mercurial/transaction.py

transaction: add a file generation mechanism

A new `transaction.addfilegenerator` function is added. It allows external code
to register files to be generated. See inline documentation for details.

It is important to gather all file creation logic on the transaction
as at some point we'll want to mimic the "pre-transaction-commit"
logic that we use for revlog. I'm refering to the logic that lets
hooks see the result of the transaction before it actually gets
committed.

# transaction.py - simple journaling 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, 2006 Matt Mackall <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from i18n import _
import errno
import error, util

def active(func):
    def _active(self, *args, **kwds):
        if self.count == 0:
            raise error.Abort(_(
                'cannot use transaction when it is already committed/aborted'))
        return func(self, *args, **kwds)
    return _active

def _playback(journal, report, opener, entries, backupentries, unlink=True):
    for f, o, ignore in entries:
        if o or not unlink:
            try:
                fp = opener(f, 'a')
                fp.truncate(o)
                fp.close()
            except IOError:
                report(_("failed to truncate %s\n") % f)
                raise
        else:
            try:
                opener.unlink(f)
            except (IOError, OSError), inst:
                if inst.errno != errno.ENOENT:
                    raise

    backupfiles = []
    for f, b, ignore in backupentries:
        filepath = opener.join(f)
        backuppath = opener.join(b)
        try:
            util.copyfile(backuppath, filepath)
            backupfiles.append(b)
        except IOError:
            report(_("failed to recover %s\n") % f)
            raise

    opener.unlink(journal)
    backuppath = "%s.backupfiles" % journal
    if opener.exists(backuppath):
        opener.unlink(backuppath)
    for f in backupfiles:
        opener.unlink(f)

class transaction(object):
    def __init__(self, report, opener, journal, after=None, createmode=None,
            onclose=None, onabort=None):
        """Begin a new transaction

        Begins a new transaction that allows rolling back writes in the event of
        an exception.

        * `after`: called after the transaction has been committed
        * `createmode`: the mode of the journal file that will be created
        * `onclose`: called as the transaction is closing, but before it is
        closed
        * `onabort`: called as the transaction is aborting, but before any files
        have been truncated
        """
        self.count = 1
        self.usages = 1
        self.report = report
        self.opener = opener
        self.after = after
        self.onclose = onclose
        self.onabort = onabort
        self.entries = []
        self.backupentries = []
        self.map = {}
        self.backupmap = {}
        self.journal = journal
        self._queue = []
        # a dict of arguments to be passed to hooks
        self.hookargs = {}

        self.backupjournal = "%s.backupfiles" % journal
        self.file = opener.open(self.journal, "w")
        self.backupsfile = opener.open(self.backupjournal, 'w')
        if createmode is not None:
            opener.chmod(self.journal, createmode & 0666)
            opener.chmod(self.backupjournal, createmode & 0666)

        # hold file generations to be performed on commit
        self._filegenerators = {}

    def __del__(self):
        if self.journal:
            self._abort()

    @active
    def startgroup(self):
        self._queue.append(([], []))

    @active
    def endgroup(self):
        q = self._queue.pop()
        self.entries.extend(q[0])
        self.backupentries.extend(q[1])

        offsets = []
        backups = []
        for f, o, _ in q[0]:
            offsets.append((f, o))

        for f, b, _ in q[1]:
            backups.append((f, b))

        d = ''.join(['%s\0%d\n' % (f, o) for f, o in offsets])
        self.file.write(d)
        self.file.flush()

        d = ''.join(['%s\0%s\0' % (f, b) for f, b in backups])
        self.backupsfile.write(d)
        self.backupsfile.flush()

    @active
    def add(self, file, offset, data=None):
        if file in self.map or file in self.backupmap:
            return
        if self._queue:
            self._queue[-1][0].append((file, offset, data))
            return

        self.entries.append((file, offset, data))
        self.map[file] = len(self.entries) - 1
        # add enough data to the journal to do the truncate
        self.file.write("%s\0%d\n" % (file, offset))
        self.file.flush()

    @active
    def addbackup(self, file, hardlink=True):
        """Adds a backup of the file to the transaction

        Calling addbackup() creates a hardlink backup of the specified file
        that is used to recover the file in the event of the transaction
        aborting.

        * `file`: the file path, relative to .hg/store
        * `hardlink`: use a hardlink to quickly create the backup
        """

        if file in self.map or file in self.backupmap:
            return
        backupfile = "%s.backup.%s" % (self.journal, file)
        if self.opener.exists(file):
            filepath = self.opener.join(file)
            backuppath = self.opener.join(backupfile)
            util.copyfiles(filepath, backuppath, hardlink=hardlink)
        else:
            self.add(file, 0)
            return

        if self._queue:
            self._queue[-1][1].append((file, backupfile))
            return

        self.backupentries.append((file, backupfile, None))
        self.backupmap[file] = len(self.backupentries) - 1
        self.backupsfile.write("%s\0%s\0" % (file, backupfile))
        self.backupsfile.flush()

    @active
    def addfilegenerator(self, genid, filenames, genfunc, order=0):
        """add a function to generates some files at transaction commit

        The `genfunc` argument is a function capable of generating proper
        content of each entry in the `filename` tuple.

        At transaction close time, `genfunc` will be called with one file
        object argument per entries in `filenames`.

        The transaction itself is responsible for the backup, creation and
        final write of such file.

        The `genid` argument is used to ensure the same set of file is only
        generated once. Call to `addfilegenerator` for a `genid` already
        present will overwrite the old entry.

        The `order` argument may be used to control the order in which multiple
        generator will be executed.
        """
        self._filegenerators[genid] = (order, filenames, genfunc)

    @active
    def find(self, file):
        if file in self.map:
            return self.entries[self.map[file]]
        if file in self.backupmap:
            return self.backupentries[self.backupmap[file]]
        return None

    @active
    def replace(self, file, offset, data=None):
        '''
        replace can only replace already committed entries
        that are not pending in the queue
        '''

        if file not in self.map:
            raise KeyError(file)
        index = self.map[file]
        self.entries[index] = (file, offset, data)
        self.file.write("%s\0%d\n" % (file, offset))
        self.file.flush()

    @active
    def nest(self):
        self.count += 1
        self.usages += 1
        return self

    def release(self):
        if self.count > 0:
            self.usages -= 1
        # if the transaction scopes are left without being closed, fail
        if self.count > 0 and self.usages == 0:
            self._abort()

    def running(self):
        return self.count > 0

    @active
    def close(self):
        '''commit the transaction'''
        # write files registered for generation
        for order, filenames, genfunc in sorted(self._filegenerators.values()):
            files = []
            try:
                for name in filenames:
                    self.addbackup(name)
                    files.append(self.opener(name, 'w', atomictemp=True))
                genfunc(*files)
            finally:
                for f in files:
                    f.close()

        if self.count == 1 and self.onclose is not None:
            self.onclose()

        self.count -= 1
        if self.count != 0:
            return
        self.file.close()
        self.backupsfile.close()
        self.entries = []
        if self.after:
            self.after()
        if self.opener.isfile(self.journal):
            self.opener.unlink(self.journal)
        if self.opener.isfile(self.backupjournal):
            self.opener.unlink(self.backupjournal)
            for f, b, _ in self.backupentries:
                self.opener.unlink(b)
        self.backupentries = []
        self.journal = None

    @active
    def abort(self):
        '''abort the transaction (generally called on error, or when the
        transaction is not explicitly committed before going out of
        scope)'''
        self._abort()

    def _abort(self):
        self.count = 0
        self.usages = 0
        self.file.close()
        self.backupsfile.close()

        if self.onabort is not None:
            self.onabort()

        try:
            if not self.entries and not self.backupentries:
                if self.journal:
                    self.opener.unlink(self.journal)
                if self.backupjournal:
                    self.opener.unlink(self.backupjournal)
                return

            self.report(_("transaction abort!\n"))

            try:
                _playback(self.journal, self.report, self.opener,
                          self.entries, self.backupentries, False)
                self.report(_("rollback completed\n"))
            except Exception:
                self.report(_("rollback failed - please run hg recover\n"))
        finally:
            self.journal = None


def rollback(opener, file, report):
    """Rolls back the transaction contained in the given file

    Reads the entries in the specified file, and the corresponding
    '*.backupfiles' file, to recover from an incomplete transaction.

    * `file`: a file containing a list of entries, specifying where
    to truncate each file.  The file should contain a list of
    file\0offset pairs, delimited by newlines. The corresponding
    '*.backupfiles' file should contain a list of file\0backupfile
    pairs, delimited by \0.
    """
    entries = []
    backupentries = []

    fp = opener.open(file)
    lines = fp.readlines()
    fp.close()
    for l in lines:
        try:
            f, o = l.split('\0')
            entries.append((f, int(o), None))
        except ValueError:
            report(_("couldn't read journal entry %r!\n") % l)

    backupjournal = "%s.backupfiles" % file
    if opener.exists(backupjournal):
        fp = opener.open(backupjournal)
        data = fp.read()
        if len(data) > 0:
            parts = data.split('\0')
            for i in xrange(0, len(parts), 2):
                f, b = parts[i:i + 1]
                backupentries.append((f, b, None))

    _playback(file, report, opener, entries, backupentries)