Query planner unwraps interface implementations incorrectly

[barthez]

Description

After updating to recent @apollo/gateway package version we started observing and error when query on a field of type of an interface cause No such type [Type], so it can't be a fragment condition. Simplified setup down below

Example (actual behavior)

Service A:

type Query {
  myField: MyInterface
}

type MyInterface {
  name: String
}

type MyTypeA implements MyInterface {
  name: String
}

type MyTypeB implements MyInterface {
  name: String
}

Service B:

interface MyInterface {
 name: String
}

type MyTypeC implements MyInterface {
  name: String
}

The query:

query {
  myField: {
    ...MyIntFragment
  }
}

fragment MyIntFragment on MyInterface {
  name
}

result in such query to service A:

query {
  myField {
    ...on MyTypeA { name }
    ...on MyTypeB { name }
    ...on MyTypeC { name }
  }
}

which cause an error

Unknown type "MyTypeC".

Expected behavior

Query to the service A doesn't include fragment for MyTypeC that isn't known for the service.

Versions when problem occurs:

@apollo/[email protected]
@apollo/[email protected]
@apollo/[email protected]

Versions when problem did not occur

@apollo/[email protected]
@apollo/[email protected]

Working example

https://github.com/barthez/query-planner-error-example

[barthez]

Hey @trevor-scheer, is there any chance you could give some feedback in here. I have tried to fix it with #816 but it turned out to break other things (when types are actually shared between services). If you could give me some hints I could try to fix it and create a PR.

[pcmanus]

The upcoming version of federation (2.0, currently in alpha) fixes this issue and I've pushed 41a9b76 with a test for it. Fixing this is on the 0.x branch is unfortunately a bit tricky (federation 2 has a laregly rewritten query planner, and one of the reasons for such rewrite was to make it easier to solve that kind of issues).

[konalegi]

@pcmanus have you seen this PR? #864, It works for our cases, could you please take a look if makes sense.