# transaction.py - simple journalling scheme for mercurial

#

# This transaction scheme is intended to gracefully handle program

# errors and interruptions. More serious failures like system crashes

# can be recovered with an fsck-like tool. As the whole repository is

# effectively log-structured, this should amount to simply truncating

# anything that isn't referenced in the changelog.

#

# Copyright 2005 Matt Mackall <mpm@selenic.com>

#

# This software may be used and distributed according to the terms

# of the GNU General Public License, incorporated herein by reference.

import os

class transaction:

    def __init__(self, opener, journal):

        self.opener = opener

        self.entries = []

        self.map = {}

        self.journal = journal

        # abort here if the journal already exists

        if os.path.exists(self.journal):

            print "journal already exists, recovering"

            self.recover()

        self.file = open(self.journal, "w")

    def __del__(self):

        if self.entries: self.abort()

        try: os.unlink(self.journal)

        except: pass

    def add(self, file, offset):

        if file in self.map: return

        self.entries.append((file, offset))

        self.map[file] = 1

        # add enough data to the journal to do the truncate

        self.file.write("%s\0%d\n" % (file, offset))

        self.file.flush()

    def close(self):

        self.file.close()

        self.entries = []

        os.unlink(self.journal)

    def abort(self):

        if not self.entries: return

        print "transaction abort!"

        for f, o in self.entries:

            self.opener(f, "a").truncate(o)

        self.entries = []

        print "rollback completed"

    def recover(self):

        for l in open(self.journal).readlines():

            f, o = l.split('\0')

            self.opener(f, "a").truncate(int(o))

        os.unlink(self.journal)