create UpdateChecker background task + constants refactoring

backend/updatechecker.go

@@ -0,0 +1,71 @@
+package backend
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type UpdateChecker struct {
+	OnUpdatedVersionFound func(releaseURL string)
+
+	foundUpdate      bool
+	latestReleaseURL string
+	appVersionTag    string
+	lastCheckedTag   *string
+}
+
+func NewUpdateChecker(appVersionTag, latestReleaseURL string, lastCheckedTag *string) UpdateChecker {
+	return UpdateChecker{
+		appVersionTag:    appVersionTag,
+		latestReleaseURL: latestReleaseURL,
+		lastCheckedTag:   lastCheckedTag,
+	}
+}
+
+func (u *UpdateChecker) Start(ctx context.Context, interval time.Duration) {
+	go func() {
+		u.checkForUpdate() // check once at startup
+		t := time.NewTicker(interval)
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-t.C:
+				u.checkForUpdate()
+			}
+		}
+	}()
+}
+
+func (u *UpdateChecker) UpdateAvailable() bool {
+	return u.foundUpdate
+}
+
+func (u *UpdateChecker) checkForUpdate() {
+	t := u.latestVersionTag()
+	if t != "" && t != *u.lastCheckedTag {
+		u.foundUpdate = true
+		*u.lastCheckedTag = t
+		if u.OnUpdatedVersionFound != nil {
+			u.OnUpdatedVersionFound(u.latestReleaseURL)
+		}
+	}
+}
+
+func (u *UpdateChecker) latestVersionTag() string {
+	resp, err := http.Head(u.latestReleaseURL)
+	if err != nil {
+		log.Printf("failed to check for newest version: %s", err.Error())
+		return ""
+	}
+	url := resp.Request.URL.String()
+	url = strings.TrimSuffix(url, "/")
+	idx := strings.LastIndex(url, "/")
+	if idx >= len(url)-1 {
+		return ""
+	}
+	return url[idx+1:]
+}