example.com/test> a schema:Person ;
+    schema:name "test"@en ;
+    schema:parent <http://example.com/test2> ;
+    schema:child [ a schema:Person ] .
+
+<http://example.com/test2> a schema:Person ;
+    schema:child <http://example.com/test> .

testsuite/oxigraph-tests/sparql/describe_output.ttl

@@ -0,0 +1,6 @@
+PREFIX schema: <http://schema.org/>
+
+<http://example.com/test> a schema:Person ;
+    schema:name "test"@en ;
+    schema:parent <http://example.com/test2> ;
+    schema:child [] .

testsuite/oxigraph-tests/sparql/describe_where.rq

@@ -0,0 +1 @@
+DESCRIBE ?s WHERE { ?s <http://schema.org/name> "test"@en }

testsuite/oxigraph-tests/sparql/manifest.ttl

@@ -8,9 +8,25 @@
     rdfs:label "Oxigraph SPARQL tests" ;
     mf:entries
     (
+    :describe
+    :describe_where
     :group_concat_with_null
     ) .
 
+:describe rdf:type mf:QueryEvaluationTest ;
+    mf:name "Simple DESCRIBE request" ;
+    mf:action
+         [ qt:query  <describe.rq> ;
+           qt:data   <describe_input.ttl> ] ;
+    mf:result  <describe_output.ttl> .
+
+:describe rdf:type mf:QueryEvaluationTest ;
+    mf:name "Simple DESCRIBE request" ;
+    mf:action
+         [ qt:query  <describe.rq> ;
+           qt:data   <describe_input.ttl> ] ;
+    mf:result  <describe_output.ttl> .
+
 :group_concat_with_null rdf:type mf:QueryEvaluationTest ;
     mf:name "GROUP_CONCAT with NULL" ;
     rdfs:comment    "GROUP_CONCAT should ignore null values" ;