Add parentType to path to avoid path ambiguity

[benjie]

Imagine you have the schema:

type A {
  int: Int
  union: C
}
type B {
  int: Int
  union: C
}
union C = A | B
type Query {
  root: C
}

You could write the query:

{
  root {
    ... on A {
      union {
        ... on A {
          alias1: union { # EXAMPLE1
            int
          }
          alias2: union {
            int
          }
        }
        ... on B {
          alias1: union {
            int
          }
          alias2: union {
            int
          }
        }
      }
    }
    ... on B {
      union {
        ... on A {
          alias1: union { # EXAMPLE2
            int
          }
          alias2: union {
            int
          }
        }
        ... on B {
          alias1: union {
            int
          }
          alias2: union {
            int
          }
        }
      }
    }
  }
}

In the resolver for A.union, in resolveInfo (the fourth argument to resolve) you would not be able to tell if you are in position EXAMPLE1 or EXAMPLE2 without looking into the loc data in fieldNodes. This makes it challenging to write certain classes of lookahead engine.

Namely in both EXAMPLE1 and EXAMPLE2, you have:

• parentType: 'A'
• path: {key: 'alias1', prev: { key: 'union', prev: { key: 'root' } } }

So the path is ambiguous. This PR adds parentType to the Path type, so these paths would become:

• EXAMPLE1: path: {key: 'alias1', parentTypeName: 'A', prev: { key: 'union', parentTypeName: 'A', prev: { key: 'root', parentTypeName: 'Query' } } }
• EXAMPLE2: path: {key: 'alias1', parentTypeName: 'A', prev: { key: 'union', parentTypeName: 'B', prev: { key: 'root', parentTypeName: 'Query' } } }

Note the subtle difference between the A/B parentTypeName in the middle of the path.

[benjie]

Ready for review; I've added a test that tests a union/list too.

[benjie]

@IvanGoncharov Just a ping in case the "request review" above didn't hit your radar.

[benjie]

Suggestion merged; I have not corrected all the other places in the file that use execute rather than executeSync.