cmd/natc: separate perPeerState from connector

Make the perPeerState objects able to function independently without a
shared reference to the connector.

We don't currently change the values from connector that perPeerState
uses at runtime. Explicitly copying them at perPeerState creation allows
us to, for example, put the perPeerState into a consensus algorithm in
the future.

Updates #14667

Signed-off-by: Fran Bull <fran@tailscale.com>

cmd/natc/natc.go

@@ -359,7 +359,7 @@ var tsMBox = dnsmessage.MustNewName("support.tailscale.com.")
 // generateDNSResponse generates a DNS response for the given request. The from
 // argument is the NodeID of the node that sent the request.
 func (c *connector) generateDNSResponse(req *dnsmessage.Message, from tailcfg.NodeID) ([]byte, error) {
-	pm, _ := c.perPeerMap.LoadOrStore(from, &perPeerState{c: c})
+	pm, _ := c.perPeerMap.LoadOrStore(from, newPerPeerState(c))
 	var addrs []netip.Addr
 	if len(req.Questions) > 0 {
 		switch req.Questions[0].Type {
@@ -509,7 +509,8 @@ func proxyTCPConn(c net.Conn, dest string) {
 
 // perPeerState holds the state for a single peer.
 type perPeerState struct {
-	c *connector
+	v6ULA netip.Prefix
+	ipset *netipx.IPSet
 
 	mu           sync.Mutex
 	addrInUse    *big.Int
@@ -517,6 +518,13 @@ type perPeerState struct {
 	addrToDomain *bart.Table[string]
 }
 
+func newPerPeerState(c *connector) *perPeerState {
+	return &perPeerState{
+		ipset: c.ipset,
+		v6ULA: c.v6ULA,
+	}
+}
+
 // domainForIP returns the domain name assigned to the given IP address and
 // whether it was found.
 func (ps *perPeerState) domainForIP(ip netip.Addr) (_ string, ok bool) {
@@ -555,7 +563,7 @@ func (ps *perPeerState) unusedIPv4Locked() netip.Addr {
 	if ps.addrInUse == nil {
 		ps.addrInUse = big.NewInt(0)
 	}
-	return allocAddr(ps.c.ipset, ps.addrInUse)
+	return allocAddr(ps.ipset, ps.addrInUse)
 }
 
 // assignAddrsLocked assigns a pair of unique IP addresses for the given domain
@@ -570,7 +578,7 @@ func (ps *perPeerState) assignAddrsLocked(domain string) []netip.Addr {
 	if !v4.IsValid() {
 		return nil
 	}
-	as16 := ps.c.v6ULA.Addr().As16()
+	as16 := ps.v6ULA.Addr().As16()
 	as4 := v4.As4()
 	copy(as16[12:], as4[:])
 	v6 := netip.AddrFrom16(as16)

cmd/natc/natc_test.go

@@ -220,7 +220,7 @@ func TestPerPeerState(t *testing.T) {
 	}
 	c.setPrefixes([]netip.Prefix{netip.MustParsePrefix("100.64.1.0/24")})
 
-	ps := &perPeerState{c: c}
+	ps := newPerPeerState(c)
 
 	addrs, err := ps.ipForDomain("example.com")
 	if err != nil {
@@ -360,7 +360,7 @@ func TestIPPoolExhaustion(t *testing.T) {
 	}
 	c.setPrefixes([]netip.Prefix{smallPrefix})
 
-	ps := &perPeerState{c: c}
+	ps := newPerPeerState(c)
 
 	assignedIPs := make(map[netip.Addr]string)