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

[Arun Isaac]

> 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)))))
>
> ?

Could we implement symlink/remove-old without catch and throw? Something
like:

--8<---------------cut here---------------start------------->8---
(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."
  (when (file-exists? link)
    (delete-file link))
  (symlink target link))
--8<---------------cut here---------------end--------------->8---

We might also inline this into switch-symlinks like so.

--8<---------------cut here---------------start------------->8---
(define (switch-symlinks link target)
  "Atomically switch LINK, a symbolic link, to point to TARGET.  Works
both when LINK already exists and when it does not."
  (let ((pivot (string-append link ".new")))
    ;; Delete pivot link in case it already exists. This can happen if
    ;; a previous switch-symlinks was interrupted.
    (when (file-exists? pivot)
      (delete-file pivot))
    (symlink target pivot)
    (rename-file pivot link)))
--8<---------------cut here---------------end--------------->8---

WDYT?