guix package: Make custom profiles actual indirect roots.

Before that, any profile generation built when '-p' is used would
effectively become a permanent GC root because the symlink in
/var/guix/gcroots/auto would point directly to /gnu/store/...-profile.

* guix/scripts/package.scm (maybe-register-gc-root): Rename to...
  (register-gc-root): ... this.  Remove conditional, and replace call to
  'canonicalize-path' with (string-append (getcwd) "/" ...).
  (guix-package): Call 'register-gc-root' only if PROFILE is different
  from %CURRENT-PROFILE.
* tests/guix-package.sh: Add test case.

guix/scripts/package.scm

@@ -661,10 +661,20 @@ removed from MANIFEST."
                (_ #f))
               options))
 
-(define (maybe-register-gc-root store profile)
-  "Register PROFILE as a GC root, unless it doesn't need it."
-  (unless (string=? profile %current-profile)
-    (add-indirect-root store (canonicalize-path profile))))
+(define (register-gc-root store profile)
+  "Register PROFILE, a profile generation symlink, as a GC root, unless it
+doesn't need it."
+  (define absolute
+    ;; We must pass the daemon an absolute file name for PROFILE.  However, we
+    ;; cannot use (canonicalize-path profile) because that would return us the
+    ;; target of PROFILE in the store; using a store item as an indirect root
+    ;; would mean that said store item will always remain live, which is not
+    ;; what we want here.
+    (if (string-prefix? "/" profile)
+        profile
+        (string-append (getcwd) "/" profile)))
+
+  (add-indirect-root store absolute))
 
 (define (readlink* file)
   "Call 'readlink' until the result is not a symlink."
@@ -857,7 +867,8 @@ more information.~%"))
                                  (count   (length entries)))
                             (switch-symlinks name prof)
                             (switch-symlinks profile name)
-                            (maybe-register-gc-root (%store) profile)
+                            (unless (string=? profile %current-profile)
+                              (register-gc-root (%store) name))
                             (format #t (N_ "~a package in profile~%"
                                            "~a packages in profile~%"
                                            count)

tests/guix-package.sh

@@ -32,7 +32,7 @@ module_dir="t-guix-package-$$"
 profile="t-profile-$$"
 rm -f "$profile"
 
-trap 'rm "$profile" "$profile-"[0-9]* ; rm -rf "$module_dir" t-home-'"$$" EXIT
+trap 'rm -f "$profile" "$profile-"[0-9]* ; rm -rf "$module_dir" t-home-'"$$" EXIT
 
 # Use `-e' with a non-package expression.
 if guix package --bootstrap -e +;
@@ -203,6 +203,18 @@ if guix package -p "$profile" --delete-generations=12m;
 then false; else true; fi
 test "`readlink_base "$profile"`" = "$generation"
 
+# Make sure $profile is a GC root at this point.
+real_profile="`readlink -f "$profile"`"
+if guix gc -d "$real_profile"
+then false; else true; fi
+test -d "$real_profile"
+
+# Now, let's remove all the symlinks to $real_profile, and make sure
+# $real_profile is no longer a GC root.
+rm "$profile" "$profile"-[0-9]-link
+guix gc -d "$real_profile"
+[ ! -d "$real_profile" ]
+
 #
 # Try with the default profile.
 #