Fast fail when save directory does not exist

Signed-off-by: Darren Stahl <darst@microsoft.com>

cli/command/image/save.go

@@ -2,6 +2,8 @@ package image
 
 import (
 	"io"
+	"os"
+	"path/filepath"
 
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
@@ -41,6 +43,10 @@ func runSave(dockerCli command.Cli, opts saveOptions) error {
 		return errors.New("cowardly refusing to save to a terminal. Use the -o flag or redirect")
 	}
 
+	if err := validateOutputPath(opts.output); err != nil {
+		return errors.Wrap(err, "failed to save image")
+	}
+
 	responseBody, err := dockerCli.Client().ImageSave(context.Background(), opts.images)
 	if err != nil {
 		return err
@@ -54,3 +60,13 @@ func runSave(dockerCli command.Cli, opts saveOptions) error {
 
 	return command.CopyToFile(opts.output, responseBody)
 }
+
+func validateOutputPath(path string) error {
+	dir := filepath.Dir(path)
+	if dir != "" && dir != "." {
+		if _, err := os.Stat(dir); os.IsNotExist(err) {
+			return errors.Errorf("unable to validate output path: directory %q does not exist", dir)
+		}
+	}
+	return nil
+}

cli/command/image/save_test.go

@@ -43,6 +43,11 @@ func TestNewSaveCommandErrors(t *testing.T) {
 				return ioutil.NopCloser(strings.NewReader("")), errors.Errorf("error saving image")
 			},
 		},
+		{
+			name:          "output directory does not exist",
+			args:          []string{"-o", "fakedir/out.tar", "arg1"},
+			expectedError: "failed to save image: unable to validate output path: directory \"fakedir\" does not exist",
+		},
 	}
 	for _, tc := range testCases {
 		cli := test.NewFakeCli(&fakeClient{imageSaveFunc: tc.imageSaveFunc})