Crash during async .start() on Swift

[ahlager]

Hello, using the latest await/async paradigm in Swift I'm running into an error.

When calling fetchMessagesOperation with .start() async inside an async function, I get an error of :0: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value. No matter if I have guards, checks, it will always hard crash.

However, running the same code but with the non-async version of .start(), it does not crash.

I have tested with other "fetch" functions/operations and I do not see the same results. Only with async fetchMessagesOperation.

Below is the code I'm using

ASYNC: This is the code that crashes

let index_set = MCOIndexSet(range: MCORange(location: UInt64(last_seen_uid) + 1, length: 20))

let fetch = safe_session.fetchMessagesOperation(withFolder: folder, requestKind: [.flags], uids: index_set)

do {
    guard let results = try await fetch?.start() else {return}  // <------ always crashes here

        // never gets here
    print(results)

        } catch let error as NSError {

            // this is never reached
            print(error.localizedDescription)
            return
}

NON-ASYNC: This does not crash

let index_set = MCOIndexSet(range: MCORange(location: UInt64(last_seen_uid) + 1, length: 20))

let fetch = safe_session.fetchMessagesOperation(withFolder: folder, requestKind: [.flags], uids: index_set)

fetch?.start({ _, messsages, _ in
     // do something with messages
})

Additionally, using the connectionlogger, the output data for each is exactly the same:

Optional("13 UID FETCH 2:22 (UID FLAGS)\r\n")
Optional("13 OK Completed (0.000 sec)\r\n")

[okferret]

session.delegateQueue.async { fetch?.start ({ *** }) }

[cliftonlabrum]

@ahlager I'm running into this same issue. Did you ever find a fix?

@imotoboy Can you elaboriate on that? Is session supposed to be an MCOIMAPSession object?

[ahlager]

@cliftonlabrum unfortunately no, this didn't work and I was not able to find a fix.

[Be-Maps]

This is a problem with XCode and mailcore works well. You cannot execute step by step as these lines go onto a different thread onto a different processor (and debugger is not able to return values properly). SOLUTION:

• put your breakpoint AFTER the await
• DO NOT have any other breakpoints before await point
• and you will have results

NOTE (XCode not always translates properly to async) For fetch messages you might need to write async extension (the below code works, and returns empty list or list of values):

extension MCOIMAPFetchMessagesOperation { func fetchMessages() async throws -> [Any] { typealias PostContinuation = CheckedContinuation <[Any], Error> return try await withCheckedThrowingContinuation { (continuation:PostContinuation) in self.start { (error: Error?, headers: [Any]?, indexSet: MCOIndexSet?) -> Void in if let error = error { print("Error downloading message UIDs: (error)") let mHeaders:[Any] = [] continuation.resume(returning: mHeaders) } else { print("Successful IMAP messages headers download") var mHeaders:[Any] = [] if (headers != nil) {mHeaders = headers!} continuation.resume(returning: mHeaders) }}}}