list-runtime-root: Fix off-by-one in 'strip-drop' call.

Fixes <https://bugs.gnu.org/29862>.
Reported by Danny Milosavljevic <dannym@scratchpost.org>.

* nix/scripts/list-runtime-roots.in (canonicalize-store-item): Define
'store' with a trailing "/".  Have the 'string-prefix?' call match the
'string-drop' call.

1 files changed, 4 insertions, 3 deletions

diff --git a/nix/scripts/list-runtime-roots.in b/nix/scripts/list-runtime-roots.in
index 48a07edf5fd..5f2660fb5e6 100644
--- a/nix/scripts/list-runtime-roots.in
+++ b/nix/scripts/list-runtime-roots.in
@@ -130,12 +130,13 @@ or the empty list."
                          (< (string->number a) (string->number b))))))
 
 (define canonicalize-store-item
-  (let ((prefix (+ 1 (string-length %store-directory))))
+  (let* ((store  (string-append %store-directory "/"))
+         (prefix (string-length store)))
     (lambda (file)
       "Return #f if FILE is not a store item; otherwise, return the store file
 name without any sub-directory components."
-      (and (string-prefix? %store-directory file)
-           (string-append %store-directory "/"
+      (and (string-prefix? store file)
+           (string-append store
                           (let ((base (string-drop file prefix)))
                             (match (string-index base #\/)
                               (#f    base)