Mail redirection causes crash

[joelshaf]

We’re seeing a reproducible crash in Notate for Citrix when mail redirection is enabled.

Crash summary:
• Citrix MAM is calling UIPasteboard from a background queue (com.citrix.openURLQueue).
• This triggers a pasteboard notification on that same queue.
• NSNotificationCenter then invokes observers (including our PDF UI code) on that background queue.
• UIKit is touched off the main thread → _dispatch_assert_queue_fail → crash.

Relevant stack frames:

dispatch_assert_queue_fail
-[UIImageView _mainQ_beginLoadingIfApplicable]
-[UIButton layoutSubviews]
-[PDFToolbarButton layoutSubviews]
__CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__
_sendPasteboardChangedNotification
-[_UIConcretePasteboard setData:forPasteboardType:]
ctxSetData_forPasteboardType  (CTXMAMContainment)
__45-[UIViewController(ctx) ctxSendMailProcedure]_block_invoke
Queue: com.citrix.openURLQueue

Root cause: ctxSetData_forPasteboardType in CTXMAMContainment performs pasteboard writes on a background Citrix queue. This causes UI-related observers to fire on that queue, violating UIKit’s main-thread requirement.

Fix needed:
Pasteboard operations inside CTXMAMContainment must be marshalled to the main queue before posting notifications or performing any work that results in UI updates.

This crash does not occur in the non-MAM build or when mail redirection is disabled.

[timliu55]

@joelshaf, thank you for reaching out.

We'd prefer to keep performing pasteboard writes on a background Citrix queue to maintain optimal performance.
To resolve the threading issue, you can specify the [NSOperationQueue mainQueue] when observing UIPasteboardChangedNotification by using the queue: parameter. This ensures all UI updates triggered by pasteboard changes execute on the main thread. For example:

   [[NSNotificationCenter defaultCenter] addObserverForName:UIPasteboardChangedNotification
                                                     object:nil
                                                      queue:[NSOperationQueue mainQueue]
                                                 usingBlock:^(NSNotification *note) {
       -[PDFToolbarButton layoutSubviews];
   }];

Please let me know if this does not help