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>

derp/derp_test.go

@@ -121,8 +121,7 @@ func TestSendRecv(t *testing.T) {
 		}
 		defer cin.Close()
 
-		ctx, cancel := context.WithCancel(context.Background())
-		defer cancel()
+		ctx := t.Context()
 
 		brwServer := bufio.NewReadWriter(bufio.NewReader(cin), bufio.NewWriter(cin))
 		go s.Accept(ctx, cin, brwServer, fmt.Sprintf("[abc::def]:%v", i))
@@ -331,8 +330,7 @@ func TestSendFreeze(t *testing.T) {
 		return c, c2
 	}
 
-	ctx, clientCtxCancel := context.WithCancel(context.Background())
-	defer clientCtxCancel()
+	ctx := t.Context()
 
 	aliceKey := key.NewNode()
 	aliceClient, aliceConn := newClient(ctx, "alice", aliceKey)
@@ -716,8 +714,7 @@ func (c *testClient) close(t *testing.T) {
 // TestWatch tests the connection watcher mechanism used by regional
 // DERP nodes to mesh up with each other.
 func TestWatch(t *testing.T) {
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	ts := newTestServer(t, ctx)
 	defer ts.close(t)
@@ -764,8 +761,7 @@ func waitConnect(t testing.TB, c *Client) {
 }
 
 func TestServerRepliesToPing(t *testing.T) {
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := t.Context()
 
 	ts := newTestServer(t, ctx)
 	defer ts.close(t)

derp/derphttp/derphttp_test.go

@@ -299,9 +299,7 @@ func TestBreakWatcherConnRecv(t *testing.T) {
 		errChan := make(chan error, 1)
 
 		// Start the watcher thread (which connects to the watched server)
-		wg.Add(1) // To avoid using t.Logf after the test ends. See https://golang.org/issue/40343
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			var peers int
 			add := func(m derp.PeerPresentMessage) {
 				t.Logf("add: %v", m.Key.ShortString())
@@ -318,7 +316,7 @@ func TestBreakWatcherConnRecv(t *testing.T) {
 			}
 
 			watcher.RunWatchConnectionLoop(ctx, serverPrivateKey1.Public(), t.Logf, add, remove, notifyErr)
-		}()
+		})
 
 		synctest.Wait()
 
@@ -381,9 +379,7 @@ func TestBreakWatcherConn(t *testing.T) {
 		errorChan := make(chan error, 1)
 
 		// Start the watcher thread (which connects to the watched server)
-		wg.Add(1) // To avoid using t.Logf after the test ends. See https://golang.org/issue/40343
-		go func() {
-			defer wg.Done()
+		wg.Go(func() {
 			var peers int
 			add := func(m derp.PeerPresentMessage) {
 				t.Logf("add: %v", m.Key.ShortString())
@@ -403,7 +399,7 @@ func TestBreakWatcherConn(t *testing.T) {
 			}
 
 			watcher1.RunWatchConnectionLoop(ctx, serverPrivateKey1.Public(), t.Logf, add, remove, notifyError)
-		}()
+		})
 
 		synctest.Wait()
 

derp/derpserver/derpserver.go

@@ -30,6 +30,7 @@ import (
 	"os"
 	"os/exec"
 	"runtime"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -71,7 +72,7 @@ func init() {
 	if keys == "" {
 		return
 	}
-	for _, keyStr := range strings.Split(keys, ",") {
+	for keyStr := range strings.SplitSeq(keys, ",") {
 		k, err := key.ParseNodePublicUntyped(mem.S(keyStr))
 		if err != nil {
 			log.Printf("ignoring invalid debug key %q: %v", keyStr, err)
@@ -1287,7 +1288,7 @@ func (c *sclient) sendPkt(dst *sclient, p pkt) error {
 	if disco.LooksLikeDiscoWrapper(p.bs) {
 		sendQueue = dst.discoSendQueue
 	}
-	for attempt := 0; attempt < 3; attempt++ {
+	for attempt := range 3 {
 		select {
 		case <-dst.done:
 			s.recordDrop(p.bs, c.key, dstKey, dropReasonGoneDisconnected)
@@ -1484,16 +1485,13 @@ func (s *Server) noteClientActivity(c *sclient) {
 	// If we saw this connection send previously, then consider
 	// the group fighting and disable them all.
 	if s.dupPolicy == disableFighters {
-		for _, prior := range dup.sendHistory {
-			if prior == c {
-				cs.ForeachClient(func(c *sclient) {
-					c.isDisabled.Store(true)
-					if cs.activeClient.Load() == c {
-						cs.activeClient.Store(nil)
-					}
-				})
-				break
-			}
+		if slices.Contains(dup.sendHistory, c) {
+			cs.ForeachClient(func(c *sclient) {
+				c.isDisabled.Store(true)
+				if cs.activeClient.Load() == c {
+					cs.activeClient.Store(nil)
+				}
+			})
 		}
 	}
 

derp/derpserver/derpserver_test.go

@@ -627,22 +627,17 @@ func BenchmarkConcurrentStreams(b *testing.B) {
 	if err != nil {
 		b.Fatal(err)
 	}
-	defer ln.Close()
 
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
+	ctx := b.Context()
 
+	acceptDone := make(chan struct{})
 	go func() {
-		for ctx.Err() == nil {
+		defer close(acceptDone)
+		for {
 			connIn, err := ln.Accept()
 			if err != nil {
-				if ctx.Err() != nil {
-					return
-				}
-				b.Error(err)
 				return
 			}
-
 			brwServer := bufio.NewReadWriter(bufio.NewReader(connIn), bufio.NewWriter(connIn))
 			go s.Accept(ctx, connIn, brwServer, "test-client")
 		}
@@ -680,6 +675,9 @@ func BenchmarkConcurrentStreams(b *testing.B) {
 			}
 		}
 	})
+
+	ln.Close()
+	<-acceptDone
 }
 
 func BenchmarkSendRecv(b *testing.B) {
@@ -769,7 +767,7 @@ func TestServeDebugTrafficUniqueSenders(t *testing.T) {
 		senderCardinality: hyperloglog.New(),
 	}
 
-	for i := 0; i < 5; i++ {
+	for range 5 {
 		c.senderCardinality.Insert(key.NewNode().Public().AppendTo(nil))
 	}
 
@@ -845,7 +843,7 @@ func TestSenderCardinality(t *testing.T) {
 		t.Errorf("EstimatedUniqueSenders() = %d, want ~10 (8-12 range)", estimate)
 	}
 
-	for i := 0; i < 5; i++ {
+	for i := range 5 {
 		c.senderCardinality.Insert(senders[i].AppendTo(nil))
 	}
 
@@ -869,7 +867,7 @@ func TestSenderCardinality100(t *testing.T) {
 	}
 
 	numSenders := 100
-	for i := 0; i < numSenders; i++ {
+	for range numSenders {
 		c.senderCardinality.Insert(key.NewNode().Public().AppendTo(nil))
 	}
 
@@ -945,7 +943,7 @@ func BenchmarkHyperLogLogInsertUnique(b *testing.B) {
 func BenchmarkHyperLogLogEstimate(b *testing.B) {
 	hll := hyperloglog.New()
 
-	for i := 0; i < 100; i++ {
+	for range 100 {
 		hll.Insert(key.NewNode().Public().AppendTo(nil))
 	}
 

derp/xdp/xdp_linux.go

@@ -62,8 +62,7 @@ func NewSTUNServer(config *STUNServerConfig, opts ...STUNServerOption) (*STUNSer
 	objs := new(bpfObjects)
 	err = loadBpfObjects(objs, nil)
 	if err != nil {
-		var ve *ebpf.VerifierError
-		if config.FullVerifierErr && errors.As(err, &ve) {
+		if ve, ok := errors.AsType[*ebpf.VerifierError](err); config.FullVerifierErr && ok {
 			err = fmt.Errorf("verifier error: %+v", ve)
 		}
 		return nil, fmt.Errorf("error loading XDP program: %w", err)