fix: TriG handling of GRAPH keyword without a graph ID (#2469)

The RDF 1.1 TriG grammar only allows the `GRAPH` keyword if it
is followed by a graph identifier
[[ref](https://www.w3.org/TR/trig/#grammar-production-block)].

This change enforces this rule so that the
<http://www.w3.org/2013/TriGTests/#trig-graph-bad-01> test passes.

---------

Co-authored-by: WhiteGobo <richardfechner@posteo.net>
Co-authored-by: Iwan Aucamp <aucampia@gmail.com>

rdflib/plugins/parsers/trig.py

@@ -69,16 +69,20 @@ def graph(self, argstr: str, i: int) -> int:
         raise Exception if it looks like a graph, but isn't.
         """
 
+        need_graphid = False
         # import pdb; pdb.set_trace()
         j = self.sparqlTok("GRAPH", argstr, i)  # optional GRAPH keyword
         if j >= 0:
             i = j
+            need_graphid = True
 
         r: MutableSequence[Any] = []
         j = self.labelOrSubject(argstr, i, r)
         if j >= 0:
             graph = r[0]
             i = j
+        elif need_graphid:
+            self.BadSyntax(argstr, i, "GRAPH keyword must be followed by graph name")
         else:
             graph = self._store.graph.identifier  # hack
 

test/test_w3c_spec/test_trig_w3c.py

@@ -173,9 +173,6 @@ def check_entry(entry: ManifestEntry) -> None:
     f"{REMOTE_BASE_IRI}#trig-syntax-bad-list-04": pytest.mark.xfail(
         reason="ignores badly formed quad"
     ),
-    f"{REMOTE_BASE_IRI}#trig-graph-bad-01": pytest.mark.xfail(
-        reason="accepts GRAPH with no name"
-    ),
 }
 
 

test_reports/rdflib_w3c_trig-HEAD.ttl

@@ -923,7 +923,7 @@
     earl:assertedBy <https://github.com/RDFLib/rdflib> ;
     earl:mode earl:automatic ;
     earl:result [ a earl:TestResult ;
-            earl:outcome earl:failed ] ;
+            earl:outcome earl:passed ] ;
     earl:subject <https://github.com/RDFLib/rdflib> ;
     earl:test <http://www.w3.org/2013/TriGTests/#trig-graph-bad-01> .