services: shepherd: Support “free-form” services.

* gnu/services/shepherd.scm (<shepherd-service>)[free-form]: New field.
[start]: Add default value.
(shepherd-service-file): Rename to…
(shepherd-service-file/regular): … this.
(shepherd-service-file/free-form): New procedure.
(shepherd-service-file): Dispatch to one of the two procedures above.
* doc/guix.texi (Shepherd Services): Document the ‘free-form’ field.

Reviewed-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

Change-Id: I206374e950ef6d1e4a996c0f507fb5fcd9cadde3

doc/guix.texi

@@ -44088,7 +44088,7 @@ Services,,, shepherd, The GNU Shepherd Manual}, for details.
 When true, this is the delay in seconds before restarting a failed
 service.
 
-@item @code{start}
+@item @code{start} (default: @code{#~(const #t)})
 @itemx @code{stop} (default: @code{#~(const #f)})
 The @code{start} and @code{stop} fields refer to the Shepherd's
 facilities to start and stop processes (@pxref{Service De- and
@@ -44107,6 +44107,30 @@ This is a list of @code{shepherd-action} objects (see below) defining
 herd @var{action} @var{service} [@var{arguments}@dots{}]
 @end example
 
+@item @code{free-form} (default: @code{#f})
+When set, this field replaces the @code{start}, @code{stop}, and
+@code{actions} fields.  It is meant to be used when the service
+definition comes from some other source, typically the service
+collection provided by the Shepherd proper (@pxref{Service Collection,,,
+shepherd, The GNU Shepherd Manual}).
+
+@cindex REPL service, for shepherd
+For example, the snippet below defines a service for the Shepherd's
+built-in @acronym{REPL, read-eval-print loop} service (@pxref{REPL
+Service,,, shepherd, The GNU Shepherd Manual}):
+
+@lisp
+(shepherd-service
+  (provision '(repl))
+  (modules '((shepherd service repl)))
+  (free-form #~(repl-service)))
+@end lisp
+
+In this case, the service object is returned by the @code{repl-service}
+procedure of the Shepherd, so all the @code{free-form} G-expression does
+is call that procedure.  Note that the @code{provision} field must be
+consistent with the actual service provision.
+
 @item @code{auto-start?} (default: @code{#t})
 Whether this service should be started automatically by the Shepherd.  If it
 is @code{#f} the service has to be started manually with @code{herd start}.

gnu/services/shepherd.scm

@@ -60,6 +60,7 @@
             shepherd-service-respawn?
             shepherd-service-start
             shepherd-service-stop
+            shepherd-service-free-form
             shepherd-service-auto-start?
             shepherd-service-modules
 
@@ -217,7 +218,10 @@ DEFAULT is given, use it as the service's default value."
                  (default #f))
   (respawn-delay shepherd-service-respawn-delay
                  (default #f))
-  (start         shepherd-service-start)               ;g-expression (procedure)
+  (free-form     shepherd-service-free-form            ;#f | g-expression (service)
+                 (default #f))
+  (start         shepherd-service-start                ;g-expression (procedure)
+                 (default #~(const #t)))
   (stop          shepherd-service-stop                 ;g-expression (procedure)
                  (default #~(const #f)))
   (actions       shepherd-service-actions              ;list of <shepherd-action>
@@ -298,8 +302,8 @@ stored."
                                provisions)
                    ".scm")))
 
-(define (shepherd-service-file service)
+(define (shepherd-service-file/regular service)
-  "Return a file defining SERVICE."
+  "Return a file defining SERVICE, a service whose 'free-form' field is #f."
   (scheme-file (shepherd-service-file-name service)
                (with-imported-modules %default-imported-modules
                  #~(begin
@@ -332,6 +336,21 @@ stored."
                                    #~(#$name #$doc #$proc)))
                                 (shepherd-service-actions service))))))))
 
+(define (shepherd-service-file/free-form service)
+  "Return a file defining SERVICE, a service whose 'free-form' field is set."
+  (scheme-file (shepherd-service-file-name service)
+               (with-imported-modules %default-imported-modules
+                 #~(begin
+                     (use-modules #$@(shepherd-service-modules service))
+
+                     #$(shepherd-service-free-form service)))))
+
+(define (shepherd-service-file service)
+  "Return a file defining SERVICE."
+  (if (shepherd-service-free-form service)
+      (shepherd-service-file/free-form service)
+      (shepherd-service-file/regular service)))
+
 (define (scm->go file shepherd)
   "Compile FILE, which contains code to be loaded by shepherd's config file,
 and return the resulting '.go' file. SHEPHERD is used as shepherd package."