Fix: Improve version polling, add thread safety and exponential backoff (#523)

Squashed commit of the following:

commit 1ed713dd8de2adb7d761e20bb8018804c2e27ea6
Author: Ralph Slooten <axllent@gmail.com>
Date:   Wed Jun 18 17:03:36 2025 +1200

    Refactor latest version caching, add console logging if update checks fails

commit bf880e583372d81a0597bc263ab22f6989e48fa9
Author: Ben Edmunds <Tigger2014@users.noreply.github.com>
Date:   Wed Jun 18 05:52:35 2025 +0100

    Fix: Improve version polling, add thread safety and exponential backoff (#523)

    * make version polling thread safe and add expo backoff

    * tidy up

internal/stats/stats.go

@@ -7,17 +7,30 @@ import (
 	"time"
 
 	"github.com/axllent/mailpit/config"
+	"github.com/axllent/mailpit/internal/logger"
 	"github.com/axllent/mailpit/internal/storage"
 	"github.com/axllent/mailpit/internal/updater"
 )
 
+// Stores cached version  along with its expiry time and error count.
+// Used to minimize repeated version lookups and track consecutive errors.
+type versionCache struct {
+	// github version string
+	value string
+	// time to expire the cache
+	expiry time.Time
+	// count of consecutive errors
+	errCount int
+}
+
 var (
-	// to prevent hammering Github for latest version
-	latestVersionCache string
+	// Version cache storing the latest GitHub version
+	vCache versionCache
 
 	// StartedAt is set to the current ime when Mailpit starts
 	startedAt time.Time
 
+	// sync mutex to prevent race condition with simultaneous requests
 	mu sync.RWMutex
 
 	smtpAccepted     uint64
@@ -62,6 +75,12 @@ type AppInformation struct {
 	}
 }
 
+// Calculates exponential backoff duration based on the error count.
+func getBackoff(errCount int) time.Duration {
+	backoff := min(time.Duration(1<<errCount)*time.Minute, 30*time.Minute)
+	return backoff
+}
+
 // Load the current statistics
 func Load() AppInformation {
 	info := AppInformation{}
@@ -78,20 +97,32 @@ func Load() AppInformation {
 	info.RuntimeStats.SMTPRejected = smtpRejected
 	info.RuntimeStats.SMTPIgnored = smtpIgnored
 
-	if latestVersionCache != "" {
-		info.LatestVersion = latestVersionCache
-	} else {
-		latest, _, _, err := updater.GithubLatest(config.Repo, config.RepoBinaryName)
-		if err == nil {
-			info.LatestVersion = latest
-			latestVersionCache = latest
+	mu.RLock()
+	cacheValid := time.Now().Before(vCache.expiry)
+	cacheValue := vCache.value
+	mu.RUnlock()
 
-			// clear latest version cache after 5 minutes
-			go func() {
-				time.Sleep(15 * time.Minute)
-				latestVersionCache = ""
-			}()
+	if cacheValid {
+		info.LatestVersion = cacheValue
+	} else {
+		mu.Lock()
+		// Re-check after acquiring write lock in case another goroutine refreshed it
+		if time.Now().Before(vCache.expiry) {
+			info.LatestVersion = vCache.value
+		} else {
+			latest, _, _, err := updater.GithubLatest(config.Repo, config.RepoBinaryName)
+			if err == nil {
+				vCache = versionCache{value: latest, expiry: time.Now().Add(15 * time.Minute)}
+				info.LatestVersion = latest
+			} else {
+				logger.Log().Errorf("Failed to fetch latest version: %v", err)
+				vCache.errCount++
+				vCache.value = ""
+				vCache.expiry = time.Now().Add(getBackoff(vCache.errCount))
+				info.LatestVersion = ""
+			}
 		}
+		mu.Unlock()
 	}
 
 	info.Database = config.Database