podman save use named pipe

podman save uses named pipe as output path, not directly using /dev/stdout.
fix containers#7017

Signed-off-by: Qi Wang <qiwan@redhat.com>

cmd/podman/images/save.go

@@ -2,10 +2,15 @@ package images
 
 import (
 	"context"
+	"io"
+	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/containers/podman/v2/libpod/define"
+	"golang.org/x/crypto/ssh/terminal"
 
 	"github.com/containers/podman/v2/cmd/podman/parse"
 	"github.com/containers/podman/v2/cmd/podman/registry"
@@ -14,7 +19,6 @@ import (
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
-	"golang.org/x/crypto/ssh/terminal"
 )
 
 var validFormats = []string{define.OCIManifestDir, define.OCIArchive, define.V2s2ManifestDir, define.V2s2Archive}
@@ -85,7 +89,10 @@ func saveFlags(flags *pflag.FlagSet) {
 
 func save(cmd *cobra.Command, args []string) error {
 	var (
-		tags []string
+		tags     []string
+		pipePath string
+		cleanup  func() error
+		err      error
 	)
 	if cmd.Flag("compress").Changed && (saveOpts.Format != define.OCIManifestDir && saveOpts.Format != define.V2s2ManifestDir && saveOpts.Format == "") {
 		return errors.Errorf("--compress can only be set when --format is either 'oci-dir' or 'docker-dir'")
@@ -95,13 +102,55 @@ func save(cmd *cobra.Command, args []string) error {
 		if terminal.IsTerminal(int(fi.Fd())) {
 			return errors.Errorf("refusing to save to terminal. Use -o flag or redirect")
 		}
-		saveOpts.Output = "/dev/stdout"
+		pipePath, cleanup, err = setupPipe()
+		if err != nil {
+			return err
+		}
+		saveOpts.Output = pipePath
 	}
 	if err := parse.ValidateFileName(saveOpts.Output); err != nil {
 		return err
 	}
 	if len(args) > 1 {
 		tags = args[1:]
 	}
-	return registry.ImageEngine().Save(context.Background(), args[0], tags, saveOpts)
+	// return registry.ImageEngine().Save(context.Background(), args[0], tags, saveOpts)
+	err = registry.ImageEngine().Save(context.Background(), args[0], tags, saveOpts)
+	if err != nil {
+		return err
+	}
+	if cleanup != nil {
+		if err = cleanup(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func setupPipe() (string, func() error, error) {
+	errc := make(chan error)
+	pipeDir, err := ioutil.TempDir(os.TempDir(), "pipeDir")
+	if err != nil {
+		return "", nil, err
+	}
+	pipePath := filepath.Join(pipeDir, "saveio")
+	err = syscall.Mkfifo(pipePath, 0600)
+	if err != nil {
+		return "", nil, errors.Wrapf(err, "error creating named pipe")
+	}
+	go func() {
+		fpipe, err := os.Open(pipePath)
+		if err != nil {
+			errc <- err
+			return
+		}
+		_, err = io.Copy(os.Stdout, fpipe)
+		fpipe.Close()
+		errc <- err
+	}()
+	return pipePath, func() error {
+		err := <-errc
+		os.RemoveAll(pipeDir)
+		return err
+	}, nil
 }