[issue1531] tarfile.open(fileobj=f) and bad metadata of the first file within the archive

[George Notaras]

New submission from George Notaras:

Assume the following situation:
- a healthy and uncompressed tar file: a.tar
- the metadata of the 1st and second files within the archive start at
positions 0 and 756 (realistic example values)

I partially damage 200 bytes of metadata (byte range 0-500) of the first
archived file:

f = open("a.tar", "rb+")
f.seek(100)
f.write("0"*200)

Now, I seek to the start of the 2nd archived file's metadata:

f.seek(756)

And I try to open the tar archive using tarfile.open() passing the
previous fileobject to it.

import tarfile
f_tar = tarfile.open(fileobj=f)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "tarfile.py", line 1143, in open
    raise ReadError("file could not be opened successfully")
tarfile.ReadError: file could not be opened successfully

Wouldn't the expected behaviour be to successfully open the tar archive
at offset 756?

It seems that tarfile.open(fileobj=f) seeks to position 0 of the
fileobject f, fails to read the 1st archived file's metadata and throws
an exception.

----------
components: Extension Modules
messages: 58026
nosy: GeorgeNotaras
severity: normal
status: open
title: tarfile.open(fileobj=f) and bad metadata of the first file within the archive
type: behavior
versions: Python 2.5

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Tracker <report at bugs.python.org>
<http://bugs.python.org/issue1531>
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