podman untag: error if tag doesn't exist

Throw an error if a specified tag does not exist.  Also make sure that
the user input is normalized as we already do for `podman tag`.

To prevent regressions, add a set of end-to-end and systemd tests.

Last but not least, update the docs and add bash completions.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

2 files changed, 13 insertions, 2 deletions

diff --git a/libpod/image/errors.go b/libpod/image/errors.go
index 4088946cb..ddbf7be4b 100644
--- a/libpod/image/errors.go
+++ b/libpod/image/errors.go
@@ -12,4 +12,6 @@ var (
 	ErrNoSuchPod = errors.New("no such pod")
 	// ErrNoSuchImage indicates the requested image does not exist
 	ErrNoSuchImage = errors.New("no such image")
+	// ErrNoSuchTag indicates the requested image tag does not exist
+	ErrNoSuchTag = errors.New("no such tag")
 )
diff --git a/libpod/image/image.go b/libpod/image/image.go
index d81f7e911..83e7467e9 100644
--- a/libpod/image/image.go
+++ b/libpod/image/image.go
@@ -559,15 +559,24 @@ func (i *Image) TagImage(tag string) error {
 	return nil
 }
 
-// UntagImage removes a tag from the given image
+// UntagImage removes the specified tag from the image.
+// If the tag does not exist, ErrNoSuchTag is returned.
 func (i *Image) UntagImage(tag string) error {
 	if err := i.reloadImage(); err != nil {
 		return err
 	}
+
+	// Normalize the tag as we do with TagImage.
+	ref, err := NormalizedTag(tag)
+	if err != nil {
+		return err
+	}
+	tag = ref.String()
+
 	var newTags []string
 	tags := i.Names()
 	if !util.StringInSlice(tag, tags) {
-		return nil
+		return errors.Wrapf(ErrNoSuchTag, "%q", tag)
 	}
 	for _, t := range tags {
 		if tag != t {