watcher: MsgHandler, EventWatcher

This package is responsible for maintaining the lifecycle of a
fifo.EventChannel.

The EventWatcher type handles a single instance of an EventChannel,
feeding messages it receives to a provided watcher.MsgHandler interface.

watcher/watcher.go

@@ -0,0 +1,73 @@
+/*
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/.
+ */
+
+package watcher
+
+import (
+	"fmt"
+
+	"git.st8l.com/luxolus/kdnotify/config"
+	"git.st8l.com/luxolus/kdnotify/fifo"
+	n "git.st8l.com/luxolus/kdnotify/schema/notify"
+)
+
+type MsgHandler interface {
+	ProcessVrrp(n.VrrpMessage) error
+}
+
+type EventWatcher struct {
+	Events fifo.EventChannel
+	Cxt    *config.LibCxt
+}
+
+func NewWatcherFifo(cxt *config.LibCxt, path string) (EventWatcher, error) {
+	events, err := fifo.NewFifoChannel(cxt, path)
+	if err != nil {
+		return EventWatcher{}, fmt.Errorf("unable to create watch fifo channel for '%s': %s", path, err)
+	}
+
+	return NewWatcher(cxt, &events), nil
+}
+
+func NewWatcher(cxt *config.LibCxt, events fifo.EventChannel) EventWatcher {
+	cxt = &config.LibCxt{
+		Context: cxt.Context,
+		Logger:  cxt.Logger.Named("watch"),
+	}
+
+	return EventWatcher{
+		Events: events,
+		Cxt:    cxt,
+	}
+}
+
+func (w *EventWatcher) Watch(h MsgHandler) error {
+	w.watchLoop(w.Events, h)
+
+	return nil
+}
+
+func (w *EventWatcher) watchLoop(events fifo.EventChannel, h MsgHandler) {
+	log := w.Cxt.Logger.Sugar()
+
+	log.Debug("starting watch")
+	for !events.Done() {
+		msg, err := events.Next()
+		if err != nil {
+			if err != fifo.EClosed {
+				log.Warnw("event channel closed abnormally", "error", err)
+			}
+			break
+		}
+		log.Debugw("incoming VRRP message", "instance", msg.Instance)
+
+		err = h.ProcessVrrp(msg)
+		if err != nil {
+			log.Warnw("handler failed to process VRRP message", "error", err)
+		}
+	}
+	log.Debug("stopping watch")
+}