mv for symlinks on dapper is not atomic

See also http://<email address hidden>/msg12594.html

Currently, mv may not be atomic. Atomic mv of symlinks is necessary for doing things like updating a symlink while ensuring the symlink always exists. Consider the case of a server that knows what directory is live via a symlink.

Upstream have removed the unlink for all moves, and this updated coreutils is in current development (and hardy).

TEST CASE: mv a symlink. The unlink syscall should not be triggered.

There is no expected regression with this patch.

marking as triaged, medium importance. Dapper is LTS.

This doesn't seem to be fixed in Intrepid either. The patch on the upstream ML isn't applied to the intrepid version.

Mark, your attached dapper patch seems to be pretty much exactly backwards to the upstream posted one. I don't understand the patch yet, that needs some closer investigation before I can judge in a qualified way, but that jumped into my eye.

Ah, you talk about symlinks, while the referenced mail talks about hardlinks. That's where my initial confusion came from.

I confirm that mv is not atomic in dapper, hardy and intrepid when using hardlinks:

$ echo foo >foo; echo bar > bar; ln foo foo.backup
$ strace mv bar foo
[...]
access("foo", W_OK) = 0
unlink("foo") = 0
rename("bar", "foo") = 0
[...]

As for symlinks, I used this test case:

$ rm -f *; ln -s file1 link; ln -s file2 link.new; strace -e access,rename,unlink mv link.new link

In Hardy and Intrepid I get the correct behaviour:

rename("link.new", "link") = 0

while in Dapper I get:

unlink("link") = 0
rename("link.new", "link") = 0

I checked the intrepid sources, and this is covered by tests/mv/atomic (with pretty much the same approach).

So for symlinks this is confirmed fixed in hardy and intrepid.

Upstream commits:
http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=f1d1ee91217d08527c6bf7682987339e38116268
http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=40b5d67c1fcf3c94b5960f69ff98a7aae253c59f

Dapper update uploaded, waiting in SRU queue for approval.

Ok for SRU, however:

Running: "accept coreutils"
Accepting ubuntu/dapper-proposed (UNAPPROVED) 1/1
---------------------------------------------------------------------------
Accepting coreutils
** coreutils could not be accepted due to The source coreutils - 5.93-5ubuntu5 is already accepted in ubuntu/edgy and you cannot upload the same version within the same distribution. You have to modify the source version and re-upload.
---------------------------------------------------------------------------

Please reupload with a distinct version number.

Accepted into dapper-proposed, please test and give feedback here. Please see https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you in advance!

I've installed the proposed coreutils, and can verify that mv is atomic for me now.

Copied to dapper-updates.