bootmisc: optionally save the previous dmesg log

X-Gentoo-Bug: 561204
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=561204

conf.d/bootmisc

@@ -8,3 +8,8 @@ wipe_tmp="YES"
 # Write the initial dmesg log into /var/log/dmesg after boot
 # This may be useful if you need the kernel boot log afterwards
 log_dmesg="YES"
+
+# Save the previous dmesg log to dmesc.old
+# This may be useful if you need to compare the current boot to the
+# previous one.
+#previous_dmesg=no

init.d/bootmisc.in

@@ -216,6 +216,9 @@ start()
 			case "$RC_SYS" in
 				VSERVER|OPENVZ|LXC|SYSTEMD-NSPAWN) ;;
 				*)
+					if yesno ${previous_dmesg:-no}; then
+						mv /var/log/dmesg /var/log/dmesg.old
+					fi
 					dmesg > /var/log/dmesg
 					chmod 640 /var/log/dmesg
 					;;