[__systemd_unit] add --unique-drop-in

[4nd3r]

From:
https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html

... equally named drop-in files further down the prefix hierarchy override those further up, i.e. foo-bar-.service.d/10-override.conf overrides foo-.service.d/10-override.conf.

It's a feature, but I consider this a bug. But oh well, fix is simple - use unique names.

This bites me when I have service template e.g. app@.service with drop-in and then also app@instance.service with drop-in too. When both drop-ins are called override.conf, then only last gets read.

This "fix" is somewhat ugly, I agree, but I couldn't think anything more saner and I don't want to deal with it manually e.g. introduce new parameter and then set it when needed.

[sideeffect42]

Hmm, this is a tough one.
If it's supposedly a feature, I'm wondering if there would be a use case where systemd's behaviour is desirable?
If there is, maybe the user should specify the name of the drop-in so that they can control the behaviour?

However, this would be a breaking change, so 🤷.

Another thing which caught my eye is the use of md5sum. Can we assume that every system running systemd has it available?
Or would it be better to just use the unit file name or generate a slug of the service name instead of the hash? Would certainly be more predictable/readable.

[sideeffect42]

I don't use systemd, so I don't really have an opinion.

[4nd3r]

Moved this to separate parameter. Makes more sense this way, also compatibility.

Also reminds me that this type is rather ugly and needs to be rewritten with all the known edge-cases. It's still in my TODO, but that will not happen today.