Re: [PATCH] Add a "-h" flag to mv

On Tue, 28 Aug 2012 10:58:09 -0400
John Baldwin <jhb_at_freebsd.org> wrote:

> I have a use case at work where I need to be able to update a symlink that 
> points to a directory atomically (so that it points to a new directory).  To 
> give a conrete example, suppose I have two directories 'foo' and 'bar', and a 
> symlink 'a' that I wish to atomically flip from 'foo' to 'bar'.
> 
> Using 'ln -shf bar a' is not atomic as it uses separate unlink() and symlink() 
> system calls, so there is a race where another thread may encounter ENOENT 
> while traversing 'a'.
> 
> The approach we used was to create a new symbolic link 'a.new' (e.g. via
> 'ln -s bar a.new') and then use rename() to rename 'a.new' on top of 'a'.
> Normally to do an atomic rename from userland one would use 'mv', but
> 'mv a.new a' doesn't do that.  Instead, it moves 'a.new' into the directory
> referenced by the 'a' symlink.  At work we have resorted to invoking python's
> os.rename() in a one-liner to handle this.
> 
> While rehashing this discussion today it occurred to me that a -h flag to
> mv would allow it to work in this case (and is very similar to how ln treats
> its -h flag).  To that end, I have a patch to add a new -h flag to mv that
> allows one to atomically update a symlink that points to a directory.  I
> could not find any other mv commands that have adopted a -h (or a different
> flag that accomplishes the same task).  Given that it functions identically
> to the -h flag for ln, -h seemed the "logical" choice.  Any objections?
> 
[snip patch]

Nope, seems like a reasonable extension to me (gj_at_).

-- 
Gary Jennejohn