nixos/stage-2: Added boot.logCommands

nixos/modules/system/boot/stage-2-init.sh

···
 mount -t tmpfs -o "mode=0755" tmpfs /var/setuid-wrappers
 
 
+# Optionally log the script output to /dev/kmsg or /run/log/stage-2-init.log.
+# Only at this point are all the necessary prerequisites ready for these commands.
+if test -n "@logCommands@"; then
+    exec {logOutFd}>&1 {logErrFd}>&2
+    if test -w /dev/kmsg; then
+        exec > >(tee -i /proc/self/fd/"$logOutFd" | while read line; do
+            if test -n "$line"; then
+                echo "stage-2-init: $line" > /dev/kmsg
+            fi
+        done) 2>&1
+    else
+        mkdir -p /run/log
+        exec > >(tee -i /run/log/stage-2-init.log) 2>&1
+    fi
+fi
+
+
 # Run the script that performs all configuration activation that does
 # not have to be done at boot time.
 echo "running activation script..."
···
 
 # Run any user-specified commands.
 @shell@ @postBootCommands@
+
+
+# Reset the logging file descriptors
+if test -n "@logCommands@"; then
+    exec 1>&$logOutFd 2>&$logErrFd
+    exec {logOutFd}>&- {logErrFd}>&-
+fi 
 
 
 # Start systemd.

nixos/modules/system/boot/stage-2.nix

···
     src = ./stage-2-init.sh;
     shellDebug = "${pkgs.bashInteractive}/bin/bash";
     isExecutable = true;
-    inherit (config.boot) devShmSize runSize;
+    inherit (config.boot) logCommands devShmSize runSize;
     inherit (config.nix) readOnlyStore;
     inherit (config.networking) useHostResolvConf;
     ttyGid = config.ids.gids.tty;
···
   options = {
 
     boot = {
+
+      logCommands = mkOption {
+        default = false;
+        type = types.bool;
+        description = ''
+          Whether to replicate command output of stage-1 booting to <filename>/dev/kmsg</filename> or <filename>/run/log/stage-2-init.log</filename> if <filename>/dev/kmsg</filename> is not writable.
+        '';
+      };
 
       postBootCommands = mkOption {
         default = "";