go: net/netip: missing odds and ends for 1.18 [freeze exception]

Now that netip has landed, I’ve started porting code to it. I’m hoping to get a bit of experience doing this before Go 1.18 in order to catch issues before it ships. So far I’ve identified six things that are missing, which I was hoping we could get into Go 1.18, as a followup to #46518.

Items with ⏳ have CLs still pending, and items with ✅ have CLs that have been submitted.

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1. MarshalBinary/UnmarshalBinary on AddrPort and Prefix ✅

Addr.MarshalBinary and Addr.UnmarshalBinary both exist. However, the same is not true for the remaining two types, AddrPort and Prefix. This makes it very hard to use with encoding/gob, because not only are these marshals missing, but the inner addr + port/cidr members are unexported. So, I propose adding these two methods, with implementations that simply concatenate Addr.MarshalBinary output with little-endian encoding of the port (uint16) for AddrPort or cidr (uint8) for Prefix.

2. Addr.AsSlice ✅

There is netip.AddrFromSlice() and netip.AddrFrom4, but there is only netip.As4 and no netip.AsSlice. This may seem kind of trifling, but it winds up being kind of annoying when coding things; I find myself writing a lot of:

var s []byte
if addr.Is4() {
    arr := addr.As4()
    s = arr[:]
} else addr.Is16() {
    arr := addr.As16()
    s = arr[:]
}

You can’t slice an rvalue, so that temporary is needed. And sometimes I just want a slice, regardless of what kind of IP it is, and having to open code dispatch for that every time is a bit of a pain. The above snippet would be simplified to:

s := addr.AsSlice()

3. IPv4Unspecified() ✅

There is IPv6Unspecified(), but not IPv4Unspecified(). This is sort of confusing, and code would be more clear and consistent if I could write netip.IPv4Unspecified() instead of netip.AddrFrom4([4]byte{}).

4. One remaining allocation in UDP path

The “net” package has one remaining allocation in the UDP packet path, which @josharian has a change to fix, so that UDP sends/receives can be allocation-free. Being alloc-free is one of the original motivations behind netip in the first place.

5. Missing UDPAddr and TCPAddr analogs ❌ (partially accepted, partially punted to go 1.19)

We have ReadFromUDPAddrPort and such, for reads and writes of UDP and TCP sockets, but the listener/dialer functions still take the old TCPAddr and UDPAddr structs. So, we’ll need new functions there, to avoid unwanted type conversions by users.

6. AddrFromSlice should return only one value ❌ (rejected)

The AddrFrom4 and AddrFrom6 functions never fail, and that makes it easy to pass them around with function composition. AddrFromSlice, however, awkwardly returns an ok value. But, when ok==false, the returned Addr isn’t valid anyway. So folks who want to check for validity can just do addr.IsValid() like normal, instead of having to learn something new and odd. So, we should drop the ok return value, especially as most users already ignore it.

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(I’ll update this issue if I find anything new.)

CC @bradfitz @crawshaw @josharian @danderson @ianlancetaylor