[dhcp_relay] Use dhcprelayd to manage critical processes

[yaqiangz]

Why I did it

Currently, dhcpv4 related processes in dhcp_relay container are managed by supervisord (Configuration file for them is generated when container start), which require restart dhcp_relay container when change.
In ipv4 dhcp_server enabled scenario (HLD: sonic-net/SONiC#1282), we need DHCP packets transferred to netdev docker0, cmds is like below:

/usr/sbin/dhcrelay -d -m discard -a %h:%p %P --name-alias-map-file /tmp/port-name-alias-map.txt -id Vlan1000 -iu docker0 240.127.1.2

It means that we need to run dhcrelay/dhcpmon processes in another format when dhcp_server feature is enabled. To avoid container restart while state of dhcp_server feature enabled, use dhcprelayd to manage dhcrelay/dhcpmon process.

Work item tracking

• Microsoft ADO (number only): 25904413

How I did it

1. Modify j2 template files in docker-dhcp-relay. Add dhcprelayd to group dhcp-relay instead of isc-dhcp-relay-VlanXXX, which would make dhcprelayd to become critical process.
2. In dhcprelayd, subscribe FEATURE table to check whether dhcp_server feature is enabled.
   2.1 If dhcp_server feature is disabled, means we need original dhcp_relay functionality, dhcprelayd would do nothing. Because dhcrelay/dhcpmon configuration is generated in supervisord configuration, they will automatically run.
   2.2 If dhcp_server feature is enabled, dhcprelayd will stop dhcpmon/dhcrelay processes started by supervisord and subscribe dhcp_server related tables in config_db to start dhcpmon/dhcrelay processes.
   2.3 While dhcprelayd running, it will regularly check feature status (by default per 5s) and would encounter below 4 state change about dhcp_server feature:
   A) disabled -> enabled
   In this scenario, dhcprelayd will subscribe dhcp_server related tables and stop dhcpmon/dhcrelay processes started by supervisord and start new pair of dhcpmon/dhcrelay processes. After this, dhcpmon/dhcrelay processes are totally managed by dhcprelayd.
   B) enabled -> enabled
   In this scenaro, dhcprelayd will monitor db changes in dhcp_server related tables to determine whether to restart dhcpmon/dhrelay processes.
   C) enabled -> disabled
   In this scenario, dhcprelayd would unsubscribe dhcp_server related tables and kill dhcpmon/dhcrelay processes started by itself. And then dhcprelayd will start dhcpmon/dhcrelay processes via supervisorctl.
   D) disabled -> disabled
   dhcprelayd will check whether dhcrelay processes running status consistent with supervisord configuration file. If they are not consistent, dhcprelayd will kill itself, then dhcp_relay container will stop because dhcprelayd is critical process.

How to verify it

1. Unit tests passed.

=============================================================================================== test session starts ===============================================================================================
platform linux -- Python 3.9.2, pytest-6.0.2, py-1.10.0, pluggy-0.13.0
rootdir: /sonic/src/sonic-dhcp-server, configfile: setup.cfg, testpaths: tests
plugins: pyfakefs-5.3.0, cov-2.10.1
collected 408 items                                                                                                                                                                                               

tests/test_dhcp_cfggen.py ..................                                                                                                                                                                [  4%]
tests/test_dhcp_db_monitor.py ............................................................................................................................................................................. [ 46%]
...................................................................                                                                                                                                         [ 63%]
tests/test_dhcp_lease.py .....                                                                                                                                                                              [ 64%]
tests/test_dhcprelayd.py ......................................................................................................                                                                             [ 89%]
tests/test_dhcpservd.py .......                                                                                                                                                                             [ 91%]
tests/test_utils.py ....................................                                                                                                                                                    [100%]

----------- coverage: platform linux, python 3.9.2-final-0 -----------
Name                                   Stmts   Miss Branch BrPart     Cover   Missing
-------------------------------------------------------------------------------------
dhcp_server/dhcprelayd/dhcprelayd.py     179     21     68      0    86.64%   94-133
dhcp_server/dhcpservd/dhcpservd.py        67      4     14      0    92.59%   91-98
dhcp_server/dhcpservd/dhcp_lease.py       95      3     30      4    94.40%   57->59, 59, 60->61, 61, 74->76, 112->134, 153
dhcp_server/dhcpservd/dhcp_cfggen.py     217      6     92      1    97.73%   201-205, 291->293, 293
dhcp_server/common/utils.py               76      1     42      1    98.31%   129->134, 134
-------------------------------------------------------------------------------------
TOTAL                                    899     35    334      6    95.54%

2 files skipped due to complete coverage.

Required test coverage of 80.0% reached. Total coverage: 95.54%

2. Build wheel and install it manually to test.
3. Run dhcp_relay tests in sonic-mgmt in t0/m0/m0-2vlan topos, all passed

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[yaqiangz]

@kellyyeh Could you pls help to review this PR?

[yaqiangz]

@yxieca Could you help to merge this PR?