filters: don't encode an empty set. update comments

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

utils/filters/parse.go

@@ -8,14 +8,12 @@ import (
 
 type Args map[string][]string
 
-/*
-Parse the argument to the filter flag. Like
-
-  `docker ps -f 'created=today' -f 'image.name=ubuntu*'`
-
-If prev map is provided, then it is appended to, and returned. By default a new
-map is created.
-*/
+// Parse the argument to the filter flag. Like
+//
+//   `docker ps -f 'created=today' -f 'image.name=ubuntu*'`
+//
+// If prev map is provided, then it is appended to, and returned. By default a new
+// map is created.
 func ParseFlag(arg string, prev Args) (Args, error) {
 	var filters Args = prev
 	if prev == nil {
@@ -37,10 +35,13 @@ func ParseFlag(arg string, prev Args) (Args, error) {
 
 var ErrorBadFormat = errors.New("bad format of filter (expected name=value)")
 
-/*
-packs the Args into an string for easy transport from client to server
-*/
+// packs the Args into an string for easy transport from client to server
 func ToParam(a Args) (string, error) {
+	// this way we don't URL encode {}, just empty space
+	if len(a) == 0 {
+		return "", nil
+	}
+
 	buf, err := json.Marshal(a)
 	if err != nil {
 		return "", err
@@ -48,11 +49,12 @@ func ToParam(a Args) (string, error) {
 	return string(buf), nil
 }
 
-/*
-unpacks the filter Args
-*/
+// unpacks the filter Args
 func FromParam(p string) (Args, error) {
 	args := Args{}
+	if len(p) == 0 {
+		return args, nil
+	}
 	err := json.Unmarshal([]byte(p), &args)
 	if err != nil {
 		return nil, err

utils/filters/parse_test.go

@@ -61,3 +61,18 @@ func TestParam(t *testing.T) {
 		}
 	}
 }
+
+func TestEmpty(t *testing.T) {
+	a := Args{}
+	v, err := ToParam(a)
+	if err != nil {
+		t.Errorf("failed to marshal the filters: %s", err)
+	}
+	v1, err := FromParam(v)
+	if err != nil {
+		t.Errorf("%s", err)
+	}
+	if len(a) != len(v1) {
+		t.Errorf("these should both be empty sets")
+	}
+}