netaddr: add initial well-known IP addresses

[mdlayher]

Signed-off-by: Matt Layher [email protected]

These are the only two "constants" I need at the moment. The standard library exposes a few more.

It'd be nice if these could be immutable but I think this is probably reasonable.

[bradfitz]

It'd be ideal if we could do this without init work. And something the linker could discard.

[mdlayher]

Yeah. I am curious if there is something more clever that could be done, but I also know that being able to check equality via == was an explicit goal with this type.

Here's a thought that comes to mind:

type WellKnown int

const (
    Unspecified WellKnown = iota
    IPv6LinkLocalAllNodes
)

func (ip IP) Is(known WellKnown) bool {}

// Usage:
ip, _ := netaddr.ParseIP("::")
fmt.Println(ip.Is(netaddr.Unspecified))
// true

[bradfitz]

No, I was thinking just laying out the value in memory to help the linker (see golang/go#19533 and golang/go#38450 (comment)). Not a new type or API.

Something like:

var ip6Unspecified = IP{v6Addr{}}
func IPv6Unspecified() IP { return ip6Unspecified }

Then:

• linker can discard it all (confirm that the unexported var can be discard to and doesn't make an init func?)
• it doesn't allocate at runtuime
• no new types
• people can't modify exported package vars

[mdlayher]

I see! I'm good with that.

[mdlayher]

Updated.

[mdlayher]

Any more thoughts? Thanks!

[mdlayher]

I'll take that as my "+2", but feel free to chime in or open issues/PRs if something's amiss! Thanks.