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

@@ -1,11 +1,18 @@
 package images
 
 import (
+	"bufio"
 	"context"
+	"io"
+	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/containers/podman/v2/libpod/define"
+	"github.com/sirupsen/logrus"
+	"golang.org/x/crypto/ssh/terminal"
 
 	"github.com/containers/podman/v2/cmd/podman/parse"
 	"github.com/containers/podman/v2/cmd/podman/registry"
@@ -14,7 +21,6 @@ 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}
@@ -95,7 +101,39 @@ 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"
+		pipeDir, err := ioutil.TempDir(os.TempDir(), "pipeDir")
+		if err != nil {
+			return err
+		}
+		saveOpts.Output = filepath.Join(pipeDir, "saveio")
+		err = syscall.Mkfifo(saveOpts.Output, 0600)
+		if err != nil {
+			return errors.Wrapf(err, "error creating named pipe")
+		}
+		defer os.RemoveAll(pipeDir)
+		errc := make(chan error)
+		fpipe, err := os.OpenFile(saveOpts.Output, os.O_RDWR, os.ModeNamedPipe)
+		if err != nil {
+			return err
+		}
+		defer fpipe.Close()
+		go func() {
+			reader := bufio.NewReader(fpipe)
+			_, err = io.Copy(os.Stdout, reader)
+			errc <- err
+			if err != nil {
+				logrus.Errorf("error copying data to /dev/stdout, %v", err)
+				return
+			}
+		}()
+		select {
+		case err := <-errc:
+			if err != nil {
+				return err
+			}
+		default:
+			break
+		}
 	}
 	if err := parse.ValidateFileName(saveOpts.Output); err != nil {
 		return err