profiles: Optionally use relative file names for symlink targets.

* guix/build/union.scm (symlink-relative): New procedure.
* guix/build/profiles.scm: Re-export it.
(build-profile): Add #:symlink and pass it to 'union-build'.
* guix/profiles.scm (profile-derivation): Add #:relative-symlinks?.
Pass #:symlink to 'build-profile'.
* tests/profiles.scm ("profile-derivation relative symlinks, one entry")
("profile-derivation relative symlinks, two entries"): New tests.

2 files changed, 17 insertions, 6 deletions

diff --git a/guix/build/profiles.scm b/guix/build/profiles.scm
index b4160fba1b..819688a913 100644
--- a/guix/build/profiles.scm
+++ b/guix/build/profiles.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -24,6 +24,7 @@
   #:use-module (ice-9 ftw)
   #:use-module (ice-9 match)
   #:use-module (ice-9 pretty-print)
+  #:re-export (symlink-relative)                  ;for convenience
   #:export (ensure-writable-directory
             build-profile))
 
@@ -129,12 +130,15 @@ instead make DIRECTORY a \"real\" directory containing symlinks."
             (apply throw args))))))
 
 (define* (build-profile output inputs
-                        #:key manifest search-paths)
-  "Build a user profile from INPUTS in directory OUTPUT.  Write MANIFEST, an
-sexp, to OUTPUT/manifest.  Create OUTPUT/etc/profile with Bash definitions for
--all the variables listed in SEARCH-PATHS."
+                        #:key manifest search-paths
+                        (symlink symlink))
+  "Build a user profile from INPUTS in directory OUTPUT, using SYMLINK to
+create symlinks.  Write MANIFEST, an sexp, to OUTPUT/manifest.  Create
+OUTPUT/etc/profile with Bash definitions for -all the variables listed in
+SEARCH-PATHS."
   ;; Make the symlinks.
   (union-build output inputs
+               #:symlink symlink
                #:log-port (%make-void-port "w"))
 
   ;; Store meta-data.
diff --git a/guix/build/union.scm b/guix/build/union.scm
index 82d6199d9e..24b366af45 100644
--- a/guix/build/union.scm
+++ b/guix/build/union.scm
@@ -29,7 +29,8 @@
 
             warn-about-collision
 
-            relative-file-name))
+            relative-file-name
+            symlink-relative))
 
 ;;; Commentary:
 ;;;
@@ -213,4 +214,10 @@ Note that this is from a purely lexical standpoint; conversely, \"..\" is
                   (finish)))))))
       file))
 
+(define (symlink-relative old new)
+  "Assuming both OLD and NEW are absolute file names, make NEW a symlink to
+OLD, but using a relative file name."
+  (symlink (relative-file-name (dirname new) old)
+           new))
+
 ;;; union.scm ends here