Return "no such user context" and "no such frame" instead "invalid argument" in "session.subscribe" and "session.unsubscribe"

[lutien]

• Return "no such user context" error instead "invalid argument" in "session.subscribe" command.
• Return "no such frame" error instead "invalid argument" in "session.subscribe" and "session.unsubscribe" commands.

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Preview | Diff

[jgraham]

This doesn't work because if a function can return error it needs to return success in the case it doesn't fail.

If you made that change you'd also need to change every other caller to either pass in the correct value for the validate argument (or set a default) and to correctly unwrap the result (even if it's using |validate navigables| = false so it's defacto infallible).

All that said, I don't love the "here's an boolean parameter that totally changes the error handling of this algorithm" approach. I think I'd actually prefer to have two different functions, like get valid navigables by ids and get navigables by ids, with the former implementing the error handling on top of the latter in the same way we do it here (i.e. comparing set sizes).

[lutien]

Ah, alright. The only thing which I don't like is that then (at least for the implementation) we will have to reverse engineer which navigable ids belong to non-existent navigables to return a proper error message. So my suggestion would be to have two functions, as you said, but not use get navigables by ids in get valid navigables by ids and use get a navigable instead, which already does a validation. It's a bit more repetition, but I think a bit easier to implement. Let me know what you think. (The PR is updated to reflect the suggestion)