Federation subgraphs skipping interface's resolveType on graphql-js schemas

[juampynr]

Issue Description

If you use graphql-js to define a schema with an interface (GraphQLInterfaceType) and then buildSubgraphSchema to build a subgraph, interface's resolveType in the schema are not being copied over, which leads to the runtime error:

"Abstract type \"Character\" must resolve to an Object type at runtime for field \"Query.character\". Either the \"Character\" type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function."

I created https://github.com/juampynr/subgraph-interface to illustrate this bug.

Link to Reproduction

https://github.com/juampynr/subgraph-interface

Reproduction Steps

Run the following commands to see the error:

git clone git@github.com:juampynr/subgraph-interface.git
cd subgraph-interface
nvm use
npm i
node index.js
curl --location 'http://localhost:4000/graphql' \
--header 'Content-Type: application/json' \
--data '{"query":"query {\n  character(id: \"1\") {\n    id\n    name\n    ... on Hero {\n      superpower\n    }\n    ... on Villain {\n      evilPlan\n    }\n  }\n}","variables":{}}'

[juampynr]

Fixed this issue by restoring the interfaces resolveType handlers after the subgraph has been built: juampynr/subgraph-interface#1.

[juampynr]

Take that back. Resolvers in nested fields in interfaces are set to undefined. There is lack of support for interface resolvers in buildSubgraphSchema().