all: use iterators over slice views more

This gets close to all of the remaining ones.

Updates #12912

Change-Id: I9c672bbed2654a6c5cab31e0cbece6c107d8c6fa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/debughttp.go

@@ -102,8 +102,7 @@ func (c *Conn) ServeHTTPDebug(w http.ResponseWriter, r *http.Request) {
 		sort.Slice(ent, func(i, j int) bool { return ent[i].pub.Less(ent[j].pub) })
 
 		peers := map[key.NodePublic]tailcfg.NodeView{}
-		for i := range c.peers.Len() {
-			p := c.peers.At(i)
+		for _, p := range c.peers.All() {
 			peers[p.Key()] = p
 		}
 

wgengine/magicsock/endpoint.go

@@ -9,6 +9,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"iter"
 	"math"
 	"math/rand/v2"
 	"net"
@@ -1384,20 +1385,18 @@ func (de *endpoint) updateFromNode(n tailcfg.NodeView, heartbeatDisabled bool, p
 }
 
 func (de *endpoint) setEndpointsLocked(eps interface {
-	Len() int
-	At(i int) netip.AddrPort
+	All() iter.Seq2[int, netip.AddrPort]
 }) {
 	for _, st := range de.endpointState {
 		st.index = indexSentinelDeleted // assume deleted until updated in next loop
 	}
 
 	var newIpps []netip.AddrPort
-	for i := range eps.Len() {
+	for i, ipp := range eps.All() {
 		if i > math.MaxInt16 {
 			// Seems unlikely.
 			break
 		}
-		ipp := eps.At(i)
 		if !ipp.IsValid() {
 			de.c.logf("magicsock: bogus netmap endpoint from %v", eps)
 			continue

wgengine/magicsock/magicsock.go

@@ -1120,8 +1120,8 @@ func (c *Conn) determineEndpoints(ctx context.Context) ([]tailcfg.Endpoint, erro
 	// re-run.
 	eps = c.endpointTracker.update(time.Now(), eps)
 
-	for i := range c.staticEndpoints.Len() {
-		addAddr(c.staticEndpoints.At(i), tailcfg.EndpointExplicitConf)
+	for _, ep := range c.staticEndpoints.All() {
+		addAddr(ep, tailcfg.EndpointExplicitConf)
 	}
 
 	if localAddr := c.pconn4.LocalAddr(); localAddr.IP.IsUnspecified() {
@@ -2360,16 +2360,14 @@ func (c *Conn) logEndpointCreated(n tailcfg.NodeView) {
 			fmt.Fprintf(w, "derp=%v%s ", regionID, code)
 		}
 
-		for i := range n.AllowedIPs().Len() {
-			a := n.AllowedIPs().At(i)
+		for _, a := range n.AllowedIPs().All() {
 			if a.IsSingleIP() {
 				fmt.Fprintf(w, "aip=%v ", a.Addr())
 			} else {
 				fmt.Fprintf(w, "aip=%v ", a)
 			}
 		}
-		for i := range n.Endpoints().Len() {
-			ep := n.Endpoints().At(i)
+		for _, ep := range n.Endpoints().All() {
 			fmt.Fprintf(w, "ep=%v ", ep)
 		}
 	}))