home: services: msmtp: Accept g-expressions for ‘password-eval’.

This allows users to specify:

  (password-eval #~(string-append
                    #$(file-append libsecret "/bin/secret-tool")
		    "…"))

* gnu/home/services/mail.scm (string-or-gexp?): New variable.
(maybe-string-or-gexp): New record type
(msmtp-configuration-serialize-string-or-gexp): New variable.
(msmtp-configuration) [password-eval]: Change type.

Change-Id: Id3016956250c447b301b40f29a6509d57058be49
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/home/services/mail.scm

@@ -49,9 +49,14 @@
             msmtp-account-name
             msmtp-account-configuration))
 
+(define (string-or-gexp? obj)
+  (or (string? obj)
+      (gexp? obj)))
+
 (define-maybe string (prefix msmtp-configuration-))
 (define-maybe boolean (prefix msmtp-configuration-))
 (define-maybe integer (prefix msmtp-configuration-))
+(define-maybe string-or-gexp (prefix msmtp-configuration-))
 
 ;; Serialization of 'msmtp'.
 (define (uglify-symbol field-name)
@@ -68,6 +73,9 @@
 (define (msmtp-configuration-serialize-string field-name value)
   #~(string-append #$(uglify-symbol field-name) " " #$value "\n"))
 
+(define msmtp-configuration-serialize-string-or-gexp
+  msmtp-configuration-serialize-string)
+
 (define (msmtp-configuration-serialize-maybe-string-no-underscore field-name value)
   #~(if #$(maybe-value-set? value)
       (string-append
@@ -148,7 +156,7 @@ unless TLS without STARTTLS is used, in which case it is 465 (\"smtps\").")
     "Set the envelope-from address.")
 
   (password-eval
-    maybe-string
+    maybe-string-or-gexp
     "Set the password for authentication to the output (stdout) of the command cmd."
     (serializer msmtp-configuration-serialize-maybe-string-no-underscore))