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>

net/dns/manager_linux.go

@@ -380,7 +380,7 @@ func isLibnssResolveUsed(env newOSConfigEnv) error {
 	if err != nil {
 		return fmt.Errorf("reading /etc/resolv.conf: %w", err)
 	}
-	for _, line := range strings.Split(string(bs), "\n") {
+	for line := range strings.SplitSeq(string(bs), "\n") {
 		fields := strings.Fields(line)
 		if len(fields) < 2 || fields[0] != "hosts:" {
 			continue

net/dns/openresolv.go

@@ -82,7 +82,7 @@ func (m openresolvManager) GetBaseConfig() (OSConfig, error) {
 
 	// Remove the "tailscale" snippet from the list.
 	args := []string{"-l"}
-	for _, f := range strings.Split(strings.TrimSpace(string(bs)), " ") {
+	for f := range strings.SplitSeq(strings.TrimSpace(string(bs)), " ") {
 		if f == "tailscale" {
 			continue
 		}

net/dns/resolver/forwarder.go

@@ -727,8 +727,7 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 	}
 
 	// If we got a truncated UDP response, return that instead of an error.
-	var trErr truncatedResponseError
-	if errors.As(err, &trErr) {
+	if trErr, ok := errors.AsType[truncatedResponseError](err); ok {
 		return trErr.res, nil
 	}
 	return nil, err

net/dns/resolver/forwarder_test.go

@@ -328,7 +328,7 @@ func runDNSServer(tb testing.TB, opts *testDNSServerOptions, response []byte, on
 		udpLn *net.UDPConn
 		err   error
 	)
-	for try := 0; try < tries; try++ {
+	for range tries {
 		if tcpLn != nil {
 			tcpLn.Close()
 			tcpLn = nil
@@ -392,9 +392,7 @@ func runDNSServer(tb testing.TB, opts *testDNSServerOptions, response []byte, on
 	var wg sync.WaitGroup
 
 	if opts == nil || !opts.SkipTCP {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			for {
 				conn, err := tcpLn.Accept()
 				if err != nil {
@@ -402,7 +400,7 @@ func runDNSServer(tb testing.TB, opts *testDNSServerOptions, response []byte, on
 				}
 				go handleConn(conn)
 			}
-		}()
+		})
 	}
 
 	handleUDP := func(addr netip.AddrPort, req []byte) {
@@ -413,9 +411,7 @@ func runDNSServer(tb testing.TB, opts *testDNSServerOptions, response []byte, on
 	}
 
 	if opts == nil || !opts.SkipUDP {
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			for {
 				buf := make([]byte, 65535)
 				n, addr, err := udpLn.ReadFromUDPAddrPort(buf)
@@ -425,7 +421,7 @@ func runDNSServer(tb testing.TB, opts *testDNSServerOptions, response []byte, on
 				buf = buf[:n]
 				go handleUDP(addr, buf)
 			}
-		}()
+		})
 	}
 
 	tb.Cleanup(func() {
@@ -684,7 +680,7 @@ func makeResponseOfSize(tb testing.TB, domain string, targetSize int, includeOPT
 	var response []byte
 	var err error
 
-	for attempt := 0; attempt < 10; attempt++ {
+	for range 10 {
 		testBuilder := dns.NewBuilder(nil, dns.Header{
 			Response:      true,
 			Authoritative: true,

net/dns/resolver/tsdns.go

@@ -16,7 +16,7 @@ import (
 	"net/netip"
 	"os"
 	"runtime"
-	"sort"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -172,7 +172,7 @@ func WriteRoutes(w *bufio.Writer, routes map[dnsname.FQDN][]*dnstype.Resolver) {
 		}
 		kk = append(kk, k)
 	}
-	sort.Slice(kk, func(i, j int) bool { return kk[i] < kk[j] })
+	slices.Sort(kk)
 	w.WriteByte('{')
 	for i, k := range kk {
 		if i > 0 {

net/dns/wsl_windows.go

@@ -172,8 +172,7 @@ func (fs wslFS) Truncate(name string) error { return fs.WriteFile(name, nil, 064
 
 func (fs wslFS) ReadFile(name string) ([]byte, error) {
 	b, err := wslCombinedOutput(fs.cmd("cat", "--", name))
-	var ee *exec.ExitError
-	if errors.As(err, &ee) && ee.ExitCode() == 1 {
+	if ee, ok := errors.AsType[*exec.ExitError](err); ok && ee.ExitCode() == 1 {
 		return nil, os.ErrNotExist
 	}
 	return b, err