[mux]: Fix mark_dhcp_packet (#9373)

- Consolidate the two [Service] sections by moving the ExecStartPre line for mark_dhcp_packet.py to the first section and removing the second.
- Make the mark_dhcp_packet.py file executable
- Also clean up mark_dhcp_packet.py
    - Remove unused imports
    - Fix spacing and line lengths to conform to PEP8
Signed-off-by: Lawrence Lee <lawlee@microsoft.com>

files/build_templates/mux.service.j2

@@ -10,6 +10,7 @@ StartLimitBurst=3
 [Service]
 User={{ sonicadmin_user }}
 ExecStartPre=/usr/local/bin/write_standby.py
+ExecStartPre=/usr/local/bin/mark_dhcp_packet.py
 ExecStartPre=/usr/bin/{{docker_container_name}}.sh start
 ExecStart=/usr/bin/{{docker_container_name}}.sh wait
 ExecStop=/usr/bin/{{docker_container_name}}.sh stop
@@ -17,15 +18,5 @@ ExecStopPost=/usr/local/bin/write_standby.py
 Restart=always
 RestartSec=30
 
-[Service]
-User={{ sonicadmin_user }}
-ExecStartPre=/usr/local/bin/mark_dhcp_packet.py
-ExecStartPre=/usr/bin/{{docker_container_name}}.sh start
-ExecStart=/usr/bin/{{docker_container_name}}.sh wait
-ExecStop=/usr/bin/{{docker_container_name}}.sh stop
-ExecStopPost=/usr/local/bin/mark_dhcp_packet.py
-Restart=always
-RestartSec=30
-
 [Install]
 WantedBy=sonic.target

files/scripts/mark_dhcp_packet.py

@@ -1,15 +1,13 @@
 #!/usr/bin/env python3
 
-import os
 import subprocess
-import sys
-import time
 
 from sonic_py_common import logger
 from swsscommon import swsscommon
 
 log = logger.Logger('mark_dhcp_packet')
 
+
 class MarkDhcpPacket(object):
     """
     Class used to configure dhcp packet mark in ebtables
@@ -38,7 +36,9 @@ class MarkDhcpPacket(object):
         Initializes the connector during the first call
         """
         if self.state_db_connector is None:
-            self.state_db_connector = swsscommon.SonicV2Connector(host='127.0.0.1')
+            self.state_db_connector = swsscommon.SonicV2Connector(
+                                                    host='127.0.0.1'
+                                                )
             self.state_db_connector.connect(self.state_db_connector.STATE_DB)
 
         return self.state_db_connector
@@ -51,7 +51,8 @@ class MarkDhcpPacket(object):
         localhost_key = self.config_db.get_keys('DEVICE_METADATA')[0]
         metadata = self.config_db.get_entry('DEVICE_METADATA', localhost_key)
 
-        return 'subtype' in metadata and 'dualtor' in metadata['subtype'].lower()
+        return 'subtype' in metadata and \
+               'dualtor' in metadata['subtype'].lower()
 
     def get_mux_intfs(self):
         """
@@ -82,10 +83,16 @@ class MarkDhcpPacket(object):
         self.run_command("sudo ebtables -F INPUT")
 
     def apply_mark_in_ebtables(self, intf, mark):
-        self.run_command("sudo ebtables -A INPUT -i {} -j mark --mark-set {}".format(intf, mark))
+        self.run_command("sudo ebtables -A INPUT -i {} -j mark --mark-set {}"
+                         .format(intf, mark))
 
     def update_mark_in_state_db(self, intf, mark):
-        self.state_db.set(self.state_db.STATE_DB, 'DHCP_PACKET_MARK|' + intf, 'mark', mark)
+        self.state_db.set(
+            self.state_db.STATE_DB,
+            'DHCP_PACKET_MARK|' + intf,
+            'mark',
+            mark
+        )
 
     def apply_marks(self):
         """
@@ -103,6 +110,7 @@ class MarkDhcpPacket(object):
 
         log.log_info("Finish marking dhcp packets in ebtables.")
 
+
 if __name__ == '__main__':
     mark_dhcp_packet = MarkDhcpPacket()
     mark_dhcp_packet.apply_marks()