# Copyright (C) 2005 by Canonical Ltd
#   Authors: Robert Collins <robert.collins@canonical.com>
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

"""This module provides a transactional facility.

Transactions provide hooks to allow data objects (i.e. inventory weaves or
the revision-history file) to be placed in a registry and retrieved later
during the same transaction. Transactions in bzr are not atomic - they
depend on data ordering of writes, so we do not have commit or rollback
facilities at the transaction level.

Read only transactions raise an assert when objects are listed as dirty
against them - preventing unintended writes.

Write transactions preserve dirty objects in the cache, though due to the
write ordering approach we use for consistency 'dirty' is a misleading term.
A dirty object is one we have modified.

Both read and write transactions *may* flush unchanged objects out of 
memory, unless they are marked as 'precious' which indicates that 
repeated reads cannot be obtained if the object is ejected, or that
the object is an expensive one for obtaining.
"""

import sys

import bzrlib.errors as errors
from bzrlib.identitymap import IdentityMap, NullIdentityMap
from bzrlib.trace import mutter


class ReadOnlyTransaction(object):
    """A read only unit of work for data objects."""

    def finish(self):
        """Clean up this transaction."""

    def __init__(self):
        super(ReadOnlyTransaction, self).__init__()
        self.map = IdentityMap()
        self._clean_objects = set()
        self._clean_queue = []
        self._limit = -1
        self._precious_objects = set()

    def is_clean(self, an_object):
        """Return True if an_object is clean."""
        return (an_object in self._clean_objects)

    def register_clean(self, an_object, precious=False):
        """Register an_object as being clean.
        
        If the precious hint is True, the object will not
        be ejected from the object identity map ever.
        """
        self._clean_objects.add(an_object)
        self._clean_queue.append(an_object)
        if precious:
            self._precious_objects.add(an_object)
        self._trim()

    def register_dirty(self, an_object):
        """Register an_object as being dirty."""
        raise errors.ReadOnlyObjectDirtiedError(an_object)

    def set_cache_size(self, size):
        """Set a new cache size."""
        assert -1 <= size
        self._limit = size
        self._trim()

    def _trim(self):
        """Trim the cache back if needed."""
        if self._limit < 0 or self._limit - len(self._clean_objects) > 0:
            return
        needed = len(self._clean_objects) - self._limit
        offset = 0
        while needed and offset < len(self._clean_objects):
            # references we know of:
            # temp passed to getrefcount in our frame
            # temp in getrefcount's frame
            # the map forward
            # the map backwards
            # _clean_objects
            # _clean_queue
            # 1 missing ?
            if (sys.getrefcount(self._clean_queue[offset]) <= 7 and
                not self._clean_queue[offset] in self._precious_objects):
                removed = self._clean_queue[offset]
                self._clean_objects.remove(removed)
                del self._clean_queue[offset]
                self.map.remove_object(removed)
                mutter('removed object %r', removed)
                needed -= 1
            else:
                offset += 1

    def writeable(self):
        """Read only transactions do not allow writes."""


class WriteTransaction(ReadOnlyTransaction):
    """A write transaction

    - caches domain objects
    - clean objects can be removed from the cache
    - dirty objects are retained.
    """

    def finish(self):
        """Clean up this transaction."""
        for thing in self._dirty_objects:
            callback = getattr(thing, 'transaction_finished', None)
            if callback is not None:
                callback()

    def __init__(self):
        super(WriteTransaction, self).__init__()
        self._dirty_objects = set()

    def is_dirty(self, an_object):
        """Return True if an_object is dirty."""
        return (an_object in self._dirty_objects)

    def register_dirty(self, an_object):
        """Register an_object as being dirty.
        
        Dirty objects are not ejected from the identity map
        until the transaction finishes and get informed
        when the transaction finishes.
        """
        self._dirty_objects.add(an_object)
        if self.is_clean(an_object):
            self._clean_objects.remove(an_object)
            del self._clean_queue[self._clean_queue.index(an_object)]
        self._trim()

    def writeable(self):
        """Write transactions allow writes."""
        return True

        
class PassThroughTransaction(object):
    """A pass through transaction
    
    - nothing is cached.
    - nothing ever gets into the identity map.
    """

    def finish(self):
        """Clean up this transaction."""
        for thing in self._dirty_objects:
            callback = getattr(thing, 'transaction_finished', None)
            if callback is not None:
                callback()

    def __init__(self):
        super(PassThroughTransaction, self).__init__()
        self.map = NullIdentityMap()
        self._dirty_objects = set()

    def register_clean(self, an_object, precious=False):
        """Register an_object as being clean.
        
        Note that precious is only a hint, and PassThroughTransaction
        ignores it.
        """

    def register_dirty(self, an_object):
        """Register an_object as being dirty.
        
        Dirty objects get informed
        when the transaction finishes.
        """
        self._dirty_objects.add(an_object)

    def set_cache_size(self, ignored):
        """Do nothing, we are passing through."""

    def writeable(self):
        """Pass through transactions allow writes."""
        return True
