services: Add 'system-service-type'.

* gnu/services.scm (system-derivation): New procedure.
  (system-service-type): New variable.
  (boot-script-entry): New procedure.
  (boot-service-type): Extend SYSTEM-SERVICE-TYPE.
  (etc-entry): New procedure.
  (etc-service-type): Extend SYSTEM-SERVICE-TYPE.
  (fold-services): Change default #:target-type to SYSTEM-SERVICE-TYPE.
* gnu/system.scm (operating-system-directory-base-entries): New procedure.
  (essential-services): Use it.  Add an instance of
  SYSTEM-SERVICE-TYPE.
  (operating-system-boot-script): Pass #:target-type to 'fold-services'.
  (operating-system-derivation): Rewrite in terms of 'fold-services'.
* gnu/system/linux-container.scm (system-container): Remove.
  (container-script): Use 'operating-system-derivation'.
* guix/scripts/system.scm (export-extension-graph): Replace
  BOOT-SERVICE-TYPE by SYSTEM-SERVICE-TYPE.
* doc/images/service-graph.dot: Add 'system' node and edges.
* doc/guix.texi (Service Composition): Mention SYSTEM-SERVICE-TYPE.
  (Service Reference): Document it.  Update 'fold-services'
  documentation.

2 files changed, 22 insertions, 9 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 74e0977db5..6ab98deef3 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -7589,8 +7589,11 @@ as arrows, a typical system might provide something like this:
 
 @image{images/service-graph,,5in,Typical service extension graph.}
 
-At the bottom, we see the @dfn{boot service}, which produces the boot
-script that is executed at boot time from the initial RAM disk.
+@cindex system service
+At the bottom, we see the @dfn{system service}, which produces the
+directory containing everything to run and boot the system, as returned
+by the @command{guix system build} command.  @xref{Service Reference},
+to learn about the other service types shown here.
 @xref{system-extension-graph, the @command{guix system extension-graph}
 command}, for information on how to generate this representation for a
 particular operating system definition.
@@ -7853,12 +7856,14 @@ Return true if @var{obj} is a service extension.
 
 At the core of the service abstraction lies the @code{fold-services}
 procedure, which is responsible for ``compiling'' a list of services
-down to a single boot script.  In essence, it propagates service
-extensions down the service graph, updating each node parameters on the
-way, until it reaches the root node.
+down to a single directory that contains everything needed to boot and
+run the system---the directory shown by the @command{guix system build}
+command (@pxref{Invoking guix system}).  In essence, it propagates
+service extensions down the service graph, updating each node parameters
+on the way, until it reaches the root node.
 
 @deffn {Scheme Procedure} fold-services @var{services} @
-                            [#:target-type @var{boot-service-type}]
+                            [#:target-type @var{system-service-type}]
 Fold @var{services} by propagating their extensions down to the root of
 type @var{target-type}; return the root service adjusted accordingly.
 @end deffn
@@ -7866,9 +7871,14 @@ type @var{target-type}; return the root service adjusted accordingly.
 Lastly, the @code{(gnu services)} module also defines several essential
 service types, some of which are listed below.
 
+@defvr {Scheme Variable} system-service-type
+This is the root of the service graph.  It produces the system directory
+as returned by the @command{guix system build} command.
+@end defvr
+
 @defvr {Scheme Variable} boot-service-type
-The type of the ``boot service'', which is the root of the service
-graph.
+The type of the ``boot service'', which produces the @dfn{boot script}.
+The boot script is what the initial RAM disk runs when booting.
 @end defvr
 
 @defvr {Scheme Variable} etc-service-type
diff --git a/doc/images/service-graph.dot b/doc/images/service-graph.dot
index 3397b878e9..04f231bb09 100644
--- a/doc/images/service-graph.dot
+++ b/doc/images/service-graph.dot
@@ -4,7 +4,8 @@ digraph "Service Type Dependencies" {
   etc [shape = box, fontname = Helvetica];
   accounts [shape = box, fontname = Helvetica];
   activation [shape = box, fontname = Helvetica];
-  boot [shape = house, fontname = Helvetica];
+  boot [shape = box, fontname = Helvetica];
+  system [shape = house, fontname = Helvetica];
   lshd -> dmd;
   lshd -> pam;
   udev -> dmd;
@@ -32,4 +33,6 @@ digraph "Service Type Dependencies" {
   guix -> dmd;
   guix -> activation;
   guix -> accounts;
+  boot -> system;
+  etc -> system;
 }