Implement resumable download queue (T-301)

core/downloader/event.go

@@ -0,0 +1,43 @@
+package downloader
+
+import "context"
+
+// EventType identifies an observer event emitted by Queue.
+type EventType string
+
+const (
+	EventStarted   EventType = "started"
+	EventProgress  EventType = "progress"
+	EventPaused    EventType = "paused"
+	EventCompleted EventType = "completed"
+	EventFailed    EventType = "failed"
+	EventCanceled  EventType = "canceled"
+)
+
+// Event reports one generation of a download task. Done is monotonic within a
+// RequestID+Attempt pair. A new Attempt permits a reset to zero when a server
+// ignores Range or an entity cannot be safely resumed.
+type Event struct {
+	Type          EventType
+	RequestID     string
+	AppID         string
+	Attempt       uint64
+	Done          int64
+	TotalKnown    bool
+	Total         int64
+	SpeedBytesSec int64
+	CompletedPath string
+	ErrorCode     string
+}
+
+// Observer receives queue events. Implementations must honor context
+// cancellation so pause/cancel can wait for the old generation to exit.
+type Observer interface {
+	PublishDownload(context.Context, Event) error
+}
+
+type discardObserver struct{}
+
+func (discardObserver) PublishDownload(context.Context, Event) error {
+	return nil
+}