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

[Danny Milosavljevic]

* 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.
---
 modules/shepherd.scm         | 10 +++++++++-
 modules/shepherd/comm.scm    | 20 ++++++++++++--------
 modules/shepherd/support.scm | 11 ++++++++++-
 3 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/modules/shepherd.scm b/modules/shepherd.scm
index 5334657..2893143 100644
--- a/modules/shepherd.scm
+++ b/modules/shepherd.scm
@@ -141,12 +141,20 @@
     ;; Enable logging as first action.
     (start-logging logfile)
 
+    (if (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)))
 
     ;; Start the 'root' service.
     (start root-service)
 
+    (set-current-error-port
+     (make-shepherd-output-port (current-error-port) (const #f)))
+
     ;; This _must_ succeed.  (We could also put the `catch' around
     ;; `main', but it is often useful to get the backtrace, and
     ;; `caught-error' does not do this yet.)
diff --git a/modules/shepherd/comm.scm b/modules/shepherd/comm.scm
index 0228f63..dc7d740 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))
 
 
 ;; Command for shepherd.
@@ -200,10 +201,16 @@ on service '~a':")
   ;; Socket of the client currently talking to the daemon.
   (make-parameter #f))
 
+(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 +223,9 @@ 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 +235,7 @@ 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 +266,3 @@ 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