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/art/stride_table_test.go

@@ -19,7 +19,7 @@ import (
 func TestInversePrefix(t *testing.T) {
 	t.Parallel()
 	for i := range 256 {
-		for len := 0; len < 9; len++ {
+		for len := range 9 {
 			addr := i & (0xFF << (8 - len))
 			idx := prefixIndex(uint8(addr), len)
 			addr2, len2 := inversePrefixIndex(idx)

net/captivedetection/captivedetection_test.go

@@ -94,8 +94,7 @@ func TestCaptivePortalRequest(t *testing.T) {
 	now := time.Now()
 	d.clock = func() time.Time { return now }
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	s := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method != "GET" {
@@ -133,8 +132,7 @@ func TestCaptivePortalRequest(t *testing.T) {
 func TestAgainstDERPHandler(t *testing.T) {
 	d := NewDetector(t.Logf)
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	s := httptest.NewServer(http.HandlerFunc(derpserver.ServeNoContent))
 	defer s.Close()

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

net/netcheck/netcheck.go

@@ -545,7 +545,7 @@ func makeProbePlanInitial(dm *tailcfg.DERPMap, ifState *netmon.State) (plan prob
 
 		var p4 []probe
 		var p6 []probe
-		for try := 0; try < 3; try++ {
+		for try := range 3 {
 			n := reg.Nodes[try%len(reg.Nodes)]
 			delay := time.Duration(try) * defaultInitialRetransmitTime
 			if n.IPv4 != "none" && ((ifState.HaveV4 && nodeMight4(n)) || n.IsTestNode()) {
@@ -975,13 +975,11 @@ func (c *Client) GetReport(ctx context.Context, dm *tailcfg.DERPMap, opts *GetRe
 				// need to close the underlying Pinger after a timeout
 				// or when all ICMP probes are done, regardless of
 				// whether the HTTPS probes have finished.
-				wg.Add(1)
-				go func() {
-					defer wg.Done()
+				wg.Go(func() {
 					if err := c.measureAllICMPLatency(ctx, rs, need); err != nil {
 						c.logf("[v1] measureAllICMPLatency: %v", err)
 					}
-				}()
+				})
 			}
 			wg.Add(len(need))
 			c.logf("netcheck: UDP is blocked, trying HTTPS")
@@ -1072,9 +1070,7 @@ func (c *Client) runHTTPOnlyChecks(ctx context.Context, last *Report, rs *report
 		if len(rg.Nodes) == 0 {
 			continue
 		}
-		wg.Add(1)
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			node := rg.Nodes[0]
 			req, _ := http.NewRequestWithContext(ctx, "HEAD", "https://"+node.HostName+"/derp/probe", nil)
 			// One warm-up one to get HTTP connection set
@@ -1099,7 +1095,7 @@ func (c *Client) runHTTPOnlyChecks(ctx context.Context, last *Report, rs *report
 			}
 			d := c.timeNow().Sub(t0)
 			rs.addNodeLatency(node, netip.AddrPort{}, d)
-		}()
+		})
 	}
 	wg.Wait()
 	return nil

net/netcheck/netcheck_test.go

@@ -42,8 +42,7 @@ func TestBasic(t *testing.T) {
 
 	c := newTestClient(t)
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	if err := c.Standalone(ctx, "127.0.0.1:0"); err != nil {
 		t.Fatal(err)
@@ -124,8 +123,7 @@ func TestWorksWhenUDPBlocked(t *testing.T) {
 
 	c := newTestClient(t)
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	r, err := c.GetReport(ctx, dm, nil)
 	if err != nil {
@@ -1038,8 +1036,7 @@ func TestNoUDPNilGetReportOpts(t *testing.T) {
 	}
 
 	c := newTestClient(t)
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	r, err := c.GetReport(ctx, dm, nil)
 	if err != nil {

net/neterror/neterror_linux.go

@@ -12,8 +12,7 @@ import (
 
 func init() {
 	shouldDisableUDPGSO = func(err error) bool {
-		var serr *os.SyscallError
-		if errors.As(err, &serr) {
+		if serr, ok := errors.AsType[*os.SyscallError](err); ok {
 			// EIO is returned by udp_send_skb() if the device driver does not
 			// have tx checksumming enabled, which is a hard requirement of
 			// UDP_SEGMENT. See:

net/netmon/state.go

@@ -812,11 +812,8 @@ func (m *Monitor) HasCGNATInterface() (bool, error) {
 		if hasCGNATInterface || !i.IsUp() || isTailscaleInterface(i.Name, pfxs) {
 			return
 		}
-		for _, pfx := range pfxs {
-			if cgnatRange.Overlaps(pfx) {
-				hasCGNATInterface = true
-				break
-			}
+		if slices.ContainsFunc(pfxs, cgnatRange.Overlaps) {
+			hasCGNATInterface = true
 		}
 	})
 	if err != nil {

net/netutil/routes.go

@@ -41,8 +41,8 @@ func CalcAdvertiseRoutes(advertiseRoutes string, advertiseDefaultRoute bool) ([]
 	routeMap := map[netip.Prefix]bool{}
 	if advertiseRoutes != "" {
 		var default4, default6 bool
-		advroutes := strings.Split(advertiseRoutes, ",")
-		for _, s := range advroutes {
+		advroutes := strings.SplitSeq(advertiseRoutes, ",")
+		for s := range advroutes {
 			ipp, err := netip.ParsePrefix(s)
 			if err != nil {
 				return nil, fmt.Errorf("%q is not a valid IP address or CIDR prefix", s)

net/socks5/socks5.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"log"
 	"net"
+	"slices"
 	"strconv"
 	"time"
 
@@ -488,10 +489,8 @@ func parseClientGreeting(r io.Reader, authMethod byte) error {
 	if err != nil {
 		return fmt.Errorf("could not read methods")
 	}
-	for _, m := range methods {
-		if m == authMethod {
-			return nil
-		}
+	if slices.Contains(methods, authMethod) {
+		return nil
 	}
 	return fmt.Errorf("no acceptable auth methods")
 }

net/socks5/socks5_test.go

@@ -180,11 +180,11 @@ func TestUDP(t *testing.T) {
 
 	const echoServerNumber = 3
 	echoServerListener := make([]net.PacketConn, echoServerNumber)
-	for i := 0; i < echoServerNumber; i++ {
+	for i := range echoServerNumber {
 		echoServerListener[i] = newUDPEchoServer()
 	}
 	defer func() {
-		for i := 0; i < echoServerNumber; i++ {
+		for i := range echoServerNumber {
 			_ = echoServerListener[i].Close()
 		}
 	}()
@@ -277,10 +277,10 @@ func TestUDP(t *testing.T) {
 	}
 	defer socks5UDPConn.Close()
 
-	for i := 0; i < echoServerNumber; i++ {
+	for i := range echoServerNumber {
 		port := echoServerListener[i].LocalAddr().(*net.UDPAddr).Port
 		addr := socksAddr{addrType: ipv4, addr: "127.0.0.1", port: uint16(port)}
-		requestBody := []byte(fmt.Sprintf("Test %d", i))
+		requestBody := fmt.Appendf(nil, "Test %d", i)
 		responseBody := sendUDPAndWaitResponse(socks5UDPConn, addr, requestBody)
 		if !bytes.Equal(requestBody, responseBody) {
 			t.Fatalf("got: %q want: %q", responseBody, requestBody)

net/stunserver/stunserver_test.go

@@ -60,8 +60,7 @@ func TestSTUNServer(t *testing.T) {
 
 func BenchmarkServerSTUN(b *testing.B) {
 	b.ReportAllocs()
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := b.Context()
 
 	s := New(ctx)
 	s.Listen("localhost:0")

net/tstun/wrap.go

@@ -1406,11 +1406,11 @@ func (t *Wrapper) InjectInboundPacketBuffer(pkt *netstack_PacketBuffer, buffs []
 			return err
 		}
 	}
-	for i := 0; i < n; i++ {
+	for i := range n {
 		buffs[i] = buffs[i][:PacketStartOffset+sizes[i]]
 	}
 	defer func() {
-		for i := 0; i < n; i++ {
+		for i := range n {
 			buffs[i] = buffs[i][:cap(buffs[i])]
 		}
 	}()

net/tstun/wrap_test.go

@@ -95,7 +95,7 @@ func tcp4syn(src, dst string, sport, dport uint16) []byte {
 
 func nets(nets ...string) (ret []netip.Prefix) {
 	for _, s := range nets {
-		if i := strings.IndexByte(s, '/'); i == -1 {
+		if found := strings.Contains(s, "/"); !found {
 			ip, err := netip.ParseAddr(s)
 			if err != nil {
 				panic(err)
@@ -122,13 +122,13 @@ func ports(s string) filter.PortRange {
 	}
 
 	var fs, ls string
-	i := strings.IndexByte(s, '-')
-	if i == -1 {
+	before, after, ok := strings.Cut(s, "-")
+	if !ok {
 		fs = s
 		ls = fs
 	} else {
-		fs = s[:i]
-		ls = s[i+1:]
+		fs = before
+		ls = after
 	}
 	first, err := strconv.ParseInt(fs, 10, 16)
 	if err != nil {

net/udprelay/server.go

@@ -977,7 +977,7 @@ func (e ErrServerNotReady) Error() string {
 // For now, we favor simplicity and reducing VNI re-use over more complex
 // ephemeral port (VNI) selection algorithms.
 func (s *Server) getNextVNILocked() (uint32, error) {
-	for i := uint32(0); i < totalPossibleVNI; i++ {
+	for range totalPossibleVNI {
 		vni := s.nextVNI
 		if vni == maxVNI {
 			s.nextVNI = minVNI

net/udprelay/server_test.go

@@ -265,7 +265,7 @@ func TestServer(t *testing.T) {
 			tcB := newTestClient(t, endpoint.VNI, tcBServerEndpointAddr, discoB, discoA.Public(), endpoint.ServerDisco)
 			defer tcB.close()
 
-			for i := 0; i < 2; i++ {
+			for range 2 {
 				// We handshake both clients twice to guarantee server-side
 				// packet reading goroutines, which are independent across
 				// address families, have seen an answer from both clients
@@ -345,7 +345,7 @@ func TestServer_getNextVNILocked(t *testing.T) {
 	s := &Server{
 		nextVNI: minVNI,
 	}
-	for i := uint64(0); i < uint64(totalPossibleVNI); i++ {
+	for range uint64(totalPossibleVNI) {
 		vni, err := s.getNextVNILocked()
 		if err != nil { // using quicktest here triples test time
 			t.Fatal(err)