Deferred query with no incremental key on incremental chunks stuck on loading

[alessbell]

With router PR apollographql/router#2184, deferred responses are no longer included in incremental chunks if the path points to an element of the response that was nullified.

This means that a previous response of

--graphql
content-type: application/json

{"data":{"allProducts":[null,null,null]},"errors":[{"message":"Cannot return null for non-nullable field Product.nonNullErrorField.","locations":[{"line":1,"column":53}],"path":["allProducts",0,"nonNullErrorField"],"extensions":{"code":"INTERNAL_SERVER_ERROR","exception":{"stacktrace":["Error: Cannot return null for non-nullable field Product.nonNullErrorField.","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:594:13)","    at executeField (/usr/src/app/node_modules/graphql/execution/execute.js:489:19)","    at executeFields (/usr/src/app/node_modules/graphql/execution/execute.js:413:20)","    at completeObjectValue (/usr/src/app/node_modules/graphql/execution/execute.js:914:10)","    at completeAbstractValue (/usr/src/app/node_modules/graphql/execution/execute.js:795:10)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:624:12)","    at /usr/src/app/node_modules/graphql/execution/execute.js:696:25","    at Function.from (<anonymous>)","    at completeListValue (/usr/src/app/node_modules/graphql/execution/execute.js:676:34)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:607:12)"]}}},{"message":"Cannot return null for non-nullable field Product.nonNullErrorField.","locations":[{"line":1,"column":53}],"path":["allProducts",1,"nonNullErrorField"],"extensions":{"code":"INTERNAL_SERVER_ERROR","exception":{"stacktrace":["Error: Cannot return null for non-nullable field Product.nonNullErrorField.","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:594:13)","    at executeField (/usr/src/app/node_modules/graphql/execution/execute.js:489:19)","    at executeFields (/usr/src/app/node_modules/graphql/execution/execute.js:413:20)","    at completeObjectValue (/usr/src/app/node_modules/graphql/execution/execute.js:914:10)","    at completeAbstractValue (/usr/src/app/node_modules/graphql/execution/execute.js:795:10)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:624:12)","    at /usr/src/app/node_modules/graphql/execution/execute.js:696:25","    at Function.from (<anonymous>)","    at completeListValue (/usr/src/app/node_modules/graphql/execution/execute.js:676:34)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:607:12)"]}}},{"message":"Cannot return null for non-nullable field Product.nonNullErrorField.","locations":[{"line":1,"column":53}],"path":["allProducts",2,"nonNullErrorField"],"extensions":{"code":"INTERNAL_SERVER_ERROR","exception":{"stacktrace":["Error: Cannot return null for non-nullable field Product.nonNullErrorField.","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:594:13)","    at executeField (/usr/src/app/node_modules/graphql/execution/execute.js:489:19)","    at executeFields (/usr/src/app/node_modules/graphql/execution/execute.js:413:20)","    at completeObjectValue (/usr/src/app/node_modules/graphql/execution/execute.js:914:10)","    at completeAbstractValue (/usr/src/app/node_modules/graphql/execution/execute.js:795:10)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:624:12)","    at /usr/src/app/node_modules/graphql/execution/execute.js:696:25","    at Function.from (<anonymous>)","    at completeListValue (/usr/src/app/node_modules/graphql/execution/execute.js:676:34)","    at completeValue (/usr/src/app/node_modules/graphql/execution/execute.js:607:12)"]}}}],"hasNext":true}
--graphql
content-type: application/json

{"hasNext":false,"incremental":[{"data":null,"path":["allProducts",0]},{"data":null,"path":["allProducts",1]},{"data":null,"path":["allProducts",2]}]}
--graphql--

now has a final chunk of

{"hasNext":false}

without an incremental array.

The test for non-nullable errors thrown outside deferred fragments is now failing in our E2E test suite when run with router versions >=1.6.0 as the query remains stuck on loading: true.

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