[Bug-tar] bug: combining --recursive-unlink with --directory

[David Lowe]

Howdy folks,

We've come across a bug when combining --recursive-unlink with
--directory... namely, that --recursive-unlink doesn't work anymore!
I'll demonstrate:

$ pwd
/home/ubuntu/tmp
## create a tarball containing a symlink 'bar'
$ mkdir foo
$ touch foo/a
$ ln -s foo bar
$ tar cf test.tar.gz bar
## now make 'bar' in the filesystem be a nonempty directory
$ rm bar
$ mv foo bar
## expected: failure to overwrite a nonempty directory with a symlink:
$ tar xf test.tar.gz
tar: bar: Cannot create symlink to `foo': File exists
tar: Exiting with failure status due to previous errors
## unexpected: failure when using --recursive-unlink + --directory:
$ (cd /; tar xf /home/ubuntu/tmp/test.tar.gz --directory
/home/ubuntu/tmp --recursive-unlink)
tar: bar: Cannot create symlink to `foo': File exists
tar: Exiting with failure status due to previous errors
## .... but it works as expected if run without --directory:
$ tar xf test.tar.gz --recursive-unlink
$ ls -l bar
lrwxrwxrwx 1 ubuntu ubuntu 3 Mar 12 21:10 bar -> foo

stracing the unexpectedly failing tar paints a fairly clear picture of
the problem:

openat(AT_FDCWD, "/home/ubuntu/tmp",
O_RDONLY|O_NOCTTY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 4
unlinkat(4, "bar", 0)                   = -1 EISDIR (Is a directory)
unlinkat(4, "bar", AT_REMOVEDIR)        = -1 ENOTEMPTY (Directory not empty)
openat(AT_FDCWD, "bar", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) =
-1 ENOENT (No such file or directory)
symlinkat("foo", 4, "bar")              = -1 EEXIST (File exists)

The problem appears to be that the second openat() call is relative to
AT_FDCWD (the process' PWD) instead of fd=4 (the --directory
argument).

FWIW:

$ tar --version
tar (GNU tar) 1.26
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by John Gilmore and Jay Fenlason.

Thanks,
David