file-systems: %base-file-systems: Add tmpfs /run.

* gnu/system/file-systems (%runtime-variable-data): New variable.
(%base-file-systems): Add it.
* doc/guix.texi (File Systems): Document it.
* gnu/services.scm (cleanup-gexp): Adjust accordingly.

Change-Id: I3a95e49d396fbb2577026aefc247cfe996c5f267
Modified-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

doc/guix.texi

@@ -18228,6 +18228,11 @@ The daemon itself is still able to write to the store: it remounts it
 read-write in its own ``name space.''
 @end defvar
 
+@defvar %runtime-variable-data
+This file system is mounted as @file{/run} and contains system
+information data describing the system since it was booted.
+@end defvar
+
 @defvar %binary-format-file-system
 The @code{binfmt_misc} file system, which allows handling of arbitrary
 executable file types to be delegated to user space.  This requires the

gnu/services.scm

@@ -632,7 +632,7 @@ information is missing, return the empty list (for channels) and possibly
     #~(begin
         (use-modules (guix build utils))
 
-        ;; Clean out /tmp, /var/run, and /run.
+        ;; Clean out /tmp and /var/run.
         ;;
         ;; XXX This needs to happen before service activations, so it
         ;; has to be here, but this also implicitly assumes that /tmp
@@ -663,15 +663,12 @@ information is missing, return the empty list (for channels) and possibly
            (setlocale LC_CTYPE "en_US.utf8")
            (delete-file-recursively "/tmp")
            (delete-file-recursively "/var/run")
-           (delete-file-recursively "/run")
 
            ;; Note: The second argument to 'mkdir' is and'ed with umask,
            ;; hence the 'chmod' calls.
            (mkdir "/tmp" #o1777)
            (chmod "/tmp" #o1777)
            (mkdir "/var/run" #o755)
-           (chmod "/var/run" #o755)
-           (mkdir "/run" #o755)
            (chmod "/var/run" #o755))))))
 
 (define cleanup-service-type

gnu/system/file-systems.scm

@@ -82,6 +82,7 @@
             %pseudo-terminal-file-system
             %tty-gid
             %immutable-store
+            %runtime-variable-data
             %control-groups
             %elogind-file-systems
 
@@ -448,6 +449,17 @@ TARGET in the other system."
     (check? #f)
     (flags '(read-only bind-mount no-atime))))
 
+(define %runtime-variable-data
+  (file-system
+    (type "tmpfs")
+    (mount-point "/run")
+    (device "tmpfs")
+    (flags '(no-suid no-dev strict-atime))
+    (options "mode=0755,nr_inodes=800k,size=20%")
+    (needed-for-boot? #t)
+    (check? #f)
+    (create-mount-point? #t)))
+
 (define %control-groups
   ;; The cgroup2 file system.
   (list (file-system
@@ -497,7 +509,8 @@ TARGET in the other system."
         %debug-file-system
         %shared-memory-file-system
         %efivars-file-system
-        %immutable-store))
+        %immutable-store
+        %runtime-variable-data))
 
 (define %base-live-file-systems
   ;; This is the bare minimum to use live file-systems.