diff -r dd5b47fb0860 -r 01b0805534bb tests/testlib/crash_transaction_late.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/testlib/crash_transaction_late.py	Fri Feb 28 03:05:52 2020 +0100
@@ -0,0 +1,32 @@
+# tiny extension to abort a transaction very late during test
+#
+# Copyright 2020 Pierre-Yves David <pierre-yves.david@octobus.net>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+from mercurial import (
+    error,
+    transaction,
+)
+
+
+def abort(fp):
+    raise error.Abort(b"This is a late abort")
+
+
+def reposetup(ui, repo):
+
+    transaction.postfinalizegenerators.add(b'late-abort')
+
+    class LateAbortRepo(repo.__class__):
+        def transaction(self, *args, **kwargs):
+            tr = super(LateAbortRepo, self).transaction(*args, **kwargs)
+            tr.addfilegenerator(
+                b'late-abort', [b'late-abort'], abort, order=9999999
+            )
+            return tr
+
+    repo.__class__ = LateAbortRepo