File: utils.py

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"""Generally useful Python code.

But strictly no third party module dependencies.
"""

#   Copyright (C) 2011 Stephen Fairchild (s-fairchild@users.sourceforge.net)
#
#   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 in the file entitled COPYING.
#   If not, see <http://www.gnu.org/licenses/>.

from __future__ import print_function

__all__ = ["Singleton", "PolicedAttributes", "FixedAttributes",
                "PathStr", "SlotObject", "string_multireplace"]

import os
import uuid
import re
import glob
import shutil
import threading
from functools import wraps

class Singleton(type):
    """Enforce the singleton pattern upon the user class."""


    def __init__(cls, name, bases, dict_):
        super(Singleton, cls).__init__(name, bases, dict_)
        cls._instance = None


    def __call__(cls, *args, **kwds):
        if cls._instance is not None:
            # Return an existing instance.
            return cls._instance

        else:
            # No existing instance so instantiate just this once.
            cls._instance = super(Singleton, cls).__call__(*args, **kwds)
            return cls._instance



def _pa_rlock(func):
    """Policed Attributes helper for thread locking."""

    @wraps(func)
    def _wrapper(cls, *args, **kwds):
        """Wrapper with locking feature. Performs rlock."""
        
        rlock = type.__getattribute__(cls, "_rlock")

        try:
            rlock.acquire()
            return func(cls, *args, **kwds)

        finally:
            rlock.release()

    return _wrapper



class FixedAttributes(type):
    """Implements a namespace class of constants."""


    def __setattr__(cls, name, value):
        raise AttributeError("attribute is locked")


    def __call__(cls, *args, **kwds):
        raise TypeError("%s object is not callable" % cls.__name__)



class PolicedAttributes(FixedAttributes):
    """Polices data access to a namespace class.

    Prevents write access to attributes after they have been read.
    Envisioned useful for the implementation of "safe" global variables.
    """

    def __new__(mcs, name, bases, dict_):
        @classmethod
        @_pa_rlock
        def peek(cls, attr, callback, *args, **kwds):
            """Allow read + write within a callback.

            Typical use might be to append to an existing string.
            No modification ban is placed or bypassed.
            """

            if attr not in type.__getattribute__(cls, "_banned"):
                new = callback(
                        super(PolicedAttributes, cls).__getattribute__(attr),
                        *args, **kwds)
                type.__setattr__(attr, new)

            else:
                raise AttributeError("attribute is locked")

        dict_["peek"] = peek
        dict_["_banned"] = set()
        dict_["_rlock"] = threading.RLock()
        return super(PolicedAttributes, mcs).__new__(mcs, name, bases, dict_)


    @_pa_rlock
    def __getattribute__(cls, name):
        type.__getattribute__(cls, "_banned").add(name)
        return type.__getattribute__(cls, name)


    @_pa_rlock
    def __setattr__(cls, name, value):
        if name in type.__getattribute__(cls, "_banned"):
            FixedAttributes.__setattr__(cls, name, value)

        type.__setattr__(cls, name, value)



class PathStrMeta(type):
    """PathStr() returns None if called with None."""
    
    def __call__(cls, arg):
        if arg is None:
            return None
        else:
            return cls.__new__(cls, arg)



class PathStr(str):
    """A data type to perform path joins using the / operator.

    In this case the higher precedence of / is unfortunate.
    """

    __metaclass__ = PathStrMeta


    def __div__(self, other):
        return PathStr(os.path.join(str(self), other))


    def __add__(self, other):
        return PathStr(str.__add__(self, other))


    def __repr__(self):
        return "PathStr('%s')" % self



class SlotObject(object):
    """A mutable object containing an immutable object."""


    __slots__ = ['value']


    def __init__(self, value):
        self.value = value


    def __str__(self):
        return str(self.value)


    def __int__(self):
        return int(self.value)


    def __float__(self):
        return float(self.value)


    def __repr__(self):
        return "SlotObject(%s)" % repr(self.value)


    def __getattr__(self, what):
        """Universal getter for get_ prefix."""
        
        def assign(value):
            """Returned by set_ prefix call. A setter function."""
            
            self.value = value

        if what.startswith("get_"):
            return lambda : self.value

        elif what.startswith("set_"):
            return assign

        else:
            object.__getattribute__(self, what)



def string_multireplace(part, table):
    """Replace multiple items in a string.

    Table is a sequence of 2 tuples of from, to strings.
    """

    if not table:
        return part

    parts = part.split(table[0][0])
    t_next = table[1:]

    for i, each in enumerate(parts):
        parts[i] = string_multireplace(each, t_next)

    return table[0][1].join(parts)



class LinkUUIDRegistry(dict):
    """Manage substitute hard links for data files."""


    __metaclass__ = Singleton

    
    link_re = re.compile(
                    "\{[a-fA-F0-9]{8}-([a-fA-F0-9]{4}-){3}[a-fA-F0-9]{12}\}")
    link_dir = None


    def add(self, uuid_, pathname):
        if os.path.exists(pathname):
            self[uuid_] = pathname
        else:
            print("LinkUUIDRegistry: pathname does not exist", pathname)


    def remove(self, uuid_):
        try:
            del self[uuid_]
        except KeyError:
            print("LinkUUIDRegisty: remove -- UUID does not exist: {%s}" % uuid_)


    def _purge(self, where):
        """Clean orphaned hard links from the links directory."""

        basedir, dirs, files = os.walk(where).next()
        for filename in files:
            match = self.link_re.match(filename)
            try:
                if match is None or str(uuid.UUID(match.group(0))) not in self:
                    os.unlink(os.path.join(basedir, filename))
            except EnvironmentError as e:
                print("LinkUUIDRegistry: link purge failed: %s" % e)


    def _save(self, where, copy):
        """Write new hard links to the links directory.
        
        Existing links are kept as they are. To unlink them could delete the
        only copy of the link source.
        """
        
        # Create the links directory as needed.
        if not os.path.isdir(where):
            try:
                os.mkdir(where)
            except EnvironmentError as e:
                print("LinkUUIDRegistry: link directory creation failed:", e)
                return

        for uuid_, source in self.iteritems():
            ext = os.path.splitext(source)[1]
            if copy:
                cmd = shutil.copyfile
            else:
                cmd = os.link

            try:
                cmd(source, os.path.join(where, "{%s}%s" % (uuid_, ext)))
            except EnvironmentError as e:
                if e.errno != 17:
                    print("LinkUUIDRegistry: link failed:", e)
            except shutil.Error:
                pass


    def update(self, where, copy=False):
        """Update the hard links in the links directory."""
        
        self._save(where, copy)
        # Purge after save because the link source may just be in the 
        # links directory itself.
        self._purge(where)
        self.link_dir = where


    def get_link_filename(self, uuid_):
        """Check in the links directory for a specific UUID filename."""
        
        if self.link_dir is not None:
            matches = glob.glob(os.path.join(self.link_dir, "{%s}.*" % uuid_))
            if len(matches) == 1:
                return os.path.basename(matches[0])

        # Link does not exist e.g. can't hard-link across filesystems
        # or was not made due to policy.
        # For a return value of None the caller must substitute the
        # pre-existing pathname to preserve functionality.
        return None