[SR-5640] JSONEncoder misrepresents UInt.max on Linux

[swift-ci]

Previous ID	SR-5640
Radar	None
Original Reporter	djones6 (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Ubuntu 16.04, DEVELOPMENT-SNAPSHOT-2017-08-03-a

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Bug
Assignee	@spevans
Priority	Medium

md5: 6cedaf4523ec8dec2eec658df2816060

Issue Description:

JSONEncoder on Linux fails to correctly encode UInt values that are greater than Int.max. For example:

import Foundation

let encoder = JSONEncoder()

struct MyValue: Codable {
  let intMin:Int
  let intMax:Int
  let uintMin:UInt
  let uintMax:UInt

  init() {
    intMin = Int.min
    intMax = Int.max
    uintMin = UInt.min
    uintMax = UInt.max
  }
}

let myValue = MyValue()
let myDict: [String:Any] = ["intMin": myValue.intMin, "intMax": myValue.intMax, "uintMin": myValue.uintMin, "uintMax": myValue.uintMax]

let result = try encoder.encode(myValue)
print("Result (JSONEncoder): \(String(data: result, encoding: .utf8) ?? "nil")")

let result2 = try JSONSerialization.data(withJSONObject: myDict)
print("Result (JSONSerialization): \(String(data: result2, encoding: .utf8) ?? "nil")")

On Ubuntu, this produces a value of -1 for UInt.max:

Result (JSONEncoder): {"uintMin":0,"intMin":-9223372036854775808,"uintMax":-1,"intMax":9223372036854775807}
Result (JSONSerialization): {"uintMin":0,"intMin":-9223372036854775808,"uintMax":18446744073709551615,"intMax":9223372036854775807}

On OSX, both methods produce the same (correct) output.

I suspect this is something to do with the boxing of UInt values to NSNumber on Linux - it appears the bytes are being interpreted as signed rather than unsigned.

[belkadan]

cc @itaiferber, @phausler

[spevans]

Ive done a workaround #1173 I think it will need someone more familiar with CoreFoundation to do a correct fix.

From the PR:

This is a workaround rather than a full fix. However it should make it easier to create the correct fix.

• NSNumber(value: UInt.max).stringValue was returning "-1"
  instead of "18446744073709551615" because NSNumber holds any
  value > Int64.max as a 128Bit quantity using a high Int and
  low UInt. It marks this type as an 'SInt128Type'.

• CFNumberFormatterCreateStringWithNumber() uses CFNumberGetType()
  to get this type however CoreFoundation wants to hide SInt128Type
  (probably for backwards compatibilty) and instead returns it
  as an SInt64Type. Thus only the low part of the NSNumber is used
  for the number and this is interpreted as an Int64.

• The workaround is simply in NSNumber.description(withLocale🙂
  to test to see if the value is of type SInt128Type and if so
  use String(format: "%@") to convert it to a string instead of
  using CFNumberFormatterCreateStringWithNumber(). This should be
  ok for most situations since it is only used for positive
  integers and there are no issues with formatting leading zeros
  or decimal points.

• For JSONWriter._serializationString(for: NSNumber) the value
  is tested to see if it is an SInt128Type and if so the .stringValue
  method is used to create a string.