git (commit|tag) atomicity

Hi,
I don't know a lot about git - I use it as a tool behind
http://cyber-dojo.com
which is an online coding dojo server.
I have a quick question...
If I do a
   git commit ....
in one thread and a
   git tag | sort -g
in another thread is the output of the git tag guaranteed to be atomic?
That is, the output will be from before the git commit or after it but
not a mixture?
Logically, I assume so but cannot find anything definitive on google
or in my git books or from the lips of my friends.
Thanks for any help
Jon Jagger

On 28.02.2012 16:40, Jon Jagger wrote:

Can a "git commit" add or remove tags? AFAIK it can't and so the two commands don't conflict in any way.

On Tue, Feb 28, 2012 at 4:46 PM, Holger Hellmuth <hellmuth@ira.uka.de> wrote:

Sorry, I failed to ask the question I really wanted to ask...

I mean in one thread
   git tag -m 'AAA' BBB HEAD
and in another thread
   git tag | sort -g

and the question is whether the output of the git tag|sort -g command is guaranteed to be from before the git tag -m... or from after the git tag -m... but not "interleaved" in any way....

Cheers Jon

Jon Jagger <jon@jaggersoft.com> writes:

Creating a tag or a commit is guaranteed to be atomic. Git first atomically adds tag or a commit to object database (atomic file write) as loose object, then atomically writes tag reference as loose tag ('.git/refs/tags/foo' file, containing SHA-1 id of newly created tag).

"git tag", which list all tags, recursively scans (reads) 'refs/tags/' directory, so it could theoretically happen that if you have very large number of loose (unpacked) tags, "git tag" might theoretically list tag 'zzz' created after start of command, but not list 'aaa' tag created after start of command.

But I am not sure... that probably depends on how opendir(3) and readdir(3) works on given filesystem wrt. updates to opened directory. I think VFS on Linux ensures that you see view of filesystem as it was on opendir().

It's a bit of a tangent, but just to be sure people don't get the wrong impression..

No, readdir() does not give you a static view of the entries in a directory as it was on opendir(). readdir() will reflect modifications that are done after opendir(). The specifics for a given situation depend on how the file system maps the readdir position (f_pos) to directory entries. You can see very different results when comparing, say, stock ext2, indexed ext[34], and btrfs.

- z
(your message probably caught my eye because telldir()/seekdir() is
*loathed* by file system designers)

On Tue, Feb 28, 2012 at 6:57 PM, Zach Brown <zab@zabbo.net> wrote:

Ok. Thanks.