[issue12800] 'tarfile.StreamError: seeking backwards is not allowed' when extract symlink

[Aurélien Dunand]

New submission from Aurélien Dunand <aurelien.dunand+python at gmail.com>:

When you extractall a tarball containing a symlink in stream mode ('r|'), an Exception happens:

Traceback (most recent call last):
    File "./test_extractall_stream_symlink.py", line 26, in <module>
    tar.extractall(path=destdir)
    File "/usr/lib/python3.2/tarfile.py", line 2134, in extractall
    self.extract(tarinfo, path, set_attrs=not tarinfo.isdir())
    File "/usr/lib/python3.2/tarfile.py", line 2173, in extract
    set_attrs=set_attrs)
    File "/usr/lib/python3.2/tarfile.py", line 2249, in _extract_member
    self.makefile(tarinfo, targetpath)
    File "/usr/lib/python3.2/tarfile.py", line 2289, in makefile
    source.seek(tarinfo.offset_data)
    File "/usr/lib/python3.2/tarfile.py", line 553, in seek
    raise StreamError("seeking backwards is not allowed")
    tarfile.StreamError: seeking backwards is not allowed

You can reproduce the bug with this snippet of code:

TEMPDIR='/tmp/pyton_test'
os.mkdir(TEMPDIR)
tempdir = os.path.join(TEMPDIR, "testsymlinks")
temparchive = os.path.join(TEMPDIR, "testsymlinks.tar")
destdir = os.path.join(TEMPDIR, "extract")
os.mkdir(tempdir)
try:
    source_file = os.path.join(tempdir,'source')
    target_file = os.path.join(tempdir,'symlink')
    with open(source_file,'w') as f:
        f.write('something\n')
    os.symlink('source', target_file)
    tar = tarfile.open(temparchive,'w')
    tar.add(target_file, arcname=os.path.basename(target_file))
    tar.add(source_file, arcname=os.path.basename(source_file))
    tar.close()
    fo = open(temparchive, 'rb')
    tar = tarfile.open(fileobj=fo, mode='r|')
    try:
        tar.extractall(path=destdir)
    finally:
        tar.close()
finally:
    os.unlink(temparchive)
    shutil.rmtree(TEMPDIR)



If source_file is added before target_file, there is no Exception raised. But it still raised when you create the same tarball with GNU tar.

----------
components: Library (Lib)
messages: 142580
nosy: adunand
priority: normal
severity: normal
status: open
title: 'tarfile.StreamError: seeking backwards is not allowed' when extract symlink
type: behavior
versions: Python 3.2

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue12800>
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