Fix "/host unmount failure" during reboot (sonic-net#4558)

Author: rkdevi27

files/build_templates/sonic_debian_extension.j2

@@ -224,6 +224,11 @@ sudo cp $IMAGE_CONFIGS/rsyslog/rsyslog.conf.j2 $FILESYSTEM_ROOT_USR_SHARE_SONIC_
 sudo cp $IMAGE_CONFIGS/rsyslog/rsyslog.d/* $FILESYSTEM_ROOT/etc/rsyslog.d/
 echo "rsyslog-config.service" | sudo tee -a $GENERATED_SERVICE_FILE
 
+# Copy syslog override files
+sudo mkdir -p $FILESYSTEM_ROOT/etc/systemd/system/syslog.socket.d
+sudo cp $IMAGE_CONFIGS/syslog/override.conf $FILESYSTEM_ROOT/etc/systemd/system/syslog.socket.d/override.conf
+sudo cp $IMAGE_CONFIGS/syslog/host_umount.sh $FILESYSTEM_ROOT/usr/bin/
+
 # Copy logrotate.d configuration files
 sudo cp -f $IMAGE_CONFIGS/logrotate/logrotate.d/* $FILESYSTEM_ROOT/etc/logrotate.d/
 

files/image_config/syslog/host_umount.sh

@@ -0,0 +1,31 @@
+#!/bin/bash
+# This script is invoked at the closure of syslog socket during reboot
+# This will stop journal services, unmount /var/log and delete loop device 
+# associated to /host to ensure proper unmount of /host
+
+journal_stop() {
+    systemctl stop systemd-journald.service
+    systemctl stop systemd-journald.socket
+    systemctl stop systemd-journald-audit.socket
+    systemctl stop systemd-journald-dev-log.socket
+}
+
+delete_loop_device() {
+    umount /var/log
+    if [[ $? -ne 0 ]]
+    then
+        exit 0
+    fi
+    losetup -d /dev/loop1
+}
+
+case "$1" in
+    journal_stop|delete_loop_device)
+        $1
+        ;;
+    *)
+        echo "Usage: $0 {journal_stop|delete_loop_device}" >&2
+        exit 1
+        ;;
+esac
+

files/image_config/syslog/override.conf

@@ -0,0 +1,6 @@
+[Unit]
+After=var-log.mount host.mount
+
+[Socket]
+ExecStopPre=/usr/bin/host_umount.sh journal_stop
+ExecStopPost=/usr/bin/host_umount.sh delete_loop_device