[PATCH] add documentation about backend naming for VMOD authors

[Dridi Boukelmoune]

>>> * If the VMOD ensures there are no duplicate vcl_names in the
>>>   first place, the vcl_name will not be touched.
>>
>>The VMOD can enforce proper uniqueness, but can't prevent against name
>>collisions from other VMODs.
>
> ... which is why the VRT function will have to step in.

Hi guys,

I just realized that a VCL_HasBackend (or VRT) function for VMODs to
"ensure" uniqueness would be racy by nature. In a scenario where the
backend name is "stolen" right after we check its availability. Since
the intention is to have a safety net in Varnish for deduplication,
the VCL_HasBackend becomes obsolete.

After a quick glance at the thread again, it seems that non-VCL
backends such as for instance unix-domain socket backend should
eventually show up in the CLI and VSM. While I have nothing against
that (quite the opposite) I don't think it can be done without
breaking both ABI and API. The current statistics should be generic
enough to suit any kind of backend, except maybe for the probe part.

The varnish-cli on the other end is too struct backend-centric, and
despite my attempt to restrict the director API to the bare minimum,
cache_backend.h made it in 4.1 and cache_director.h wasn't stripped
off of the VDI_ functions.

I have the feeling that Geoff started a 4.1 thread and that we are
also discussing things that belong post-4.1.

Best,
Dridi