pull: Turn ~/.config/guix/current into a symlink to /var/guix/profiles.

This is more consistent with what 'guix package' does, more pleasant for
users (we no longer clobber ~/.config/guix), and more
cluster-friendly (since /var/guix/profiles is usually an NFS share
already.)

* guix/scripts/pull.scm (%current-profile, %user-profile-directory): New
variables.
(migrate-generations, ensure-default-profile): New procedures.
(guix-pull): Use %CURRENT-PROFILE by default.  Call
'ensure-default-profile'.
* doc/guix.texi (Invoking guix pull): Adjust 'guix package -p
~/.config/guix/current' example.
* guix/scripts.scm (warn-about-old-distro): Check %PROFILE-DIRECTORY
"/current-guix".

doc/guix.texi

@@ -2831,7 +2831,7 @@ generation---i.e., the previous Guix---and so on:
 $ guix package -p ~/.config/guix/current --roll-back
 switched from generation 3 to 2
 $ guix package -p ~/.config/guix/current --delete-generations=1
-deleting /home/charlie/.config/guix/current-1-link
+deleting /var/guix/profiles/per-user/charlie/current-guix-1-link
 @end example
 
 The @command{guix pull} command is usually invoked with no arguments,

guix/scripts.scm

@@ -26,6 +26,7 @@ (define-module (guix scripts)
   #:use-module (guix monads)
   #:use-module (guix packages)
   #:use-module (guix derivations)
+  #:use-module ((guix profiles) #:select (%profile-directory))
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-37)
@@ -169,8 +170,7 @@ (define (seconds->days seconds)
 
     (define age
       (match (false-if-not-found
-              (lstat (string-append (config-directory #:ensure? #f)
-                                    "/current")))
+              (lstat (string-append %profile-directory "/current-guix")))
         (#f    #f)
         (stat  (- (time-second (current-time time-utc))
                   (stat:mtime stat)))))

guix/scripts/pull.scm

@@ -225,6 +225,53 @@ (define-syntax-rule (with-git-error-handling body ...)
     (lambda (key err)
       (report-git-error err))))
 
+
+;;;
+;;; Profile.
+;;;
+
+(define %current-profile
+  ;; The "real" profile under /var/guix.
+  (string-append %profile-directory "/current-guix"))
+
+(define %user-profile-directory
+  ;; The user-friendly name of %CURRENT-PROFILE.
+  (string-append (config-directory #:ensure? #f) "/current"))
+
+(define (migrate-generations profile directory)
+  "Migration the generations of PROFILE to DIRECTORY."
+  (format (current-error-port)
+          (G_ "Migrating profile generations to '~a'...~%")
+          %profile-directory)
+  (for-each (lambda (generation)
+              (let ((source (generation-file-name profile generation))
+                    (target (string-append directory "/current-guix-"
+                                           (number->string generation)
+                                           "-link")))
+                (rename-file source target)))
+            (profile-generations profile)))
+
+(define (ensure-default-profile)
+  (ensure-profile-directory)
+
+  ;; In 0.15.0+ we'd create ~/.config/guix/current-[0-9]*-link symlinks.  Move
+  ;; them to %PROFILE-DIRECTORY.
+  (unless (string=? %profile-directory
+                    (dirname (canonicalize-profile %user-profile-directory)))
+    (migrate-generations %user-profile-directory %profile-directory))
+
+  ;; Make sure ~/.config/guix/current points to /var/guix/profiles/….
+  (let ((link %user-profile-directory))
+    (unless (equal? (false-if-exception (readlink link))
+                    %current-profile)
+      (catch 'system-error
+        (lambda ()
+          (false-if-exception (delete-file link))
+          (symlink %current-profile link))
+        (lambda args
+          (leave (G_ "while creating symlink '~a': ~a~%")
+                 link (strerror (system-error-errno args))))))))
+
 
 ;;;
 ;;; Queries.
@@ -438,9 +485,8 @@ (define (guix-pull . args)
                                           (list %default-options)))
             (cache    (string-append (cache-directory) "/pull"))
             (channels (channel-list opts))
-            (profile  (or (assoc-ref opts 'profile)
-                          (string-append (config-directory) "/current"))))
-
+            (profile  (or (assoc-ref opts 'profile) %current-profile)))
+       (ensure-default-profile)
        (cond ((assoc-ref opts 'query)
               (process-query opts profile))
              ((assoc-ref opts 'dry-run?)