[SR-13784] Exception during Measurement serialization with a Unit that uses custom UnitConverter

[swift-ci]

Previous ID	SR-13784
Radar	rdar://problem/70730985
Original Reporter	Rostyslav_D (JIRA User)
Type	Bug

Environment

Swift 5.3 (swiftlang-1200.0.29.2 clang-1200.0.30.1)
Xcode 12.0 GM

Additional Detail from JIRA

Votes	0
Component/s	Foundation, Standard Library
Labels	Bug
Assignee	None
Priority	Medium

md5: 39638118a3a39937e75933935b5cef3d

Issue Description:

In my project I extended a UnitAngle with a custom unit that relies on UnitConverter subclass:

extension UnitAngle {
   static let cosine = UnitAngle(symbol: "cosφ", converter: CosineUnitConverter())
}

class CosineUnitConverter: UnitConverter, NSSecureCoding, Codable {
    static var supportsSecureCoding: Bool { true }
    required convenience init?(coder: NSCoder) {
         self.init()
    }
    func encode(with coder: NSCoder) { }
    override func baseUnitValue(fromValue value: Double) -> Double {
         let rad = acos(value)
         return rad / .pi * 180
    }
    override func value(fromBaseUnitValue baseUnitValue: Double) -> Double {
         let rad = baseUnitValue * .pi / 180
         return cos(rad)
    }
}

In that converter I added conformance to NSSecureCoding similarly as UnitConverterLinear does.
During attempt to serialize such a measurement with NSKeyedArchiver I get a runtime exception EXC_BAD_INSTRUCTION:

let archiver = NSKeyedArchiver(requiringSecureCoding: false)
let measurement = Measurement<UnitAngle>(value: 1, unit: .cosine)
try archiver.encodeEncodable(measurement, forKey: NSKeyedArchiveRootObjectKey)

[typesanitizer]

@swift-ci create

[swift-ci]

Comment by Rostyslav Dovhaliuk (JIRA)

Upon further investigation I noticed that Swift implementation of Measurement struct explicitly checks for converter class and if it's not UnitConverterLinear, it triggers a precondition failure. Considering that almost all types of Measurements and Units framework conform to NSSecureCoding protocol maybe it is reasonable to add such a conformance to UnitConverter base class and then encode and decode it with NSKeyed(Un)archiver like this:

@available(macOS 10.12, iOS 10.0, watchOS 3.0, tvOS 10.0, *)
extension Measurement : Codable {
    private enum CodingKeys : Int, CodingKey {
        case value
        case unit
    }

    private enum UnitCodingKeys : Int, CodingKey {
        case symbol
        case converter
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let value = try container.decode(Double.self, forKey: .value)

        let unitContainer = try container.nestedContainer(keyedBy: UnitCodingKeys.self, forKey: .unit)
        let symbol = try unitContainer.decode(String.self, forKey: .symbol)

        let unit: UnitType
        if UnitType.self is Dimension.Type {
            let converterData = try unitContainer.decode(Data.self, forKey: .converter)
            let converter = try NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(converterData) as? UnitConverter
            let unitMetaType = (UnitType.self as! Dimension.Type)
            if let converter = converter {
                unit = (unitMetaType.init(symbol: symbol, converter: converter) as! UnitType)
            } else {
                unit = UnitType(symbol: symbol)
            }
        } else {
            unit = UnitType(symbol: symbol)
        }

        self.init(value: value, unit: unit)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(self.value, forKey: .value)

        var unitContainer = container.nestedContainer(keyedBy: UnitCodingKeys.self, forKey: .unit)
        try unitContainer.encode(self.unit.symbol, forKey: .symbol)

        if UnitType.self is Dimension.Type {
            let converter = (self.unit as! Dimension).converter
            let converterData = try NSKeyedArchiver.archivedData(withRootObject: converter, requiringSecureCoding: false)
            try unitContainer.encode(converterData, forKey: .converter)
        }
    }
}

Any thoughts about such a solution?