Thanks for that. It doesn't look like there is an open bug report for this yet.

I understand your commit description but I'm still a bit puzzled by it. cursor_to_update shouldn't have a deeper stack than cursor, even if it was a cursor on a subdatabase. All the references to the subdatabase are in the subcursor, and that has no bearing on the main cursor's stack.

The original code with the bug was added for ITS#8406, commit 37081325f7356587c5e6ce4c1f36c3b303fa718c on 2016-04-18. Definitely a rare occurrence since it's uncommon to write from multiple cursors on the same DB. Fixed now in ITS#10396.