feat(scheduler): marshal task properties as empty JSON not null

Task properties typing is problematic. Within SM server codebase,
they are json.RawMessage. When returned to sctool or apps using SM
client, they are interface{} which is carelessly cast to map[string]any.
The problem is that nil task properties can't be cast to map[string]any.
This results in unexpected errors and panics on sctool or apps using SM
client side. Within SM codebase, we also make those such assumptions
and sometimes ad-hoc replace nil properties with json.RawMessage("").

This commit doesn't fix anything on its own, but it makes it less
likely that sctool or app using SM client breaks in those silly
places by marshaling nil task properties as empty JSON object.

Author: Michal-Leszczynski

pkg/service/scheduler/model.go

@@ -259,7 +259,7 @@ type Task struct {
 	Deleted    bool              `json:"deleted,omitempty"`
 	Sched      Schedule          `json:"schedule"`
 	Properties json.RawMessage   `json:"properties,omitempty"`
-	Tags       []string
+	Tags       []string          `json:"tags,omitempty"`
 
 	Status       Status     `json:"status"`
 	SuccessCount int        `json:"success_count"`
@@ -268,6 +268,18 @@ type Task struct {
 	LastError    *time.Time `json:"last_error"`
 }
 
+// MarshalJSON is a wrapper around default JSON marshaller ensuring
+// that Task.Properties are marshaled as an empty JSON object instead
+// of null. This makes it easier and safer on SM client side.
+func (t Task) MarshalJSON() ([]byte, error) {
+	type Alias Task // To avoid infinite recursion
+	taskWithFilledProps := Alias(t)
+	if len(taskWithFilledProps.Properties) == 0 || string(taskWithFilledProps.Properties) == "null" {
+		taskWithFilledProps.Properties = json.RawMessage("{}")
+	}
+	return json.Marshal(taskWithFilledProps)
+}
+
 func (t *Task) String() string {
 	return fmt.Sprintf("%s/%s", t.Type, t.ID)
 }