Limnoria/src/utils/transaction.py

###
# Copyright (c) 2005, Jeremiah Fincher
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###

"""
Defines a Transaction class for multi-file transactions.
"""

import os
import shutil
import os.path

from . import error, file as File, python

# 'txn' is used as an abbreviation for 'transaction' in the following source.

class FailedAcquisition(error.Error):
    def __init__(self, txnDir, e=None):
        self.txnDir = txnDir
        msg = 'Could not acquire transaction directory: %s.' % self.txnDir
        error.Error.__init__(self, msg, e)
        
class InProgress(error.Error):
    def __init__(self, inProgress, e=None):
        self.inProgress = inProgress
        msg = 'Transaction appears to be in progress already: %s exists.' % \
              self.inProgress
        error.Error.__init__(self, msg, e)
        

class InvalidCwd(Exception):
    pass
class TransactionMixin(python.Object):
    JOURNAL = 'journal'
    ORIGINALS = 'originals'
    INPROGRESS = '.inProgress'
    REPLACEMENTS = 'replacements'
    # expects a self.dir.  used by Transaction and Rollback.
    def __init__(self, txnDir):
        self.txnDir = txnDir
        self.dir = self.txnDir + self.INPROGRESS
        self._journalName = self.dirize(self.JOURNAL)

    def escape(self, filename):
        return os.path.abspath(filename)[1:]
    
    def dirize(self, *args):
        return os.path.join(self.dir, *args)

    def _original(self, filename):
        return self.dirize(self.ORIGINALS, self.escape(filename))
    
    def _replacement(self, filename):
        return self.dirize(self.REPLACEMENTS, self.escape(filename))
    
    def _checkCwd(self):
        expected = File.contents(self.dirize('cwd'))
        if os.getcwd() != expected:
            raise InvalidCwd(expected)
        
    def _journalCommands(self):
        journal = open(self._journalName)
        for line in journal:
            line = line.rstrip('\n')
            (command, rest) = line.split(None, 1)
            args = rest.split()
            yield (command, args)
        journal.close()
        

class Transaction(TransactionMixin):
    # XXX Transaction needs to be made threadsafe.
    def __init__(self, *args, **kwargs):
        """Transaction(txnDir) -> None

        txnDir is the directory that will hold the transaction's working files
        and such.  If it can't be renamed, there is probably an active
        transaction.
        """
        TransactionMixin.__init__(self, *args, **kwargs)
        if os.path.exists(self.dir):
            raise InProgress(self.dir)
        if not os.path.exists(self.txnDir):
            raise FailedAcquisition(self.txnDir)
        try:
            os.rename(self.txnDir, self.dir)
        except EnvironmentError as e:
            raise FailedAcquisition(self.txnDir, e)
        os.mkdir(self.dirize(self.ORIGINALS))
        os.mkdir(self.dirize(self.REPLACEMENTS))
        self._journal = open(self._journalName, 'a')
        cwd = open(self.dirize('cwd'), 'w')
        cwd.write(os.getcwd())
        cwd.close()

    def _journalCommand(self, command, *args):
        File.writeLine(self._journal,
                       '%s %s' % (command, ' '.join(map(str, args))))
        self._journal.flush()

    def _makeOriginal(self, filename):
        File.copy(filename, self._original(filename))

    # XXX There needs to be a way, given a transaction, to get a
    #     "sub-transaction", which:
    #
    #     1. Doesn't try to grab the txnDir and move it, but instead is just
    #        given the actual directory being used and uses that.
    #     2. Acquires the lock of the original transaction, only releasing it
    #        when its .commit method is called (assuming Transaction is
    #        threadsafe).
    #     3. Has a no-op .commit method (i.e., doesn't commit).
    #
    #     This is so that, for instance, an object with an active Transaction
    #     can give other objects a Transaction-ish object without worrying that
    #     the transaction will be committed, while still allowing those objects
    #     to work properly with real transactions (i.e., they still call
    #     as they would on a normal Transaction, it just has no effect with a
    #     sub-transaction).
    #     The method that returns a subtransaction should be called "child."
    def child(self):
        raise NotImplementedError
    
    # XXX create, replace, etc. return file objects.  This class should keep a
    #     list of such file descriptors and only allow a commit if all of them
    #     are closed.  Trying to commit with open file objects should raise an
    #     exception.
    def create(self, filename):
        """
        Returns a file object for a filename that should be created (with
        the contents as they were written to the filename) when the transaction
        is committed.
        """
        raise NotImplementedError # XXX.

    def mkdir(self, filename):
        raise NotImplementedError # XXX

    def delete(self, filename):
        raise NotImplementedError # XXX

    def replace(self, filename):
        """
        Returns a file object for a filename that should be replaced by the
        contents written to the file object when the transaction is committed.
        """
        self._checkCwd()
        self._makeOriginal(filename)
        self._journalCommand('replace', filename)
        return File.open(self._replacement(filename))

    def append(self, filename):
        self._checkCwd()
        length = os.stat(filename).st_size
        self._journalCommand('append', filename, length)
        replacement = self._replacement(filename)
        File.copy(filename, replacement)
        return open(replacement, 'a')

    def commit(self, removeWhenComplete=True):
        self._journal.close()
        self._checkCwd()
        File.touch(self.dirize('commit'))
        for (command, args) in self._journalCommands():
            methodName = 'commit%s' % command.capitalize()
            getattr(self, methodName)(*args)
        File.touch(self.dirize('committed'))
        if removeWhenComplete:
            shutil.rmtree(self.dir)

    def commitReplace(self, filename):
        shutil.copy(self._replacement(filename), filename)

    def commitAppend(self, filename, length):
        shutil.copy(self._replacement(filename), filename)

    # XXX need to be able to rename files transactionally. (hard; especially
    #     with renames that depend on one another.  It might be easier to do
    #     rename separate from relocate.)


class Rollback(TransactionMixin):
    def rollback(self, removeWhenComplete=True):
        self._checkCwd()
        if not os.path.exists(self.dirize('commit')):
            return # No action taken; commit hadn't begun.
        for (command, args) in self._journalCommands():
            methodName = 'rollback%s' % command.capitalize()
            getattr(self, methodName)(*args)
        if removeWhenComplete:
            shutil.rmtree(self.dir)

    def rollbackReplace(self, filename):
        shutil.copy(self._original(filename), filename)

    def rollbackAppend(self, filename, length):
        fd = open(filename, 'a')
        fd.truncate(int(length))
        fd.close()
        

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