file-systems: Allow users to specify file system UUIDs as strings.

Fixes <http://bugs.gnu.org/19778>.
Reported by Mark H Weaver <mhw@netris.org>.

* gnu/system/file-systems.scm (%uuid-rx): New variable.
  (string->uuid): New procedure.
  (uuid): New macro.
* tests/file-systems.scm: New file.
* Makefile.am (SCM_TESTS): Add it.
* doc/guix.texi (File Systems): Give an example of UUID.

Makefile.am

@@ -203,6 +203,7 @@ SCM_TESTS =					\
   tests/lint.scm				\
   tests/publish.scm				\
   tests/size.scm				\
+  tests/file-systems.scm			\
   tests/containers.scm
 
 if HAVE_GUILE_JSON

doc/guix.texi

@@ -4963,8 +4963,24 @@ interpreted as a file name; when it is @code{label}, then @code{device}
 is interpreted as a partition label name; when it is @code{uuid},
 @code{device} is interpreted as a partition unique identifier (UUID).
 
+UUIDs may be converted from their string representation (as shown by the
+@command{tune2fs -l} command) using the @code{uuid} form, like this:
+
+@example
+(file-system
+  (mount-point "/home")
+  (type "ext4")
+  (title 'uuid)
+  (device (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
+@end example
+
 The @code{label} and @code{uuid} options offer a way to refer to disk
-partitions without having to hard-code their actual device name.
+partitions without having to hard-code their actual device
+name@footnote{Note that, while it is tempting to use
+@file{/dev/disk/by-uuid} and similar device names to achieve the same
+result, this is not recommended: These special device nodes are created
+by the udev daemon and may be unavailable at the time the device is
+mounted.}.
 
 However, when a file system's source is a mapped device (@pxref{Mapped
 Devices}), its @code{device} field @emph{must} refer to the mapped

gnu/system/file-systems.scm

@@ -18,9 +18,13 @@
 
 (define-module (gnu system file-systems)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 regex)
   #:use-module (guix gexp)
   #:use-module (guix records)
   #:use-module (guix store)
+  #:use-module (rnrs bytevectors)
+  #:use-module ((gnu build file-systems) #:select (uuid->string))
+  #:re-export (uuid->string)
   #:export (<file-system>
             file-system
             file-system?
@@ -35,6 +39,8 @@
             file-system-create-mount-point?
 
             file-system->spec
+            string->uuid
+            uuid
 
             %fuse-control-file-system
             %binary-format-file-system
@@ -106,6 +112,57 @@ initrd code."
     (($ <file-system> device title mount-point type flags options _ check?)
      (list device title mount-point type flags options check?))))
 
+(define %uuid-rx
+  ;; The regexp of a UUID.
+  (make-regexp "^([[:xdigit:]]{8})-([[:xdigit:]]{4})-([[:xdigit:]]{4})-([[:xdigit:]]{4})-([[:xdigit:]]{12})$"))
+
+(define (string->uuid str)
+  "Parse STR as a DCE UUID (see <https://tools.ietf.org/html/rfc4122>) and
+return its contents as a 16-byte bytevector.  Return #f if STR is not a valid
+UUID representation."
+  (and=> (regexp-exec %uuid-rx str)
+         (lambda (match)
+           (letrec-syntax ((hex->number
+                            (syntax-rules ()
+                              ((_ index)
+                               (string->number (match:substring match index)
+                                               16))))
+                           (put!
+                            (syntax-rules ()
+                              ((_ bv index (number len) rest ...)
+                               (begin
+                                 (bytevector-uint-set! bv index number
+                                                       (endianness big) len)
+                                 (put! bv (+ index len) rest ...)))
+                              ((_ bv index)
+                               bv))))
+             (let ((time-low  (hex->number 1))
+                   (time-mid  (hex->number 2))
+                   (time-hi   (hex->number 3))
+                   (clock-seq (hex->number 4))
+                   (node      (hex->number 5))
+                   (uuid      (make-bytevector 16)))
+               (put! uuid 0
+                     (time-low 4) (time-mid 2) (time-hi 2)
+                     (clock-seq 2) (node 6)))))))
+
+(define-syntax uuid
+  (lambda (s)
+    "Return the bytevector corresponding to the given UUID representation."
+    (syntax-case s ()
+      ((_ str)
+       (string? (syntax->datum #'str))
+       ;; A literal string: do the conversion at expansion time.
+       (with-syntax ((bv (string->uuid (syntax->datum #'str))))
+         #''bv))
+      ((_ str)
+       #'(string->uuid str)))))
+
+
+;;;
+;;; Common file systems.
+;;;
+
 (define %fuse-control-file-system
   ;; Control file system for Linux' file systems in user-space (FUSE).
   (file-system

tests/file-systems.scm

@@ -0,0 +1,46 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2015 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (test-file-systems)
+  #:use-module (gnu system file-systems)
+  #:use-module (srfi srfi-64)
+  #:use-module (rnrs bytevectors))
+
+;; Test the (gnu system file-systems) module.
+
+(test-begin "file-systems")
+
+(test-equal "uuid->string"
+  "c5307e6b-d1ba-499d-89c5-cb0b143577c4"
+  (uuid->string
+   #vu8(197 48 126 107 209 186 73 157 137 197 203 11 20 53 119 196)))
+
+(test-equal "string->uuid"
+  '(16 "4dab5feb-d176-45de-b287-9b0a6e4c01cb")
+  (let ((uuid (string->uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
+    (list (bytevector-length uuid) (uuid->string uuid))))
+
+(test-assert "uuid"
+  (let ((str "4dab5feb-d176-45de-b287-9b0a6e4c01cb"))
+    (bytevector=? (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")
+                  (string->uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb"))))
+
+(test-end)
+
+
+(exit (= (test-runner-fail-count (test-runner-current)) 0))