cp: allow trailing slash in non-existant destination

Signed-off-by: Tibor Vass <tibor@docker.com>

cli/command/utils.go

@@ -130,7 +130,7 @@ func AddPlatformFlag(flags *pflag.FlagSet, target *string) {
 
 // ValidateOutputPath validates the output paths of the `export` and `save` commands.
 func ValidateOutputPath(path string) error {
-	dir := filepath.Dir(path)
+	dir := filepath.Dir(filepath.Clean(path))
 	if dir != "" && dir != "." {
 		if _, err := os.Stat(dir); os.IsNotExist(err) {
 			return errors.Errorf("invalid output path: directory %q does not exist", dir)

cli/command/utils_test.go

@@ -1,8 +1,12 @@
 package command
 
 import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
 	"testing"
 
+	"github.com/pkg/errors"
 	"gotest.tools/assert"
 )
 
@@ -31,3 +35,39 @@ func TestStringSliceReplaceAt(t *testing.T) {
 	assert.Assert(t, !ok)
 	assert.DeepEqual(t, []string{"foo"}, out)
 }
+
+func TestValidateOutputPath(t *testing.T) {
+	basedir, err := ioutil.TempDir("", "TestValidateOutputPath")
+	assert.NilError(t, err)
+	defer os.RemoveAll(basedir)
+	dir := filepath.Join(basedir, "dir")
+	notexist := filepath.Join(basedir, "notexist")
+	err = os.MkdirAll(dir, 0755)
+	assert.NilError(t, err)
+	file := filepath.Join(dir, "file")
+	err = ioutil.WriteFile(file, []byte("hi"), 0644)
+	assert.NilError(t, err)
+	var testcases = []struct {
+		path string
+		err  error
+	}{
+		{basedir, nil},
+		{file, nil},
+		{dir, nil},
+		{dir + string(os.PathSeparator), nil},
+		{notexist, nil},
+		{notexist + string(os.PathSeparator), nil},
+		{filepath.Join(notexist, "file"), errors.New("does not exist")},
+	}
+
+	for _, testcase := range testcases {
+		t.Run(testcase.path, func(t *testing.T) {
+			err := ValidateOutputPath(testcase.path)
+			if testcase.err == nil {
+				assert.NilError(t, err)
+			} else {
+				assert.ErrorContains(t, err, testcase.err.Error())
+			}
+		})
+	}
+}