[Bug-tar] [bug] multi-volume archive not compatible with --sparse

[Marek Marczykowski-Górecki]

Hi,

I've noticed that sparse file header can't be splitted across multiple archive
volume. So if you try to make a multi-volume archive of a big sparse file
(with a lot of holes), it can happen that a file header (list of the holes)
will not fit on the same archive volume. Tar then continues such a header on
the next volume, but *without any tar archive header*. If even second volume
isn't enough, it looks like the header got truncated there and tar starts the
file data (with a proper header this time) on the next volume.
When you try to extract such a file from an archive, tar complains that the
second volume doesn't look like tar archive. When you concatenate all the
volumes and extract this archive, it will look better (apart from "tar:
Skipping to next header" message), but the file will be broken.

The worst thing here - tar pretends that the archive was successfully created,
the user do not get any error message during creation of such archive.

Tested on tar 1.26 (Fedora 20), confirmed on 1.28 compiled from sources.

Steps to reproduce:
1. Create sparse file:
for i in `seq 10000`; do
  yes | dd bs=512 count=1
  dd if=/dev/zero bs=1024 count=20
done |cp --sparse=always /dev/stdin sparse.bin

1a. Check the file:
ls -lhs sparse.bin
81M -rw------- 1 user user 201M Sep 15 04:10 sparse.bin
md5sum sparse.bin
11a7794e3b5650d6c6779cbb9cd24c68  sparse.bin

2. Create multi-volume archive:
tar cf sparse.tar.000 --tape-length 128 --sparse sparse.bin
(enter new names like sparse.tar.001 etc when prompted for next volume)

3. Try to extract:
$ tar xMf sparse.tar.000
Prepare volume #2 for `sparse.tar.000' and hit return: n sparse.tar.001
tar: `sparse.bin' is not continued on this volume
Prepare volume #2 for `sparse.tar.001' and hit return:

4. Try to concatenate volumes and extract:
$ cat sparse.tar.* |tar xM
tar: Skipping to next header
tar: Exiting with failure status due to previous errors

5. Verify extracted file:
ls -lhs sparse.bin
40M -rw------- 1 user user 201M Sep 15 04:10 sparse.bin
md5sum sparse.bin
081154090bd6862985960a433a7a06a9  sparse.bin
It's different than original file!

-- 
Pozdrawiam / Best Regards,
Marek Marczykowski-Górecki | RLU #390519
marmarek at mimuw edu pl   | xmpp:marmarek at staszic waw pl

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