feat: Add flags helpers for lists flags

  Simplify the usage of lists flags with typed return values.

  Lists flags are defined like this:

  ```json
  "<name>": { "list": ["<value>", ...] }
  ```

model/feature/flag.go

@@ -71,6 +71,38 @@ func (f *Flags) UnmarshalJSON(bytes []byte) error {
 	return nil
 }
 
+func (f *Flags) GetList(name string) ([]interface{}, error) {
+	if f.M[name] == nil {
+		return []interface{}{}, nil
+	}
+
+	value, ok := f.M[name].(map[string]interface{})
+	if !ok {
+		return nil, fmt.Errorf("Flag %s is not a list flag", name)
+	}
+
+	list, ok := value["list"].([]interface{})
+	if !ok {
+		return nil, fmt.Errorf("Flag %s is not a list flag", name)
+	}
+
+	return list, nil
+}
+
+func (f *Flags) HasListItem(name, item string) (bool, error) {
+	list, err := f.GetList(name)
+	if err != nil {
+		return false, err
+	}
+
+	for _, i := range list {
+		if i == item {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
 // GetFlags returns the list of feature flags for the given instance.
 func GetFlags(inst *instance.Instance) (*Flags, error) {
 	sources := make([]*Flags, 0)

model/feature/flag_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/cozy/cozy-stack/model/instance"
 	"github.com/gofrs/uuid/v5"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func uuidv7() string {
@@ -38,3 +39,40 @@ func TestFeatureFlagRatio(t *testing.T) {
 	assert.InDelta(t, 4000, results[float64(4)], 100)
 	assert.InDelta(t, 3000, results[nil], 100)
 }
+
+func TestFeatureFlagList(t *testing.T) {
+	var flags Flags
+	err := json.Unmarshal(
+		[]byte(`{
+		  "flag1": { "list": ["other", "val"] },
+		  "flag2": { "list": ["other"] },
+		  "flag3": { "list": [] },
+		  "flag4": ["val"],
+		  "flag5": "val"
+		}`),
+		&flags,
+	)
+	require.NoError(t, err)
+
+	// GetList
+	list, err := flags.GetList("flag1")
+	require.NoError(t, err)
+	assert.EqualValues(t, []interface{}{"other", "val"}, list)
+	_, err = flags.GetList("flag4")
+	assert.Error(t, err)
+	_, err = flags.GetList("flag5")
+	assert.Error(t, err)
+
+	// HasListItem
+	hasVal, err := flags.HasListItem("flag1", "val")
+	require.NoError(t, err)
+	assert.True(t, hasVal)
+	hasVal, _ = flags.HasListItem("flag2", "val")
+	assert.False(t, hasVal)
+	hasVal, _ = flags.HasListItem("flag3", "val")
+	assert.False(t, hasVal)
+	_, err = flags.HasListItem("flag4", "val")
+	assert.Error(t, err)
+	_, err = flags.HasListItem("flag5", "val")
+	assert.Error(t, err)
+}