file-systems: Remove dependency on (guix store).

(gnu system file-systems) is used on the "build" side since commit
5970e8e248.

* gnu/system/file-systems.scm: Remove dependency on (guix store).
(%store-prefix): New procedure.
* tests/file-systems.scm ("does not pull (guix config)"): New test.

gnu/system/file-systems.scm

@@ -19,7 +19,6 @@
 (define-module (gnu system file-systems)
   #:use-module (ice-9 match)
   #:use-module (guix records)
-  #:use-module (guix store)
   #:use-module ((gnu build file-systems)
                 #:select (string->uuid uuid->string))
   #:re-export (string->uuid
@@ -97,6 +96,20 @@
   (dependencies     file-system-dependencies      ; list of <file-system>
                     (default '())))               ; or <mapped-device>
 
+;; Note: This module is used both on the build side and on the host side.
+;; Arrange not to pull (guix store) and (guix config) because the latter
+;; differs from user to user.
+(define (%store-prefix)
+  "Return the store prefix."
+  (cond ((resolve-module '(guix store) #:ensure #f)
+         =>
+         (lambda (store)
+           ((module-ref store '%store-prefix))))
+        ((getenv "NIX_STORE")
+         => identity)
+        (else
+         "/gnu/store")))
+
 (define %not-slash
   (char-set-complement (char-set #\/)))
 

tests/file-systems.scm

@@ -18,6 +18,7 @@
 
 (define-module (test-file-systems)
   #:use-module (guix store)
+  #:use-module (guix modules)
   #:use-module (gnu system file-systems)
   #:use-module (srfi srfi-64)
   #:use-module (rnrs bytevectors))
@@ -72,4 +73,11 @@
                    (device "/foo")
                    (flags '(bind-mount read-only)))))))))
 
+(test-assert "does not pull (guix config)"
+  ;; This module is meant both for the host side and "build side", so make
+  ;; sure it doesn't pull in (guix config), which depends on the user's
+  ;; config.
+  (not (member '(guix config)
+               (source-module-closure '((gnu system file-systems))))))
+
 (test-end)