Deadlock getting ARTRealtimeChannel between two dispatch_sync (ui queue vs ably main queue)

[eungkyu]

Which version of the Ably SDK are you using?

1.1.23

On which platform does the issue happen?

iOS12

Are you using Carthage?

no

Are you using Cocoapods?

yes, 1.9.3

Which version of Xcode are you using?

Xcode 11.5 Build version 11E608c

What did you do?

Describe the problem.
I called

let ablyChannel = ably.channels.get("connected-user:\(myUid)")

from UI queue (viewDidLoad)
but it stuck by deadlock in ARTChannel.m:83 which is

dispatch_sync(_queue, ^{
    channel = [self _getChannel:name options:options addPrefix:true];
});
    return channel;
}

other queue (io.ably.main) is stuck in
ARTPush.m:110

            dispatch_sync(dispatch_get_main_queue(), ^{
                delegate = UIApplication.sharedApplication.delegate;
            });

What did you expect to happen?

ably.channels.get function does not return by deadlock

What happened instead?

ably.channels.get function should return

Tip: set the log level to debug. E.g. realtime.logger.logLevel = .debug and take a look at our troubleshooting guide

Link to sample project

Feel free to link to a sample project that showcases the issue.

P.S. If you are an Ably Enterprise customer, please get in touch with a link to this issue to get prioritized support.

[QuintinWillison]

Hello @eungkyu and thanks for your detailed bug report. We really appreciate the effort you've gone to in explaining the problem you are encountering.

We're having a look now and either myself or one of our other engineers on the iOS side of things will report back when we either have some questions for you, a solution or can confirm this is a bug. 😁

[ricardopereira]

Hello @eungkyu. Does this happens occasionally or can you reproduce the deadlock every time you access the channel?

[eungkyu]

This happens occasionally. It's hard to reproduce on purpose, but I've seen two or three times randomly.