fix(incremental): prevent error completions for unannounced fragments

[yaacovCR]

Follow-up to merged PR #4868. Thanks to Duckki Oe (@duckki) for identifying these issues and providing an initial fix in #4859.

PR #4868 fixed successful shared results. A nested fragment can now deliver its ready data, including a shared stream, as soon as its parent finishes. Another fragment's separate failure or slower fields no longer cause that data to be lost or delayed.

Previously, even after that fix, a shared non-null field failure could complete a nested deferred fragment before it was announced, although its parent's separate fields succeeded. This was incorrect: the response referred to an ID that the client had never received in pending.

This PR waits until the nested fragment is announced before sending its error completion. The client therefore receives its pending entry before its completion, including when both appear in the same response. If several shared fields have already failed, the fragment completes once with one shared error. Its parent's successful data is still delivered.

Late outcomes and descendants belonging only to failed fragments are discarded. Shared work needed by a surviving fragment remains available.

This enables the remaining regression tests introduced in #4859 and adds integration coverage for error ordering, late outcomes, and shared streams whose remaining fragment fails.

@defer: the shared field fails while the parent succeeds

{
  hero {
    ... @defer(label: "R") {
      nonNullName
    }
    ... @defer(label: "P") {
      name
      ... @defer(label: "C") {
        nonNullName
      }
    }
  }
}

nonNullName returns null, failing R and C. P's separate name returns "Luke" successfully. Both results are available immediately.

Both versions start with:

{
  "data": {"hero": {}},
  "pending": [
    {"id": "0", "path": ["hero"], "label": "R"},
    {"id": "1", "path": ["hero"], "label": "P"}
  ],
  "hasNext": true
}

Before this fix:

Previously, C completed as ID "2" even though only IDs "0" and "1" had been announced.

Response 2:

{
  "hasNext": false,
  "incremental": [
    {"id": "1", "data": {"name": "Luke"}}
  ],
  "completed": [
    {
      "id": "0",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 9, "column": 9}
          ],
          "path": ["hero", "nonNullName"]
        }
      ]
    },
    {
      "id": "2",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 9, "column": 9}
          ],
          "path": ["hero", "nonNullName"]
        }
      ]
    },
    {"id": "1"}
  ]
}

After this fix:

This commit announces C as ID "2" before sending its error completion, so the client can identify the failed fragment. P still returns its successful name.

Response 2:

{
  "hasNext": false,
  "pending": [
    {"id": "2", "path": ["hero"], "label": "C"}
  ],
  "incremental": [
    {"id": "1", "data": {"name": "Luke"}}
  ],
  "completed": [
    {
      "id": "0",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 9, "column": 9}
          ],
          "path": ["hero", "nonNullName"]
        }
      ]
    },
    {"id": "1"},
    {
      "id": "2",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 9, "column": 9}
          ],
          "path": ["hero", "nonNullName"]
        }
      ]
    }
  ]
}

@defer: two shared fields have already failed

{
  hero {
    ... @defer(label: "R1") {
      first: nonNullName
    }
    ... @defer(label: "R2") {
      second: nonNullName
    }
    ... @defer(label: "P") {
      name
      ... @defer(label: "C") {
        first: nonNullName
        second: nonNullName
      }
    }
  }
}

name returns "Luke". Both first and second return null. C shares first with R1 and second with R2; both have failed by the time P completes.

Both versions start with:

{
  "data": {"hero": {}},
  "pending": [
    {"id": "0", "path": ["hero"], "label": "R1"},
    {"id": "1", "path": ["hero"], "label": "R2"},
    {"id": "2", "path": ["hero"], "label": "P"}
  ],
  "hasNext": true
}

Before this fix:

Previously, C completed as ID "3" without a pending entry.

Response 2:

{
  "hasNext": false,
  "incremental": [
    {"id": "2", "data": {"name": "Luke"}}
  ],
  "completed": [
    {
      "id": "0",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 12, "column": 9}
          ],
          "path": ["hero", "first"]
        }
      ]
    },
    {
      "id": "3",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 12, "column": 9}
          ],
          "path": ["hero", "first"]
        }
      ]
    },
    {
      "id": "1",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 7, "column": 7},
            {"line": 13, "column": 9}
          ],
          "path": ["hero", "second"]
        }
      ]
    },
    {"id": "2"}
  ]
}

After this fix:

This PR announces C before completing it once with a shared error. In this example, its completion reports the first error. R1 and R2 each report their own error, and P delivers name.

Response 2:

{
  "hasNext": false,
  "pending": [
    {"id": "3", "path": ["hero"], "label": "C"}
  ],
  "incremental": [
    {"id": "2", "data": {"name": "Luke"}}
  ],
  "completed": [
    {
      "id": "0",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 12, "column": 9}
          ],
          "path": ["hero", "first"]
        }
      ]
    },
    {
      "id": "1",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 7, "column": 7},
            {"line": 13, "column": 9}
          ],
          "path": ["hero", "second"]
        }
      ]
    },
    {"id": "2"},
    {
      "id": "3",
      "errors": [
        {
          "message": "Cannot return null for non-nullable field Hero.nonNullName.",
          "locations": [
            {"line": 4, "column": 7},
            {"line": 12, "column": 9}
          ],
          "path": ["hero", "first"]
        }
      ]
    }
  ]
}

[vercel]

@yaacovCR is attempting to deploy a commit to the The GraphQL Foundation Team on Vercel.

A member of the Team first needs to authorize it.