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, 31 insertions, 1 deletions

diff --git a/tests/services.scm b/tests/services.scm
index ca32b565c49..b146a0dec2a 100644
--- a/tests/services.scm
+++ b/tests/services.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2015, 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -122,6 +122,36 @@
       (fold-services (list s) #:target-type t1)
       #f)))
 
+(test-assert "instantiate-missing-services"
+  (let* ((t1 (service-type (name 't1) (extensions '())
+                           (default-value 'dflt)
+                           (compose concatenate)
+                           (extend cons)))
+         (t2 (service-type (name 't2)
+                           (extensions
+                            (list (service-extension t1 list)))))
+         (s1 (service t1 'hey!))
+         (s2 (service t2 42)))
+    (and (lset= equal?
+                (list (service t1) s2)
+                (instantiate-missing-services (list s2)))
+         (equal? (list s1 s2)
+                 (instantiate-missing-services (list s1 s2))))))
+
+(test-assert "instantiate-missing-services, no default value"
+  (let* ((t1 (service-type (name 't1) (extensions '())))
+         (t2 (service-type (name 't2)
+                           (extensions
+                            (list (service-extension t1 list)))))
+         (s  (service t2 42)))
+    (guard (c ((missing-target-service-error? c)
+               (and (eq? (missing-target-service-error-target-type c)
+                         t1)
+                    (eq? (missing-target-service-error-service c)
+                         s))))
+      (instantiate-missing-services (list s))
+      #f)))
+
 (test-assert "shepherd-service-lookup-procedure"
   (let* ((s1 (shepherd-service (provision '(s1 s1b)) (start #f)))
          (s2 (shepherd-service (provision '(s2 s2b)) (start #f)))