[PATCH] guix package: Add '--switch-generation' option.

[Alex Kost <alezost@gmail.com>]

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Ludovic Courtès (2014-10-05 00:23 +0400) wrote:

> Alex Kost <alezost@gmail.com> skribis:>

[...]

>> - (Not related to this patch, but still …)  Currently with “roll-back”,
>>   we can only switch to the previous generation.  What about adding a
>>   possibility to switch to any generation?  So that we could use
>>   something like this:
>>
>>     guix package --switch-generation=7
>>
>>   Also such functionality can be added to Emacs UI: for example pressing
>>   "C" on a generation in *Guix Generation List* will make this
>>   generation the current one.
>>
>>   So ‘roll-back’ procedure may become a special case of the
>>   ‘switch-generation’ one.  WDYT?
>
> I think it’s a good idea!  (I think it was suggested in earlier
> discussions, but never implemented.)

Andreas Enge (2014-10-05 18:44 +0400) wrote:

[...]

> Actually, sometimes I would like to switch to the next generation ("-roll-
> forward", in a sense): I install something, go back with "--roll-back",
> and might like to go just one forward again.
>
> So how about the following:
> --switch-generation=1
> goes to generation 1;
> --switch-generation=-1
> goes 1 generation back;
> --switch-generation=+1
> goes one generation forward.

Thanks for the great idea!

————————————————————————————————————————————————————————————————
A patch is attached.  Some comments:

- ‘shitted-generation’ is not a very good name, I think.  Ideas?

- ‘previous-generation-number’ may use ‘shifted-generation’ now:


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(define* (previous-generation-number profile #:optional
                                     (number (generation-number profile)))
  "Return the number of the generation before generation NUMBER of
PROFILE, or 0 if none exists.  It could be NUMBER - 1, but it's not the
case when generations have been deleted (there are \"holes\")."
  (or (shifted-generation profile -1 number)
      0))

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  Worth changing?

- Perhaps it would be better to make 2 commits (?): one for adding
  ‘shifted-generation’ and ‘switch-to-generation’ procedures to (guix
  profiles) and another is for adding the “--switch-generation” option
  itself.

- Also I made a couple of cosmetic changes in “guix/scripts/package.scm”:
  * ‘filter-map’ was replaced by 'for-each' because it was called only for
    side effects there;
  * ‘begin’ was removed from ‘cond’.
  I think these changes do not deserve a separate commit and may stay in
  this patch.  Is it OK?


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From 3cc52d1aade5e9723c38c0af5fa4437cbdf1a9b6 Mon Sep 17 00:00:00 2001
From: Alex Kost <alezost@gmail.com>
Date: Mon, 6 Oct 2014 17:35:51 +0400
Subject: [PATCH] guix package: Add '--switch-generation' option.

* doc/guix.texi (Invoking guix package): Update documentation.
* guix/profiles.scm (shifted-generation, switch-to-generation): New
  procedures.
* guix/scripts/package.scm: Add '--switch-generation' option.
  (switch-to-previous-generation): Use 'switch-to-generation'.
---
 doc/guix.texi            | 15 +++++++++++++++
 guix/profiles.scm        | 36 +++++++++++++++++++++++++++++++++++-
 guix/scripts/package.scm | 44 +++++++++++++++++++++++++++++++++-----------
 3 files changed, 83 insertions(+), 12 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index f6357bd..c6921b1 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -784,6 +784,21 @@ Installing, removing, or upgrading packages from a generation that has
 been rolled back to overwrites previous future generations.  Thus, the
 history of a profile's generations is always linear.
 
+@item --switch-generation=@var{pattern}
+@itemx -S @var{pattern}
+Switch to a particular generation defined by @var{pattern}.
+
+@var{pattern} may be either a generation number or a number prefixed
+with ``+'' or ``-''.  The latter means: move forward/backward by a
+specified number of generations.  For example, if you want to return to
+the latest generation after @code{--roll-back}, use
+@code{--switch-generation=+1}.
+
+The difference between @code{--roll-back} and
+@code{--switch-generation=-1} is that @code{--switch-generation} will
+not make a zeroth generation, so if a specified generation does not
+exist, the current generation will not be changed.
+
 @item --search-paths
 @cindex search paths
 Report environment variable definitions, in Bash syntax, that may be
diff --git a/guix/profiles.scm b/guix/profiles.scm
index 18733a6..589402e 100644
--- a/guix/profiles.scm
+++ b/guix/profiles.scm
@@ -71,9 +71,11 @@
             generation-number
             generation-numbers
             profile-generations
+            shifted-generation
             previous-generation-number
             generation-time
-            generation-file-name))
+            generation-file-name
+            switch-to-generation))
 
 ;;; Commentary:
 ;;;
@@ -569,6 +571,21 @@ former profiles were found."
         '()
         generations)))
 
+(define* (shifted-generation profile shift
+                             #:optional (current (generation-number profile)))
+  "Return PROFILE's generation shifted from the CURRENT generation by SHIFT.
+SHIFT is a positive or negative number.
+Return #f if there is no such generation."
+  (let* ((abs-shift (abs shift))
+         (numbers (profile-generations profile))
+         (from-current (memq current
+                             (if (negative? shift)
+                                 (reverse numbers)
+                                 numbers))))
+    (and from-current
+         (< abs-shift (length from-current))
+         (list-ref from-current abs-shift))))
+
 (define (previous-generation-number profile number)
   "Return the number of the generation before generation NUMBER of
 PROFILE, or 0 if none exists.  It could be NUMBER - 1, but it's not the
@@ -589,4 +606,21 @@ case when generations have been deleted (there are \"holes\")."
   (make-time time-utc 0
              (stat:ctime (stat (generation-file-name profile number)))))
 
+(define (switch-to-generation profile number)
+  "Atomically switch PROFILE to the generation NUMBER."
+  (let ((current (generation-number profile))
+        (file (generation-file-name profile number)))
+    (cond ((not (file-exists? profile))
+           (format (current-error-port)
+                   (_ "profile '~a' does not exist~%")
+                   profile))
+          ((not (file-exists? file))
+           (format (current-error-port)
+                   (_ "generation ~a does not exist~%")
+                   number))
+          (else
+           (format #t (_ "switching from generation ~a to ~a~%")
+                   current number)
+           (switch-symlinks profile file)))))
+
 ;;; profiles.scm ends here
diff --git a/guix/scripts/package.scm b/guix/scripts/package.scm
index fc9c37b..b071029 100644
--- a/guix/scripts/package.scm
+++ b/guix/scripts/package.scm
@@ -96,12 +96,9 @@ return PROFILE unchanged.  The goal is to treat '-p ~/.guix-profile' as if
 
 (define (switch-to-previous-generation profile)
   "Atomically switch PROFILE to the previous generation."
-  (let* ((number              (generation-number profile))
-         (previous-number     (previous-generation-number profile number))
-         (previous-generation (generation-file-name profile previous-number)))
-    (format #t (_ "switching from generation ~a to ~a~%")
-            number previous-number)
-    (switch-symlinks profile previous-generation)))
+  (let* ((current  (generation-number profile))
+         (previous (previous-generation-number profile current)))
+    (switch-to-generation profile previous)))
 
 (define (roll-back store profile)
   "Roll back to the previous generation of PROFILE."
@@ -409,6 +406,9 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
   -d, --delete-generations[=PATTERN]
                          delete generations matching PATTERN"))
   (display (_ "
+  -S, --switch-generation=PATTERN
+                         switch to a generation matching PATTERN"))
+  (display (_ "
   -p, --profile=PROFILE  use PROFILE instead of the user's default profile"))
   (newline)
   (display (_ "
@@ -488,6 +488,10 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
                    (values (alist-cons 'delete-generations (or arg "")
                                        result)
                            #f)))
+         (option '(#\S "switch-generation") #t #f
+                 (lambda (opt name arg result arg-handler)
+                   (values (alist-cons 'switch-generation arg result)
+                           #f)))
          (option '("search-paths") #f #f
                  (lambda (opt name arg result arg-handler)
                    (values (cons `(query search-paths) result)
@@ -713,13 +717,31 @@ more information.~%"))
       (generation-number profile))
 
     ;; First roll back if asked to.
-    (cond ((and (assoc-ref opts 'roll-back?) (not dry-run?))
-           (begin
-             (roll-back (%store) profile)
-             (process-actions (alist-delete 'roll-back? opts))))
+    (cond ((and (assoc-ref opts 'roll-back?)
+                (not dry-run?))
+           (roll-back (%store) profile)
+           (process-actions (alist-delete 'roll-back? opts)))
+          ((and (assoc-ref opts 'switch-generation)
+                (not dry-run?))
+           (for-each
+            (match-lambda
+              (('switch-generation . pattern)
+               (let* ((number (string->number pattern))
+                      (number (and number
+                                   (case (string-ref pattern 0)
+                                     ((#\+ #\-)
+                                      (shifted-generation profile number))
+                                     (else number)))))
+                 (if number
+                     (switch-to-generation profile number)
+                     (format (current-error-port)
+                             "Cannot switch to generation '~a'~%" pattern)))
+               (process-actions (alist-delete 'switch-generation opts)))
+              (_ #f))
+            opts))
           ((and (assoc-ref opts 'delete-generations)
                 (not dry-run?))
-           (filter-map
+           (for-each
             (match-lambda
              (('delete-generations . pattern)
               (cond ((not (file-exists? profile)) ; XXX: race condition
-- 
2.1.2


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-- 
Thanks,
Alex