services: nscd: Provide an 'activate' script to make /var/run/nscd.

* gnu/services/base.scm (nscd-service): Add 'activate' field.
* guix/build/install.scm (directives): Remove /var/run/nscd; add
  /var/run.
* doc/guix.texi (Defining Services): Add 'activate' field in example.
  Document it.

doc/guix.texi

@@ -3299,6 +3299,9 @@ like:
     (return (service
              (documentation "Run libc's name service cache daemon.")
              (provision '(nscd))
+             (activate #~(begin
+                           (use-modules (guix build utils))
+                           (mkdir-p "/var/run/nscd")))
              (start #~(make-forkexec-constructor
                        (string-append #$glibc "/sbin/nscd")
                        "-f" "/dev/null" "--foreground"))
@@ -3307,12 +3310,16 @@ like:
 @end lisp
 
 @noindent
-The @code{start} and @code{stop} fields are G-expressions
-(@pxref{G-Expressions}).  They refer to dmd's facilities to start and
-stop processes (@pxref{Service De- and Constructors,,, dmd, GNU dmd
-Manual}).  The @code{provision} field specifies the name under which
-this service is known to dmd, and @code{documentation} specifies on-line
-documentation.  Thus, the commands @command{deco start ncsd},
+The @code{activate}, @code{start}, and @code{stop} fields are G-expressions
+(@pxref{G-Expressions}).  The @code{activate} field contains a script to
+run at ``activation'' time; it makes sure that the @file{/var/run/nscd}
+directory exists before @command{nscd} is started.
+
+The @code{start} and @code{stop} fields refer to dmd's facilities to
+start and stop processes (@pxref{Service De- and Constructors,,, dmd,
+GNU dmd Manual}).  The @code{provision} field specifies the name under
+which this service is known to dmd, and @code{documentation} specifies
+on-line documentation.  Thus, the commands @command{deco start ncsd},
 @command{deco stop nscd}, and @command{deco doc nscd} will do what you
 would expect (@pxref{Invoking deco,,, dmd, GNU dmd Manual}).
 

gnu/services/base.scm

@@ -225,6 +225,11 @@ stopped before 'kill' is called."
              (documentation "Run libc's name service cache daemon (nscd).")
              (provision '(nscd))
              (requirement '(user-processes))
+
+             (activate #~(begin
+                           (use-modules (guix build utils))
+                           (mkdir-p "/var/run/nscd")))
+
              (start
               #~(make-forkexec-constructor (string-append #$glibc "/sbin/nscd")
                                            "-f" "/dev/null"

guix/build/install.scm

@@ -76,9 +76,9 @@ STORE."
   `((directory ,store 0 0)
     (directory "/etc")
     (directory "/var/log")                          ; for dmd
-    (directory "/var/run/nscd")
     (directory "/var/guix/gcroots")
     (directory "/var/empty")                        ; for no-login accounts
+    (directory "/var/run")
     (directory "/run")
     ("/var/guix/gcroots/booted-system" -> "/run/booted-system")
     ("/var/guix/gcroots/current-system" -> "/run/current-system")