fix(sort): Only filter out nodes with positive offsets. (#8077)

[joshua-goldstein]

Cherry pick from #8077.

Steps to reproduce:

make image-local
cd contrib/local-test && make up
make schema-dql
make load-data-dql-rdf
make query-dql
curl: (52) Empty reply from server
make: *** [query-dql] Error 52
<alpha will crash and restart>

with the following minimal schema / data / query (should be placed in contrib/local-test):

schema.dql:

type Person {
    name
    age
    friend
}

name: string @index(term)  .
age: int @index(int) .
friend: [uid] @count .

dql-data.rdf

{
  set {
    _:michael <name> "Michael" .
    _:michael <dgraph.type> "Person" .
    _:michael <age> "314" .
    _:michael <friend> _:alice .
    _:michael <friend> _:bob .
    _:michael <friend> _:charlie .

    _:alice <name> "Alice" .
    _:alice <dgraph.type> "Person" .
    _:alice <age> "1" .

    _:bob <name> "Bob" .
    _:bob <dgraph.type> "Person" .
    _:bob <age> "2" .

    _:charlie <name> "Charlie" .
    _:charlie <dgraph.type> "Person" .
  }
}

query.dql

{
  michael_friends_first(func: allofterms(name, "Michael")) {
    name
    age
    friend (orderasc: age, offset: -1) {
      name
      age
    }
  }
}

Remarks

• The cherry-pick does resolve the above crash
• This issue exists in main
• Why do we allow negative offsets? If they are not useful, we should return an error when a user attempt to query using a negative offset.

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[coveralls]

Coverage decreased (-0.001%) to 37.181% when pulling fae2ad4 on cherry-pick-8077 into 7d7c02f on main.

[matthewmcneely]

Other tests in the past have given a full link to the issue, i.e.

// Regression test for https://github.com/dgraph-io/dgraph/issues/3657.

[meghalims]

Approving this Joshua. Please go ahead and merge

[damonfeldman]

Note this fixes errors reported as panic: runtime error: slice bounds out of range [-4:] where a negative offset is used, a predicate is indexed, and nodes are included that do not have the predicate. It is questionable why or when negative offsets are useful, but this can happen.