feature/conn25: add expiresAt field to addrs

And use it to allow overwrites of old address assignments in the conn25 client.

The magic and transit address pools from which the addresses come are limited
resources and we want to reuse them. This commit is a small part of that bigger
need.

We expect to follow soon:
 * Extending expiry if assignments are still in use.
 * Returning expired addresses back to the pools so they can be reallocated.

Updates tailscale/corp#39975

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

feature/conn25/conn25.go

@@ -19,6 +19,7 @@ import (
 	"slices"
 	"strings"
 	"sync"
+	"time"
 
 	"go4.org/netipx"
 	"golang.org/x/net/dns/dnsmessage"
@@ -32,6 +33,7 @@ import (
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tstun"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tstime"
 	"tailscale.com/types/appctype"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
@@ -327,8 +329,9 @@ func (c *Conn25) isConfigured() bool {
 func newConn25(logf logger.Logf) *Conn25 {
 	c := &Conn25{
 		client: &client{
-			logf:    logf,
-			addrsCh: make(chan addrs, 64),
+			logf:        logf,
+			addrsCh:     make(chan addrs, 64),
+			assignments: addrAssignments{clock: tstime.StdClock{}},
 		},
 		connector: &connector{logf: logf},
 	}
@@ -1160,11 +1163,12 @@ func (c *connector) reconfig(newCfg config) {
 }
 
 type addrs struct {
-	dst     netip.Addr
-	magic   netip.Addr
-	transit netip.Addr
-	domain  dnsname.FQDN
-	app     string
+	dst       netip.Addr
+	magic     netip.Addr
+	transit   netip.Addr
+	domain    dnsname.FQDN
+	app       string
+	expiresAt time.Time
 }
 
 func (c addrs) isValid() bool {
@@ -1187,22 +1191,38 @@ type addrAssignments struct {
 	byMagicIP   map[netip.Addr]addrs
 	byTransitIP map[netip.Addr]addrs
 	byDomainDst map[domainDst]addrs
+	clock       tstime.Clock
 }
 
+const defaultExpiry = 48 * time.Hour
+
 func (a *addrAssignments) insert(as addrs) error {
-	// we likely will want to allow overwriting in the future when we
-	// have address expiry, but for now this should not happen
-	if _, ok := a.byMagicIP[as.magic]; ok {
-		return errors.New("byMagicIP key exists")
+	return a.insertWithExpiry(as, defaultExpiry)
+}
+
+func (a *addrAssignments) insertWithExpiry(as addrs, d time.Duration) error {
+	if !as.expiresAt.IsZero() {
+		return errors.New("expiresAt already set")
+	}
+	now := a.clock.Now()
+	as.expiresAt = now.Add(d)
+	// we don't expect for addresses to be reused before expiry
+	if existing, ok := a.byMagicIP[as.magic]; ok {
+		if !existing.expiresAt.Before(now) {
+			return errors.New("byMagicIP key exists")
+		}
 	}
 	ddst := domainDst{domain: as.domain, dst: as.dst}
-	if _, ok := a.byDomainDst[ddst]; ok {
-		return errors.New("byDomainDst key exists")
+	if existing, ok := a.byDomainDst[ddst]; ok {
+		if !existing.expiresAt.Before(now) {
+			return errors.New("byDomainDst key exists")
+		}
 	}
-	if _, ok := a.byTransitIP[as.transit]; ok {
-		return errors.New("byTransitIP key exists")
+	if existing, ok := a.byTransitIP[as.transit]; ok {
+		if !existing.expiresAt.Before(now) {
+			return errors.New("byTransitIP key exists")
+		}
 	}
-
 	mak.Set(&a.byMagicIP, as.magic, as)
 	mak.Set(&a.byTransitIP, as.transit, as)
 	mak.Set(&a.byDomainDst, ddst, as)
@@ -1211,17 +1231,26 @@ func (a *addrAssignments) insert(as addrs) error {
 
 func (a *addrAssignments) lookupByDomainDst(domain dnsname.FQDN, dst netip.Addr) (addrs, bool) {
 	v, ok := a.byDomainDst[domainDst{domain: domain, dst: dst}]
-	return v, ok
+	if !ok || v.expiresAt.Before(a.clock.Now()) {
+		return addrs{}, false
+	}
+	return v, true
 }
 
 func (a *addrAssignments) lookupByMagicIP(mip netip.Addr) (addrs, bool) {
 	v, ok := a.byMagicIP[mip]
-	return v, ok
+	if !ok || v.expiresAt.Before(a.clock.Now()) {
+		return addrs{}, false
+	}
+	return v, true
 }
 
 func (a *addrAssignments) lookupByTransitIP(tip netip.Addr) (addrs, bool) {
 	v, ok := a.byTransitIP[tip]
-	return v, ok
+	if !ok || v.expiresAt.Before(a.clock.Now()) {
+		return addrs{}, false
+	}
+	return v, true
 }
 
 // insertTransitConnMapping adds an entry to the byConnKey map