[Data masking] Error when adding unmask warnings on interface types

[jerelmiller]

Issue Description

Take the following query from the Spotify Showcase:

query PlaybackStateSubscriberQuery {
  me {
    player {
      playbackState {
        ...PlaybackStateFragment @unmask(mode: "migrate")
      }
    }
  }
}

fragment PlaybackStateFragment on PlaybackState {
  isPlaying
  repeatState
  shuffleState
  actions {
    disallows
  }
  context {
    uri
  }
  device {
    id
    name
    type
    volumePercent
  }
  item {
    id

    ... on Track {
      album {
        id
        images {
          url
        }
      }
    }

    ... on Episode {
      show {
        id
        images {
          url
        }
      }
    }
  }

  ...Playbar_playbackState @unmask(mode: "migrate")
}

When executing this query with data masking enabled, there is an error thrown:

Uncaught TypeError: Cannot read properties of undefined (reading 'id')

This happens when the item field returns a Track type. This is because the masking algorithm fails to check the inline fragment condition on the item before trying to add accessor warnings. In this case, the masking algorithm is trying to add warnings for the show field which should not happen because the fragment condition is unmatched.

We already handle this in the main part of the masking algorithm so we should be able to apply something similar here.

For now the workaround is to remove the mode: "migrate" argument from the @unmask directive to get this to work until we have a fix.

Link to Reproduction

https://github.com/apollographql/spotify-showcase/compare/jerel/masking-playground

Reproduction Steps

• Run the spotify showcase with the above branch
• Enable data masking in the client
• Observe that the home page cannot load properly

@apollo/client version

3.12.0-rc.0

[jerelmiller]

Fixed with #12130

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