Fix SparqlHandler to refrain from returning DELETE DATA with variables

[andrewzhurov]

Closes #352

[andrewzhurov]

@jeswr Let me know if there are any adjustments you'd like to have.

[jeswr]

Thanks for your work @andrewzhurov ! Just have a couple of comments.

[jeswr]

IIRC there is nothing stopping us from instantiating the subject as a variable - can you check this and if so also include the DATA when the subject is a variable.

[andrewzhurov]

From what I gather, according to the spec INSERT DATA must not allow variables in quad patterns, so when there is a subject variable it's not INSERT DATA but INSERT.

When there are no where and predicate objects are specified - there are no vars and we're safe to use INSERT DATA, it seems.

[jeswr]

Suggested change

	return predicateObjects2.filter(({ predicate, objects }) => objects === null || predicate === null).length === 0;
	return predicateObjects2.some(({ predicate, objects }) => objects === null || predicate === null);

[jeswr]

Why is this a function rather than just assigning const variablePredicateOrObject = predicateObjects.some(({ predicate, objects }) => objects === null || predicate === null); and using that const later on

Furthermore objects is a list so you may need to check if each element in that array could be a variable. I'm not currently in a position to check for myself if it is possible for those objects to be variables - so could you quickly investigate this yourself?

[andrewzhurov]

It's a function and not a const in order to not execute it ahead of time, but only after where.length === 0 predicate passes, to give us a bit of performance. And it's a function and not an inline expression to give a bit of readability.
There is some! Gotta use that. JS is not my first language.:)

I'll give objects a look, thanks for showing the gotcha

[andrewzhurov]

Regarding objects, it seems we either have them undefined altogether, or values present (a value cannot be undefined).

[andrewzhurov]

An expression like this db.fov.set(90), results in such SPARQL

DELETE {
  <http://example.com/db> <ex:fov> ?fov.
}
;
INSERT DATA {
  <http://example.com/db> <ex:fov> "90"^^<http://www.w3.org/2001/XMLSchema#integer>.
}

which is, interestingly, consider invalid by sparql.js

SparqlParser.js:675 Uncaught (in promise) Error: Parse error on line 3:
...b> <ex:fov> ?fov.};INSERT DATA {  <h
---------------------^
Expecting 'WHERE', 'INSERT', 'USING', got ';'
    at Parser.parseError (SparqlParser.js:675:25)
    at Parser.parse (SparqlParser.js:742:22)
    at Parser.parser.parse (sparql.js:37:37)
    at ActorQueryParseSparql.run (ActorQueryParseSparql.js:24:37)
    at ActorQueryParseSparql.runObservable (Actor.js:58:29)
    at MediatorRace.mediate (Mediator.js:80:22)
    at async QueryEngine.queryOrExplain (QueryEngineBase.js:105:38)
    at async QueryEngine.query (QueryEngineBase.js:43:24)
    at async QueryEngine.queryOfType (QueryEngineBase.js:30:24)

Although it seems to be valid, according to the spec