Completion for host name by service-type (mdns, bonjour and dns)

[sirnewton01]

For commands like ssh and sftp the service type is known in context. For others like curl or xdg-open that accept URLs, the context might be available from the URL prefix (e.g. http://, https://). This information could be used to complete using either (or both) avahi and dig commands.

The avahi completion is already there, but it only triggers on _workstation._tcp. The idea would be to allow different commands to provide the context of the type of service that they are operating on and discover the hostnames based on that.

[scop]

I can see the point, but it raises some questions:

• When asked for a service type, what do we do with generic things like /etc/hosts?
• Does it make a difference to what to do with such things if e.g. something that is actually able to some service based discovery exists, but is not enabled? I think we shouldn't make services which might be slow, contact the network etc unconditionally enabled, e.g. avahi-browse, dig, etc. (We do have ruptime enabled unconditionally if available, should probably provide way to disable it.)
• Willing to take a look at contributions that come with test cases, but not likely to work on this myself any time soon.

[sirnewton01]

I was thinking that some commands, such as ping, might not use a service-based approach while others like ssh/sftp might and other like curl, might do so based on URL protocol. I'm not sure if the service-based ones should ignore /etc/hosts, ssh known hosts, etc. Perhaps it could be configured, or perhaps that's too much configuration bloat.

Yes, I see now that avahi-browse is guarded with an environment variable to avoid unnecessary slowdowns in certain environments. I suspect that should be the case here too. However, there should probably be a distinction between mdns (avahi, dns-sd) and dns (dig, host) cases because the latter is generally considered to be very fast in most environments since virtually every command or completion hits dns one or more times on a regular basis. So, perhaps mdns is opt-in like the existing host lookup, while dns is opt-out.

I understand that I may need to make this contribution myself if I can make the code general enough to work in most environments. Otherwise, I completely understand if you decide to close this issue. I just thought I'd report the idea here in case there is enough of a benefit to others to consider it.