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#
# Copyright (c) 2001, 2002 Zope Foundation and Contributors.
# All Rights Reserved.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
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"""Persistence Interfaces
"""

from zope.interface import Interface
from zope.interface import Attribute

from persistent._compat import use_c_impl


# Allowed values for _p_state. Use the C constants if available (which
# are defined in cPersistence --- there is no cInterfaces --- so we
# need to pass the corresponding ``mod_name``), otherwise define some
# values here.
GHOST = use_c_impl(-1, 'GHOST', mod_name='persistent.persistence')
UPTODATE = use_c_impl(0, 'UPTODATE', mod_name='persistent.persistence')
CHANGED = use_c_impl(1, 'CHANGED', mod_name='persistent.persistence')
STICKY = use_c_impl(2, 'STICKY', mod_name='persistent.persistence')


OID_TYPE = SERIAL_TYPE = bytes

class IPersistent(Interface):
    """Python persistent interface

    A persistent object can be in one of several states:

    - Unsaved

      The object has been created but not saved in a data manager.

      In this state, the _p_changed attribute is non-None and false
      and the _p_jar attribute is None.

    - Saved

      The object has been saved and has not been changed since it was saved.

      In this state, the _p_changed attribute is non-None and false
      and the _p_jar attribute is set to a data manager.

    - Sticky

      This state is identical to the saved state except that the
      object cannot transition to the ghost state.  This is a special
      state used by C methods of persistent objects to make sure that
      state is not unloaded in the middle of computation.

      In this state, the _p_changed attribute is non-None and false
      and the _p_jar attribute is set to a data manager.

      There is no Python API for detecting whether an object is in the
      sticky state.

    - Changed

      The object has been changed.

      In this state, the _p_changed attribute is true
      and the _p_jar attribute is set to a data manager.

    - Ghost

      the object is in memory but its state has not been loaded from
      the database (or its state has been unloaded).  In this state,
      the object doesn't contain any application data.

      In this state, the _p_changed attribute is None, and the _p_jar
      attribute is set to the data manager from which the object was
      obtained.

    In all the above, _p_oid (the persistent object id) is set when
    _p_jar first gets set.

    The following state transitions are possible:

    - Unsaved -> Saved

      This transition occurs when an object is saved in the
      database.  This usually happens when an unsaved object is added
      to (e.g. as an attribute or item of) a saved (or changed) object
      and the transaction is committed.

    - Saved  -> Changed
      Sticky -> Changed
      Ghost  -> Changed

      This transition occurs when someone sets an attribute or sets
      _p_changed to a true value on a saved, sticky or ghost object.  When
      the transition occurs, the persistent object is required to call the
      register() method on its data manager, passing itself as the
      only argument.

      Prior to ZODB 3.6, setting _p_changed to a true value on a ghost object
      was ignored (the object remained a ghost, and getting its _p_changed
      attribute continued to return None).

    - Saved -> Sticky

      This transition occurs when C code marks the object as sticky to
      prevent its deactivation.

    - Saved -> Ghost

      This transition occurs when a saved object is deactivated or
      invalidated.  See discussion below.

    - Sticky -> Saved

      This transition occurs when C code unmarks the object as sticky to
      allow its deactivation.

    - Changed -> Saved

      This transition occurs when a transaction is committed.  After
      saving the state of a changed object during transaction commit,
      the data manager sets the object's _p_changed to a non-None false
      value.

    - Changed -> Ghost

      This transition occurs when a transaction is aborted.  All changed
      objects are invalidated by the data manager by an abort.

    - Ghost -> Saved

      This transition occurs when an attribute or operation of a ghost
      is accessed and the object's state is loaded from the database.

    Note that there is a separate C API that is not included here.
    The C API requires a specific data layout and defines the sticky
    state.


    About Invalidation, Deactivation and the Sticky & Ghost States

    The sticky state is intended to be a short-lived state, to prevent
    an object's state from being discarded while we're in C routines.  It
    is an error to invalidate an object in the sticky state.

    Deactivation is a request that an object discard its state (become
    a ghost).  Deactivation is an optimization, and a request to
    deactivate may be ignored.  There are two equivalent ways to
    request deactivation:

          - call _p_deactivate()
          - set _p_changed to None

    There are two ways to invalidate an object:  call the
    _p_invalidate() method (preferred) or delete its _p_changed
    attribute.  This cannot be ignored, and is used when semantics
    require invalidation.  Normally, an invalidated object transitions
    to the ghost state.  However, some objects cannot be ghosts.  When
    these objects are invalidated, they immediately reload their state
    from their data manager, and are then in the saved state.

    reprs

    By default, persistent objects include the reprs of their
    _p_oid and _p_jar, if any, in their repr. If a subclass implements
    the optional method ``_p_repr``, it will be called and its results returned
    instead of the default repr; if this method raises an exception, that
    exception will be caught and its repr included in the default repr.

    """

    _p_jar = Attribute(
        """The data manager for the object.

        The data manager should implement IPersistentDataManager (note that
        this constraint is not enforced).

        If there is no data manager, then this is None.

        Once assigned to a data manager, an object cannot be re-assigned
        to another.
        """)

    _p_oid = Attribute(
        """The object id.

        It is up to the data manager to assign this.

        The special value None is reserved to indicate that an object
        id has not been assigned.  Non-None object ids must be non-empty
        strings.  The 8-byte string consisting of 8 NUL bytes
        ('\x00\x00\x00\x00\x00\x00\x00\x00') is reserved to identify the
        database root object.

        Once assigned an OID, an object cannot be re-assigned another.
        """)

    _p_changed = Attribute(
        """The persistent state of the object.

        This is one of:

        None -- The object is a ghost.

        false but not None -- The object is saved (or has never been saved).

        true -- The object has been modified since it was last saved.

        The object state may be changed by assigning or deleting this
        attribute; however, assigning None is ignored if the object is
        not in the saved state, and may be ignored even if the object is
        in the saved state.

        At and after ZODB 3.6, setting _p_changed to a true value for a ghost
        object activates the object; prior to 3.6, setting _p_changed to a
        true value on a ghost object was ignored.

        Note that an object can transition to the changed state only if
        it has a data manager.  When such a state change occurs, the
        'register' method of the data manager must be called, passing the
        persistent object.

        Deleting this attribute forces invalidation independent of
        existing state, although it is an error if the sticky state is
        current.
        """)

    _p_serial = Attribute(
        """The object serial number.

        This member is used by the data manager to distiguish distinct
        revisions of a given persistent object.

        This is an 8-byte string (not Unicode).
        """)

    _p_mtime = Attribute(
        """The object's modification time (read-only).

        This is a float, representing seconds since the epoch (as returned
        by time.time).
        """)

    _p_state = Attribute(
        """The object's persistence state token.

        Must be one of GHOST, UPTODATE, CHANGED, or STICKY.
        """)

    _p_estimated_size = Attribute(
        """An estimate of the object's size in bytes.

        May be set by the data manager.
        """)

    # Attribute access protocol
    def __getattribute__(name):
        """ Handle activating ghosts before returning an attribute value.

        "Special" attributes and '_p_*' attributes don't require activation.
        """

    def __setattr__(name, value):
        """ Handle activating ghosts before setting an attribute value.

        "Special" attributes and '_p_*' attributes don't require activation.
        """

    def __delattr__(name):
        """ Handle activating ghosts before deleting an attribute value.

        "Special" attributes and '_p_*' attributes don't require activation.
        """

    # Pickling protocol.
    def __getstate__():
        """Get the object data.

        The state should not include persistent attributes ("_p_name").
        The result must be picklable.
        """

    def __setstate__(state):
        """Set the object data.
        """

    def __reduce__():
        """Reduce an object to contituent parts for serialization.
        """

    # Custom methods
    def _p_activate():
        """Activate the object.

        Change the object to the saved state if it is a ghost.
        """

    def _p_deactivate():
        """Deactivate the object.

        Possibly change an object in the saved state to the
        ghost state.  It may not be possible to make some persistent
        objects ghosts, and, for optimization reasons, the implementation
        may choose to keep an object in the saved state.
        """

    def _p_invalidate():
        """Invalidate the object.

        Invalidate the object.  This causes any data to be thrown
        away, even if the object is in the changed state.  The object
        is moved to the ghost state; further accesses will cause
        object data to be reloaded.
        """

    def _p_getattr(name):
        """Test whether the base class must handle the name

        The method unghostifies the object, if necessary.
        The method records the object access, if necessary.

        This method should be called by subclass __getattribute__
        implementations before doing anything else. If the method
        returns True, then __getattribute__ implementations must delegate
        to the base class, Persistent.
        """

    def _p_setattr(name, value):
        """Save persistent meta data

        This method should be called by subclass __setattr__ implementations
        before doing anything else.  If it returns true, then the attribute
        was handled by the base class.

        The method unghostifies the object, if necessary.
        The method records the object access, if necessary.
        """