cmd/strelaypoolsrv: Fix relay shuffling (fixes #6936) (#6935)

When cap(permanentRelays) >= len(permanentRelays) + len(knownRelays),

	append(permanentRelays, knownRelays...)

returns a slice of the array underlying permanentRelays. The subsequent
rand.Shuffle then mixes the permanent and known relays. Sequential
requests may cause strelaypoolsrv to forget its permanent relays. Worse,
concurrent requests may cause shuffling of the same slice on multiple
processors concurrently.

Co-authored-by: greatroar <@>

cmd/strelaypoolsrv/main.go

@@ -314,8 +314,11 @@ func handleRequest(w http.ResponseWriter, r *http.Request) {
 
 func handleGetRequest(rw http.ResponseWriter, r *http.Request) {
 	rw.Header().Set("Content-Type", "application/json; charset=utf-8")
+
 	mut.RLock()
-	relays := append(permanentRelays, knownRelays...)
+	relays := make([]*relay, len(permanentRelays)+len(knownRelays))
+	n := copy(relays, permanentRelays)
+	copy(relays[n:], knownRelays)
 	mut.RUnlock()
 
 	// Shuffle