services: Missing services are automatically instantiated.

This simplifies OS configuration: users no longer need to be aware of
what a given service depends on.

See the discussion at
<https://lists.gnu.org/archive/html/guix-devel/2018-01/msg00114.html>.

* gnu/services.scm (missing-target-error): New procedure.
(service-back-edges): Use it.
(instantiate-missing-services): New procedure.
* gnu/system.scm (operating-system-services): Call
'instantiate-missing-services'.
* tests/services.scm ("instantiate-missing-services")
("instantiate-missing-services, no default value"): New tests.
* gnu/services/version-control.scm (cgit-service-type)[extensions]: Add
FCGIWRAP-SERVICE-TYPE.
* gnu/tests/version-control.scm (%cgit-os): Remove NGINX-SERVICE-TYPE
and FCGIWRAP-SERVICE-TYPE instances.
* doc/guix.texi (Log Rotation): Remove 'mcron-service-type' in example.
(Miscellaneous Services): Remove 'nginx-service-type' and
'fcgiwrap-service-type' in Cgit example.

1 files changed, 2 insertions, 5 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 1ecdcd2182..58b9675a3f 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -10342,9 +10342,8 @@ with the default settings, for commonly encountered log files.
 
 (operating-system
   ;; @dots{}
-  (services (cons* (service mcron-service-type)
-                   (service rottlog-service-type)
-                   %base-services)))
+  (services (cons (service rottlog-service-type)
+                  %base-services)))
 @end lisp
 
 @defvr {Scheme Variable} rottlog-service-type
@@ -18269,8 +18268,6 @@ The following example will configure the service with default values.
 By default, Cgit can be accessed on port 80 (@code{http://localhost:80}).
 
 @example
-(service nginx-service-type)
-(service fcgiwrap-service-type)
 (service cgit-service-type)
 @end example