Using variables in a query on a sub graph causes an error

[maartenkools]

Product

Hot Chocolate

Version

14.3.0

Link to minimal reproduction

https://github.com/maartenkools/hc-fusion-duplicate-vars/tree/main

Steps to reproduce

Start the Server and Gateway projects from the sample repo, and execute the following query:

query getItems($groupId: String! = "groupId") {
  items(groupId: $groupId) {
    values(groupId: $groupId)
  }
}

The server has the following schema:

type Query {
    items(groupId: String): [Item!]!
}

type Item {
    values(groupId: String!): [String!]!
}

Key point is that groupId is optional for the items query, while it's mandatory for values.

What is expected?

The query executes successfully (in case of the repo, an empty array is returned)

What is actually happening?

An error occurs:

Using HC 13.9.7:

{
  "errors": [
    {
      "message": "An item with the same key has already been added. Key: groupId",
      "extensions": {
        "code": "HC0012"
      }
    }
  ]
}

Using HC 14.3.0 and up:

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "items"
      ],
      "extensions": {
        "code": "HC0018"
      }
    },
    {
      "message": "An item with the same key has already been added. Key: groupId",
      "extensions": {
        "code": "HC0012"
      }
    }
  ]
}

In both cases the following query plan is generated:

{
    "queryPlan": {
        "document": "query getItems($groupId: String! = \"groupId\") { items(groupId: $groupId) { values(groupId: $groupId) } }",
        "operation": "getItems",
        "rootNode": {
            "type": "Sequence",
            "nodes": [{
                    "type": "Resolve",
                    "subgraph": "Subgraph",
                    "document": "query getItems_1($groupId: String! = \"groupId\", $groupId: String = \"groupId\") { items(groupId: $groupId) { values(groupId: $groupId) } }",
                    "selectionSetId": 0,
                    "forwardedVariables": [{
                            "variable": "groupId"
                        }, {
                            "variable": "groupId"
                        }
                    ]
                }, {
                    "type": "Compose",
                    "selectionSetIds": [
                        0
                    ]
                }
            ]
        }
    }
}

Relevant log output

Additional context

Possibly related to #7365?

[michaelstaib]

This issue is fixed with the new variable batching in fusion 15.

[michaelstaib]

Hot Chocolate 14 already implements this new protocol but the current fusion implementation is not yet taking advantage of this.