image: Add sanitizers for 'format' and 'partition-table-type'.

This allows common mistakes to be diagnosed early.

* gnu/image.scm (define-set-sanitizer): New macro.
(validate-image-format, validate-partition-table-type): New sanitizers.
(<image>)[format, partition-table-type]: Add 'sanitize' property.

gnu/image.scm

@@ -19,6 +19,10 @@
 (define-module (gnu image)
   #:use-module (guix platform)
   #:use-module (guix records)
+  #:use-module (guix diagnostics)
+  #:use-module (guix i18n)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:export (partition
             partition?
             partition-device
@@ -77,12 +81,31 @@
 ;;; Image record.
 ;;;
 
+(define-syntax-rule (define-set-sanitizer name field set)
+  "Define NAME as a procedure or macro that raises an error if passed a value
+that is not in SET, mentioning FIELD in the error message."
+  (define-with-syntax-properties (name (value properties))
+    (unless (memq value 'set)
+      (raise
+       (make-compound-condition
+        (condition
+         (&error-location
+          (location (source-properties->location properties))))
+        (formatted-message (G_ "~s: invalid '~a' value") value 'field))))
+    value))
+
+(define-set-sanitizer validate-image-format format
+  (disk-image compressed-qcow2 docker iso9660))
+(define-set-sanitizer validate-partition-table-type partition-table-type
+  (mbr gpt))
+
 (define-record-type* <image>
   image make-image
   image?
   (name               image-name ;symbol
                       (default #f))
-  (format             image-format) ;symbol
+  (format             image-format                ;symbol
+                      (sanitize validate-image-format))
   (platform           image-platform ;<platform>
                       (default #f))
   (size               image-size  ;size in bytes as integer
@@ -90,7 +113,8 @@
   (operating-system   image-operating-system  ;<operating-system>
                       (default #f))
   (partition-table-type image-partition-table-type ; 'mbr or 'gpt
-                      (default 'mbr))
+                      (default 'mbr)
+                      (sanitize validate-partition-table-type))
   (partitions         image-partitions ;list of <partition>
                       (default '()))
   (compression?       image-compression? ;boolean