cpython/Lib/tarfile.py

#!/usr/bin/env python
# -*- coding: iso-8859-1 -*-
#-------------------------------------------------------------------
# tarfile.py
#-------------------------------------------------------------------
# Copyright (C) 2002 Lars Gust<73>bel <lars@gustaebel.de>
# All rights reserved.
#
# Permission  is  hereby granted,  free  of charge,  to  any person
# obtaining a  copy of  this software  and associated documentation
# files  (the  "Software"),  to   deal  in  the  Software   without
# restriction,  including  without limitation  the  rights to  use,
# copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies  of  the  Software,  and to  permit  persons  to  whom the
# Software  is  furnished  to  do  so,  subject  to  the  following
# conditions:
#
# The above copyright  notice and this  permission notice shall  be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS  IS", WITHOUT WARRANTY OF ANY  KIND,
# EXPRESS OR IMPLIED, INCLUDING  BUT NOT LIMITED TO  THE WARRANTIES
# OF  MERCHANTABILITY,  FITNESS   FOR  A  PARTICULAR   PURPOSE  AND
# NONINFRINGEMENT.  IN  NO  EVENT SHALL  THE  AUTHORS  OR COPYRIGHT
# HOLDERS  BE LIABLE  FOR ANY  CLAIM, DAMAGES  OR OTHER  LIABILITY,
# WHETHER  IN AN  ACTION OF  CONTRACT, TORT  OR OTHERWISE,  ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
# OTHER DEALINGS IN THE SOFTWARE.
#
"""Read from and write to tar format archives.
"""

__version__ = "$Revision$"
# $Source$

version     = "0.6.4"
__author__  = "Lars Gust<73>bel (lars@gustaebel.de)"
__date__    = "$Date$"
__cvsid__   = "$Id$"
__credits__ = "Gustavo Niemeyer, Niels Gust<73>bel, Richard Townsend."

#---------
# Imports
#---------
import sys
import os
import shutil
import stat
import errno
import time
import struct

if sys.platform == 'mac':
    # This module needs work for MacOS9, especially in the area of pathname
    # handling. In many places it is assumed a simple substitution of / by the
    # local os.path.sep is good enough to convert pathnames, but this does not
    # work with the mac rooted:path:name versus :nonrooted:path:name syntax
    raise ImportError, "tarfile does not work for platform==mac"

try:
    import grp, pwd
except ImportError:
    grp = pwd = None

# from tarfile import *
__all__ = ["TarFile", "TarInfo", "is_tarfile", "TarError"]

#---------------------------------------------------------
# tar constants
#---------------------------------------------------------
NUL        = "\0"               # the null character
BLOCKSIZE  = 512                # length of processing blocks
RECORDSIZE = BLOCKSIZE * 20     # length of records
MAGIC      = "ustar"            # magic tar string
VERSION    = "00"               # version number

LENGTH_NAME    = 100            # maximum length of a filename
LENGTH_LINK    = 100            # maximum length of a linkname
LENGTH_PREFIX  = 155            # maximum length of the prefix field
MAXSIZE_MEMBER = 077777777777L  # maximum size of a file (11 octal digits)

REGTYPE  = "0"                  # regular file
AREGTYPE = "\0"                 # regular file
LNKTYPE  = "1"                  # link (inside tarfile)
SYMTYPE  = "2"                  # symbolic link
CHRTYPE  = "3"                  # character special device
BLKTYPE  = "4"                  # block special device
DIRTYPE  = "5"                  # directory
FIFOTYPE = "6"                  # fifo special device
CONTTYPE = "7"                  # contiguous file

GNUTYPE_LONGNAME = "L"          # GNU tar extension for longnames
GNUTYPE_LONGLINK = "K"          # GNU tar extension for longlink
GNUTYPE_SPARSE   = "S"          # GNU tar extension for sparse file

#---------------------------------------------------------
# tarfile constants
#---------------------------------------------------------
SUPPORTED_TYPES = (REGTYPE, AREGTYPE, LNKTYPE,  # file types that tarfile
                   SYMTYPE, DIRTYPE, FIFOTYPE,  # can cope with.
                   CONTTYPE, CHRTYPE, BLKTYPE,
                   GNUTYPE_LONGNAME, GNUTYPE_LONGLINK,
                   GNUTYPE_SPARSE)

REGULAR_TYPES = (REGTYPE, AREGTYPE,             # file types that somehow
                 CONTTYPE, GNUTYPE_SPARSE)      # represent regular files

#---------------------------------------------------------
# Bits used in the mode field, values in octal.
#---------------------------------------------------------
S_IFLNK = 0120000        # symbolic link
S_IFREG = 0100000        # regular file
S_IFBLK = 0060000        # block device
S_IFDIR = 0040000        # directory
S_IFCHR = 0020000        # character device
S_IFIFO = 0010000        # fifo

TSUID   = 04000          # set UID on execution
TSGID   = 02000          # set GID on execution
TSVTX   = 01000          # reserved

TUREAD  = 0400           # read by owner
TUWRITE = 0200           # write by owner
TUEXEC  = 0100           # execute/search by owner
TGREAD  = 0040           # read by group
TGWRITE = 0020           # write by group
TGEXEC  = 0010           # execute/search by group
TOREAD  = 0004           # read by other
TOWRITE = 0002           # write by other
TOEXEC  = 0001           # execute/search by other

#---------------------------------------------------------
# Some useful functions
#---------------------------------------------------------
def nts(s):
    """Convert a null-terminated string buffer to a python string.
    """
    return s.rstrip(NUL)

def calc_chksum(buf):
    """Calculate the checksum for a member's header. It's a simple addition
       of all bytes, treating the chksum field as if filled with spaces.
       buf is a 512 byte long string buffer which holds the header.
    """
    chk = 256                           # chksum field is treated as blanks,
                                        # so the initial value is 8 * ord(" ")
    for c in buf[:148]: chk += ord(c)   # sum up all bytes before chksum
    for c in buf[156:]: chk += ord(c)   # sum up all bytes after chksum
    return chk

def copyfileobj(src, dst, length=None):
    """Copy length bytes from fileobj src to fileobj dst.
       If length is None, copy the entire content.
    """
    if length == 0:
        return
    if length is None:
        shutil.copyfileobj(src, dst)
        return

    BUFSIZE = 16 * 1024
    blocks, remainder = divmod(length, BUFSIZE)
    for b in xrange(blocks):
        buf = src.read(BUFSIZE)
        if len(buf) < BUFSIZE:
            raise IOError, "end of file reached"
        dst.write(buf)

    if remainder != 0:
        buf = src.read(remainder)
        if len(buf) < remainder:
            raise IOError, "end of file reached"
        dst.write(buf)
    return

filemode_table = (
    ((S_IFLNK,      "l"),
     (S_IFREG,      "-"),
     (S_IFBLK,      "b"),
     (S_IFDIR,      "d"),
     (S_IFCHR,      "c"),
     (S_IFIFO,      "p")),

    ((TUREAD,       "r"),),
    ((TUWRITE,      "w"),),
    ((TUEXEC|TSUID, "s"),
     (TSUID,        "S"),
     (TUEXEC,       "x")),

    ((TGREAD,       "r"),),
    ((TGWRITE,      "w"),),
    ((TGEXEC|TSGID, "s"),
     (TSGID,        "S"),
     (TGEXEC,       "x")),

    ((TOREAD,       "r"),),
    ((TOWRITE,      "w"),),
    ((TOEXEC|TSVTX, "t"),
     (TSVTX,        "T"),
     (TOEXEC,       "x"))
)

def filemode(mode):
    """Convert a file's mode to a string of the form
       -rwxrwxrwx.
       Used by TarFile.list()
    """
    perm = []
    for table in filemode_table:
        for bit, char in table:
            if mode & bit == bit:
                perm.append(char)
                break
        else:
            perm.append("-")
    return "".join(perm)

if os.sep != "/":
    normpath = lambda path: os.path.normpath(path).replace(os.sep, "/")
else:
    normpath = os.path.normpath

class TarError(Exception):
    """Base exception."""
    pass
class ExtractError(TarError):
    """General exception for extract errors."""
    pass
class ReadError(TarError):
    """Exception for unreadble tar archives."""
    pass
class CompressionError(TarError):
    """Exception for unavailable compression methods."""
    pass
class StreamError(TarError):
    """Exception for unsupported operations on stream-like TarFiles."""
    pass

#---------------------------
# internal stream interface
#---------------------------
class _LowLevelFile:
    """Low-level file object. Supports reading and writing.
       It is used instead of a regular file object for streaming
       access.
    """

    def __init__(self, name, mode):
        mode = {
            "r": os.O_RDONLY,
            "w": os.O_WRONLY | os.O_CREAT | os.O_TRUNC,
        }[mode]
        if hasattr(os, "O_BINARY"):
            mode |= os.O_BINARY
        self.fd = os.open(name, mode)

    def close(self):
        os.close(self.fd)

    def read(self, size):
        return os.read(self.fd, size)

    def write(self, s):
        os.write(self.fd, s)

class _Stream:
    """Class that serves as an adapter between TarFile and
       a stream-like object.  The stream-like object only
       needs to have a read() or write() method and is accessed
       blockwise.  Use of gzip or bzip2 compression is possible.
       A stream-like object could be for example: sys.stdin,
       sys.stdout, a socket, a tape device etc.

       _Stream is intended to be used only internally.
    """

    def __init__(self, name, mode, type, fileobj, bufsize):
        """Construct a _Stream object.
        """
        self._extfileobj = True
        if fileobj is None:
            fileobj = _LowLevelFile(name, mode)
            self._extfileobj = False

        self.name    = name or ""
        self.mode    = mode
        self.type    = type
        self.fileobj = fileobj
        self.bufsize = bufsize
        self.buf     = ""
        self.pos     = 0L
        self.closed  = False

        if type == "gz":
            try:
                import zlib
            except ImportError:
                raise CompressionError, "zlib module is not available"
            self.zlib = zlib
            self.crc = zlib.crc32("")
            if mode == "r":
                self._init_read_gz()
            else:
                self._init_write_gz()

        if type == "bz2":
            try:
                import bz2
            except ImportError:
                raise CompressionError, "bz2 module is not available"
            if mode == "r":
                self.dbuf = ""
                self.cmp = bz2.BZ2Decompressor()
            else:
                self.cmp = bz2.BZ2Compressor()

    def __del__(self):
        if not self.closed:
            self.close()

    def _init_write_gz(self):
        """Initialize for writing with gzip compression.
        """
        self.cmp = self.zlib.compressobj(9, self.zlib.DEFLATED,
                                            -self.zlib.MAX_WBITS,
                                            self.zlib.DEF_MEM_LEVEL,
                                            0)
        timestamp = struct.pack("<L", long(time.time()))
        self.__write("\037\213\010\010%s\002\377" % timestamp)
        if self.name.endswith(".gz"):
            self.name = self.name[:-3]
        self.__write(self.name + NUL)

    def write(self, s):
        """Write string s to the stream.
        """
        if self.type == "gz":
            self.crc = self.zlib.crc32(s, self.crc)
        self.pos += len(s)
        if self.type != "tar":
            s = self.cmp.compress(s)
        self.__write(s)

    def __write(self, s):
        """Write string s to the stream if a whole new block
           is ready to be written.
        """
        self.buf += s
        while len(self.buf) > self.bufsize:
            self.fileobj.write(self.buf[:self.bufsize])
            self.buf = self.buf[self.bufsize:]

    def close(self):
        """Close the _Stream object. No operation should be
           done on it afterwards.
        """
        if self.closed:
            return

        if self.mode == "w" and self.type != "tar":
            self.buf += self.cmp.flush()
        if self.mode == "w" and self.buf:
            self.fileobj.write(self.buf)
            self.buf = ""
            if self.type == "gz":
                self.fileobj.write(struct.pack("<l", self.crc))
                self.fileobj.write(struct.pack("<L", self.pos & 0xffffFFFFL))

        if not self._extfileobj:
            self.fileobj.close()

        self.closed = True

    def _init_read_gz(self):
        """Initialize for reading a gzip compressed fileobj.
        """
        self.cmp = self.zlib.decompressobj(-self.zlib.MAX_WBITS)
        self.dbuf = ""

        # taken from gzip.GzipFile with some alterations
        if self.__read(2) != "\037\213":
            raise ReadError, "not a gzip file"
        if self.__read(1) != "\010":
            raise CompressionError, "unsupported compression method"

        flag = ord(self.__read(1))
        self.__read(6)

        if flag & 4:
            xlen = ord(self.__read(1)) + 256 * ord(self.__read(1))
            self.read(xlen)
        if flag & 8:
            while True:
                s = self.__read(1)
                if not s or s == NUL:
                    break
        if flag & 16:
            while True:
                s = self.__read(1)
                if not s or s == NUL:
                    break
        if flag & 2:
            self.__read(2)

    def tell(self):
        """Return the stream's file pointer position.
        """
        return self.pos

    def seek(self, pos=0):
        """Set the stream's file pointer to pos. Negative seeking
           is forbidden.
        """
        if pos - self.pos >= 0:
            blocks, remainder = divmod(pos - self.pos, self.bufsize)
            for i in xrange(blocks):
                self.read(self.bufsize)
            self.read(remainder)
        else:
            raise StreamError, "seeking backwards is not allowed"
        return self.pos

    def read(self, size=None):
        """Return the next size number of bytes from the stream.
           If size is not defined, return all bytes of the stream
           up to EOF.
        """
        if size is None:
            t = []
            while True:
                buf = self._read(self.bufsize)
                if not buf:
                    break
                t.append(buf)
            buf = "".join(t)
        else:
            buf = self._read(size)
        self.pos += len(buf)
        return buf

    def _read(self, size):
        """Return size bytes from the stream.
        """
        if self.type == "tar":
            return self.__read(size)

        c = len(self.dbuf)
        t = [self.dbuf]
        while c < size:
            buf = self.__read(self.bufsize)
            if not buf:
                break
            buf = self.cmp.decompress(buf)
            t.append(buf)
            c += len(buf)
        t = "".join(t)
        self.dbuf = t[size:]
        return t[:size]

    def __read(self, size):
        """Return size bytes from stream. If internal buffer is empty,
           read another block from the stream.
        """
        c = len(self.buf)
        t = [self.buf]
        while c < size:
            buf = self.fileobj.read(self.bufsize)
            if not buf:
                break
            t.append(buf)
            c += len(buf)
        t = "".join(t)
        self.buf = t[size:]
        return t[:size]
# class _Stream

#------------------------
# Extraction file object
#------------------------
class ExFileObject(object):
    """File-like object for reading an archive member.
       Is returned by TarFile.extractfile(). Support for
       sparse files included.
    """

    def __init__(self, tarfile, tarinfo):
        self.fileobj = tarfile.fileobj
        self.name    = tarinfo.name
        self.mode    = "r"
        self.closed  = False
        self.offset  = tarinfo.offset_data
        self.size    = tarinfo.size
        self.pos     = 0L
        self.linebuffer = ""
        if tarinfo.issparse():
            self.sparse = tarinfo.sparse
            self.read = self._readsparse
        else:
            self.read = self._readnormal

    def __read(self, size):
        """Overloadable read method.
        """
        return self.fileobj.read(size)

    def readline(self, size=-1):
        """Read a line with approx. size. If size is negative,
           read a whole line. readline() and read() must not
           be mixed up (!).
        """
        if size < 0:
            size = sys.maxint

        nl = self.linebuffer.find("\n")
        if nl >= 0:
            nl = min(nl, size)
        else:
            size -= len(self.linebuffer)
            while (nl < 0 and size > 0):
                buf = self.read(min(size, 100))
                if not buf:
                    break
                self.linebuffer += buf
                size -= len(buf)
                nl = self.linebuffer.find("\n")
            if nl == -1:
                s = self.linebuffer
                self.linebuffer = ""
                return s
        buf = self.linebuffer[:nl]
        self.linebuffer = self.linebuffer[nl + 1:]
        while buf[-1:] == "\r":
            buf = buf[:-1]
        return buf + "\n"

    def readlines(self):
        """Return a list with all (following) lines.
        """
        result = []
        while True:
            line = self.readline()
            if not line: break
            result.append(line)
        return result

    def _readnormal(self, size=None):
        """Read operation for regular files.
        """
        if self.closed:
            raise ValueError, "file is closed"
        self.fileobj.seek(self.offset + self.pos)
        bytesleft = self.size - self.pos
        if size is None:
            bytestoread = bytesleft
        else:
            bytestoread = min(size, bytesleft)
        self.pos += bytestoread
        return self.__read(bytestoread)

    def _readsparse(self, size=None):
        """Read operation for sparse files.
        """
        if self.closed:
            raise ValueError, "file is closed"

        if size is None:
            size = self.size - self.pos

        data = []
        while size > 0:
            buf = self._readsparsesection(size)
            if not buf:
                break
            size -= len(buf)
            data.append(buf)
        return "".join(data)

    def _readsparsesection(self, size):
        """Read a single section of a sparse file.
        """
        section = self.sparse.find(self.pos)

        if section is None:
            return ""

        toread = min(size, section.offset + section.size - self.pos)
        if isinstance(section, _data):
            realpos = section.realpos + self.pos - section.offset
            self.pos += toread
            self.fileobj.seek(self.offset + realpos)
            return self.__read(toread)
        else:
            self.pos += toread
            return NUL * toread

    def tell(self):
        """Return the current file position.
        """
        return self.pos

    def seek(self, pos, whence=0):
        """Seek to a position in the file.
        """
        self.linebuffer = ""
        if whence == 0:
            self.pos = min(max(pos, 0), self.size)
        if whence == 1:
            if pos < 0:
                self.pos = max(self.pos + pos, 0)
            else:
                self.pos = min(self.pos + pos, self.size)
        if whence == 2:
            self.pos = max(min(self.size + pos, self.size), 0)

    def close(self):
        """Close the file object.
        """
        self.closed = True
#class ExFileObject

#------------------
# Exported Classes
#------------------
class TarInfo(object):
    """Informational class which holds the details about an
       archive member given by a tar header block.
       TarInfo objects are returned by TarFile.getmember(),
       TarFile.getmembers() and TarFile.gettarinfo() and are
       usually created internally.
    """

    def __init__(self, name=""):
        """Construct a TarInfo object. name is the optional name
           of the member.
        """

        self.name     = name       # member name (dirnames must end with '/')
        self.mode     = 0666       # file permissions
        self.uid      = 0          # user id
        self.gid      = 0          # group id
        self.size     = 0          # file size
        self.mtime    = 0          # modification time
        self.chksum   = 0          # header checksum
        self.type     = REGTYPE    # member type
        self.linkname = ""         # link name
        self.uname    = "user"     # user name
        self.gname    = "group"    # group name
        self.devmajor = 0          #-
        self.devminor = 0          #-for use with CHRTYPE and BLKTYPE
        self.prefix   = ""         # prefix to filename or holding information
                                   # about sparse files

        self.offset   = 0          # the tar header starts here
        self.offset_data = 0       # the file's data starts here

    def __repr__(self):
        return "<%s %r at %#x>" % (self.__class__.__name__,self.name,id(self))

    @classmethod
    def frombuf(cls, buf):
        """Construct a TarInfo object from a 512 byte string buffer.
        """
        tarinfo = cls()
        tarinfo.name   = nts(buf[0:100])
        tarinfo.mode   = int(buf[100:108], 8)
        tarinfo.uid    = int(buf[108:116],8)
        tarinfo.gid    = int(buf[116:124],8)

        # There are two possible codings for the size field we
        # have to discriminate, see comment in tobuf() below.
        if buf[124] != chr(0200):
            tarinfo.size = long(buf[124:136], 8)
        else:
            tarinfo.size = 0L
            for i in range(11):
                tarinfo.size <<= 8
                tarinfo.size += ord(buf[125 + i])

        tarinfo.mtime  = long(buf[136:148], 8)
        tarinfo.chksum = int(buf[148:156], 8)
        tarinfo.type   = buf[156:157]
        tarinfo.linkname = nts(buf[157:257])
        tarinfo.uname  = nts(buf[265:297])
        tarinfo.gname  = nts(buf[297:329])
        try:
            tarinfo.devmajor = int(buf[329:337], 8)
            tarinfo.devminor = int(buf[337:345], 8)
        except ValueError:
            tarinfo.devmajor = tarinfo.devmajor = 0
        tarinfo.prefix = buf[345:500]

        # The prefix field is used for filenames > 100 in
        # the POSIX standard.
        # name = prefix + '/' + name
        if tarinfo.type != GNUTYPE_SPARSE:
            tarinfo.name = normpath(os.path.join(nts(tarinfo.prefix), tarinfo.name))

        # Directory names should have a '/' at the end.
        if tarinfo.isdir() and tarinfo.name[-1:] != "/":
            tarinfo.name += "/"
        return tarinfo

    def tobuf(self):
        """Return a tar header block as a 512 byte string.
        """
        # Prefer the size to be encoded as 11 octal ascii digits
        # which is the most portable. If the size exceeds this
        # limit (>= 8 GB), encode it as an 88-bit value which is
        # a GNU tar feature.
        if self.size <= MAXSIZE_MEMBER:
            size = "%011o" % self.size
        else:
            s = self.size
            size = ""
            for i in range(11):
                size = chr(s & 0377) + size
                s >>= 8
            size = chr(0200) + size

        # The following code was contributed by Detlef Lannert.
        parts = []
        for value, fieldsize in (
                (self.name, 100),
                ("%07o" % (self.mode & 07777), 8),
                ("%07o" % self.uid, 8),
                ("%07o" % self.gid, 8),
                (size, 12),
                ("%011o" % self.mtime, 12),
                ("        ", 8),
                (self.type, 1),
                (self.linkname, 100),
                (MAGIC, 6),
                (VERSION, 2),
                (self.uname, 32),
                (self.gname, 32),
                ("%07o" % self.devmajor, 8),
                ("%07o" % self.devminor, 8),
                (self.prefix, 155)
            ):
            l = len(value)
            parts.append(value[:fieldsize] + (fieldsize - l) * NUL)

        buf = "".join(parts)
        chksum = calc_chksum(buf)
        buf = buf[:148] + "%06o\0" % chksum + buf[155:]
        buf += (BLOCKSIZE - len(buf)) * NUL
        self.buf = buf
        return buf

    def isreg(self):
        return self.type in REGULAR_TYPES
    def isfile(self):
        return self.isreg()
    def isdir(self):
        return self.type == DIRTYPE
    def issym(self):
        return self.type == SYMTYPE
    def islnk(self):
        return self.type == LNKTYPE
    def ischr(self):
        return self.type == CHRTYPE
    def isblk(self):
        return self.type == BLKTYPE
    def isfifo(self):
        return self.type == FIFOTYPE
    def issparse(self):
        return self.type == GNUTYPE_SPARSE
    def isdev(self):
        return self.type in (CHRTYPE, BLKTYPE, FIFOTYPE)
# class TarInfo

class TarFile(object):
    """The TarFile Class provides an interface to tar archives.
    """

    debug = 0                   # May be set from 0 (no msgs) to 3 (all msgs)

    dereference = False         # If true, add content of linked file to the
                                # tar file, else the link.

    ignore_zeros = False        # If true, skips empty or invalid blocks and
                                # continues processing.

    errorlevel = 0              # If 0, fatal errors only appear in debug
                                # messages (if debug >= 0). If > 0, errors
                                # are passed to the caller as exceptions.

    posix = False               # If True, generates POSIX.1-1990-compliant
                                # archives (no GNU extensions!)

    fileobject = ExFileObject

    def __init__(self, name=None, mode="r", fileobj=None):
        """Open an (uncompressed) tar archive `name'. `mode' is either 'r' to
           read from an existing archive, 'a' to append data to an existing
           file or 'w' to create a new file overwriting an existing one. `mode'
           defaults to 'r'.
           If `fileobj' is given, it is used for reading or writing data. If it
           can be determined, `mode' is overridden by `fileobj's mode.
           `fileobj' is not closed, when TarFile is closed.
        """
        self.name = name

        if len(mode) > 1 or mode not in "raw":
            raise ValueError, "mode must be 'r', 'a' or 'w'"
        self._mode = mode
        self.mode = {"r": "rb", "a": "r+b", "w": "wb"}[mode]

        if not fileobj:
            fileobj = file(self.name, self.mode)
            self._extfileobj = False
        else:
            if self.name is None and hasattr(fileobj, "name"):
                self.name = fileobj.name
            if hasattr(fileobj, "mode"):
                self.mode = fileobj.mode
            self._extfileobj = True
        self.fileobj = fileobj

        # Init datastructures
        self.closed      = False
        self.members     = []       # list of members as TarInfo objects
        self._loaded     = False    # flag if all members have been read
        self.offset      = 0L       # current position in the archive file
        self.inodes      = {}       # dictionary caching the inodes of
                                    # archive members already added

        if self._mode == "r":
            self.firstmember = None
            self.firstmember = self.next()

        if self._mode == "a":
            # Move to the end of the archive,
            # before the first empty block.
            self.firstmember = None
            while True:
                try:
                    tarinfo = self.next()
                except ReadError:
                    self.fileobj.seek(0)
                    break
                if tarinfo is None:
                    self.fileobj.seek(- BLOCKSIZE, 1)
                    break

        if self._mode in "aw":
            self._loaded = True

    #--------------------------------------------------------------------------
    # Below are the classmethods which act as alternate constructors to the
    # TarFile class. The open() method is the only one that is needed for
    # public use; it is the "super"-constructor and is able to select an
    # adequate "sub"-constructor for a particular compression using the mapping
    # from OPEN_METH.
    #
    # This concept allows one to subclass TarFile without losing the comfort of
    # the super-constructor. A sub-constructor is registered and made available
    # by adding it to the mapping in OPEN_METH.

    @classmethod
    def open(cls, name=None, mode="r", fileobj=None, bufsize=20*512):
        """Open a tar archive for reading, writing or appending. Return
           an appropriate TarFile class.

           mode:
           'r'          open for reading with transparent compression
           'r:'         open for reading exclusively uncompressed
           'r:gz'       open for reading with gzip compression
           'r:bz2'      open for reading with bzip2 compression
           'a' or 'a:'  open for appending
           'w' or 'w:'  open for writing without compression
           'w:gz'       open for writing with gzip compression
           'w:bz2'      open for writing with bzip2 compression
           'r|'         open an uncompressed stream of tar blocks for reading
           'r|gz'       open a gzip compressed stream of tar blocks
           'r|bz2'      open a bzip2 compressed stream of tar blocks
           'w|'         open an uncompressed stream for writing
           'w|gz'       open a gzip compressed stream for writing
           'w|bz2'      open a bzip2 compressed stream for writing
        """

        if not name and not fileobj:
            raise ValueError, "nothing to open"

        if ":" in mode:
            filemode, comptype = mode.split(":", 1)
            filemode = filemode or "r"
            comptype = comptype or "tar"

            # Select the *open() function according to
            # given compression.
            if comptype in cls.OPEN_METH:
                func = getattr(cls, cls.OPEN_METH[comptype])
            else:
                raise CompressionError, "unknown compression type %r" % comptype
            return func(name, filemode, fileobj)

        elif "|" in mode:
            filemode, comptype = mode.split("|", 1)
            filemode = filemode or "r"
            comptype = comptype or "tar"

            if filemode not in "rw":
                raise ValueError, "mode must be 'r' or 'w'"

            t = cls(name, filemode,
                    _Stream(name, filemode, comptype, fileobj, bufsize))
            t._extfileobj = False
            return t

        elif mode == "r":
            # Find out which *open() is appropriate for opening the file.
            for comptype in cls.OPEN_METH:
                func = getattr(cls, cls.OPEN_METH[comptype])
                try:
                    return func(name, "r", fileobj)
                except (ReadError, CompressionError):
                    continue
            raise ReadError, "file could not be opened successfully"

        elif mode in "aw":
            return cls.taropen(name, mode, fileobj)

        raise ValueError, "undiscernible mode"

    @classmethod
    def taropen(cls, name, mode="r", fileobj=None):
        """Open uncompressed tar archive name for reading or writing.
        """
        if len(mode) > 1 or mode not in "raw":
            raise ValueError, "mode must be 'r', 'a' or 'w'"
        return cls(name, mode, fileobj)

    @classmethod
    def gzopen(cls, name, mode="r", fileobj=None, compresslevel=9):
        """Open gzip compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if len(mode) > 1 or mode not in "rw":
            raise ValueError, "mode must be 'r' or 'w'"

        try:
            import gzip
            gzip.GzipFile
        except (ImportError, AttributeError):
            raise CompressionError, "gzip module is not available"

        pre, ext = os.path.splitext(name)
        pre = os.path.basename(pre)
        if ext == ".tgz":
            ext = ".tar"
        if ext == ".gz":
            ext = ""
        tarname = pre + ext

        if fileobj is None:
            fileobj = file(name, mode + "b")

        if mode != "r":
            name = tarname

        try:
            t = cls.taropen(tarname, mode,
                gzip.GzipFile(name, mode, compresslevel, fileobj)
            )
        except IOError:
            raise ReadError, "not a gzip file"
        t._extfileobj = False
        return t

    @classmethod
    def bz2open(cls, name, mode="r", fileobj=None, compresslevel=9):
        """Open bzip2 compressed tar archive name for reading or writing.
           Appending is not allowed.
        """
        if len(mode) > 1 or mode not in "rw":
            raise ValueError, "mode must be 'r' or 'w'."

        try:
            import bz2
        except ImportError:
            raise CompressionError, "bz2 module is not available"

        pre, ext = os.path.splitext(name)
        pre = os.path.basename(pre)
        if ext == ".tbz2":
            ext = ".tar"
        if ext == ".bz2":
            ext = ""
        tarname = pre + ext

        if fileobj is not None:
            raise ValueError, "no support for external file objects"

        try:
            t = cls.taropen(tarname, mode, bz2.BZ2File(name, mode, compresslevel=compresslevel))
        except IOError:
            raise ReadError, "not a bzip2 file"
        t._extfileobj = False
        return t

    # All *open() methods are registered here.
    OPEN_METH = {
        "tar": "taropen",   # uncompressed tar
        "gz":  "gzopen",    # gzip compressed tar
        "bz2": "bz2open"    # bzip2 compressed tar
    }

    #--------------------------------------------------------------------------
    # The public methods which TarFile provides:

    def close(self):
        """Close the TarFile. In write-mode, two finishing zero blocks are
           appended to the archive.
        """
        if self.closed:
            return

        if self._mode in "aw":
            self.fileobj.write(NUL * (BLOCKSIZE * 2))
            self.offset += (BLOCKSIZE * 2)
            # fill up the end with zero-blocks
            # (like option -b20 for tar does)
            blocks, remainder = divmod(self.offset, RECORDSIZE)
            if remainder > 0:
                self.fileobj.write(NUL * (RECORDSIZE - remainder))

        if not self._extfileobj:
            self.fileobj.close()
        self.closed = True

    def getmember(self, name):
        """Return a TarInfo object for member `name'. If `name' can not be
           found in the archive, KeyError is raised. If a member occurs more
           than once in the archive, its last occurence is assumed to be the
           most up-to-date version.
        """
        tarinfo = self._getmember(name)
        if tarinfo is None:
            raise KeyError, "filename %r not found" % name
        return tarinfo

    def getmembers(self):
        """Return the members of the archive as a list of TarInfo objects. The
           list has the same order as the members in the archive.
        """
        self._check()
        if not self._loaded:    # if we want to obtain a list of
            self._load()        # all members, we first have to
                                # scan the whole archive.
        return self.members

    def getnames(self):
        """Return the members of the archive as a list of their names. It has
           the same order as the list returned by getmembers().
        """
        return [tarinfo.name for tarinfo in self.getmembers()]

    def gettarinfo(self, name=None, arcname=None, fileobj=None):
        """Create a TarInfo object for either the file `name' or the file
           object `fileobj' (using os.fstat on its file descriptor). You can
           modify some of the TarInfo's attributes before you add it using
           addfile(). If given, `arcname' specifies an alternative name for the
           file in the archive.
        """
        self._check("aw")

        # When fileobj is given, replace name by
        # fileobj's real name.
        if fileobj is not None:
            name = fileobj.name

        # Building the name of the member in the archive.
        # Backward slashes are converted to forward slashes,
        # Absolute paths are turned to relative paths.
        if arcname is None:
            arcname = name
        arcname = normpath(arcname)
        drv, arcname = os.path.splitdrive(arcname)
        while arcname[0:1] == "/":
            arcname = arcname[1:]

        # Now, fill the TarInfo object with
        # information specific for the file.
        tarinfo = TarInfo()

        # Use os.stat or os.lstat, depending on platform
        # and if symlinks shall be resolved.
        if fileobj is None:
            if hasattr(os, "lstat") and not self.dereference:
                statres = os.lstat(name)
            else:
                statres = os.stat(name)
        else:
            statres = os.fstat(fileobj.fileno())
        linkname = ""

        stmd = statres.st_mode
        if stat.S_ISREG(stmd):
            inode = (statres.st_ino, statres.st_dev)
            if inode in self.inodes and not self.dereference:
                # Is it a hardlink to an already
                # archived file?
                type = LNKTYPE
                linkname = self.inodes[inode]
            else:
                # The inode is added only if its valid.
                # For win32 it is always 0.
                type = REGTYPE
                if inode[0]:
                    self.inodes[inode] = arcname
        elif stat.S_ISDIR(stmd):
            type = DIRTYPE
            if arcname[-1:] != "/":
                arcname += "/"
        elif stat.S_ISFIFO(stmd):
            type = FIFOTYPE
        elif stat.S_ISLNK(stmd):
            type = SYMTYPE
            linkname = os.readlink(name)
        elif stat.S_ISCHR(stmd):
            type = CHRTYPE
        elif stat.S_ISBLK(stmd):
            type = BLKTYPE
        else:
            return None

        # Fill the TarInfo object with all
        # information we can get.
        tarinfo.name  = arcname
        tarinfo.mode  = stmd
        tarinfo.uid   = statres.st_uid
        tarinfo.gid   = statres.st_gid
        if stat.S_ISDIR(stmd):
            # For a directory, the size must be 0
            tarinfo.size  = 0
        else:
            tarinfo.size = statres.st_size
        tarinfo.mtime = statres.st_mtime
        tarinfo.type  = type
        tarinfo.linkname = linkname
        if pwd:
            try:
                tarinfo.uname = pwd.getpwuid(tarinfo.uid)[0]
            except KeyError:
                pass
        if grp:
            try:
                tarinfo.gname = grp.getgrgid(tarinfo.gid)[0]
            except KeyError:
                pass

        if type in (CHRTYPE, BLKTYPE):
            if hasattr(os, "major") and hasattr(os, "minor"):
                tarinfo.devmajor = os.major(statres.st_rdev)
                tarinfo.devminor = os.minor(statres.st_rdev)
        return tarinfo

    def list(self, verbose=True):
        """Print a table of contents to sys.stdout. If `verbose' is False, only
           the names of the members are printed. If it is True, an `ls -l'-like
           output is produced.
        """
        self._check()

        for tarinfo in self:
            if verbose:
                print filemode(tarinfo.mode),
                print "%s/%s" % (tarinfo.uname or tarinfo.uid,
                                 tarinfo.gname or tarinfo.gid),
                if tarinfo.ischr() or tarinfo.isblk():
                    print "%10s" % ("%d,%d" \
                                    % (tarinfo.devmajor, tarinfo.devminor)),
                else:
                    print "%10d" % tarinfo.size,
                print "%d-%02d-%02d %02d:%02d:%02d" \
                      % time.localtime(tarinfo.mtime)[:6],

            print tarinfo.name,

            if verbose:
                if tarinfo.issym():
                    print "->", tarinfo.linkname,
                if tarinfo.islnk():
                    print "link to", tarinfo.linkname,
            print

    def add(self, name, arcname=None, recursive=True):
        """Add the file `name' to the archive. `name' may be any type of file
           (directory, fifo, symbolic link, etc.). If given, `arcname'
           specifies an alternative name for the file in the archive.
           Directories are added recursively by default. This can be avoided by
           setting `recursive' to False.
        """
        self._check("aw")

        if arcname is None:
            arcname = name

        # Skip if somebody tries to archive the archive...
        if self.name is not None \
            and os.path.abspath(name) == os.path.abspath(self.name):
            self._dbg(2, "tarfile: Skipped %r" % name)
            return

        # Special case: The user wants to add the current
        # working directory.
        if name == ".":
            if recursive:
                if arcname == ".":
                    arcname = ""
                for f in os.listdir("."):
                    self.add(f, os.path.join(arcname, f))
            return

        self._dbg(1, name)

        # Create a TarInfo object from the file.
        tarinfo = self.gettarinfo(name, arcname)

        if tarinfo is None:
            self._dbg(1, "tarfile: Unsupported type %r" % name)
            return

        # Append the tar header and data to the archive.
        if tarinfo.isreg():
            f = file(name, "rb")
            self.addfile(tarinfo, f)
            f.close()

        if tarinfo.type in (LNKTYPE, SYMTYPE, FIFOTYPE, CHRTYPE, BLKTYPE):
            tarinfo.size = 0L
            self.addfile(tarinfo)

        if tarinfo.isdir():
            self.addfile(tarinfo)
            if recursive:
                for f in os.listdir(name):
                    self.add(os.path.join(name, f), os.path.join(arcname, f))

    def addfile(self, tarinfo, fileobj=None):
        """Add the TarInfo object `tarinfo' to the archive. If `fileobj' is
           given, tarinfo.size bytes are read from it and added to the archive.
           You can create TarInfo objects using gettarinfo().
           On Windows platforms, `fileobj' should always be opened with mode
           'rb' to avoid irritation about the file size.
        """
        self._check("aw")

        tarinfo.name = normpath(tarinfo.name)
        if tarinfo.isdir():
            # directories should end with '/'
            tarinfo.name += "/"

        if tarinfo.linkname:
            tarinfo.linkname = normpath(tarinfo.linkname)

        if tarinfo.size > MAXSIZE_MEMBER:
            if self.posix:
                raise ValueError, "file is too large (>= 8 GB)"
            else:
                self._dbg(2, "tarfile: Created GNU tar largefile header")


        if len(tarinfo.linkname) > LENGTH_LINK:
            if self.posix:
                raise ValueError, "linkname is too long (>%d)" \
                                  % (LENGTH_LINK)
            else:
                self._create_gnulong(tarinfo.linkname, GNUTYPE_LONGLINK)
                tarinfo.linkname = tarinfo.linkname[:LENGTH_LINK -1]
                self._dbg(2, "tarfile: Created GNU tar extension LONGLINK")

        if len(tarinfo.name) > LENGTH_NAME:
            if self.posix:
                prefix = tarinfo.name[:LENGTH_PREFIX + 1]
                while prefix and prefix[-1] != "/":
                    prefix = prefix[:-1]

                name = tarinfo.name[len(prefix):]
                prefix = prefix[:-1]

                if not prefix or len(name) > LENGTH_NAME:
                    raise ValueError, "name is too long (>%d)" \
                                      % (LENGTH_NAME)

                tarinfo.name   = name
                tarinfo.prefix = prefix
            else:
                self._create_gnulong(tarinfo.name, GNUTYPE_LONGNAME)
                tarinfo.name = tarinfo.name[:LENGTH_NAME - 1]
                self._dbg(2, "tarfile: Created GNU tar extension LONGNAME")

        self.fileobj.write(tarinfo.tobuf())
        self.offset += BLOCKSIZE

        # If there's data to follow, append it.
        if fileobj is not None:
            copyfileobj(fileobj, self.fileobj, tarinfo.size)
            blocks, remainder = divmod(tarinfo.size, BLOCKSIZE)
            if remainder > 0:
                self.fileobj.write(NUL * (BLOCKSIZE - remainder))
                blocks += 1
            self.offset += blocks * BLOCKSIZE

        self.members.append(tarinfo)

    def extract(self, member, path=""):
        """Extract a member from the archive to the current working directory,
           using its full name. Its file information is extracted as accurately
           as possible. `member' may be a filename or a TarInfo object. You can
           specify a different directory using `path'.
        """
        self._check("r")

        if isinstance(member, TarInfo):
            tarinfo = member
        else:
            tarinfo = self.getmember(member)

        # Prepare the link target for makelink().
        if tarinfo.islnk():
            tarinfo._link_target = os.path.join(path, tarinfo.linkname)

        try:
            self._extract_member(tarinfo, os.path.join(path, tarinfo.name))
        except EnvironmentError, e:
            if self.errorlevel > 0:
                raise
            else:
                if e.filename is None:
                    self._dbg(1, "tarfile: %s" % e.strerror)
                else:
                    self._dbg(1, "tarfile: %s %r" % (e.strerror, e.filename))
        except ExtractError, e:
            if self.errorlevel > 1:
                raise
            else:
                self._dbg(1, "tarfile: %s" % e)

    def extractfile(self, member):
        """Extract a member from the archive as a file object. `member' may be
           a filename or a TarInfo object. If `member' is a regular file, a
           file-like object is returned. If `member' is a link, a file-like
           object is constructed from the link's target. If `member' is none of
           the above, None is returned.
           The file-like object is read-only and provides the following
           methods: read(), readline(), readlines(), seek() and tell()
        """
        self._check("r")

        if isinstance(member, TarInfo):
            tarinfo = member
        else:
            tarinfo = self.getmember(member)

        if tarinfo.isreg():
            return self.fileobject(self, tarinfo)

        elif tarinfo.type not in SUPPORTED_TYPES:
            # If a member's type is unknown, it is treated as a
            # regular file.
            return self.fileobject(self, tarinfo)

        elif tarinfo.islnk() or tarinfo.issym():
            if isinstance(self.fileobj, _Stream):
                # A small but ugly workaround for the case that someone tries
                # to extract a (sym)link as a file-object from a non-seekable
                # stream of tar blocks.
                raise StreamError, "cannot extract (sym)link as file object"
            else:
                # A (sym)link's file object is it's target's file object.
                return self.extractfile(self._getmember(tarinfo.linkname,
                                                        tarinfo))
        else:
            # If there's no data associated with the member (directory, chrdev,
            # blkdev, etc.), return None instead of a file object.
            return None

    def _extract_member(self, tarinfo, targetpath):
        """Extract the TarInfo object tarinfo to a physical
           file called targetpath.
        """
        # Fetch the TarInfo object for the given name
        # and build the destination pathname, replacing
        # forward slashes to platform specific separators.
        if targetpath[-1:] == "/":
            targetpath = targetpath[:-1]
        targetpath = os.path.normpath(targetpath)

        # Create all upper directories.
        upperdirs = os.path.dirname(targetpath)
        if upperdirs and not os.path.exists(upperdirs):
            ti = TarInfo()
            ti.name  = upperdirs
            ti.type  = DIRTYPE
            ti.mode  = 0777
            ti.mtime = tarinfo.mtime
            ti.uid   = tarinfo.uid
            ti.gid   = tarinfo.gid
            ti.uname = tarinfo.uname
            ti.gname = tarinfo.gname
            try:
                self._extract_member(ti, ti.name)
            except:
                pass

        if tarinfo.islnk() or tarinfo.issym():
            self._dbg(1, "%s -> %s" % (tarinfo.name, tarinfo.linkname))
        else:
            self._dbg(1, tarinfo.name)

        if tarinfo.isreg():
            self.makefile(tarinfo, targetpath)
        elif tarinfo.isdir():
            self.makedir(tarinfo, targetpath)
        elif tarinfo.isfifo():
            self.makefifo(tarinfo, targetpath)
        elif tarinfo.ischr() or tarinfo.isblk():
            self.makedev(tarinfo, targetpath)
        elif tarinfo.islnk() or tarinfo.issym():
            self.makelink(tarinfo, targetpath)
        elif tarinfo.type not in SUPPORTED_TYPES:
            self.makeunknown(tarinfo, targetpath)
        else:
            self.makefile(tarinfo, targetpath)

        self.chown(tarinfo, targetpath)
        if not tarinfo.issym():
            self.chmod(tarinfo, targetpath)
            self.utime(tarinfo, targetpath)

    #--------------------------------------------------------------------------
    # Below are the different file methods. They are called via
    # _extract_member() when extract() is called. They can be replaced in a
    # subclass to implement other functionality.

    def makedir(self, tarinfo, targetpath):
        """Make a directory called targetpath.
        """
        try:
            os.mkdir(targetpath)
        except EnvironmentError, e:
            if e.errno != errno.EEXIST:
                raise

    def makefile(self, tarinfo, targetpath):
        """Make a file called targetpath.
        """
        source = self.extractfile(tarinfo)
        target = file(targetpath, "wb")
        copyfileobj(source, target)
        source.close()
        target.close()

    def makeunknown(self, tarinfo, targetpath):
        """Make a file from a TarInfo object with an unknown type
           at targetpath.
        """
        self.makefile(tarinfo, targetpath)
        self._dbg(1, "tarfile: Unknown file type %r, " \
                     "extracted as regular file." % tarinfo.type)

    def makefifo(self, tarinfo, targetpath):
        """Make a fifo called targetpath.
        """
        if hasattr(os, "mkfifo"):
            os.mkfifo(targetpath)
        else:
            raise ExtractError, "fifo not supported by system"

    def makedev(self, tarinfo, targetpath):
        """Make a character or block device called targetpath.
        """
        if not hasattr(os, "mknod") or not hasattr(os, "makedev"):
            raise ExtractError, "special devices not supported by system"

        mode = tarinfo.mode
        if tarinfo.isblk():
            mode |= stat.S_IFBLK
        else:
            mode |= stat.S_IFCHR

        os.mknod(targetpath, mode,
                 os.makedev(tarinfo.devmajor, tarinfo.devminor))

    def makelink(self, tarinfo, targetpath):
        """Make a (symbolic) link called targetpath. If it cannot be created
          (platform limitation), we try to make a copy of the referenced file
          instead of a link.
        """
        linkpath = tarinfo.linkname
        try:
            if tarinfo.issym():
                os.symlink(linkpath, targetpath)
            else:
                # See extract().
                os.link(tarinfo._link_target, targetpath)
        except AttributeError:
            if tarinfo.issym():
                linkpath = os.path.join(os.path.dirname(tarinfo.name),
                                        linkpath)
                linkpath = normpath(linkpath)

            try:
                self._extract_member(self.getmember(linkpath), targetpath)
            except (EnvironmentError, KeyError), e:
                linkpath = os.path.normpath(linkpath)
                try:
                    shutil.copy2(linkpath, targetpath)
                except EnvironmentError, e:
                    raise IOError, "link could not be created"

    def chown(self, tarinfo, targetpath):
        """Set owner of targetpath according to tarinfo.
        """
        if pwd and hasattr(os, "geteuid") and os.geteuid() == 0:
            # We have to be root to do so.
            try:
                g = grp.getgrnam(tarinfo.gname)[2]
            except KeyError:
                try:
                    g = grp.getgrgid(tarinfo.gid)[2]
                except KeyError:
                    g = os.getgid()
            try:
                u = pwd.getpwnam(tarinfo.uname)[2]
            except KeyError:
                try:
                    u = pwd.getpwuid(tarinfo.uid)[2]
                except KeyError:
                    u = os.getuid()
            try:
                if tarinfo.issym() and hasattr(os, "lchown"):
                    os.lchown(targetpath, u, g)
                else:
                    if sys.platform != "os2emx":
                        os.chown(targetpath, u, g)
            except EnvironmentError, e:
                raise ExtractError, "could not change owner"

    def chmod(self, tarinfo, targetpath):
        """Set file permissions of targetpath according to tarinfo.
        """
        if hasattr(os, 'chmod'):
            try:
                os.chmod(targetpath, tarinfo.mode)
            except EnvironmentError, e:
                raise ExtractError, "could not change mode"

    def utime(self, tarinfo, targetpath):
        """Set modification time of targetpath according to tarinfo.
        """
        if not hasattr(os, 'utime'):
            return
        if sys.platform == "win32" and tarinfo.isdir():
            # According to msdn.microsoft.com, it is an error (EACCES)
            # to use utime() on directories.
            return
        try:
            os.utime(targetpath, (tarinfo.mtime, tarinfo.mtime))
        except EnvironmentError, e:
            raise ExtractError, "could not change modification time"

    #--------------------------------------------------------------------------

    def next(self):
        """Return the next member of the archive as a TarInfo object, when
           TarFile is opened for reading. Return None if there is no more
           available.
        """
        self._check("ra")
        if self.firstmember is not None:
            m = self.firstmember
            self.firstmember = None
            return m

        # Read the next block.
        self.fileobj.seek(self.offset)
        while True:
            buf = self.fileobj.read(BLOCKSIZE)
            if not buf:
                return None
            try:
                tarinfo = TarInfo.frombuf(buf)
            except ValueError:
                if self.ignore_zeros:
                    if buf.count(NUL) == BLOCKSIZE:
                        adj = "empty"
                    else:
                        adj = "invalid"
                    self._dbg(2, "0x%X: %s block" % (self.offset, adj))
                    self.offset += BLOCKSIZE
                    continue
                else:
                    # Block is empty or unreadable.
                    if self.offset == 0:
                        # If the first block is invalid. That does not
                        # look like a tar archive we can handle.
                        raise ReadError,"empty, unreadable or compressed file"
                    return None
            break

        # We shouldn't rely on this checksum, because some tar programs
        # calculate it differently and it is merely validating the
        # header block. We could just as well skip this part, which would
        # have a slight effect on performance...
        if tarinfo.chksum != calc_chksum(buf):
            self._dbg(1, "tarfile: Bad Checksum %r" % tarinfo.name)

        # Set the TarInfo object's offset to the current position of the
        # TarFile and set self.offset to the position where the data blocks
        # should begin.
        tarinfo.offset = self.offset
        self.offset += BLOCKSIZE

        # Check if the TarInfo object has a typeflag for which a callback
        # method is registered in the TYPE_METH. If so, then call it.
        if tarinfo.type in self.TYPE_METH:
            return self.TYPE_METH[tarinfo.type](self, tarinfo)

        tarinfo.offset_data = self.offset
        if tarinfo.isreg() or tarinfo.type not in SUPPORTED_TYPES:
            # Skip the following data blocks.
            self.offset += self._block(tarinfo.size)

        if tarinfo.isreg() and tarinfo.name[:-1] == "/":
            # some old tar programs don't know DIRTYPE
            tarinfo.type = DIRTYPE

        self.members.append(tarinfo)
        return tarinfo

    #--------------------------------------------------------------------------
    # Below are some methods which are called for special typeflags in the
    # next() method, e.g. for unwrapping GNU longname/longlink blocks. They
    # are registered in TYPE_METH below. You can register your own methods
    # with this mapping.
    # A registered method is called with a TarInfo object as only argument.
    #
    # During its execution the method MUST perform the following tasks:
    # 1. set tarinfo.offset_data to the position where the data blocks begin,
    #    if there is data to follow.
    # 2. set self.offset to the position where the next member's header will
    #    begin.
    # 3. append the tarinfo object to self.members, if it is supposed to appear
    #    as a member of the TarFile object.
    # 4. return tarinfo or another valid TarInfo object.

    def proc_gnulong(self, tarinfo):
        """Evaluate the blocks that hold a GNU longname
           or longlink member.
        """
        buf = ""
        count = tarinfo.size
        while count > 0:
            block = self.fileobj.read(BLOCKSIZE)
            buf += block
            self.offset += BLOCKSIZE
            count -= BLOCKSIZE

        # Fetch the next header
        next = self.next()

        next.offset = tarinfo.offset
        if tarinfo.type == GNUTYPE_LONGNAME:
            next.name = nts(buf)
        elif tarinfo.type == GNUTYPE_LONGLINK:
            next.linkname = nts(buf)

        return next

    def proc_sparse(self, tarinfo):
        """Analyze a GNU sparse header plus extra headers.
        """
        buf = tarinfo.tobuf()
        sp = _ringbuffer()
        pos = 386
        lastpos = 0L
        realpos = 0L
        # There are 4 possible sparse structs in the
        # first header.
        for i in xrange(4):
            try:
                offset = int(buf[pos:pos + 12], 8)
                numbytes = int(buf[pos + 12:pos + 24], 8)
            except ValueError:
                break
            if offset > lastpos:
                sp.append(_hole(lastpos, offset - lastpos))
            sp.append(_data(offset, numbytes, realpos))
            realpos += numbytes
            lastpos = offset + numbytes
            pos += 24

        isextended = ord(buf[482])
        origsize = int(buf[483:495], 8)

        # If the isextended flag is given,
        # there are extra headers to process.
        while isextended == 1:
            buf = self.fileobj.read(BLOCKSIZE)
            self.offset += BLOCKSIZE
            pos = 0
            for i in xrange(21):
                try:
                    offset = int(buf[pos:pos + 12], 8)
                    numbytes = int(buf[pos + 12:pos + 24], 8)
                except ValueError:
                    break
                if offset > lastpos:
                    sp.append(_hole(lastpos, offset - lastpos))
                sp.append(_data(offset, numbytes, realpos))
                realpos += numbytes
                lastpos = offset + numbytes
                pos += 24
            isextended = ord(buf[504])

        if lastpos < origsize:
            sp.append(_hole(lastpos, origsize - lastpos))

        tarinfo.sparse = sp

        tarinfo.offset_data = self.offset
        self.offset += self._block(tarinfo.size)
        tarinfo.size = origsize

        self.members.append(tarinfo)
        return tarinfo

    # The type mapping for the next() method. The keys are single character
    # strings, the typeflag. The values are methods which are called when
    # next() encounters such a typeflag.
    TYPE_METH = {
        GNUTYPE_LONGNAME: proc_gnulong,
        GNUTYPE_LONGLINK: proc_gnulong,
        GNUTYPE_SPARSE:   proc_sparse
    }

    #--------------------------------------------------------------------------
    # Little helper methods:

    def _block(self, count):
        """Round up a byte count by BLOCKSIZE and return it,
           e.g. _block(834) => 1024.
        """
        blocks, remainder = divmod(count, BLOCKSIZE)
        if remainder:
            blocks += 1
        return blocks * BLOCKSIZE

    def _getmember(self, name, tarinfo=None):
        """Find an archive member by name from bottom to top.
           If tarinfo is given, it is used as the starting point.
        """
        # Ensure that all members have been loaded.
        members = self.getmembers()

        if tarinfo is None:
            end = len(members)
        else:
            end = members.index(tarinfo)

        for i in xrange(end - 1, -1, -1):
            if name == members[i].name:
                return members[i]

    def _load(self):
        """Read through the entire archive file and look for readable
           members.
        """
        while True:
            tarinfo = self.next()
            if tarinfo is None:
                break
        self._loaded = True

    def _check(self, mode=None):
        """Check if TarFile is still open, and if the operation's mode
           corresponds to TarFile's mode.
        """
        if self.closed:
            raise IOError, "%s is closed" % self.__class__.__name__
        if mode is not None and self._mode not in mode:
            raise IOError, "bad operation for mode %r" % self._mode

    def __iter__(self):
        """Provide an iterator object.
        """
        if self._loaded:
            return iter(self.members)
        else:
            return TarIter(self)

    def _create_gnulong(self, name, type):
        """Write a GNU longname/longlink member to the TarFile.
           It consists of an extended tar header, with the length
           of the longname as size, followed by data blocks,
           which contain the longname as a null terminated string.
        """
        name += NUL

        tarinfo = TarInfo()
        tarinfo.name = "././@LongLink"
        tarinfo.type = type
        tarinfo.mode = 0
        tarinfo.size = len(name)

        # write extended header
        self.fileobj.write(tarinfo.tobuf())
        self.offset += BLOCKSIZE
        # write name blocks
        self.fileobj.write(name)
        blocks, remainder = divmod(tarinfo.size, BLOCKSIZE)
        if remainder > 0:
            self.fileobj.write(NUL * (BLOCKSIZE - remainder))
            blocks += 1
        self.offset += blocks * BLOCKSIZE

    def _dbg(self, level, msg):
        """Write debugging output to sys.stderr.
        """
        if level <= self.debug:
            print >> sys.stderr, msg
# class TarFile

class TarIter:
    """Iterator Class.

       for tarinfo in TarFile(...):
           suite...
    """

    def __init__(self, tarfile):
        """Construct a TarIter object.
        """
        self.tarfile = tarfile
    def __iter__(self):
        """Return iterator object.
        """
        return self
    def next(self):
        """Return the next item using TarFile's next() method.
           When all members have been read, set TarFile as _loaded.
        """
        tarinfo = self.tarfile.next()
        if not tarinfo:
            self.tarfile._loaded = True
            raise StopIteration
        return tarinfo

# Helper classes for sparse file support
class _section:
    """Base class for _data and _hole.
    """
    def __init__(self, offset, size):
        self.offset = offset
        self.size = size
    def __contains__(self, offset):
        return self.offset <= offset < self.offset + self.size

class _data(_section):
    """Represent a data section in a sparse file.
    """
    def __init__(self, offset, size, realpos):
        _section.__init__(self, offset, size)
        self.realpos = realpos

class _hole(_section):
    """Represent a hole section in a sparse file.
    """
    pass

class _ringbuffer(list):
    """Ringbuffer class which increases performance
       over a regular list.
    """
    def __init__(self):
        self.idx = 0
    def find(self, offset):
        idx = self.idx
        while True:
            item = self[idx]
            if offset in item:
                break
            idx += 1
            if idx == len(self):
                idx = 0
            if idx == self.idx:
                # End of File
                return None
        self.idx = idx
        return item

#---------------------------------------------
# zipfile compatible TarFile class
#---------------------------------------------
TAR_PLAIN = 0           # zipfile.ZIP_STORED
TAR_GZIPPED = 8         # zipfile.ZIP_DEFLATED
class TarFileCompat:
    """TarFile class compatible with standard module zipfile's
       ZipFile class.
    """
    def __init__(self, file, mode="r", compression=TAR_PLAIN):
        if compression == TAR_PLAIN:
            self.tarfile = TarFile.taropen(file, mode)
        elif compression == TAR_GZIPPED:
            self.tarfile = TarFile.gzopen(file, mode)
        else:
            raise ValueError, "unknown compression constant"
        if mode[0:1] == "r":
            members = self.tarfile.getmembers()
            for i in xrange(len(members)):
                m = members[i]
                m.filename = m.name
                m.file_size = m.size
                m.date_time = time.gmtime(m.mtime)[:6]
    def namelist(self):
        return map(lambda m: m.name, self.infolist())
    def infolist(self):
        return filter(lambda m: m.type in REGULAR_TYPES,
                      self.tarfile.getmembers())
    def printdir(self):
        self.tarfile.list()
    def testzip(self):
        return
    def getinfo(self, name):
        return self.tarfile.getmember(name)
    def read(self, name):
        return self.tarfile.extractfile(self.tarfile.getmember(name)).read()
    def write(self, filename, arcname=None, compress_type=None):
        self.tarfile.add(filename, arcname)
    def writestr(self, zinfo, bytes):
        try:
            from cStringIO import StringIO
        except ImportError:
            from StringIO import StringIO
        import calendar
        zinfo.name = zinfo.filename
        zinfo.size = zinfo.file_size
        zinfo.mtime = calendar.timegm(zinfo.date_time)
        self.tarfile.addfile(zinfo, StringIO(bytes))
    def close(self):
        self.tarfile.close()
#class TarFileCompat

#--------------------
# exported functions
#--------------------
def is_tarfile(name):
    """Return True if name points to a tar archive that we
       are able to handle, else return False.
    """
    try:
        t = open(name)
        t.close()
        return True
    except TarError:
        return False

open = TarFile.open