Re: [gentoo-user] Atomically swap directories

[Alan McKinnon]

On Sun, 22 Jul 2012 10:59:46 -0400
Michael Mol <mikemol@×××××.com> wrote:

> On Sun, Jul 22, 2012 at 9:53 AM, Florian Philipp
> <lists@×××××××××××.net> wrote:
> > Hi list!
> >
> > This is more a general POSIX question but I guess here I have the
> > best chance to get a definite answer.
> >
> > If I want to replace a file with another file without removing the
> > first one and without having a moment in time at which the file
> > name does not exist, I can use the following sequence:
> >
> > # swap $file1 with $file2 on the same file system
> > dir=$(dirname "$file1")
> > tmp=$(mktemp -u -d "$dir") # [1]
> > ln "$file1" "$tmp"
> > mv "$file2" "$file1"
> > mv "$tmp" "$file2"
> >
> > This works because mv does not affect the hardlink to $file1 and a
> > rename operation on a single file system is atomic. This is a handy
> > procedure when you have a background process which occasionally
> > opens $file1 and you don't want it to fail just because of bad
> > timing.
> >
> > Now my question is: How can I do a similar thing for a directory? I
> > cannot usually create hardlinks on directories (are there file
> > systems where this actually works?) and I cannot use mv to
> > overwrite one directory with another.
> >
> > The only idea I currently have is to create a level of indirection
> > via symlinks and then atomically overwrite the symlink. Is there
> > any other way?
>
> I'd be very, very wary of doing something like this without knowing
> exactly what programs might be accessing files inside the folder
> you're swapping out. In order to avoid a race where some poor process
> winds up with open file handles to some content in both your old and
> new folders, you'd really need a way to:
>
> 1) lock the folder so no programs can gain new handles on it or any
> file or folder inside
> 2) wait until all other open file handles to the folder and its
> contents are closed
> 3) swap out the folder
> 4) unlock
>
> (1) might be doable with flock() on a parent directory.
> (2) you'll need to use fuser to find the processes which have open
> handles and get them to release them.
> (3) mv a a_tmp; mv b a; mv a_tmp b
> (4) flock -u
>


I'd argue that what the OP wants is fundamentally impossible - there is
no such thing to my knowledge as a directory locking mechanism that is
guaranteed to always work in all cases - it would have to be
kernel-based to do that and I've personally never heard of such a
mechanism.

The fact is, that directory operations are not atomic wrt the directory
contents, and the OP needs to find a different way to solve his problem.

As for the OPs question re: hard-linking directories, Linux never allows
this as a user action - it can cause un-solveable loops when traversing
directory trees. Most other Unixes have the same rule for the same
reason. Some do allow it; but I forget which ones and I'm too lazy on a
Sunday afternoon to Google it :-)

The kernel is of course perfectly able to hard-link directories - it
has to to be able to create . and .. at all, but that code only runs as
part of what mkdir does. In all other cases the kernel is hard-coded to
just refuse to do it.

-- 
Alan McKinnon
alan.mckinnon@×××××.com