types/dnstype: introduce new package for Resolver

So the type can be used in net/dns without introducing a tailcfg
dependency.

For #2596

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

types/dnstype/dnstype.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package dnstype defines types for working with DNS.
+package dnstype
+
+//go:generate go run tailscale.com/cmd/cloner --type=Resolver --clonefunc=true --output=dnstype_clone.go
+
+import "inet.af/netaddr"
+
+// Resolver is the configuration for one DNS resolver.
+type Resolver struct {
+	// Addr is the address of the DNS resolver, one of:
+	//  - A plain IP address for a "classic" UDP+TCP DNS resolver
+	//  - [TODO] "tls://resolver.com" for DNS over TCP+TLS
+	//  - [TODO] "https://resolver.com/query-tmpl" for DNS over HTTPS
+	Addr string `json:",omitempty"`
+
+	// BootstrapResolution is an optional suggested resolution for the
+	// DoT/DoH resolver, if the resolver URL does not reference an IP
+	// address directly.
+	// BootstrapResolution may be empty, in which case clients should
+	// look up the DoT/DoH server using their local "classic" DNS
+	// resolver.
+	BootstrapResolution []netaddr.IP `json:",omitempty"`
+}

types/dnstype/dnstype_clone.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Code generated by tailscale.com/cmd/cloner -type Resolver; DO NOT EDIT.
+
+package dnstype
+
+import (
+	"inet.af/netaddr"
+)
+
+// Clone makes a deep copy of Resolver.
+// The result aliases no memory with the original.
+func (src *Resolver) Clone() *Resolver {
+	if src == nil {
+		return nil
+	}
+	dst := new(Resolver)
+	*dst = *src
+	dst.BootstrapResolution = append(src.BootstrapResolution[:0:0], src.BootstrapResolution...)
+	return dst
+}
+
+// A compilation failure here means this code must be regenerated, with command:
+//   tailscale.com/cmd/cloner -type Resolver
+var _ResolverNeedsRegeneration = Resolver(struct {
+	Addr                string
+	BootstrapResolution []netaddr.IP
+}{})
+
+// Clone duplicates src into dst and reports whether it succeeded.
+// To succeed, <src, dst> must be of types <*T, *T> or <*T, **T>,
+// where T is one of Resolver.
+func Clone(dst, src interface{}) bool {
+	switch src := src.(type) {
+	case *Resolver:
+		switch dst := dst.(type) {
+		case *Resolver:
+			*dst = *src.Clone()
+			return true
+		case **Resolver:
+			*dst = src.Clone()
+			return true
+		}
+	}
+	return false
+}