Crash in SwiftyJSON during string comparison: swift_unknownRetain - swift

Question

I'm switching on a flag from the server in order to determine what type of object to instantiate. Each type is a subclass of the return type (Snack, in the example.) My guess is that the whole subclass thing is irrelevant to the main issue, but I include it for completeness.
The trouble is that I'm pretty consistently getting a crash reported from Crashlytics on the case "chips": line. In order to simplify parsing in my initializers, I'm wrapping the server response in a SwiftyJSON JSON. This all worked fine in testing.
class func fromJSON(json: JSON) -> Snack {
switch json["SnackName"] {
case "chips": // CRASH OCCURS HERE
return BagOChips(json: json)
case "apple":
return Apple(json: json)
default:
return Spam(json: json)
}
}
Specifically, the crash is occurring at "SwiftyJSON.swift:1013" (marked below). Crashlytics describes it as "EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x0000000093a4bec8" and "swift_unknownRetain + 32".
public func ==(lhs: JSON, rhs: JSON) -> Bool {
switch (lhs.type, rhs.type) {
case (.Number, .Number):
return (lhs.object as NSNumber) == (rhs.object as NSNumber)
case (.String, .String):
return (lhs.object as String) == (rhs.object as String) // CRASH REALLY OCCURS HERE
case (.Bool, .Bool):
return (lhs.object as Bool) == (rhs.object as Bool)
case (.Array, .Array):
return (lhs.object as NSArray) == (rhs.object as NSArray)
case (.Dictionary, .Dictionary):
return (lhs.object as NSDictionary) == (rhs.object as NSDictionary)
case (.Null, .Null):
return true
default:
return false
}
}
Any idea why this is failing and what I might be able to do to correct it in our next release?

Answer 1

Found the problem, and boy was it obscure!
TL;DR - Avoid SwiftyJSON's == function entirely by replacing
switch json["SnackName"]
with
switch json["SnackName"].stringValue
That's probably a good idea in general, but the reason it's necessary appears to be a bug deep in the bowels of how Swift + Foundation handle strings. I've filed an open radar here.
All it takes to reproduce this problem is Xcode 6.1, SwiftyJSON, and the following sample code that I submitted to Apple:
let d = NSDictionary(dictionary: ["foo": "bar"])
let j = JSON(d)
switch (j["foo"]) {
case "bar":
println("> No crash!")
default:
println("> default")
}
Then throw in these logging statements in your copy of SwiftyJSON.
public func ==(lhs: JSON, rhs: JSON) -> Bool {
// Next 2 lines added just for SwiftyCrasher test project.
println( "> Left: \(_stdlib_getTypeName(lhs.object))" )
println( "> Right: \(_stdlib_getTypeName(rhs.object))" )
switch (lhs.type, rhs.type) {
case (.Number, .Number):
return (lhs.object as NSNumber) == (rhs.object as NSNumber)
case (.String, .String):
...
}
This shows the following console output, just before the crash:
> Left: _TtCSs19_NSContiguousString
> Right: _TtCSs19_NSContiguousString
Again, in Debug mode, this doesn't crash. Boxing "foo" and "bar" in NSString, or changing j["foo"] to j["foo"].stringValue also prevent a crash.