Add HLD for Reboot support BlockingMode in SONiCDev/lityu/local

doc/reboot/Reboot_BlockingMode_HLD.md

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+# Reboot support BlockingMode in SONiC
+
+## Table of Content
+
+- [Overview](#overview)
+- [Background](#background)
+- [Function Design](#function-design)
+- [Test Plan](#functional-test)
+
+## Revision
+
+| Revision | Date       | Author     | Change Description |
+| -------- | ---------- | ---------- | ------------------ |
+| 1.0      | June 12 2025 | Litao Yu | Initial proposal   |
+
+## Overview
+
+We are trying to introduce blocking mode for reboot script, which currently is in non-blocking mode. This will make automation system easier to identify whether the reboot operation is success.
+
+## Background
+
+Currently the reboot script uses linux command `systemctl reboot` to reboot the SONIC system. And the linux command is asynchronous. For detail you can refer `reboot` section in [Linux Man Doc for systemctl](https://www.man7.org/linux/man-pages/man1/systemctl.1.html).
+
+As the result, the output from user batch side will be unpredictable. It displays as 2 kinds of behavior randomly:
+
+- [C] The user bash stopped by systemd before `systemctl reboot` returns. Then the automation system will see extra user prompt. The time sequence is as following:
+![MAN Reboot](./img/background-C.png)
+The console output will like the following:
+```
+admin@sonic:~$ sudo reboot
+Error response from daemon: Container e6a136aee372584ae6e64d1c55ae49352d842d76daff13a782a4ecfa0262d0e5 is not running
+/var/log: 3.9 GiB (4138283008 bytes) trimmed on /dev/loop1
+/host: 8 GiB (8547295232 bytes) trimmed on /dev/sda2
+Fri 20 Oct 2023 06:03:33 AM UTC Issuing OS-level reboot ...
+admin@sonic:~$ Connection to 10.150.22.134 closed by remote host.
+Connection to 10.150.22.134 closed.
+```
+
+- [C'] The user bash stopped by systemd after `systemctl reboot` returns. Then the automation system will not see extra user prompt. The time sequence is as following:
+![MAN Reboot](./img/background-C'.png)
+The console output will like the following:
+```
+admin@sonic:~$ sudo reboot
+Error response from daemon: Container e6a136aee372584ae6e64d1c55ae49352d842d76daff13a782a4ecfa0262d0e5 is not running
+/var/log: 3.9 GiB (4138283008 bytes) trimmed on /dev/loop1
+/host: 8 GiB (8547295232 bytes) trimmed on /dev/sda2
+Fri 20 Oct 2023 06:03:33 AM UTC Issuing OS-level reboot ...
+Connection to 10.150.22.134 closed by remote host.
+Connection to 10.150.22.134 closed.
+```
+
+We want to unify the behavior to `C'` to make sure the automation system always get the same result instead of a random result.
+
+## Function Design
+
+We don't want to make a break change to SONIC reboot command, so we introduce 2 types of inputs to identify we are trying to run on blocking-mode:
+
+- Paramter: Use command parameter `-b` to enable blocking mode for reboot script. The whole command will looks like `reboot -b`.
+
+- Config File: The reboot script will check the config file in `/etc/sonic/reboot.conf`. If the file contains the follow config, SONIC reboot will default use blocking mode.
+
+```
+blocking_mode=true
+```
+
+And to identify the reboot script is still running, we will print dots every 10 seconds and change line every 50 dots. The console output will like the following:
+
+```
+admin@sonic:~$ sudo reboot
+Error response from daemon: Container e6a136aee372584ae6e64d1c55ae49352d842d76daff13a782a4ecfa0262d0e5 is not running
+/var/log: 3.9 GiB (4138283008 bytes) trimmed on /dev/loop1
+/host: 8 GiB (8547295232 bytes) trimmed on /dev/sda2
+Fri 20 Oct 2023 06:03:33 AM UTC Issuing OS-level reboot ...
+..................................................
+..................................................
+..............................Connection to 10.150.22.134 closed by remote host.
+Connection to 10.150.22.134 closed.
+```
+
+The first requirement to enable the dots printing is enable blocking mode. And the second requirement is one of the below options:
+- Parameter: Use command parameter `-v` to enable Verbose mode for reboot script. The whole command will looks like `reboot -v -b`
+
+- Config file: The reboot script will check the config file in `/etc/sonic/reboot.conf`. The file needs to contain the config `print_process_in_blocking_mode=true`. The whole config file will show as the following:
+
+```
+blocking_mode=true
+print_process_in_blocking_mode=true
+```
+
+## Functional Test
+
+Functional test plan will be published in [sonic-net/sonic-mgmt](https://github.com/sonic-net/sonic-mgmt).