guix system: Report wrong file system 'device' fields.

Previously, if you wrote (device "my-label") without (title 'label),
you'd get:

  guix system: error: stat: No such file or directory: "my-label"

Now you get a proper error and a hint.

Reported by Pierre-Antoine Rouby.

* guix/scripts/system.scm (check-file-system-availability)[literal]: New
variable.  Loop over LITERAL.
* gnu/system/file-systems.scm (%pseudo-file-system-types): New variable.
* guix/ui.scm (display-hint): Make public.

gnu/system/file-systems.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -47,6 +47,7 @@
             spec->file-system
             specification->file-system-mapping
 
+            %pseudo-file-system-types
             %fuse-control-file-system
             %binary-format-file-system
             %shared-memory-file-system
@@ -203,6 +204,12 @@ TARGET in the other system."
 ;;; Common file systems.
 ;;;
 
+(define %pseudo-file-system-types
+  ;; List of know pseudo file system types.  This is used when validating file
+  ;; system definitions.
+  '("binfmt_misc" "cgroup" "devpts" "devtmpfs" "fusectl"
+    "proc" "sysfs" "tmpfs"))
+
 (define %fuse-control-file-system
   ;; Control file system for Linux' file systems in user-space (FUSE).
   (file-system

guix/scripts/system.scm

@@ -583,7 +583,8 @@ any, are available.  Raise an error if they're not."
   (define relevant
     (filter (lambda (fs)
               (and (file-system-mount? fs)
-                   (not (string=? "tmpfs" (file-system-type fs)))
+                   (not (member (file-system-type fs)
+                                %pseudo-file-system-types))
                    (not (memq 'bind-mount (file-system-flags fs)))))
             file-systems))
 
@@ -592,6 +593,11 @@ any, are available.  Raise an error if they're not."
               (eq? (file-system-title fs) 'label))
             relevant))
 
+  (define literal
+    (filter (lambda (fs)
+              (eq? (file-system-title fs) 'device))
+            relevant))
+
   (define uuid
     (filter (lambda (fs)
               (eq? (file-system-title fs) 'uuid))
@@ -610,6 +616,22 @@ any, are available.  Raise an error if they're not."
                            (set! fail? #t)
                            (format (current-error-port)
                                    args ...))))))
+    (for-each (lambda (fs)
+                (catch 'system-error
+                  (lambda ()
+                    (stat (file-system-device fs)))
+                  (lambda args
+                    (let ((errno  (system-error-errno args))
+                          (device (file-system-device fs)))
+                      (error (G_ "~a: error: device '~a' not found: ~a~%")
+                             (file-system-location* fs) device
+                             (strerror errno))
+                      (unless (string-prefix? "/" device)
+                        (display-hint (format #f (G_ "If '~a' is a file system
+label, you need to add @code{(title 'label)} to your @code{file-system}
+definition.")
+                                              device)))))))
+              literal)
     (for-each (lambda (fs)
                 (unless (find-partition-by-label (file-system-device fs))
                   (error (G_ "~a: error: file system with label '~a' not found~%")

guix/ui.scm

@@ -60,6 +60,7 @@
   #:use-module (texinfo string-utils)
   #:re-export (G_ N_ P_)                          ;backward compatibility
   #:export (report-error
+            display-hint
             leave
             make-user-module
             load*