guix package: Never remove the current generation and warn about it.

Fixes <http://bugs.gnu.org/19978>.
Reported by taylanbayirli@gmail.com (Taylan Ulrich Bayırlı/Kammer).

* guix/scripts/package.scm (delete-matching-generations): Warn when
  CURRENT is in NUMBERS, and always remove it before calling
  'delete-generations'.
* tests/guix-package.sh: Add --switch-generation=2 invocation before
  --delete-generations=3 invocation.
  Add --delete-generations=1.. test case.

guix/scripts/package.scm

@@ -254,9 +254,16 @@ denote ranges as interpreted by 'matching-derivations'."
                                  #:duration-relation >)
            =>
            (lambda (numbers)
-             (if (null-list? numbers)
-                 (exit 1)
-                 (delete-generations (%store) profile numbers))))
+             (when (memv current numbers)
+               (warning (_ "not removing generation ~a, which is current~%")
+                        current))
+
+             ;; Make sure we don't inadvertently remove the current
+             ;; generation.
+             (let ((numbers (delv current numbers)))
+               (if (null-list? numbers)
+                   (exit 1)
+                   (delete-generations (%store) profile numbers)))))
           (else
            (leave (_ "invalid syntax: ~a~%") pattern)))))
 

tests/guix-package.sh

@@ -161,6 +161,9 @@ then
     guix package --bootstrap -p "$profile" -i guile-bootstrap -i gcc-bootstrap
     guix package --search-paths -p "$profile" | grep LIBRARY_PATH
 
+    # Roll back so we can delete #3 below.
+    guix package -p "$profile" --switch-generation=2
+
     # Delete the third generation and check that it was actually deleted.
     guix package -p "$profile" --delete-generations=3
     test -z "`guix package -p "$profile" -l 3`"
@@ -212,6 +215,14 @@ if guix package -p "$profile" --delete-generations=12m;
 then false; else true; fi
 test "`readlink_base "$profile"`" = "$generation"
 
+# The following command should not delete the current generation, even though
+# it matches the given pattern (see <http://bugs.gnu.org/19978>.)  And since
+# there's nothing else to delete, it should just fail.
+guix package --list-generations -p "$profile"
+if guix package --bootstrap -p "$profile" --delete-generations=1..
+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"