logs: support --follow

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

README.md

@@ -299,6 +299,9 @@ Fetch the logs of a container.
 
 :warning: Currently, only containers created with `nerdctl run -d` are supported.
 
+Flags:
+- :whale: `--f, --follow`: Follow log output
+
 ### :whale: nerdctl port
 List port mappings or a specific mapping for the container.
 

logs.go

@@ -19,10 +19,13 @@ package main
 
 import (
 	"context"
+	"io"
 	"os"
+	"os/exec"
 
 	"github.com/AkihiroSuda/nerdctl/pkg/idutil/containerwalker"
 	"github.com/AkihiroSuda/nerdctl/pkg/logging/jsonfile"
+	"github.com/containerd/containerd"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli/v2"
@@ -34,6 +37,13 @@ var logsCommand = &cli.Command{
 	ArgsUsage:    "[flags] CONTAINER",
 	Action:       logsAction,
 	BashComplete: logsBashComplete,
+	Flags: []cli.Flag{
+		&cli.BoolFlag{
+			Name:    "follow",
+			Aliases: []string{"f"},
+			Usage:   "Follow log output",
+		},
+	},
 }
 
 func logsAction(clicontext *cli.Context) error {
@@ -65,12 +75,32 @@ func logsAction(clicontext *cli.Context) error {
 				return errors.Errorf("ambiguous ID %q", found.Req)
 			}
 			logJSONFilePath := jsonfile.Path(dataStore, ns, found.Container.ID())
-			f, err := os.Open(logJSONFilePath)
-			if err != nil {
+			if _, err := os.Stat(logJSONFilePath); err != nil {
 				return errors.Wrapf(err, "failed to open %q, container is not created with `nerdctl run -d`?", logJSONFilePath)
 			}
-			defer f.Close()
-			return jsonfile.Decode(clicontext.App.Writer, clicontext.App.ErrWriter, f)
+			task, err := found.Container.Task(ctx, nil)
+			if err != nil {
+				return err
+			}
+			status, err := task.Status(ctx)
+			if err != nil {
+				return err
+			}
+			var reader io.Reader
+			if clicontext.Bool("follow") && status.Status == containerd.Running {
+				reader, err = newTailReader(ctx, task, logJSONFilePath)
+				if err != nil {
+					return err
+				}
+			} else {
+				f, err := os.Open(logJSONFilePath)
+				if err != nil {
+					return err
+				}
+				defer f.Close()
+				reader = f
+			}
+			return jsonfile.Decode(clicontext.App.Writer, clicontext.App.ErrWriter, reader)
 		},
 	}
 	req := clicontext.Args().First()
@@ -92,3 +122,24 @@ func logsBashComplete(clicontext *cli.Context) {
 	// show container names (TODO: only show containers with logs)
 	bashCompleteContainerNames(clicontext)
 }
+
+func newTailReader(ctx context.Context, task containerd.Task, filePath string) (io.Reader, error) {
+	cmd := exec.CommandContext(ctx, "tail", "-f", "-n", "+0", filePath)
+	cmd.Stderr = os.Stderr
+	r, err := cmd.StdoutPipe()
+	if err != nil {
+		return nil, err
+	}
+	if err := cmd.Start(); err != nil {
+		return nil, err
+	}
+	waitCh, err := task.Wait(ctx)
+	if err != nil {
+		return nil, err
+	}
+	go func() {
+		<-waitCh
+		cmd.Process.Kill()
+	}()
+	return r, nil
+}

logs_test.go

@@ -0,0 +1,49 @@
+/*
+   Copyright (C) nerdctl authors.
+   Copyright (C) containerd authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package main
+
+import (
+	"testing"
+
+	"github.com/AkihiroSuda/nerdctl/pkg/testutil"
+)
+
+func TestLogs(t *testing.T) {
+	base := testutil.NewBase(t)
+	const containerName = "nerdctl-test-logs"
+	defer base.Cmd("rm", containerName).Run()
+	base.Cmd("run", "-d", "--name", containerName, testutil.AlpineImage,
+		"sh", "-euxc", "echo foo; echo bar").AssertOK()
+	base.Cmd("logs", "-f", containerName).AssertOut("bar")
+	// Run logs twice, make sure that the logs are not removed
+	base.Cmd("logs", "-f", containerName).AssertOut("foo")
+	base.Cmd("rm", "-f", containerName).AssertOK()
+}
+
+func TestLogsWithFailingContainer(t *testing.T) {
+	base := testutil.NewBase(t)
+	const containerName = "nerdctl-test-logs"
+	defer base.Cmd("rm", containerName).Run()
+	base.Cmd("run", "-d", "--name", containerName, testutil.AlpineImage,
+		"sh", "-euxc", "echo foo; echo bar; exit 42; echo baz").AssertOK()
+	// AssertOut also asserts that the exit code of the logs command == 0,
+	// even when the container is failing
+	base.Cmd("logs", "-f", containerName).AssertOut("bar")
+	base.Cmd("logs", "-f", containerName).AssertNoOut("baz")
+	base.Cmd("rm", "-f", containerName).AssertOK()
+}