Add a libgcrypt-based implementation of `sha256'.

* guix/utils.scm (sha256): Add a libgcrypt-based implementation using
  the FFI.

guix/utils.scm

@@ -30,6 +30,7 @@
   #:autoload   (ice-9 rdelim) (read-line)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 match)
+  #:autoload   (system foreign) (pointer->procedure)
   #:export (bytevector-quintet-length
             bytevector->base32-string
             bytevector->nix-base32-string
@@ -381,14 +382,36 @@ starting from the right of S."
 ;;; Hash.
 ;;;
 
-(define (sha256 bv)
+(define sha256
+  (cond
+   ((compile-time-value
+     (false-if-exception (dynamic-link "libgcrypt")))
+    ;; Using libgcrypt.
+    (let ((hash   (pointer->procedure void
+                                      (dynamic-func "gcry_md_hash_buffer"
+                                                    (dynamic-link "libgcrypt"))
+                                      `(,int * * ,size_t)))
+          (sha256 8))                           ; GCRY_MD_SHA256, as of 1.5.0
+      (lambda (bv)
         "Return the SHA256 of BV as a bytevector."
-  (if (compile-time-value
+        (let ((digest (make-bytevector (/ 256 8))))
+          (hash sha256 (bytevector->pointer digest)
+                (bytevector->pointer bv) (bytevector-length bv))
+          digest))))
+
+   ((compile-time-value
      (false-if-exception (resolve-interface '(chop hash))))
+    ;; Using libchop.
     (let ((bytevector-hash    (@ (chop hash) bytevector-hash))
           (hash-method/sha256 (@ (chop hash) hash-method/sha256)))
-        (bytevector-hash hash-method/sha256 bv))
-      ;; XXX: Slow, poor programmer's implementation that uses Coreutils.
+      (lambda (bv)
+        "Return the SHA256 of BV as a bytevector."
+        (bytevector-hash hash-method/sha256 bv))))
+
+   (else
+    ;; Slow, poor programmer's implementation that uses Coreutils.
+    (lambda (bv)
+      "Return the SHA256 of BV as a bytevector."
       (let ((in  (pipe))
             (out (pipe))
             (pid (primitive-fork)))
@@ -410,7 +433,7 @@ starting from the right of S."
                 (close (car out))
                 (and (and=> (status:exit-val (cdr (waitpid pid)))
                             zero?)
-                     (base16-string->bytevector line))))))))
+                     (base16-string->bytevector line))))))))))