Re: Move switch-symlinks to (guix build utils)

[bokr@bokr.com]

Hi,

I am not expert on kernel link internals, but if
you need/prefer atomic change to a specific link,
does my log [1] below suggest a way?

On +2022-06-03 21:30:39 +0200, Ludovic Courtès wrote:
> Hi,
> 
> Maxime Devos <maximedevos@telenet.be> skribis:
> 
> >, Maybe replace
> >
> >    (symlink target pivot)
> >
> > by (symlink/remove-old target pivot)
> >
> > where
> >
> > (define (symlink/remove-old target link)
> >   "Make a symbolic link named LINK pointing to TARGET.
> > If LINK already exists, it will be removed first.
> > This is not an atomic operation."
> >   (catch 'system-error
> >     (lambda ()
> >       (symlink target link))
> >     (lambda stuff
> >       (if (= (system-error-errno stuff) EEXIST)
> >           (begin
> >             ;; remove old link and retry
> >             (delete-file link)
> >             (symlink/remove-old link target))
> >           (apply throw stuff)))))
> 
> Alright, SGTM (this procedure would be kept private).
> 
> So Arun, the floor is yours!  :-)
> 
> Ludo’.
>

[0]: (some "<--= note" notes edited in)
BTW: notice that Size: is number of chars in TARGET name, not the LINK name
--8<---------------cut here---------------start------------->8---
$ ln -sT TARGET LINK
$ stat LINK
  File: LINK -> TARGET
  Size: 6               Blocks: 0          IO Block: 4096   symbolic link
Device: fe00h/65024d    Inode: 19401553    Links: 1   # <--= note inode number: mv will not change it
Access: (0777/lrwxrwxrwx)  Uid: ( 1000/    bokr)   Gid: ( 1000/    bokr)
Access: 2022-06-04 00:57:47.680832044 +0200
Modify: 2022-06-04 00:57:47.680832044 +0200
Change: 2022-06-04 00:57:47.680832044 +0200
 Birth: -
$ mv -v LINK WAS-LINK
renamed 'LINK' -> 'WAS-LINK'
$ stat WAS-LINK
  File: WAS-LINK -> TARGET
  Size: 6               Blocks: 0          IO Block: 4096   symbolic link # <--= TARGET is still 6 chars :)
Device: fe00h/65024d    Inode: 19401553    Links: 1    # <--= note inode number: did not change with mv
Access: (0777/lrwxrwxrwx)  Uid: ( 1000/    bokr)   Gid: ( 1000/    bokr)
Access: 2022-06-04 00:57:47.680832044 +0200
Modify: 2022-06-04 00:57:47.680832044 +0200
Change: 2022-06-04 00:58:33.617240104 +0200
 Birth: -
$ ln -sT NEW-TARGET LINK
$ stat LINK 
  File: LINK -> NEW-TARGET
  Size: 10              Blocks: 0          IO Block: 4096   symbolic link # <--= NEW-TARGET is 10 chars :)
Device: fe00h/65024d    Inode: 19401638    Links: 1    <--= note inode number: new for new use of LINK name
Access: (0777/lrwxrwxrwx)  Uid: ( 1000/    bokr)   Gid: ( 1000/    bokr)
Access: 2022-06-04 01:00:10.630121441 +0200
Modify: 2022-06-04 01:00:10.630121441 +0200
Change: 2022-06-04 01:00:10.630121441 +0200
 Birth: -
$ rm -v WAS-LINK      <--= note no OS objection to removing renamed old LINK
removed 'WAS-LINK'
$ stat *LINK*
  File: LINK -> NEW-TARGET
  Size: 10              Blocks: 0          IO Block: 4096   symbolic link
Device: fe00h/65024d    Inode: 19401638    Links: 1
Access: (0777/lrwxrwxrwx)  Uid: ( 1000/    bokr)   Gid: ( 1000/    bokr)
Access: 2022-06-04 01:00:10.630121441 +0200
Modify: 2022-06-04 01:00:10.630121441 +0200
Change: 2022-06-04 01:00:10.630121441 +0200
 Birth: -
$ 
--8<---------------cut here---------------end--------------->8---

I'm guessing the inode rewrite involved in mv -v LINK WAS-LINK should be atomic?
Maybe some kernels have to struggle without good hardware support, but still atomic?

Maybe a kernel insider will chime in?
HTH some way.
--
Regards,
Bengt Richter