services: web: Improve nginx formatting for extra-content.

When extra-content is a list, add 4 space indentation and a newline to every
line. If it's a string, continue inserting it directly. This makes the list
serialization behavior more consistent with other services.

* gnu/services/web (default-nginx-config): Support lists.
* doc/guix.texi (Web Services)[nginx-configuration]: Document it.

Change-Id: Iec8614ba3cfc37292a566197e8d39b352b04846a
Signed-off-by: Christopher Baines <mail@cbaines.net>

doc/guix.texi

@@ -32073,8 +32073,17 @@ configuration.  Values may themselves be association lists.
 @end lisp
 
 @item @code{extra-content} (default: @code{""})
-Extra content for the @code{http} block.  Should be string or a string
+Additional content to be appended to the @code{http} block.  Can either
-valued G-expression.
+be a value that can be lowered into a string or a list of such values.
+In the former case, it is inserted directly.  In the latter, it is
+prefixed with indentation and suffixed with a newline.  Nested lists are
+flattened into one line.
+
+@lisp
+(extra-content "include /etc/nginx/custom-config.conf;")
+(extra-content `("include /etc/nginx/custom-config.conf;"
+                 ("include " ,%custom-config.conf ";")))
+@end lisp
 
 @end table
 @end deftp

gnu/services/web.scm

@@ -771,7 +771,14 @@ of index files."
            "\n"
            (map emit-nginx-upstream-config upstream-blocks)
            (map emit-nginx-server-config server-blocks)
-           extra-content
+           (match extra-content
+             ((? list? extra-content)
+              (map (lambda (line)
+                     `("    " ,line "\n"))
+                   extra-content))
+             ;; XXX: For compatibility strings and gexp's are inserted
+             ;; directly.
+             (_ extra-content))
            "\n}\n"))))
 
 (define %nginx-accounts