tarwriter: use new PAX format, instead of old GNU for sparse files

The old format have many issues and is discouraged by tar developers. In
this case the most important one is header with possible non-ASCII
characters, which will result in UnicodeDecodeError (tarfile module
require header parts in utf-8).
PAX format is much cleaner, as it use standard mechanism for extended
headers.

qubes/tarwriter.py

@@ -19,6 +19,7 @@
 # 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 import argparse
 import functools
+import os
 import subprocess
 import tarfile
 import io
@@ -29,12 +30,21 @@ class TarSparseInfo(tarfile.TarInfo):
     def __init__(self, name="", sparsemap=None):
         super(TarSparseInfo, self).__init__(name)
         if sparsemap is not None:
-            self.type = tarfile.GNUTYPE_SPARSE
-            self.sparsemap = list(sparsemap)
+            self.type = tarfile.REGTYPE
+            self.sparsemap = sparsemap
+            self.sparsemap_buf = self.format_sparse_map()
             # compact size
-            self.size = functools.reduce(lambda x, y: x+y[1], sparsemap, 0)
+            self.size = functools.reduce(lambda x, y: x+y[1], sparsemap,
+                0) + len(self.sparsemap_buf)
+            self.pax_headers['GNU.sparse.major'] = '1'
+            self.pax_headers['GNU.sparse.minor'] = '0'
+            self.pax_headers['GNU.sparse.name'] = name
+            self.pax_headers['GNU.sparse.realsize'] = str(self.realsize)
+            self.name = '{}/GNUSparseFile.{}/{}'.format(
+                os.path.dirname(name), os.getpid(), os.path.basename(name))
         else:
             self.sparsemap = []
+            self.sparsemap_buf = b''
 
     @property
     def realsize(self):
@@ -42,56 +52,22 @@ class TarSparseInfo(tarfile.TarInfo):
             return self.sparsemap[-1][0] + self.sparsemap[-1][1]
         return self.size
 
-    def sparse_header_chunk(self, index):
-        if index < len(self.sparsemap):
-            return b''.join([
-                tarfile.itn(self.sparsemap[index][0], 12, tarfile.GNU_FORMAT),
-                tarfile.itn(self.sparsemap[index][1], 12, tarfile.GNU_FORMAT),
-            ])
-        return b'\0' * 12 * 2
+    def format_sparse_map(self):
+        sparsemap_txt = (str(len(self.sparsemap)) + '\n' +
+            ''.join('{}\n{}\n'.format(*entry) for entry in self.sparsemap))
+        sparsemap_txt_len = len(sparsemap_txt)
+        if sparsemap_txt_len % tarfile.BLOCKSIZE:
+            padding = '\0' * (tarfile.BLOCKSIZE -
+                              sparsemap_txt_len % tarfile.BLOCKSIZE)
+        else:
+            padding = ''
+        return (sparsemap_txt + padding).encode()
 
-    def get_gnu_header(self):
-        '''Part placed in 'prefix' field of posix header'''
-
-        parts = [
-            tarfile.itn(self.mtime, 12, tarfile.GNU_FORMAT),  # atime
-            tarfile.itn(self.mtime, 12, tarfile.GNU_FORMAT),  # ctime
-            tarfile.itn(0, 12, tarfile.GNU_FORMAT),  # offset
-            tarfile.stn('', 4, tarfile.ENCODING, 'surrogateescape'),  #longnames
-            b'\0',  # unused_pad2
-        ]
-        parts += [self.sparse_header_chunk(i) for i in range(4)]
-        parts += [
-            b'\1' if len(self.sparsemap) > 4 else b'\0',  # isextended
-            tarfile.itn(self.realsize, 12, tarfile.GNU_FORMAT),  # realsize
-        ]
-        return b''.join(parts)
-
-    def get_info(self):
-        info = super(TarSparseInfo, self).get_info()
-        # place GNU extension into
-        info['prefix'] = self.get_gnu_header().decode(tarfile.ENCODING)
-        return info
-
-    def tobuf(self, format=tarfile.DEFAULT_FORMAT, encoding=tarfile.ENCODING,
+    def tobuf(self, format=tarfile.PAX_FORMAT, encoding=tarfile.ENCODING,
             errors="strict"):
         # pylint: disable=redefined-builtin
         header_buf = super(TarSparseInfo, self).tobuf(format, encoding, errors)
-        if len(self.sparsemap) > 4:
-            return header_buf + b''.join(self.create_ext_sparse_headers())
-        return header_buf
-
-    def create_ext_sparse_headers(self):
-        for ext_hdr in range(4, len(self.sparsemap), 21):
-            sparse_parts = [
-                self.sparse_header_chunk(i).decode(
-                    tarfile.ENCODING, 'surrogateescape')
-                for i in range(ext_hdr, ext_hdr+21)]
-            sparse_parts.append(
-                '\1' if ext_hdr+21 < len(self.sparsemap) else '\0')
-            yield tarfile.stn(''.join(sparse_parts), 512,
-                    tarfile.ENCODING, 'surrogateescape')
-
+        return header_buf + self.sparsemap_buf
 
 def get_sparse_map(input_file):
     '''
@@ -190,7 +166,7 @@ def main(args=None):
         output = compress.stdin
     else:
         compress = None
-    output.write(tar_info.tobuf(tarfile.GNU_FORMAT))
+    output.write(tar_info.tobuf(tarfile.PAX_FORMAT))
     copy_sparse_data(input_file, output, sparse_map)
     finalize(output)
     input_file.close()

qubes/tests/tarwriter.py

@@ -1,7 +1,7 @@
 #
 # The Qubes OS Project, http://www.qubes-os.org
 #
-# Copyright (C) 2016 Marek Marczykowski-Górecki 
+# Copyright (C) 2016 Marek Marczykowski-Górecki
 #                               <marmarek@invisiblethingslab.com>
 #
 # This program is free software; you can redistribute it and/or modify