From e964a3af2ed608d818c6679abce6c835deebdb09 Mon Sep 17 00:00:00 2001
From: Djordje Lukic <djordje.lukic@docker.com>
Date: Mon, 4 May 2020 16:38:02 +0200
Subject: [PATCH] Implement `docker logs --tal <N>`

---
 azure/backend.go   | 21 ++++++++++++++++++---
 cli/cmd/logs.go    | 16 +++++++++++++---
 containers/api.go  |  8 +++++++-
 example/backend.go |  4 ++--
 4 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/azure/backend.go b/azure/backend.go
index 9b0aec7d..03de06f9 100644
--- a/azure/backend.go
+++ b/azure/backend.go
@@ -4,6 +4,8 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"strconv"
+	"strings"
 
 	"github.com/Azure/azure-sdk-for-go/services/containerinstance/mgmt/2018-10-01/containerinstance"
 	"github.com/Azure/go-autorest/autorest/azure/auth"
@@ -139,11 +141,24 @@ func (cs *containerService) Exec(ctx context.Context, name string, command strin
 	)
 }
 
-func (cs *containerService) Logs(ctx context.Context, name string, writer io.Writer, follow bool) error {
-	logs, err := getACIContainerLogs(ctx, cs.ctx, name, name)
+func (cs *containerService) Logs(ctx context.Context, containerName string, req containers.LogsRequest) error {
+	logs, err := getACIContainerLogs(ctx, cs.ctx, containerName, containerName)
 	if err != nil {
 		return err
 	}
-	_, err = fmt.Fprint(writer, logs)
+	if req.Tail != "all" {
+		tail, err := strconv.Atoi(req.Tail)
+		if err != nil {
+			return err
+		}
+		lines := strings.Split(logs, "\n")
+
+		// If asked for less lines than exist, take only those lines
+		if tail <= len(lines) {
+			logs = strings.Join(lines[len(lines)-tail:], "\n")
+		}
+	}
+
+	_, err = fmt.Fprint(req.Writer, logs)
 	return err
 }
diff --git a/cli/cmd/logs.go b/cli/cmd/logs.go
index 739873c8..16a39ffe 100644
--- a/cli/cmd/logs.go
+++ b/cli/cmd/logs.go
@@ -4,13 +4,16 @@ import (
 	"context"
 	"os"
 
-	"github.com/docker/api/client"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
+
+	"github.com/docker/api/client"
+	"github.com/docker/api/containers"
 )
 
 type logsOpts struct {
 	Follow bool
+	Tail   string
 }
 
 func LogsCommand() *cobra.Command {
@@ -25,15 +28,22 @@ func LogsCommand() *cobra.Command {
 	}
 
 	cmd.Flags().BoolVarP(&opts.Follow, "follow", "f", false, "Follow log outut")
+	cmd.Flags().StringVar(&opts.Tail, "tail", "all", "Number of lines to show from the end of the logs")
 
 	return cmd
 }
 
-func runLogs(ctx context.Context, name string, opts logsOpts) error {
+func runLogs(ctx context.Context, containerName string, opts logsOpts) error {
 	c, err := client.New(ctx)
 	if err != nil {
 		return errors.Wrap(err, "cannot connect to backend")
 	}
 
-	return c.ContainerService().Logs(ctx, name, os.Stdout, opts.Follow)
+	req := containers.LogsRequest{
+		Follow: opts.Follow,
+		Tail:   opts.Tail,
+		Writer: os.Stdout,
+	}
+
+	return c.ContainerService().Logs(ctx, containerName, req)
 }
diff --git a/containers/api.go b/containers/api.go
index a0fbb489..d437ddb5 100644
--- a/containers/api.go
+++ b/containers/api.go
@@ -37,6 +37,12 @@ type ContainerConfig struct {
 	Ports []Port
 }
 
+type LogsRequest struct {
+	Follow bool
+	Tail   string
+	Writer io.Writer
+}
+
 // ContainerService interacts with the underlying container backend
 type ContainerService interface {
 	// List returns all the containers
@@ -46,5 +52,5 @@ type ContainerService interface {
 	// Exec executes a command inside a running container
 	Exec(ctx context.Context, containerName string, command string, reader io.Reader, writer io.Writer) error
 	// Logs returns all the logs of a container
-	Logs(ctx context.Context, name string, writer io.Writer, follow bool) error
+	Logs(ctx context.Context, containerName string, request LogsRequest) error
 }
diff --git a/example/backend.go b/example/backend.go
index 5b145c5f..03cff36c 100644
--- a/example/backend.go
+++ b/example/backend.go
@@ -44,7 +44,7 @@ func (cs *containerService) Exec(ctx context.Context, name string, command strin
 	return nil
 }
 
-func (cs *containerService) Logs(ctx context.Context, name string, writer io.Writer, follow bool) error {
-	fmt.Fprintf(writer, "Following logs for container %q", name)
+func (cs *containerService) Logs(ctx context.Context, containerName string, request containers.LogsRequest) error {
+	fmt.Fprintf(request.Writer, "Following logs for container %q", containerName)
 	return nil
 }