How should a sync layer react to CKError codes: partial failures, retry-after, conflicts and expired tokens?

Predict first

What does each error ask for?

import Foundation
import CloudKit

func advice(_ e: CKError) -> String {
    switch e.code {
    case .requestRateLimited, .zoneBusy, .serviceUnavailable:
        return "retry after \(e.retryAfterSeconds ?? 0)s"
    case .networkUnavailable, .networkFailure: return "wait for the network, then retry"
    case .serverRecordChanged: return "merge with the server record, save again"
    case .changeTokenExpired: return "drop the token, refetch everything"
    case .zoneNotFound: return "create the zone, then retry"
    case .quotaExceeded: return "tell the user; do not retry"
    case .notAuthenticated: return "ask the user to sign in to iCloud"
    case .unknownItem: return "the record is gone; delete locally"
    case .limitExceeded: return "split the batch and retry"
    case .partialFailure: return "inspect each item"
    default: return "log and surface"
    }
}
let id1 = CKRecord.ID(recordName: "a"), id2 = CKRecord.ID(recordName: "b")
let partial = CKError(.partialFailure, userInfo: [CKPartialErrorsByItemIDKey: [
    id1: CKError(.unknownItem),
    id2: CKError(.serverRecordChanged)]])
print("top level:", partial.code.rawValue, advice(partial))
for (id, e) in partial.partialErrorsByItemID!.sorted(by: { ($0.key as! CKRecord.ID).recordName < ($1.key as! CKRecord.ID).recordName }) {
    let err = e as! CKError
    print((id as! CKRecord.ID).recordName, err.code.rawValue, advice(err))
}
print(advice(CKError(.requestRateLimited, userInfo: [CKErrorRetryAfterKey: 7.5])))
for c in [CKError.Code.changeTokenExpired, .zoneNotFound, .quotaExceeded, .limitExceeded, .notAuthenticated] { print(c.rawValue, advice(CKError(c))) }

• Partial failure inspects items: a is deleted locally, b merged; rate limit waits 7.5 s; expired token refetches
• Every error is retried at once
• quotaExceeded is retried