container export: implement file-write with atomicwriter

Same functionality, but implemented with atomicwriter. There's a slight
difference in error-messages produced (but can be adjusted if we want).

Before:

    docker container export -o ./no/such/foo mycontainer
    failed to export container: invalid output path: directory "no/such" does not exist

    docker container export -o /no/permissions mycontainer
    failed to export container: stat /no/permissions: permission denied

After:

    docker container export -o ./no/such/foo mycontainer
    failed to export container: invalid file path: stat no/such: no such file or directory

    docker container export -o /no/permissions mycontainer
    failed to export container: failed to stat output path: lstat /no/permissions: permission denied

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/container/export.go

@@ -7,6 +7,7 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/completion"
+	"github.com/moby/sys/atomicwriter"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 )
@@ -41,27 +42,28 @@ func NewExportCommand(dockerCli command.Cli) *cobra.Command {
 	return cmd
 }
 
-func runExport(ctx context.Context, dockerCli command.Cli, opts exportOptions) error {
+func runExport(ctx context.Context, dockerCLI command.Cli, opts exportOptions) error {
-	if opts.output == "" && dockerCli.Out().IsTerminal() {
+	var output io.Writer
-		return errors.New("cowardly refusing to save to a terminal. Use the -o flag or redirect")
+	if opts.output == "" {
+		if dockerCLI.Out().IsTerminal() {
+			return errors.New("cowardly refusing to save to a terminal. Use the -o flag or redirect")
+		}
+		output = dockerCLI.Out()
+	} else {
+		writer, err := atomicwriter.New(opts.output, 0o600)
+		if err != nil {
+			return errors.Wrap(err, "failed to export container")
+		}
+		defer writer.Close()
+		output = writer
 	}
 
-	if err := command.ValidateOutputPath(opts.output); err != nil {
+	responseBody, err := dockerCLI.Client().ContainerExport(ctx, opts.container)
-		return errors.Wrap(err, "failed to export container")
-	}
-
-	clnt := dockerCli.Client()
-
-	responseBody, err := clnt.ContainerExport(ctx, opts.container)
 	if err != nil {
 		return err
 	}
 	defer responseBody.Close()
 
-	if opts.output == "" {
+	_, err = io.Copy(output, responseBody)
-		_, err := io.Copy(dockerCli.Out(), responseBody)
+	return err
-		return err
-	}
-
-	return command.CopyToFile(opts.output, responseBody)
 }

cli/command/container/export_test.go

@@ -42,8 +42,6 @@ func TestContainerExportOutputToIrregularFile(t *testing.T) {
 	cmd.SetErr(io.Discard)
 	cmd.SetArgs([]string{"-o", "/dev/random", "container"})
 
-	err := cmd.Execute()
+	const expected = `failed to export container: cannot write to a character device file`
-	assert.Assert(t, err != nil)
+	assert.Error(t, cmd.Execute(), expected)
-	expected := `"/dev/random" must be a directory or a regular file`
-	assert.ErrorContains(t, err, expected)
 }