cmd/strelaypoolsrv: Fix race condition in caching (#6496)

Successful LRU cache lookups modify the cache's recency list, so
RWMutex.RLock isn't enough protection.

Secondarily, multiple concurrent lookups with the same key should not
create separate rate limiters, so release the lock only when presence
of the key in the cache has been ascertained.

Co-authored-by: greatroar <@>

cmd/strelaypoolsrv/main.go

@@ -116,10 +116,10 @@ var (
 	requestQueueLen   = 10
 	requestProcessors = 1
 
-	getMut      = sync.NewRWMutex()
+	getMut      = sync.NewMutex()
 	getLRUCache *lru.Cache
 
-	postMut      = sync.NewRWMutex()
+	postMut      = sync.NewMutex()
 	postLRUCache *lru.Cache
 
 	requests chan request
@@ -583,22 +583,21 @@ func evict(relay *relay) func() {
 	}
 }
 
-func limit(addr string, cache *lru.Cache, lock sync.RWMutex, intv time.Duration, burst int) bool {
+func limit(addr string, cache *lru.Cache, lock sync.Mutex, intv time.Duration, burst int) bool {
 	if host, _, err := net.SplitHostPort(addr); err == nil {
 		addr = host
 	}
 
-	lock.RLock()
+	lock.Lock()
 	bkt, ok := cache.Get(addr)
-	lock.RUnlock()
 	if ok {
+		lock.Unlock()
 		bkt := bkt.(*rate.Limiter)
 		if !bkt.Allow() {
 			// Rate limit
 			return true
 		}
 	} else {
-		lock.Lock()
 		cache.Add(addr, rate.NewLimiter(rate.Every(intv), burst))
 		lock.Unlock()
 	}