tail: preserve pending lines during log rotation

CHANGELOG.md

@@ -133,6 +133,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 
+- `tt log -f`: possible line loss/duplication on rename, hanging after
+  a watched log directory is removed, and lines written just as the
+  file was read to the end not showing up until the next write.
+
 ## [2.13.0] - 2026-05-21
 
 This release adds cluster worker configuration management and fixes

cli/cmd/log.go

@@ -6,7 +6,9 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"sync"
+	"time"
 
 	"github.com/spf13/cobra"
 	"github.com/tarantool/tt/cli/cmd/internal"
@@ -20,6 +22,8 @@ var logOpts struct {
 	follow bool // Follow logs output.
 }
 
+const logRootCheckInterval = 100 * time.Millisecond
+
 // NewLogCmd creates log command.
 func NewLogCmd() *cobra.Command {
 	logCmd := &cobra.Command{
@@ -60,10 +64,46 @@ func printLines(ctx context.Context, in <-chan string) error {
 	}
 }
 
+type logRootContext struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+}
+
+func logRoot(inst running.InstanceCtx) string {
+	if inst.SingleApp {
+		return inst.LogDir
+	}
+	return filepath.Dir(inst.LogDir)
+}
+
+func monitorLogRoot(ctx context.Context, root string, cancel context.CancelFunc) {
+	ticker := time.NewTicker(logRootCheckInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			if _, err := os.Stat(root); errors.Is(err, os.ErrNotExist) {
+				cancel()
+				return
+			}
+		}
+	}
+}
+
 func follow(instances []running.InstanceCtx, n int) error {
 	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt)
 	defer stop()
 
+	rootContexts := make(map[string]logRootContext)
+	defer func() {
+		for _, rootCtx := range rootContexts {
+			rootCtx.cancel()
+		}
+	}()
+
 	nextColor := tail.DefaultColorPicker()
 	color := nextColor()
 	const logLinesChannelCapacity = 64
@@ -72,9 +112,18 @@ func follow(instances []running.InstanceCtx, n int) error {
 	// Wait group to wait for completion of all log reading routines to close the channel once.
 	var wg sync.WaitGroup
 	for _, inst := range instances {
-		if err := tail.Follow(ctx, logLines,
+		root := logRoot(inst)
+		rootCtx, ok := rootContexts[root]
+		if !ok {
+			rootCtx.ctx, rootCtx.cancel = context.WithCancel(ctx)
+			rootContexts[root] = rootCtx
+			go monitorLogRoot(rootCtx.ctx, root, rootCtx.cancel)
+		}
+
+		err := tail.Follow(rootCtx.ctx, logLines,
 			tail.NewLogFormatter(running.GetAppInstanceName(inst)+": ", color),
-			inst.Log, n, &wg); err != nil {
+			inst.Log, n, &wg)
+		if err != nil {
 			if errors.Is(err, os.ErrNotExist) {
 				continue
 			}

cli/cmd/log_test.go

@@ -0,0 +1,49 @@
+package cmd
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+	"github.com/tarantool/tt/cli/running"
+)
+
+func TestLogRoot(t *testing.T) {
+	t.Run("multi-instance application", func(t *testing.T) {
+		inst := running.InstanceCtx{
+			LogDir: filepath.Join("app", "var", "log", "instance"),
+		}
+		expected := filepath.Join("app", "var", "log")
+		actual := logRoot(inst)
+		require.Equal(t, expected, actual)
+	})
+
+	t.Run("single-instance application", func(t *testing.T) {
+		inst := running.InstanceCtx{
+			LogDir:    filepath.Join("var", "log"),
+			SingleApp: true,
+		}
+		actual := logRoot(inst)
+		require.Equal(t, inst.LogDir, actual)
+	})
+}
+
+func TestMonitorLogRoot(t *testing.T) {
+	root := t.TempDir()
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go monitorLogRoot(ctx, root, cancel)
+
+	err := os.Remove(root)
+	require.NoError(t, err)
+
+	select {
+	case <-ctx.Done():
+	case <-time.After(time.Second):
+		require.Fail(t, "log root removal did not cancel the follow context")
+	}
+}

cli/tail/follow.go

@@ -10,7 +10,7 @@ import (
 	"time"
 
 	"github.com/apex/log"
-	"github.com/nxadm/tail"
+	"github.com/tarantool/go-tail"
 )
 
 const (