services: Add ‘system-log’ Shepherd service.

* gnu/services/shepherd.scm (gexp-or-integer?)
(gexp-or-string?, gexp-or-string-or-false?): New procedures.
(system-log-configuration): New record type.
(shepherd-system-log-service-type): New variable.
* doc/guix.texi (Shepherd Services): Document it.

Reviewed-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>
Change-Id: I1e29fe0977eb4f8026ee6a2d2f282e269c8c00b4

doc/guix.texi

@@ -46047,6 +46047,62 @@ herd schedule timer at 07:00 -- mpg123 Music/alarm.mp3
 on the @code{timer} service.
 @end defvar
 
+@cindex system log service, from Shepherd
+@cindex syslogd, Shepherd system log service
+@defvar shepherd-system-log-service-type
+This service implements a @dfn{system log}, reading messages
+applications send to @file{/dev/log} and writing them to files or
+terminals according to user-defined rules.  It provides functionality
+traditionally implemented by @command{syslogd} programs.
+
+The value of services of this type must be a
+@code{system-log-configuration} record, as described below.
+@end defvar
+
+@c %start of fragment
+
+@deftp {Data Type} system-log-configuration
+Available @code{system-log-configuration} fields are:
+
+@table @asis
+@item @code{provision} (default: @code{(system-log syslogd)}) (type: list-of-symbols)
+The name(s) of the system log service.
+
+@item @code{requirement} (default: @code{(root-file-system)}) (type: list-of-symbols)
+Dependencies of the system log service.
+
+@item @code{kernel-log-file} (type: gexp-or-string-or-false)
+File from which kernel messages are read, @file{/dev/kmsg} by default.
+
+@item @code{message-destination} (default: @code{#f}) (type: gexp-or-false)
+This gexp must evaluate to a procedure that, when passed a log message,
+returns the list of files to write it to; @code{#f} is equivalent to
+using @code{(default-message-destination-procedure)}.  @xref{System Log
+Service,,,shepherd,The GNU Shepherd Manual}, for information on how to
+write that procedure.
+
+@item @code{date-format} (type: gexp-or-string)
+String or string-valued gexp specifying how to format timestamps in log
+file.  It must be a valid string for @code{strftime} (@pxref{Time,,,
+guile,GNU Guile Reference Manual}), including delimiting space---e.g.,
+@code{"%c "} for a format identical to that of traditional syslogd
+implementations.
+
+@item @code{history-size} (type: gexp-or-integer)
+Number of logging messages kept in memory for the purposes of making
+them available to @command{herd status system-log}.
+
+@item @code{max-silent-time} (type: gexp-or-integer)
+Time after which a mark is written to log files if nothing was logged
+during that time frame.
+
+@end table
+
+@end deftp
+
+
+@c %end of fragment
+
 @defvar %shepherd-root-service
 This service represents PID@tie{}1.
 @end defvar

gnu/services/shepherd.scm

@@ -31,6 +31,7 @@
   #:use-module ((guix diagnostics)
                 #:select (define-with-syntax-properties formatted-message))
   #:use-module (gnu services)
+  #:use-module (gnu services configuration)
   #:use-module (gnu services herd)
   #:use-module (gnu packages admin)
   #:use-module (ice-9 match)
@@ -84,6 +85,17 @@
             shepherd-timer-service-type
             shepherd-transient-service-type
 
+            system-log-configuration
+            system-log-configuration?
+            system-log-configuration-provision
+            system-log-configuration-requirement
+            system-log-configuration-message-destination
+            system-log-configuration-date-format
+            system-log-configuration-history-size
+            system-log-configuration-max-silent-time
+
+            shepherd-system-log-service-type
+
             assert-valid-graph))
 
 ;;; Commentary:
@@ -658,3 +670,87 @@ herd spawn transient -E SSH_AUTH_SOCK=$SSH_AUTH_SOCK -- \\
 
 This runs @command{rsync} in the background, as a service that you can inspect
 with @command{herd status} and stop with @command{herd stop}.")))
+
+
+;;;
+;;; System log.
+;;;
+
+(define (gexp-or-false? x)
+  (or (gexp? x) (not x)))
+
+(define (gexp-or-integer? x)
+  (or (gexp? x) (integer? x)))
+
+(define (gexp-or-string? x)
+  (or (gexp? x) (string? x)))
+
+(define (gexp-or-string-or-false? x)
+  (or (gexp-or-string? x) (not x)))
+
+(define-configuration/no-serialization system-log-configuration
+  (provision
+   (list-of-symbols '(system-log syslogd))
+   "The name(s) of the system log service.")
+  (requirement
+   (list-of-symbols '(root-file-system))
+   "Dependencies of the system log service.")
+  (kernel-log-file
+   (gexp-or-string-or-false #~(kernel-log-file))
+   "File from which kernel messages are read, @file{/dev/kmsg} by default.")
+  (message-destination
+   (gexp-or-false #f)
+   "This gexp must evaluate to a procedure that, when passed a log message,
+returns the list of files to write it to; @code{#f} is equivalent to using
+@code{(default-message-destination-procedure)}.  @xref{System Log Service,,,
+shepherd, The GNU Shepherd Manual}, for information on how to write that
+procedure.")
+  (date-format
+   (gexp-or-string #~default-logfile-date-format)
+   "String or string-valued gexp specifying how to format timestamps in log
+file.  It must be a valid string for @code{strftime} (@pxref{Time,,, guile,
+GNU Guile Reference Manual}), including delimiting space---e.g., @code{\"%c
+\"} for a format identical to that of traditional syslogd implementations.")
+  (history-size
+   (gexp-or-integer #~(default-log-history-size))
+   "Number of logging messages kept in memory for the purposes of making them
+available to @command{herd status system-log}.")
+  (max-silent-time
+   (gexp-or-integer #~(default-max-silent-time))
+   "Time after which a mark is written to log files if nothing was logged
+during that time frame."))
+
+(define shepherd-system-log-service-type
+  (shepherd-service-type
+   'shepherd-system-log
+   (lambda (config)
+     (match-record config <system-log-configuration>
+       (provision requirement message-destination
+        date-format history-size max-silent-time)
+       (shepherd-service
+        (documentation "Shepherd's built-in system log (syslogd).")
+        (provision (system-log-configuration-provision config))
+        (requirement (system-log-configuration-requirement config))
+        (modules '((shepherd service system-log)
+                   ((shepherd support) #:select (default-logfile-date-format))
+                   ((shepherd logger) #:select (default-log-history-size))))
+        (free-form
+         #~(system-log-service #:provision '#$provision
+                               #:requirement '#$requirement
+
+                               ;; XXX: As of Shepherd 1.0.1,
+                               ;; 'default-message-destination-procedure' is not
+                               ;; exported, hence this conditional.
+                               #$@(match message-destination
+                                    (#f #~())
+                                    (value #~(#:message-destination #$value)))
+
+                               #:date-format #$date-format
+                               #:history-size #$history-size
+                               #:max-silent-time #$max-silent-time)))))
+   (system-log-configuration)
+   (description
+    "The Shepherd's @code{system-log} service plays the role of traditional
+@command{syslogd} program, reading data logged by daemons to @file{/dev/log}
+and writing it to several files in @file{/var/log} according to user-provided
+dispatching rules.")))