diff -r 8c7895db8955 -r becd16690cbe mercurial/merge.py
--- a/mercurial/merge.py	Wed Jan 11 16:51:37 2023 +0400
+++ b/mercurial/merge.py	Fri Jan 06 16:01:31 2023 +0000
@@ -1142,6 +1142,8 @@
             followcopies,
         )
         _checkunknownfiles(repo, wctx, mctx, force, mresult, mergeforce)
+        if repo.ui.configbool(b'devel', b'debug.abort-update'):
+            exit(1)
 
     else:  # only when merge.preferancestor=* - the default
         repo.ui.note(