improve template handling

[kayrus]

When you submit template@.service and start template@{1..10}.service fleet writes these unit files into the file system:

ls -la /run/fleet/units/ 
total 40
drwxr-xr-x 2 root root 240 Mar 16 16:51 .
drwxr-xr-x 3 root root  60 Mar 16 11:47 ..
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@1.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@10.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@2.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@3.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@4.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@5.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@6.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@7.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@8.service
-rw-r--r-- 1 root root  97 Mar 16 16:51 hello@9.service

Also it writes down unnecessary duplicated data inside the etcd registry.

With the new --replace feature you have to update each "templated" unit file instead of only one template.

Also when you start units using local template, which is not stored into the registry (i.e. fleetctl start template@{1..10}.service), fleet just creates copies of this template. But for some reason doesn't store the origin template into the registry. I suppose it should.

So the idea is to create a reference to the origin template inside the registry instead of copying template's data. This will allow you to update only one template using the --replace feature.

[antrik]

@kayrus how would you replace a template? AIUI, fleet commands always work on actual instances. If you do, say fleetctl start --replace template@3.service template@7.service, it should replace these two instances with ones created from the current (possible changed) template; but not the others. I don't see how you would express "update all instances of this template"...

[kayrus]

@antrik it shouldn't allow to replace template's reference. like it is not possible to start template itself.

[tixxdz]

@kayrus on your argument "With the new --replace feature you have to update each "templated" unit file instead of only one template."
I don't see any problem on this ? if you update a template then "submit --replace" to updated into the registry, that's it. We don't start units from a template on behalf of the user as I guess we don't want to automatically replace instantiated units on behalf of the user, and it's pretty easy just after submit --replace, do start --replace template@{1..100}

[kayrus]

@tixxdz start --replace template@{1..100} will create at least 100 write events into etcd. if you rewrite only one template it will create only N etcd read events, where N is amount of machines in cluster.