transaction: support for callbacks during abort Previous transaction work added callbacks to be called during regular transaction commit/close. As part of refactoring Mozilla's pushlog extension (an extension that opens a SQLite database and tries to tie its transaction semantics to Mercurial's transaction), I discovered that the new transaction APIs were insufficient to avoid monkeypatching transaction instance internals. Adding a callback that is called during transaction abort removes the necessity for monkeypatching and completes the API.