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bpo-12800: 'tarfile.StreamError: seeking backwards is not allowed' when extract symlink #20972

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bpo-12800: 'tarfile.StreamError: seeking backwards is not allowed' when extract symlink #20972

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bpo-12800: Extracting a symlink from a tarball using the tarfile
websurfer5 May 9, 2019
bce13d6
📜🤖 Added by blurb_it.
blurb-it[bot] May 9, 2019
700c3f0
Fix tests as per review comments
catlee Jun 18, 2020
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3 changes: 3 additions & 0 deletions Lib/tarfile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2232,6 +2232,9 @@ def makelink(self, tarinfo, targetpath):
try:
# For systems that support symbolic and hard links.
if tarinfo.issym():
if os.path.islink(targetpath) or os.path.isfile(targetpath):
os.unlink(targetpath)

os.symlink(tarinfo.linkname, targetpath)
else:
# See extract().
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41 changes: 41 additions & 0 deletions Lib/test/test_tarfile.py
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Expand Up @@ -2580,6 +2580,47 @@ def test_partial_input_bz2(self):
self._test_partial_input("r:bz2")


class SymlinkOverwriteTest(unittest.TestCase):
# The testcase checks for correct overwriting of an
# existing symlink (issue #12800)

@support.skip_unless_symlink
def test_overwrite_symlink(self):
with support.temp_cwd('overwrite_symlink'):
source = 'source'
link = 'link'
with open(source, 'wb'):
pass
os.symlink(source, link)
with tarfile.open(tmpname, 'w') as tar:
tar.add(source, arcname=os.path.basename(source))
tar.add(link, arcname=os.path.basename(link))

with open(tmpname, 'rb') as fileobj:
with tarfile.open(fileobj=fileobj, mode='r|') as tar:
tar.extractall(path=support.SAVEDCWD)

@support.skip_unless_symlink
def test_overwrite_file_with_symlink(self):
with support.temp_cwd('overwrite_file_with_symlink'):
source = 'source'
link = 'link'
with open(source, 'wb'):
pass
os.symlink(source, link)
with tarfile.open(tmpname, 'w') as tar:
tar.add(source, arcname=os.path.basename(source))
tar.add(link, arcname=os.path.basename(link))

os.unlink(link)

with open(link, 'wb'):
pass

with open(tmpname, 'rb') as fileobj:
with tarfile.open(fileobj=fileobj, mode='r|') as tar:
tar.extractall(path=support.SAVEDCWD)

def root_is_uid_gid_0():
try:
import pwd, grp
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2019-05-09-18-37-37.bpo-12800.TyjRQq.rst
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@@ -0,0 +1 @@
Extracting a symlink from a tarball using the tarfile module in stream mode ('r|') fails with an exception when a file or symlink of the same name already exists. The fix is to remove the existing file or symlink before extraction.