NSUndoManager throws NSInternalInconsistencyException in endUndoGroupRemovingIfEmpty if removeAllActions is called inside handler

Number:rdar://46395110 Date Originated:01-Dec-2018 11:46 AM
Status:Open Resolved:
Product:iOS + SDK Product Version:12.1
Classification:Crash/Hang/Data Loss Reproducible:Always
Please see attached test case. When removeAllActions is called inside an undo handler and a group is open, NSUndoManager throws "NSInternalInconsistencyException", "_endUndoGroupRemovingIfEmpty:: NSUndoManager 0x600001026620 is in invalid state, endUndoGrouping called with no matching begin

_endUndoGroupRemovingIfEmpty checks if a group needs to be closed, but does that before invoking the undoHandler, and then not checking again.

This is an example where you might quickly say "API misuse". In a real-world application with 20 layers and user-facing API, it is MUCH harder to prevent calls from various model change callbacks that don't affect the undo manager or might trigger a call to removeAllActions. It's an extremely hard to reproduce case, we took weeks with a customer to track it down to exactly this trivial case.

NSUndoManager could add an additional check for the groupingCount before trying to close a group after invoking the undo handler to fix this.

Steps to Reproduce:
Run test case, observe exception.

Expected Results:
No exception. NSUndoManager should be robust enough to deal with the call order we throw at it.

Actual Results:
NSUndoManager’s redo logic gets confused and subsequently throws.


We currently work around it with a custom NSUndoManager subclass that includes additional checks in endUndoGrouping, however this seems suboptimal.


Workaround in Swift 4.2 (call InstallWorkaroundForRdar46395110() somewhere early)



See https://github.com/PSPDFKit-labs/radar.apple.com/tree/master/46395110%20-%20NSUndoManager%20invalid%20state for sample project

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