Fuseki currently does not support FROM/FROM NAMED clauses returning a HTTP 400 error if a user tries to make use of them
The attached patch provides for optional enabling of this feature by a user specified --from or --allowFrom switch when the server is started or programmtically via ARQ.getContext().set(Fuseki.enableFromClauses, true). Therefore unless a user turns this feature on by default the old behavior remains in place.
When enabled Fuseki will not reject queries with FROM/FROM NAMED in them and will use DynamicDatasets.dynamicDataset() to build the dataset for queries, there is minimal overhead for queries without FROM/FROM NAMED clauses as the code in DynamicDatasets appears to leave the default dataset unchanged if no dataset description is present in the query AFAICT
A small number of tests has been added to TestQuery.java to test the behaviour, more tests might be desirable and I can probably supply those if necessary but the ones given test all the obvious combinations with the trivial test data used by TestQuery