all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

control/controlclient/controlclient_test.go

@@ -38,8 +38,8 @@ import (
 )
 
 func fieldsOf(t reflect.Type) (fields []string) {
-	for i := range t.NumField() {
-		if name := t.Field(i).Name; name != "_" {
+	for field := range t.Fields() {
+		if name := field.Name; name != "_" {
 			fields = append(fields, name)
 		}
 	}
@@ -214,12 +214,12 @@ func TestRetryableErrors(t *testing.T) {
 }
 
 type retryableForTest interface {
+	error
 	Retryable() bool
 }
 
 func isRetryableErrorForTest(err error) bool {
-	var ae retryableForTest
-	if errors.As(err, &ae) {
+	if ae, ok := errors.AsType[retryableForTest](err); ok {
 		return ae.Retryable()
 	}
 	return false

control/controlclient/direct.go

@@ -1484,7 +1484,7 @@ func (c *Direct) answerPing(pr *tailcfg.PingRequest) {
 		}
 		return
 	}
-	for _, t := range strings.Split(pr.Types, ",") {
+	for t := range strings.SplitSeq(pr.Types, ",") {
 		switch pt := tailcfg.PingType(t); pt {
 		case tailcfg.PingTSMP, tailcfg.PingDisco, tailcfg.PingICMP, tailcfg.PingPeerAPI:
 			go doPingerPing(c.logf, httpc, pr, c.pinger, pt)

control/controlclient/map.go

@@ -617,12 +617,12 @@ func (ms *mapSession) patchifyPeersChanged(resp *tailcfg.MapResponse) {
 
 var nodeFields = sync.OnceValue(getNodeFields)
 
-// getNodeFields returns the fails of tailcfg.Node.
+// getNodeFields returns the fields of tailcfg.Node.
 func getNodeFields() []string {
 	rt := reflect.TypeFor[tailcfg.Node]()
-	ret := make([]string, rt.NumField())
-	for i := range rt.NumField() {
-		ret[i] = rt.Field(i).Name
+	ret := make([]string, 0, rt.NumField())
+	for f := range rt.Fields() {
+		ret = append(ret, f.Name)
 	}
 	return ret
 }