fix(sub): coalesce external subscription refreshes (#6139)

* fix(sub): coalesce external subscription refreshes

Limit concurrent cache misses to one upstream request per URL and evict the oldest entries once the cache reaches its bounded capacity.

* fix(sub): preserve shared stale refresh results

Release every in-flight waiter on panic or error, carry the leader outcome to waiters, and strengthen cache capacity and stale fallback coverage.

---------

Co-authored-by: PathGao <gaoyanbo@gaoyanbodeMacBook-Air.local>

internal/sub/external_subscription.go

@@ -16,8 +16,9 @@ import (
 // with a short timeout so a slow/dead provider can't stall a client's sub.
 
 const (
-	subscriptionCacheTTL = 5 * time.Minute
-	subscriptionMaxBytes = 2 << 20 // 2 MiB
+	subscriptionCacheTTL      = 5 * time.Minute
+	subscriptionMaxBytes      = 2 << 20 // 2 MiB
+	subscriptionCacheCapacity = 256
 )
 
 var subscriptionHTTPClient = &http.Client{Timeout: 6 * time.Second}
@@ -27,10 +28,19 @@ type subscriptionCacheEntry struct {
 	fetchedAt time.Time
 }
 
+type subscriptionFetch struct {
+	done  chan struct{}
+	links []string
+}
+
 var subscriptionCache = struct {
 	sync.Mutex
-	m map[string]subscriptionCacheEntry
-}{m: make(map[string]subscriptionCacheEntry)}
+	m        map[string]subscriptionCacheEntry
+	inflight map[string]*subscriptionFetch
+}{
+	m:        make(map[string]subscriptionCacheEntry),
+	inflight: make(map[string]*subscriptionFetch),
+}
 
 // fetchSubscriptionLinks returns the share links contained in a remote
 // subscription URL, using a short-lived cache. On any failure it returns the
@@ -44,24 +54,59 @@ func fetchSubscriptionLinks(rawURL string) []string {
 
 	subscriptionCache.Lock()
 	cached, ok := subscriptionCache.m[rawURL]
-	subscriptionCache.Unlock()
 	if ok && time.Since(cached.fetchedAt) < subscriptionCacheTTL {
+		subscriptionCache.Unlock()
 		return cached.links
 	}
+	if fetch, waiting := subscriptionCache.inflight[rawURL]; waiting {
+		subscriptionCache.Unlock()
+		<-fetch.done
+		return fetch.links
+	}
+	fetch := &subscriptionFetch{done: make(chan struct{})}
+	subscriptionCache.inflight[rawURL] = fetch
+	subscriptionCache.Unlock()
+	defer func() {
+		subscriptionCache.Lock()
+		close(fetch.done)
+		delete(subscriptionCache.inflight, rawURL)
+		subscriptionCache.Unlock()
+	}()
 
 	links, err := doFetchSubscriptionLinks(rawURL)
 	if err != nil {
-		// Serve stale on error rather than dropping the client's configs.
 		if ok {
-			return cached.links
+			fetch.links = cached.links
 		}
-		return nil
+		return fetch.links
 	}
 
 	subscriptionCache.Lock()
 	subscriptionCache.m[rawURL] = subscriptionCacheEntry{links: links, fetchedAt: time.Now()}
+	trimSubscriptionCacheLocked(rawURL)
 	subscriptionCache.Unlock()
-	return links
+	fetch.links = links
+	return fetch.links
+}
+
+func trimSubscriptionCacheLocked(keep string) {
+	for len(subscriptionCache.m) > subscriptionCacheCapacity {
+		var oldestURL string
+		var oldest time.Time
+		for rawURL, entry := range subscriptionCache.m {
+			if rawURL == keep {
+				continue
+			}
+			if oldestURL == "" || entry.fetchedAt.Before(oldest) {
+				oldestURL = rawURL
+				oldest = entry.fetchedAt
+			}
+		}
+		if oldestURL == "" {
+			return
+		}
+		delete(subscriptionCache.m, oldestURL)
+	}
 }
 
 func doFetchSubscriptionLinks(rawURL string) ([]string, error) {