Add support for systemd notify (start, watchdog)

[mths1]

Proposed Changes

systemd support is currently not optimal. As described in #1514 it is possible that systemd does not recognize a state change and thus e.g. does not restart the service accordingly. This change proposes:

• Use systemd library to support "notify" service type
• Change the mainpid to run_erl (supervises) by systemd
• Use systemd-notify instead of pid files

Example system-d unit:

systemd unit file

[Unit]
Description=VerneMQ Server

[Service]
Type=notify
NotifyAccess=all
LimitNOFILE=100000
SuccessExitStatus=1
[...]
WatchdogSec = 30s
Restart = always

Types of Changes

What types of changes does your code introduce to this project?
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• Bugfix (non-breaking change which fixes issue SystemD service is not align with VerneMQ state #1514 )
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (correction or otherwise)
• Cosmetics (whitespace, styles...)
• DevOps (Build scripts, pipelines...)

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• I have added tests that prove my fix is effective or that my feature works
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Further Comments

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considered, etc.

[ioolkos]

Cool stuff, thanks! I wonder whether we could use this to have some cheap "boot states" instead of just "ready". (think "listeners started", "subscriptions loaded").

[mths1]

systemd allows to set any status (here "custom status"). We would just need to know where to set the information and if it is ok to just have the latest status there indefinately or find a good place to really signal boot complete.

[ioolkos]

@mths1 I see, I'm looking into what makes sense to add.

[mths1]

@ioolkos : This should be "good enough" as a first version. There surely is room for improvement, but I am currently not working on this active anymore