Add -n/--tail support to lstk logs

cmd/logs.go

@@ -3,6 +3,7 @@ package cmd
 import (
 	"fmt"
 	"os"
+	"strconv"
 
 	"github.com/localstack/lstk/internal/config"
 	"github.com/localstack/lstk/internal/container"
@@ -17,7 +18,7 @@ func newLogsCmd(cfg *env.Env) *cobra.Command {
 	cmd := &cobra.Command{
 		Use:     "logs",
 		Short:   "Show emulator logs",
-		Long:    "Show logs from the emulator. Use --follow to stream in real-time.",
+		Long:    "Show logs from the emulator. Use --follow to stream in real-time and --tail to limit output to the last N lines.",
 		PreRunE: initConfig(nil),
 		RunE: func(cmd *cobra.Command, args []string) error {
 			follow, err := cmd.Flags().GetBool("follow")
@@ -28,6 +29,13 @@ func newLogsCmd(cfg *env.Env) *cobra.Command {
 			if err != nil {
 				return err
 			}
+			tail, err := cmd.Flags().GetString("tail")
+			if err != nil {
+				return err
+			}
+			if err := validateTail(tail); err != nil {
+				return err
+			}
 			rt, err := runtime.NewDockerRuntime(cfg.DockerHost)
 			if err != nil {
 				return err
@@ -37,12 +45,23 @@ func newLogsCmd(cfg *env.Env) *cobra.Command {
 				return fmt.Errorf("failed to get config: %w", err)
 			}
 			if isInteractiveMode(cfg) {
-				return ui.RunLogs(cmd.Context(), rt, appConfig.Containers, follow, verbose)
+				return ui.RunLogs(cmd.Context(), rt, appConfig.Containers, follow, tail, verbose)
 			}
-			return container.Logs(cmd.Context(), rt, output.NewPlainSink(os.Stdout), appConfig.Containers, follow, verbose)
+			return container.Logs(cmd.Context(), rt, output.NewPlainSink(os.Stdout), appConfig.Containers, follow, tail, verbose)
 		},
 	}
 	cmd.Flags().BoolP("follow", "f", false, "Follow log output")
 	cmd.Flags().BoolP("verbose", "v", false, "Show all log output without filtering")
+	cmd.Flags().StringP("tail", "n", "all", "Number of lines to show from the end of the logs")
 	return cmd
 }
+
+func validateTail(tail string) error {
+	if tail == "all" {
+		return nil
+	}
+	if n, err := strconv.Atoi(tail); err != nil || n < 0 {
+		return fmt.Errorf("invalid --tail value %q: expected a non-negative integer or \"all\"", tail)
+	}
+	return nil
+}

internal/container/logs.go

@@ -11,7 +11,7 @@ import (
 	"github.com/localstack/lstk/internal/runtime"
 )
 
-func Logs(ctx context.Context, rt runtime.Runtime, sink output.Sink, containers []config.ContainerConfig, follow bool, verbose bool) error {
+func Logs(ctx context.Context, rt runtime.Runtime, sink output.Sink, containers []config.ContainerConfig, follow bool, tail string, verbose bool) error {
 	if err := rt.IsHealthy(ctx); err != nil {
 		rt.EmitUnhealthyError(sink, err)
 		return output.NewSilentError(fmt.Errorf("runtime not healthy: %w", err))
@@ -27,7 +27,7 @@ func Logs(ctx context.Context, rt runtime.Runtime, sink output.Sink, containers
 	pr, pw := io.Pipe()
 	errCh := make(chan error, 1)
 	go func() {
-		err := rt.StreamLogs(ctx, c.Name(), pw, follow)
+		err := rt.StreamLogs(ctx, c.Name(), pw, follow, tail)
 		pw.CloseWithError(err)
 		errCh <- err
 	}()

internal/runtime/docker.go

@@ -371,12 +371,15 @@ func (d *DockerRuntime) Logs(ctx context.Context, containerID string, tail int)
 	return string(logs), nil
 }
 
-func (d *DockerRuntime) StreamLogs(ctx context.Context, containerID string, out io.Writer, follow bool) error {
+func (d *DockerRuntime) StreamLogs(ctx context.Context, containerID string, out io.Writer, follow bool, tail string) error {
+	if tail == "" {
+		tail = "all"
+	}
 	reader, err := d.client.ContainerLogs(ctx, containerID, client.ContainerLogsOptions{
 		ShowStdout: true,
 		ShowStderr: true,
 		Follow:     follow,
-		Tail:       "all",
+		Tail:       tail,
 	})
 	if err != nil {
 		if errdefs.IsNotFound(err) {

internal/runtime/runtime.go

@@ -63,7 +63,7 @@ type Runtime interface {
 	ContainerStartedAt(ctx context.Context, containerName string) (time.Time, error)
 	ContainerEnv(ctx context.Context, containerName string) ([]string, error)
 	Logs(ctx context.Context, containerID string, tail int) (string, error)
-	StreamLogs(ctx context.Context, containerID string, out io.Writer, follow bool) error
+	StreamLogs(ctx context.Context, containerID string, out io.Writer, follow bool, tail string) error
 	GetImageVersion(ctx context.Context, imageName string) (string, error)
 	// ImageExists reports whether the given image is already present locally.
 	ImageExists(ctx context.Context, image string) (bool, error)

internal/ui/run_logs.go

@@ -12,7 +12,7 @@ import (
 	"github.com/localstack/lstk/internal/runtime"
 )
 
-func RunLogs(parentCtx context.Context, rt runtime.Runtime, containers []config.ContainerConfig, follow bool, verbose bool) error {
+func RunLogs(parentCtx context.Context, rt runtime.Runtime, containers []config.ContainerConfig, follow bool, tail string, verbose bool) error {
 	ctx, cancel := context.WithCancel(parentCtx)
 	defer cancel()
 
@@ -21,7 +21,7 @@ func RunLogs(parentCtx context.Context, rt runtime.Runtime, containers []config.
 	runErrCh := make(chan error, 1)
 
 	go func() {
-		err := container.Logs(ctx, rt, output.NewTUISink(programSender{p: p}), containers, follow, verbose)
+		err := container.Logs(ctx, rt, output.NewTUISink(programSender{p: p}), containers, follow, tail, verbose)
 		runErrCh <- err
 		if err != nil && !errors.Is(err, context.Canceled) {
 			p.Send(runErrMsg{err: err})

test/integration/logs_test.go

@@ -2,6 +2,9 @@ package integration_test
 
 import (
 	"bufio"
+	"context"
+	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -60,6 +63,129 @@ func TestLogsCommandFailsWhenNotRunning(t *testing.T) {
 	assertCommandTelemetry(t, events, "logs", 1)
 }
 
+// writeNumberedLogLines writes tail-marker-1..tail-marker-<count> to PID 1's
+// stdout inside the test container and waits until the last one is visible in
+// docker logs, so tail assertions don't race the writes.
+func writeNumberedLogLines(t *testing.T, ctx context.Context, count int) {
+	t.Helper()
+
+	execResp, err := dockerClient.ExecCreate(ctx, containerName, client.ExecCreateOptions{
+		Cmd: []string{"sh", "-c", fmt.Sprintf("for i in $(seq 1 %d); do echo tail-marker-$i; done >/proc/1/fd/1", count)},
+	})
+	require.NoError(t, err, "failed to create exec")
+	_, err = dockerClient.ExecStart(ctx, execResp.ID, client.ExecStartOptions{Detach: true})
+	require.NoError(t, err, "failed to start exec")
+
+	lastMarker := fmt.Sprintf("tail-marker-%d", count)
+	require.Eventually(t, func() bool {
+		reader, err := dockerClient.ContainerLogs(ctx, containerName, client.ContainerLogsOptions{
+			ShowStdout: true,
+			ShowStderr: true,
+			Tail:       "all",
+		})
+		if err != nil {
+			return false
+		}
+		defer func() { _ = reader.Close() }()
+		data, err := io.ReadAll(reader)
+		return err == nil && strings.Contains(string(data), lastMarker)
+	}, 10*time.Second, 100*time.Millisecond, "log lines did not appear in docker logs")
+}
+
+func TestLogsTailLimitsOutput(t *testing.T) {
+	requireDocker(t)
+	cleanup()
+	t.Cleanup(cleanup)
+
+	ctx := testContext(t)
+	startTestContainer(t, ctx)
+	writeNumberedLogLines(t, ctx, 10)
+
+	configFile := writeAwsConfig(t)
+
+	for _, flags := range [][]string{{"--tail", "3"}, {"-n", "3"}} {
+		args := append([]string{"--config", configFile, "logs"}, flags...)
+		stdout, stderr, err := runLstk(t, ctx, "", env.Without(), args...)
+		require.NoError(t, err, "lstk logs %s should exit cleanly, stderr: %s", strings.Join(flags, " "), stderr)
+		for i := 8; i <= 10; i++ {
+			assert.Contains(t, stdout, fmt.Sprintf("tail-marker-%d", i), "last 3 lines should be shown with %s", strings.Join(flags, " "))
+		}
+		for i := 1; i <= 7; i++ {
+			assert.NotContains(t, stdout, fmt.Sprintf("tail-marker-%d\n", i), "older lines should be cut off with %s", strings.Join(flags, " "))
+		}
+	}
+}
+
+func TestLogsWithoutTailShowsAllLines(t *testing.T) {
+	requireDocker(t)
+	cleanup()
+	t.Cleanup(cleanup)
+
+	ctx := testContext(t)
+	startTestContainer(t, ctx)
+	writeNumberedLogLines(t, ctx, 10)
+
+	configFile := writeAwsConfig(t)
+	stdout, stderr, err := runLstk(t, ctx, "", env.Without(), "--config", configFile, "logs")
+	require.NoError(t, err, "lstk logs should exit cleanly, stderr: %s", stderr)
+	for i := 1; i <= 10; i++ {
+		assert.Contains(t, stdout, fmt.Sprintf("tail-marker-%d", i), "all lines should be shown without --tail")
+	}
+}
+
+func TestLogsTailRejectsInvalidValue(t *testing.T) {
+	t.Parallel()
+
+	configFile := writeAwsConfig(t)
+	_, stderr, err := runLstk(t, testContext(t), "", env.Without(), "--config", configFile, "logs", "--tail", "bogus")
+	require.Error(t, err, "expected lstk logs --tail bogus to fail")
+	requireExitCode(t, 1, err)
+	assert.Contains(t, stderr, "bogus", "error should name the invalid value")
+	assert.Contains(t, stderr, "--tail", "error should name the flag")
+}
+
+func TestLogsTailWithFollowStartsFromTail(t *testing.T) {
+	requireDocker(t)
+	cleanup()
+	t.Cleanup(cleanup)
+
+	ctx := testContext(t)
+	startTestContainer(t, ctx)
+	writeNumberedLogLines(t, ctx, 10)
+
+	configFile := writeAwsConfig(t)
+	// Uses StdoutPipe for streaming — cannot use runLstk.
+	logsCmd := exec.CommandContext(ctx, binaryPath(), "--config", configFile, "logs", "--follow", "--tail", "3")
+	logsCmd.Env = env.Without()
+	stdout, err := logsCmd.StdoutPipe()
+	require.NoError(t, err, "failed to get stdout pipe")
+
+	err = logsCmd.Start()
+	require.NoError(t, err, "failed to start lstk logs --follow --tail 3")
+	t.Cleanup(func() { _ = logsCmd.Process.Kill() })
+
+	// The backlog is capped at the last 3 lines, so the first line streamed
+	// must be tail-marker-8; seeing an older marker first means tail was ignored.
+	firstLine := make(chan string, 1)
+	go func() {
+		scanner := bufio.NewScanner(stdout)
+		for scanner.Scan() {
+			line := scanner.Text()
+			if strings.Contains(line, "tail-marker-") {
+				firstLine <- line
+				return
+			}
+		}
+	}()
+
+	select {
+	case line := <-firstLine:
+		assert.Contains(t, line, "tail-marker-8", "follow should start from the last 3 lines")
+	case <-ctx.Done():
+		t.Fatal("no marker appeared in lstk logs --follow --tail output within timeout")
+	}
+}
+
 func TestLogsFollowStreamsOutput(t *testing.T) {
 	requireDocker(t)
 	cleanup()