New submission from David Nesting :
When opening a tarfile with mode "r|" (streaming mode), extractfile("filename")
and extractfile(mytarfile.getmembers()[0]) raise "tarfile.StreamError: seeking
backwards is not allowed". extractfile(mytarfile.next()) succeeds. A more
complete test case:
"""
import tarfile
import StringIO
# Create a simple tar file in memory. This could easily be a real tar file
# though.
data = StringIO.StringIO()
tf = tarfile.open(fileobj=data, mode="w")
tarinfo = tarfile.TarInfo(name="testfile")
filedata = StringIO.StringIO("test data")
tarinfo.size = len(filedata.getvalue())
tf.addfile(tarinfo, fileobj=filedata)
tf.close()
data.seek(0)
# Open as an uncompressed stream
tf = tarfile.open(fileobj=data, mode="r|")
#f = tf.extractfile("testfile")
#print "%s: %s" % (f.name, f.read())
#
#Traceback (most recent call last):
# File "./bug.py", line 19, in
#print "%s: %s" % (f.name, f.read())
# File "/usr/lib/python2.7/tarfile.py", line 815, in read
#buf += self.fileobj.read()
# File "/usr/lib/python2.7/tarfile.py", line 735, in read
#return self.readnormal(size)
# File "/usr/lib/python2.7/tarfile.py", line 742, in readnormal
#self.fileobj.seek(self.offset + self.position)
# File "/usr/lib/python2.7/tarfile.py", line 554, in seek
#raise StreamError("seeking backwards is not allowed")
#tarfile.StreamError: seeking backwards is not allowed
#for member in tf.getmembers():
# f = tf.extractfile(member)
# print "%s: %s" % (f.name, f.read())
#
# Same traceback
while True:
member = tf.next()
if member is None:
break
f = tf.extractfile(member)
print "%s: %s" % (f.name, f.read())
# This works.
"""
It appears that extractfile("filename") invokes getmember("filename"), which
invokes getmembers(). getmembers() scans the entire file before returning
results, and by doing so, it's read past and discarded the actual file data,
which makes it impossible for us to actually extract it.
If this is accurate, this seems tricky to completely fix. You could make
getmembers() a generator that doesn't read too far ahead so that the file's
contents are still available if someone wants to retrieve them for each file
yielded. getmember("filename") could just scan forward through the file until
it hits a match, but you'd still lose the ability to do a getmember("filename")
on a file that we skipped over.
If nothing else, document that extractfile("filename"), getmember() and
getmembers() won't work reliably in streaming mode, and possibly raise an
exception whenever someone tries just to make behavior consistent.
--
components: Library (Lib)
messages: 121308
nosy: David.Nesting
priority: normal
severity: normal
status: open
title: tarfile.extractfile in "r|" stream mode fails with filenames or members
from getmembers()
type: behavior
versions: Python 2.6, Python 2.7
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<http://bugs.python.org/issue10436>
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