On Sun, 22 Jul 2012 10:59:46 -0400 Michael Mol <[email protected]> wrote:
> On Sun, Jul 22, 2012 at 9:53 AM, Florian Philipp > <[email protected]> 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 [email protected]
