[swift-dev] Foundation Data Behavior Different (read: crashes) in Swift 3.1 and Swift 3.2

[Philippe Hausler]

This is expected behaviors of collections and their slices. The previous result was a bug.

I agree that collections in general and their slices need more documentation.

Sent from my iPhone

> On Aug 24, 2017, at 11:28 AM, Ryan Lovelett <swift-dev at ryan.lovelett.me> wrote:
> 
> I think your onto it Philippe but then why is it behaving that way?
> 
> I updated my earlier script (attached) and added 2 things.
> 
>   1. print("Start: \(data.startIndex), End: \(data.endIndex), Count: \(data.count)") after the data.removeFirst(4).
>   2. The startIndex offset you suggested.
> 
> It no longer crashes. Hooray!
> 
> Swift 3.1:
> 
> Start: 0, End: 2, Count: 2
> Base64: QWE=, String: Aa
> 
> Swift 3.2:
> 
> Start: 4, End: 6, Count: 2
> Base64: QWE=, String: Aa
> 
> Oh wait. Woah. That's a really subtle change that has some very real consequences.
> 
> Is this expected and/or documented? Because I certainly would not have expected that drastic of a change. I'll obviously need to go read a lot more of the changes between Swift 3.1 and Swift 3.2 if these sorts of changes are in scope.
> 
> 
>> On Thu, Aug 24, 2017, at 02:10 PM, Philippe Hausler wrote:
>> I see the issue.. the latest version traps (appropriately so).
>> 
>> let str = String(bytes: data[0..<2], encoding: .utf8)!
>> 
>> The sub-range of the slice you have is incorrectly indexed. Try this out (I am presuming this is what you mean):
>> 
>> let str = String(bytes: data[data.startIndex..<(data.startIndex + 2)], encoding: .utf8)!
>> 
>>> On Aug 24, 2017, at 11:07 AM, Philippe Hausler <phausler at apple.com> wrote:
>>> 
>>> Is there a radar or bugs.swift.org ticket filed on this?
>>> 
>>> I presume because of the import this is a Darwin thing and not a linux thing.
>>> 
>>>> On Aug 24, 2017, at 11:05 AM, Michael Gottesman via swift-dev <swift-dev at swift.org> wrote:
>>>> 
>>>> 
>>>>> On Aug 24, 2017, at 10:47 AM, Ryan Lovelett via swift-dev <swift-dev at swift.org> wrote:
>>>>> 
>>>>> I've found what I believe is a bug. Though I'm unclear if the bug is in
>>>>> Swift 3.1 or Swift 3.2/4.0. All I can say for sure is the behavior is
>>>>> quite drastically different between the two.
>>>>> 
>>>>> For the code below (and attached):
>>>>> 
>>>>>  import Cocoa
>>>>> 
>>>>>  var data = Data(bytes: [0x50, 0x4B, 0x01, 0x02, 0x41, 0x61])
>>>>>  data.removeFirst(4)
>>>>>  let base64 = data.base64EncodedString()
>>>>>  let str = String(bytes: data[0..<2], encoding: .utf8)!
>>>>>  print("Base64: \(base64), String: \(str)")
>>>>> 
>>>>> If I compile and run that with the Swift included in Xcode 8.3.3 (e.g.,
>>>>> swift ./data-bug.swift) it outputs: Base64: QWE=, String: Aa. Which is
>>>>> what I expect.
>>>>> 
>>>>> With the Swift that is included with Xcode 9.0 beta 6 (9M214v) (e.g.,
>>>>> swift -swift-version 3 ./data-bug.swift). It performs an illegal
>>>>> hardware instruction and crashes. It also does this if I use use the
>>>>> version 4 of the compiler.
>>>>> 
>>>>> Is this a bug? If so where is the bug? Was this always meant to not work
>>>>> and Swift 3.1 just happened to work or is there now an issue in the
>>>>> Swift 3.2 implementation?
>>>> 
>>>> I have not engaged my brain with the particulars of the rest of the email, but high level question: does this happen without optimization? Or does it happen only with optimization?
>>>> 
>>>> Michael
>>>> 
>>>>> <data-bug.swift>_______________________________________________
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>>>> 
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> 
> <data-bug.swift>
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