[SR-10478] EXC_BAD_ACCESS of NSError via caught error.localizedDescription, when trying to match it to other error type.

[swift-ci]

Previous ID	SR-10478
Radar	rdar://problem/49906841
Original Reporter	fbeeper (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Running on Xcode Version 10.2 (10E125).

Failing on both a Swift 4.2 project and Swift 5.0 Playground.

Compiling with Optimization Level None [-Onone].

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, RunTimeCrash
Assignee	None
Priority	Medium

md5: 8669e5a47c8541304bb7e62da864518a

duplicates:

• SR-9207 Crash casting NSError to URLError in Swift 4.x

Issue Description:

I ran into an unexpected EXC_BAD_ACCESS on an error I catch in my code. The crash happens when accessing localizedDescription, but only when I try to compare the error with some other error type before that.

There are many ways to code a working alternative, but this crash certainly looked interesting to me. Hope this report is somehow useful.

I reproduced my issue in this nonsensical but enough-to-proof-the-crash sample code:

struct MyObject: Codable {
    let id: Int
}

enum MyError: CustomNSError {
    case tomato
}

do {
    _ = try JSONDecoder().decode(MyObject.self, from: "akldnlknglfkng".data(using: .utf8)!)
}
catch let error as NSError {
    
    if let myError = error as? MyError {
        print("MyError: \(myError.localizedDescription)")
    }
    else {
        print(error.localizedDescription)
    }
}

As I mention above, the crash happens in the execution of localizedDescription. But, if I remove the if-statement trying to match the catched error to MyError it will not crash anymore.

I defined MyError to be a CustomNSError because mine was, but making it an Error causes the same issue.

While somehow similar to the issue described on SR-7493, I am having this issue running the project on Xcode, Debug config.

[belkadan]

Doesn't seem to be a regression (i.e. the same crash is observed in 4.1), but definitely seems bad.

[swift-ci]

Comment by Ferran Poveda (JIRA)

Let me add that the following scenarios will not end up in the crash described above:

do {
    _ = try JSONDecoder().decode(MyObject.self, from: "akldnlknglfkng".data(using: .utf8)!)
}
catch let error as MyError {
    print("MyError: \(error.localizedDescription)")
}
catch {
    print(error.localizedDescription)
}

do {
    _ = try JSONDecoder().decode(MyObject.self, from: "akldnlknglfkng".data(using: .utf8)!)
}
catch {
    if let myError = error as? MyError {
        print("MyError: \(myError.localizedDescription)")
    }
    else {
        print(error.localizedDescription)
    }
}

[swift-ci]

Comment by Slava (JIRA)

I have a similar issue with the following code. In my case Error object is released after being deallocated:

enum ErrorA: Error {
    case A
    case B
}


enum ErrorB: Error {
    case A
    case B
}


func test() {
    let error = ErrorA.A as NSError
    print(error as? ErrorB)
}


test()

call stack:

#0 objc_release ()

#1 test()

#2 main

#3 start ()

log with Zombies enabled:

-[__SwiftNativeNSError release]: message sent to deallocated instance 0x10330cf10

[DougGregor]

We have a fix for this, which is tracked by the original SR-9207