A handful of my customers have reported a consistent freeze in my app (smells like either a thread deadlock or an infinite recursion to me). It's working properly for most people, but if 3 people are reporting it to me that says to me that probably 300 are experiencing it but staying quiet.
It happens every time they take a certain kind of action. But it never happens to me so I can't figure out what is breaking.
No crash is happening, so I'm not getting any reports via Firebase.
It doesn't appear to be related to their data, because they can have someone else use a different device, login to their account, download all of their data, and then successfully do the thing that freezes 100% of the time on their original device.
I've got one of these customers set up on TestFlight so I can send them test versions of the app with some debug collection code. I created my own version of a tracelog and write a line to a file whenever I start/end a function. I've added those tracelog calls to a bunch of methods and classes (but not all).
The action that causes the freeze is when they tap Edit on a record. What's supposed to happen is that I modally present a nav controller containing an EditBlahViewController.
The trace logs show that the EditBlahViewController gets through viewDidLoad, viewWillAppear, two rounds of viewWillLayoutSubviews/viewDidLayoutSubviews... but then never gets to viewDidAppear.
I've tried updating more and more functions/classes with the tracelog calls, but I can't find any infinite recursions. I've added tracelogs to all calls involving dispatching to other threads (especially synchronous dispatches to the main thread) but I don't see any deadlocks. Every dispatch to main starts and ends properly and then goes on to the next thing.
User is on iOS 16.6.1 with an iPhone 12.
I don't know what else to try. How can I debug this at a distance and figure out where the problem is?
