EagleCast

internal/events/events.go

@@ -0,0 +1,100 @@
+// Package events implements a simple event broadcasting mechanism
+// for usage in broadcasting error messages, postbacks etc. various
+// channels.
+package events
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"sync"
+)
+
+const (
+	TypeError = "error"
+)
+
+// Event represents a single event in the system.
+type Event struct {
+	ID       string   `json:"id"`
+	Type     string   `json:"type"`
+	Message  string   `json:"message"`
+	Data     any      `json:"data"`
+	Channels []string `json:"-"`
+}
+
+type Events struct {
+	subs map[string]chan Event
+	sync.RWMutex
+}
+
+// New returns a new instance of Events.
+func New() *Events {
+	return &Events{
+		subs: make(map[string]chan Event),
+	}
+}
+
+// Subscribe returns a channel to which the given event `types` are streamed.
+// id is the unique identifier for the caller. A caller can only register
+// for subscription once.
+func (ev *Events) Subscribe(id string) (chan Event, error) {
+	ev.Lock()
+	defer ev.Unlock()
+
+	if ch, ok := ev.subs[id]; ok {
+		return ch, nil
+	}
+
+	ch := make(chan Event, 100)
+	ev.subs[id] = ch
+
+	return ch, nil
+}
+
+// Unsubscribe unsubscribes a subscriber (obviously).
+func (ev *Events) Unsubscribe(id string) {
+	ev.Lock()
+	defer ev.Unlock()
+	delete(ev.subs, id)
+}
+
+// Publish publishes an event to all subscribers.
+func (ev *Events) Publish(e Event) error {
+	ev.Lock()
+	defer ev.Unlock()
+
+	for _, ch := range ev.subs {
+		select {
+		case ch <- e:
+		default:
+			return fmt.Errorf("event queue full for type: %s", e.Type)
+		}
+	}
+
+	return nil
+}
+
+// This implements an io.Writer specifically for receiving error messages
+// mirrored (io.MultiWriter) from error log writing.
+type wri struct {
+	ev *Events
+}
+
+func (w *wri) Write(b []byte) (n int, err error) {
+	// Only broadcast error messages.
+	if !bytes.Contains(b, []byte("error")) {
+		return 0, nil
+	}
+
+	w.ev.Publish(Event{
+		Type:    TypeError,
+		Message: string(b),
+	})
+
+	return len(b), nil
+}
+
+func (ev *Events) ErrWriter() io.Writer {
+	return &wri{ev: ev}
+}