Add Terraform CIDR based functions

[bartoszj]

I've added a few more functions to the net namespace:

• net.CidrHost to calculate host IP address from CIDR and host number, based on Terraform cidrhost function
• net.CidrNetmask to return CIDR netmask in dotted-decimal IPv4 address syntax, based on Terraform cidrnetmask function
• net.CidrSubnets to return a list of subnets from give CIDR and new netmask bits, based on Terraform cidrsubnet function
• net.CidrSubnetSizes to calculate a sequence of consecutive IP address ranges within a particular CIDR prefix, based on Terraform cidrsubnets function

[hairyhenderson]

Thanks for taking the time to submit this, @bartoszj!

For the future, it's a good idea to file an issue before spending time sending a PR (see CONTRIBUTING.md for details)

I'm curious about why the current net.Parse* functions not adequate for your needs? I had actually been planning to swap them out for the new net/netip functions that were added to the stdlib in Go 1.18 - my understanding is the new package is almost the same as the inet.af/netaddr package - some API changed, and some is missing, though I'm not sure how much impact there is there.

[bartoszj]

Hello. Sorry for that missing issue ticket :(

Regarding net.Parse*, I'm using github.com/apparentlymart/go-cidr/cidr package, which works with net types. But after writing this PR I think I can remove net.StdParse* methods, I don't need them. For me only the net.Cidr* methods are important and it is possible to reuse inet.af/netaddr types and probably the netip types.

If you want I can remove net.StdParse* methods and reimplement parsing in net.Cidr* using the methods which you will choose.

[hairyhenderson]

Thanks for the extra detail.

Can you explain in concrete terms what you're planning on using the net.Cidr* functions for?

A bit of a tangent:

It's a bit unfortunate that neither inet.af/netaddr nor net/netip have better support for netmasks... It seems to me that something definitely missing is a function that can take a prefix length and format it as an IPv4 netmask. There's a bit of ambiguity as I might want a v6 mask, but I've never seen a v6 mask written as anything other than prefix length...

Also - I looked a bit more at the difference between inet.af/netaddr and net/netip, and there are methods missing from net/netip - there's no equivalent for inet.af/netaddr#IPPrefix.IPNet, for example. So if I were to simply migrate the package over, that would technically be a breaking change, which I'd like to reserve for gomplate v4. Not a big deal, but adds a bit of an extra complicating factor...

[bartoszj]

Hello. Regarding net.Cidr* methods. I need to prepare network configuration from a network like 10.0.0.0/23. I can use net.CidrSubnetSizes to make a basic network setup:

$ gomplate -i '{{ net.CidrSubnetSizes 1 2 3 3 "10.0.0.0/23" -}}'
[10.0.0.0/24 10.0.1.0/25 10.0.1.128/26 10.0.1.192/26]

The first and second subnet is later divided into 4 or 8 smaller subnets. net.CidrSubnets can be used to make those networks:

$ gomplate -i '{{ net.CidrSubnets 3 "10.0.0.0/24" -}}'
[10.0.0.0/27 10.0.0.32/27 10.0.0.64/27 10.0.0.96/27 10.0.0.128/27 10.0.0.160/27 10.0.0.192/27 10.0.0.224/27]

$ gomplate -i '{{ net.CidrSubnets 2 "10.1.0.0/25" -}}'
[10.1.0.0/27 10.1.0.32/27 10.1.0.64/27 10.1.0.96/27]

Sometimes there is also a need to get the 10th host IP from a given subnet like:

$ gomplate -i '{{ net.CidrHost 10 "10.0.0.32/27" -}}'
10.0.0.42

I don't have yet usage for net.CidrNetmask but I've added it since it was the last method from Terraform. Maybe later it will be useful.

I'll try to find a method to convert inet.af/netaddr#IPPerfix and net/netip#Prefix to net#IPNet.

[bartoszj]

I've removed net.StdParse* methods, since I'm not using them. Also I've converted all return types to use inet.af/netaddr types. Added support for parsing net/netip types and two helper methods to convert from net.IP and net.IPNet to net/netip.Addr and net/netip.Prefix types. So once, you will move to net/netip the net.Cidr* methods will work without any problems.

[hairyhenderson]

Thanks @bartoszj, and sorry for the delay... I'm going to merge this, though I think the names should be net.CIDR* rather than net.Cidr* - because CIDR is an acronym (compare with net.LookupCNAME or conv.URL).

I'll take care of changing these though - I might do this in this PR, or I might merge it first and then make the change in a followup...

[hairyhenderson]

I've reworked this a bit to move to using net/netip instead on these new functions. Also, I've marked them as experimental for now - as I'm fairly certain I'm going to want to change the names to fit better with some new net/netip-based functions I plan on adding before v4. But I want to get this merged sooner rather than later so it can go into an upcoming v3.11 release.