CONSTRUCT query with empty result on persistent dataset raises an error

[scossu]

Issue: When performing a CONSTRUCT query on a triplestore-backed Dataset, an error is raised if the result set is empty.

Environment: Linux, Python 3.5.3, rdflib 4.2.2, Fuseki/TDB 3.4.0 back end

To reproduce:

>>> from rdflib.plugins.stores.sparqlstore import SPARQLUpdateStore
>>> from rdflib import Dataset, Graph
>>> 
>>> store = SPARQLUpdateStore(queryEndpoint='http://localhost:3030/my_dataset/query',
...         update_endpoint='http://localhost:3030/my_dataset/update',
...         autocommit=False)
>>> ds = Dataset(store, default_union=True)
>>> 
>>> # This is expected:
>>> qres = ds.query('SELECT * WHERE { <urn:res:a> ?p ?o . }')
>>> len(qres)
22
>>> # This is expected:
>>> qres = ds.query('CONSTRUCT WHERE { <urn:res:a> ?p ?o . }')
>>> len(qres)
22
>>> # This is expected:
>>> qres = ds.query('SELECT * WHERE { <urn:bogus:0> ?p ?o . }')
>>> len(qres)
0
>>> # This is NOT expected:
>>> qres = ds.query('CONSTRUCT WHERE { <urn:bogus:0> ?p ?o . }')
Traceback (most recent call last):
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/plugins/sparql/results/xmlresults.py", line 64, in __init__
    raise
RuntimeError: No active exception to reraise

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/graph.py", line 1079, in query
    **kwargs)
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/plugins/stores/sparqlstore.py", line 616, in query
    return SPARQLStore.query(self, *args, **kwargs)
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/plugins/stores/sparqlstore.py", line 320, in query
    return Result.parse(res)
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/query.py", line 197, in parse
    return parser.parse(source, **kwargs)
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/plugins/sparql/results/xmlresults.py", line 34, in parse
    return XMLResult(source)
  File "/home/scossu/code/my_prj/virtualenv/lib/python3.5/site-packages/rdflib/plugins/sparql/results/xmlresults.py", line 69, in __init__
    "No RDF Graph, result-bindings or boolean answer found!")
rdflib.query.ResultException: No RDF Graph, result-bindings or boolean answer found! 

[gromgull]

This is maybe fixed in #744 ?

[scossu]

@gromgull I am working on the master branch of RDFlib. When this gets merged I will be happy to test against my code.

[ghost]

Unable to reproduce, no longer an issue.