apply go fix modernizations across the workspace

`go fix` carries the modernize analyzers now, and the tree had drifted behind
them. This is the mechanical result, reviewed rather than trusted: the tool is
capable of rewriting code into something that no longer tests or does what it
did, so every non-test change was read individually and the concurrency-bearing
packages were re-run under -race.

Production code, four changes, all semantics-preserving:

  - leases/manager.go: wg.Add(1) + go + defer wg.Done() becomes wg.Go. The
    comment above that function turns on Add happening before the goroutine
    starts, so that a Wait cannot return before the worker has run. wg.Go does
    the Add synchronously on the calling goroutine, so the invariant it
    describes still holds.
  - auth/token/handler.go: strings.Fields -> strings.FieldsSeq, same splitting,
    iterated rather than allocated.
  - hold/gc/gc.go: a hand-written map copy -> maps.Copy.
  - hold/pds/scan_broadcaster.go: three-clause loop -> range over int.

The rest are tests. The one worth naming is carstore_contention_test.go, where a
careless rewrite could have quietly stopped exercising contention: go fix
converted the reader and side-table goroutines to loopWG.Go but correctly
declined to touch the writer loop, which passes its index as a parameter. The
writer/reader/side-table shape and the stop channel are unchanged, so the test
still contends over the same carstore transactions.

Verified: go build for hold and appview, `make lint` 0 issues, the deploy and
credential-helper modules 0 issues, `make test` green across all 43 packages,
and -race green on leases, hold/pds, hold/gc and auth/token. The scanner module's
two lint findings are unchanged from HEAD and are in files go fix never touched.

Kept separate from the HTTP/2 commit so that one stays readable, and so this can
be reverted on its own if a modernization turns out to matter.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TA9D4DjaLZTvzQ7dJbu4eg

deploy/upcloud/provision.go

@@ -1522,7 +1522,7 @@ func writeRemoteCloudInit(ip, script string) error {
 // for cloud-init to complete before returning.
 func waitForSetup(ip, name string) error {
 	fmt.Printf("  %s (%s): waiting for SSH...\n", name, ip)
-	for i := 0; i < 30; i++ {
+	for i := range 30 {
 		_, err := runSSH(ip, "echo ssh_ready", false)
 		if err == nil {
 			break

deploy/upcloud/status.go

@@ -76,13 +76,13 @@ func cmdStatus(token string) error {
 			if err != nil {
 				fmt.Printf("          Service: unreachable\n")
 			} else {
-				lines := strings.Split(strings.TrimSpace(output), "\n")
-				for _, line := range lines {
+				lines := strings.SplitSeq(strings.TrimSpace(output), "\n")
+				for line := range lines {
 					line = strings.TrimSpace(line)
 					if line == "active" || line == "inactive" {
 						fmt.Printf("          Service: %s\n", line)
-					} else if strings.HasPrefix(line, "health:") {
-						fmt.Printf("          Health:  %s\n", strings.TrimPrefix(line, "health:"))
+					} else if after, ok := strings.CutPrefix(line, "health:"); ok {
+						fmt.Printf("          Health:  %s\n", after)
 					}
 				}
 			}
@@ -101,13 +101,13 @@ func cmdStatus(token string) error {
 			if err != nil {
 				fmt.Printf("          Service: unreachable\n")
 			} else {
-				lines := strings.Split(strings.TrimSpace(output), "\n")
-				for _, line := range lines {
+				lines := strings.SplitSeq(strings.TrimSpace(output), "\n")
+				for line := range lines {
 					line = strings.TrimSpace(line)
 					if line == "active" || line == "inactive" {
 						fmt.Printf("          Service: %s\n", line)
-					} else if strings.HasPrefix(line, "health:") {
-						fmt.Printf("          Health:  %s\n", strings.TrimPrefix(line, "health:"))
+					} else if after, ok := strings.CutPrefix(line, "health:"); ok {
+						fmt.Printf("          Health:  %s\n", after)
 					}
 				}
 			}

pkg/appview/db/device_store_scan_test.go

@@ -31,7 +31,7 @@ func seedIndexedDeviceRows(t *testing.T, store *DeviceStore, did, handle string,
 		t.Fatalf("GenerateFromPassword: %v", err)
 	}
 
-	for i := 0; i < n; i++ {
+	for i := range n {
 		hash := append([]byte(nil), base...)
 		// Vary the tail so secret_hash stays UNIQUE, keeping the base64 alphabet.
 		hash[len(hash)-1] = byte('a' + (i % 26))

pkg/appview/db/device_store_test.go

@@ -723,7 +723,7 @@ func TestDeviceStore_LegacyDeviceBackfills(t *testing.T) {
 func TestDeviceStore_ValidateDoesNotScanIndexedRows(t *testing.T) {
 	store := setupTestDB(t)
 	createTestUser(t, store, "did:plc:alice123", "alice.bsky.social")
-	for i := 0; i < 5; i++ {
+	for i := range 5 {
 		newDeviceForTest(t, store, "did:plc:alice123", "alice.bsky.social", fmt.Sprintf("Device %d", i))
 	}
 

pkg/appview/holdpurge/queue_test.go

@@ -282,7 +282,7 @@ func TestSubmitShedsWhenQueueIsFull(t *testing.T) {
 	// One job occupies the worker, one fills the single buffer slot, the
 	// third has nowhere to go.
 	accepted := 0
-	for i := 0; i < 3; i++ {
+	for i := range 3 {
 		if q.Submit(testRequest(fmt.Sprintf("at://did:plc:testuser/io.atcr.manifest/%d", i))) {
 			accepted++
 		}
@@ -315,7 +315,7 @@ func TestWaitDrainsAndDoesNotLeak(t *testing.T) {
 		return nil
 	})
 
-	for i := 0; i < 4; i++ {
+	for i := range 4 {
 		if !q.Submit(testRequest(fmt.Sprintf("at://did:plc:testuser/io.atcr.manifest/drain%d", i))) {
 			t.Fatalf("Submit %d rejected", i)
 		}

pkg/appview/installscript/installscript_test.go

@@ -209,7 +209,7 @@ func TestCredHelpersKeyIsRegistryNotSite(t *testing.T) {
 
 func offendingLines(s, needle string) string {
 	var out []string
-	for _, line := range strings.Split(s, "\n") {
+	for line := range strings.SplitSeq(s, "\n") {
 		if strings.Contains(line, needle) {
 			out = append(out, "  "+line)
 		}

pkg/appview/leases/manager.go

@@ -126,11 +126,9 @@ var errNotAcquired = errors.New("leases: not acquired")
 // could then return before the worker has even started, let alone released its
 // lease.
 func (m *Manager) Go(ctx context.Context, name string, fn func(context.Context) error) {
-	m.wg.Add(1)
-	go func() {
-		defer m.wg.Done()
+	m.wg.Go(func() {
 		m.Run(ctx, name, fn)
-	}()
+	})
 }
 
 // Wait blocks until every worker started with Go has stopped and released its

pkg/auth/token/handler.go

@@ -391,7 +391,7 @@ func collectScopes(values []string) []string {
 	var scopes []string
 	seen := make(map[string]bool)
 	for _, value := range values {
-		for _, scope := range strings.Fields(value) {
+		for scope := range strings.FieldsSeq(value) {
 			if seen[scope] {
 				continue
 			}

pkg/credhelper/config_safety_test.go

@@ -258,7 +258,7 @@ func TestMigrateV2toV3_DIDCollisionIsDeterministic(t *testing.T) {
 	}`
 
 	// Repeat: a single pass can pass by luck when the bug is map-order dependent.
-	for i := 0; i < 50; i++ {
+	for i := range 50 {
 		dir := setupConfigDir(t)
 		writeConfigFile(t, dir, v2)
 
@@ -297,7 +297,7 @@ func TestMigrateV2toV3_DIDCollisionIsDeterministic(t *testing.T) {
 func TestMigrateV2toV3_CollisionDonatesSecret(t *testing.T) {
 	// Looped: with map-order-dependent code this is a coin flip, so a single
 	// pass can pass by luck.
-	for i := 0; i < 50; i++ {
+	for i := range 50 {
 		testCollisionDonatesSecret(t, i)
 	}
 }
@@ -394,7 +394,7 @@ func TestFindIsDeterministic(t *testing.T) {
 	if want == nil {
 		t.Fatal("find returned nil for a known handle")
 	}
-	for i := 0; i < 50; i++ {
+	for i := range 50 {
 		if got := newReg().find("shared.com"); got.DID != want.DID {
 			t.Fatalf("iteration %d: find returned %s, want %s consistently", i, got.DID, want.DID)
 		}

pkg/hold/db/libsql_open_test.go

@@ -22,7 +22,7 @@ func assertPoolSettings(t *testing.T, label string, db *sql.DB, wantJournal stri
 			_ = c.Close()
 		}
 	}()
-	for i := 0; i < n; i++ {
+	for i := range n {
 		c, err := db.Conn(ctx)
 		if err != nil {
 			t.Fatalf("%s: open conn %d: %v", label, i, err)

pkg/hold/db/sqlite_store_contention_test.go

@@ -31,8 +31,8 @@ func makeBlocks(t *testing.T, n int, salt string) (cid.Cid, map[cid.Cid]blockfor
 	t.Helper()
 	blks := make(map[cid.Cid]blockformat.Block, n)
 	var root cid.Cid
-	for i := 0; i < n; i++ {
-		b := blockformat.NewBlock([]byte(fmt.Sprintf("%s-block-%d-%s", salt, i, strings.Repeat("x", 2048))))
+	for i := range n {
+		b := blockformat.NewBlock(fmt.Appendf(nil, "%s-block-%d-%s", salt, i, strings.Repeat("x", 2048)))
 		blks[b.Cid()] = b
 		if i == 0 {
 			root = b.Cid()
@@ -50,11 +50,11 @@ func writeShards(t *testing.T, sqs *SQLiteStore, writers, iters int) error {
 	var mu sync.Mutex
 	var firstErr error
 
-	for w := 0; w < writers; w++ {
+	for w := range writers {
 		wg.Add(1)
 		go func(w int) {
 			defer wg.Done()
-			for i := 0; i < iters; i++ {
+			for i := range iters {
 				salt := fmt.Sprintf("w%d-i%d", w, i)
 				root, blks := makeBlocks(t, 12, salt)
 				rev := fmt.Sprintf("rev-%03d-%03d", w, i)

pkg/hold/gc/gc.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"maps"
 	"net"
 	"net/http"
 	"net/url"
@@ -136,9 +137,7 @@ func (c *auxOrphanClock) snapshot() map[string]time.Time {
 	defer c.mu.Unlock()
 
 	out := make(map[string]time.Time, len(c.since))
-	for k, v := range c.since {
-		out[k] = v
-	}
+	maps.Copy(out, c.since)
 	return out
 }
 

pkg/hold/pds/carstore_contention_test.go

@@ -75,11 +75,11 @@ func TestHoldPDSConcurrentRepoWritesAndSideTable(t *testing.T) {
 	stop := make(chan struct{})
 
 	// Repo record writers.
-	for w := 0; w < writers; w++ {
+	for w := range writers {
 		writeWG.Add(1)
 		go func(w int) {
 			defer writeWG.Done()
-			for i := 0; i < iters; i++ {
+			for i := range iters {
 				rkey := fmt.Sprintf("w%d-layer-%03d", w, i)
 				rec := atproto.NewLayerRecord(
 					fmt.Sprintf("sha256:%064x", w*1000+i),
@@ -97,10 +97,8 @@ func TestHoldPDSConcurrentRepoWritesAndSideTable(t *testing.T) {
 	}
 
 	// Repo readers, hitting the carstore's read transactions concurrently.
-	for r := 0; r < 2; r++ {
-		loopWG.Add(1)
-		go func() {
-			defer loopWG.Done()
+	for range 2 {
+		loopWG.Go(func() {
 			for {
 				select {
 				case <-stop:
@@ -112,13 +110,11 @@ func TestHoldPDSConcurrentRepoWritesAndSideTable(t *testing.T) {
 					return
 				}
 			}
-		}()
+		})
 	}
 
 	// The side subsystem, writing its own table on its own goroutine.
-	loopWG.Add(1)
-	go func() {
-		defer loopWG.Done()
+	loopWG.Go(func() {
 		for i := 0; ; i++ {
 			select {
 			case <-stop:
@@ -133,7 +129,7 @@ func TestHoldPDSConcurrentRepoWritesAndSideTable(t *testing.T) {
 				return
 			}
 		}
-	}()
+	})
 
 	writersDone := make(chan struct{})
 	go func() {

pkg/hold/pds/events_test.go

@@ -872,7 +872,7 @@ func TestSendBackfillMsg_AbortsOnDisconnect(t *testing.T) {
 // The subscriber uses a 1-slot buffer with no reader so the backfill is
 // reliably blocked in the send when Unsubscribe fires.
 func TestBackfillFromDatabase_ConcurrentUnsubscribe(t *testing.T) {
-	for i := 0; i < 25; i++ {
+	for range 25 {
 		func() {
 			dbPath := t.TempDir() + "/events.db"
 			broadcaster := NewEventBroadcaster("did:web:hold.example.com", 500, dbPath)
@@ -880,7 +880,7 @@ func TestBackfillFromDatabase_ConcurrentUnsubscribe(t *testing.T) {
 
 			ctx := context.Background()
 			testCID, _ := cid.Decode("bafyreib2rxk3rkhh5ylyxj3x3gathxt3s32qvwj2lf3qg4kmzr6b7teqke")
-			for j := 0; j < 50; j++ {
+			for range 50 {
 				broadcaster.Broadcast(ctx, &RepoEvent{
 					NewRoot:   testCID,
 					Rev:       "rev",

pkg/hold/pds/scan_broadcaster.go

@@ -905,7 +905,7 @@ func (sb *ScanBroadcaster) selectSubscriberLocked() *ScanSubscriber {
 
 	best := -1
 	bestScore := 0.0
-	for i := 0; i < n; i++ {
+	for i := range n {
 		idx := (sb.nextIdx + i) % n
 		sub := sb.subscribers[idx]
 		capacity := sub.effectiveCapacity()

pkg/hold/pds/scan_broadcaster_concurrency_test.go

@@ -761,7 +761,7 @@ func TestScanDrain_StopsAtTheSubscriberCapacity(t *testing.T) {
 	sub.capacity = 2
 
 	var seqs []int64
-	for i := 0; i < 5; i++ {
+	for range 5 {
 		seqs = append(seqs, seedJob(t, sb, "sha256:backlog", originProactive))
 	}
 

pkg/hold/pds/scan_broadcaster_stuck_test.go

@@ -520,7 +520,7 @@ func TestScanDispatchQueue_ReturnsUndeliverableJobsToPending(t *testing.T) {
 	sub.capacity = 4                       // the send buffer, not capacity, is the constraint here
 
 	var seqs []int64
-	for i := 0; i < 4; i++ {
+	for range 4 {
 		seqs = append(seqs, seedJobWithDigest(t, sb, "sha256:burst"))
 	}
 

pkg/hold/pds/scan_broadcaster_test.go

@@ -114,7 +114,7 @@ func newTestScanSubscriber(t *testing.T, sb *ScanBroadcaster, bufSize int) *Scan
 func seedPendingJobs(t *testing.T, sb *ScanBroadcaster, n int) {
 	t.Helper()
 
-	for i := 0; i < n; i++ {
+	for i := range n {
 		_, err := sb.db.Exec(`
 			INSERT INTO scan_jobs
 				(manifest_digest, repository, tag, user_did, user_handle,
@@ -226,7 +226,7 @@ func TestScanUnsubscribe_MarksItsOwnJobsOnce(t *testing.T) {
 // The subscriber uses a 1-slot buffer with no reader so the drain is reliably
 // blocked in the send when Unsubscribe fires.
 func TestScanDrainPendingJobs_ConcurrentUnsubscribe(t *testing.T) {
-	for i := 0; i < 25; i++ {
+	for range 25 {
 		func() {
 			sb := newTestScanBroadcaster(t)
 			sub := newTestScanSubscriber(t, sb, 1)