fncache: move fncache writing to be in a transaction Previously the fncache was written at lock.release time. This meant it was not tracked by a transaction, and if an error occurred during the fncache write it would fail to update the fncache, but would not rollback the transaction, resulting in an fncache that was not in sync with the files on disk (which causes verify to fail, and causes streaming clones to not copy all the revlogs). This uses the new transaction backup mechanism to make the fncache transacted. It also moves the fncache from being written at lock.release time, to being written at transaction.close time.

transaction: add support for non-append files This adds support for normal, non-append-only files in transactions. For example, .hg/store/fncache and .hg/store/phaseroots should be written as part of the transaction, but are not append only files. This adds a journal.backupfiles along side the normal journal. This tracks which files have been backed up as part of the transaction. transaction.addbackup() creates a backup of the file (using a hardlink), which is later used to recover in the event of the transaction failing. Using a seperate journal allows the repository to still be used by older versions of Mercurial. A future patch will use this functionality and add tests for it.

transaction: add onclose/onabort hook for pre-close logic Adds an optional onclose parameter to transactions that gets called just before the transaction is committed. This allows things that build up data over the course of the transaction (like the fncache) to commit their data. Also adds onabort. It's not used, but will allow extensions to hook into onclose and onabort to provide transaction support.

clone: put streaming clones in a transaction Streaming clones were writing to files outside of a transaction. Soon the fncache will be written at transaction close time, so we need streaming clones to be in a transaction.