This still needs to be deleted.

Not for this PR, but this get-set can introduce a race condition. We'd have to use an API like Atomic.write that takes the inout and writes it inside of the lock.

This should be called after calling the completion handler (since the operation is still technically running).
You could wrap this into a defer:

see andy's comment: #1534 (comment)

do we want both to be in the defer then?

Not sure I understand the comment, the completion here has to be called for both cases, and the response handler as well. My comment was regarding the fact that the operation shouldn't complete until we've called the completion handler.

@aboedo 👀 ^^

I think both suggestions are compatible - like Nacho said, we should call completion as the last step in the method, and what I suggested was to do it by having both this code path and the early return look similar, i.e.:

guard let appUserID = try? self.configuration.appUserID.escapedOrError() else {
    defer { completion() } 
    self.responseHandler?(.missingAppUserID())
}

/// ...

{ (response: HTTPResponse<HTTPEmptyResponseBody>.Result) in
    defer { completion() } 
    self.responseHandler?(response.error.map(BackendError.networkError))
}

or

guard let appUserID = try? self.configuration.appUserID.escapedOrError() else {
    self.responseHandler?(.missingAppUserID())
    completion()
}

/// ...

{ (response: HTTPResponse<HTTPEmptyResponseBody>.Result) in
    self.responseHandler?(response.error.map(BackendError.networkError))
    completion()
}

ahhh i totally misunderstood your original comment, my bad. thank you for clarifying!

These 3 are duplicated in all of these tests.
Just a suggestion, feel free to ignore, but you could extract a BaseAttributionPosterTests that has the properties and setUp in this class that AttributionPosterTests and a new AdServicesAttributionPosterTests class can inherit.
Then you can put this inside of a shared setUp as well as making that whole class:

@available(iOS 14.3, macOS 11.1, macCatalyst 14.3, *)