[bug#30498] [WIP v2 shepherd] shepherd: If /dev/kmsg is writable, use it for logging.

[Danny Milosavljevic <dannym@scratchpost.org>]

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Hi Ludo,

thanks for the review!  Does that mean that I should push to shepherd?

If not, new patch attached :)

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From 40426570679e83fff25eadbcc20476ebc321740f Mon Sep 17 00:00:00 2001
From: Danny Milosavljevic <dannym@scratchpost.org>
Date: Sat, 17 Feb 2018 17:44:34 +0100
Subject: [PATCH v2] shepherd: If /dev/kmsg is writable, use it for logging.
Tags: patch

* modules/shepherd.scm (main): If /dev/kmsg is used, don't log to console
again - use only /dev/kmsg.  Also redirect stderr to /dev/kmsg in
that case.
* modules/shepherd/comm.scm (%current-logfile-date-format): New variable.
(make-shepherd-output-port): Use it.  Export.
* modules/shepherd/support.scm (default-logfile-date-format): New variable.
(default-logfile): Use /dev/kmsg if writable.
(default-logfile-date-format): Drop duplicate timestamp.
* doc/shepherd.texi (logging): Document /dev/kmsg.
---
 doc/shepherd.texi            |  3 ++-
 modules/shepherd.scm         | 11 ++++++++++-
 modules/shepherd/comm.scm    | 22 ++++++++++++++--------
 modules/shepherd/support.scm | 11 ++++++++++-
 4 files changed, 36 insertions(+), 11 deletions(-)

diff --git a/doc/shepherd.texi b/doc/shepherd.texi
index 810336c..3c02d92 100644
--- a/doc/shepherd.texi
+++ b/doc/shepherd.texi
@@ -405,7 +405,8 @@ permissions are not as expected.
 @cindex logging
 @cindex log file
 Log output into @var{file}, or if @var{file} is not given,
-@code{/var/log/shepherd.log} when running as superuser,
+@code{/dev/kmsg} when running as superuser (except when this is not possible --
+then it uses @code{/var/log/shepherd.log}),
 @code{$XDG_CONFIG_HOME/shepherd/shepherd.log} otherwise.
 
 @item --pid[=@var{file}]
diff --git a/modules/shepherd.scm b/modules/shepherd.scm
index 5334657..6acd41e 100644
--- a/modules/shepherd.scm
+++ b/modules/shepherd.scm
@@ -141,8 +141,17 @@
     ;; Enable logging as first action.
     (start-logging logfile)
 
+    (when (string=? logfile "/dev/kmsg")
+      ;; Prevent duplicate messages.
+      (set-current-output-port (%make-void-port "w")))
+
     ;; Send output to log and clients.
-    (set-current-output-port shepherd-output-port)
+    (set-current-output-port
+     (make-shepherd-output-port (current-output-port)))
+
+    ;; Send errors to log.
+    (set-current-error-port
+     (make-shepherd-output-port (current-error-port) (const #f)))
 
     ;; Start the 'root' service.
     (start root-service)
diff --git a/modules/shepherd/comm.scm b/modules/shepherd/comm.scm
index 0228f63..99b8d04 100644
--- a/modules/shepherd/comm.scm
+++ b/modules/shepherd/comm.scm
@@ -51,7 +51,8 @@
             start-logging
             stop-logging
             %current-client-socket
-            shepherd-output-port))
+            %current-logfile-date-format
+            make-shepherd-output-port))
 
 \f
 ;; Command for shepherd.
@@ -200,10 +201,18 @@ on service '~a':")
   ;; Socket of the client currently talking to the daemon.
   (make-parameter #f))
 
+;; Every entry in the logfile is prefixed with
+;; (strftime %current-logfile-date-format).
+(define %current-logfile-date-format
+  (make-parameter default-logfile-date-format))
+
 ;; We provide our own output mechanism, because we have certain
 ;; special needs; most importantly, we want to send output to herd
 ;; sometimes.
-(define (make-shepherd-output-port original-output-port)
+(define* (make-shepherd-output-port original-output-port
+                                   #:optional
+                                   (current-client-socket-thunk
+                                    %current-client-socket))
   (make-soft-port
    (vector
 
@@ -216,9 +225,9 @@ on service '~a':")
       (lambda (str)
         ;; When herd is connected, send it the output; otherwise, in the
         ;; unlikely case nobody is listening, send to the standard output.
-        (if (%current-client-socket)
+        (if (current-client-socket-thunk)
             (catch-system-error
-             (display str (%current-client-socket)))
+             (display str (current-client-socket-thunk)))
             (display str original-output-port))
 
         ;; Logfile, buffer line-wise and output time for each
@@ -228,7 +237,7 @@ on service '~a':")
             (let* ((log (lambda (x)
                           (display x log-output-port)))
                    (init-line (lambda ()
-                                (log (strftime "%Y-%m-%d %H:%M:%S "
+                                (log (strftime (%current-logfile-date-format)
                                                (localtime (current-time)))))))
               (init-line)
               (for-each log (reverse buffer))
@@ -259,6 +268,3 @@ on service '~a':")
 
    ;; It's an output-only port.
    "w"))
-
-(define shepherd-output-port
-  (make-shepherd-output-port (current-output-port)))
diff --git a/modules/shepherd/support.scm b/modules/shepherd/support.scm
index bb01edc..585aef9 100644
--- a/modules/shepherd/support.scm
+++ b/modules/shepherd/support.scm
@@ -22,6 +22,7 @@
 (define-module (shepherd support)
   #:use-module (shepherd config)
   #:use-module (ice-9 match)
+  #:use-module (ice-9 format)
   #:export (call/ec
             caught-error
             assert
@@ -43,6 +44,7 @@
 
             user-homedir
             default-logfile
+            default-logfile-date-format
             default-config-file
             default-socket-dir
             default-socket-file
@@ -282,9 +284,16 @@ TARGET should be a string representing a filepath + name."
 ;; Logfile.
 (define default-logfile
   (if (zero? (getuid))
-      (string-append %localstatedir "/log/shepherd.log")
+      (if (access? "/dev/kmsg" W_OK)
+          "/dev/kmsg"
+          (string-append %localstatedir "/log/shepherd.log"))
       (string-append %user-config-dir "/shepherd.log")))
 
+(define default-logfile-date-format
+   (if (and (zero? (getuid)) (string=? default-logfile "/dev/kmsg"))
+       (format #f "shepherd[~d]: " (getpid))
+       "%Y-%m-%d %H:%M:%S "))
+
 ;; Configuration file.
 (define (default-config-file)
   "Return the default configuration file---either the user's file, or the