Fix "response position" definition; clarify sibling errors on propagation

[benjie]

If a response position resolves to {null} because of an execution error which
has already been added to the {"errors"} list in the execution result, the
{"errors"} list must not be further affected. That is, only one error should be
added to the errors list per response position.

The response position mentioned at the beginning of this paragraph is different to the response position from which the error originated; therefore "only one error should be added to the errors list per response position" is kind of moot. Actually what we mean here is that the response path of the error (i.e. the "path" entry in the error object) should be unique.

This edit clarifies that it's the "path" of the error that matters (and should be unique), not the position that re-raised it.

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[benjie]

Plan from today's WG:

1. open a new PR that clarifies "response position" without renaming to "execution position" (which could be ambiguous w.r.t. visiting fragments?)
2. rebase this change on that PR, so this becomes just the one sentence change
3. see how hard this would be to implement in GraphQL.js and a few other implementations
4. if hard, reject
5. in graphql-toe, throw the last error that matches the path (just reverse the array?) or throw an aggregate error

[yaacovCR]

graphql/graphql-js#4458 not sure if this is the implementation you want to test, I thought it’s the implementation for: #1184

[benjie]

Editorial change for definition of response position extracted to here:

• Clarify definition of response position #1238

[martinbonnin]

If a _response position_ resolves to {null} because of an execution error which
has already been added to the {"errors"} list in the _execution result_, the
{"errors"} list must not be further affected. That is, only one error should be
added to the errors list per _response position_.

FWIW, I don't necessarily read this as "the response position at the beginning of the paragraph and at the end must be the same". So the curernt text was ok-ish to me.

Nevertheless, the new text is more explicit so all good 👍

[leebyron]

Nice. I like that this cleans up things since the response since the response position might not exist in the response thanks to bubbling.