lib/connections: Use own KCP fork, move listener setup earlier (ref #4446)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4452

vendor/github.com/AudriusButkevicius/kcp-go/blacklist.go

@@ -0,0 +1,56 @@
+package kcp
+
+import (
+	"sync"
+	"time"
+)
+
+var (
+	BlacklistDuration time.Duration
+	blacklist         = blacklistMap{
+		entries: make(map[sessionKey]time.Time),
+	}
+)
+
+// a global map for blacklisting conversations
+type blacklistMap struct {
+	entries map[sessionKey]time.Time
+	reapAt  time.Time
+	mut     sync.Mutex
+}
+
+func (m *blacklistMap) add(address string, conv uint32) {
+	if BlacklistDuration == 0 {
+		return
+	}
+	m.mut.Lock()
+	timeout := time.Now().Add(BlacklistDuration)
+	m.entries[sessionKey{
+		addr:   address,
+		convID: conv,
+	}] = timeout
+	m.reap()
+	m.mut.Unlock()
+}
+
+func (m *blacklistMap) has(address string, conv uint32) bool {
+	if BlacklistDuration == 0 {
+		return false
+	}
+	m.mut.Lock()
+	t, ok := m.entries[sessionKey{
+		addr:   address,
+		convID: conv,
+	}]
+	m.mut.Lock()
+	return ok && t.After(time.Now())
+}
+
+func (m *blacklistMap) reap() {
+	now := time.Now()
+	for k, t := range m.entries {
+		if t.Before(now) {
+			delete(m.entries, k)
+		}
+	}
+}

vendor/github.com/AudriusButkevicius/kcp-go/sess.go

@@ -151,6 +151,7 @@ func newUDPSession(conv uint32, dataShards, parityShards int, l *Listener, conn
 		}
 	})
 	sess.kcp.SetMtu(IKCP_MTU_DEF - sess.headerSize)
+	blacklist.add(remote.String(), conv)
 
 	// add current session to the global updater,
 	// which periodically calls sess.update()
@@ -751,7 +752,7 @@ func (l *Listener) monitor() {
 					}
 
 					if !ok { // new session
-						if len(l.chAccepts) < cap(l.chAccepts) && len(l.sessions) < 4096 { // do not let new session overwhelm accept queue and connection count
+						if !blacklist.has(from.String(), conv) && len(l.chAccepts) < cap(l.chAccepts) && len(l.sessions) < 4096 { // do not let new session overwhelm accept queue and connection count
 							s := newUDPSession(conv, l.dataShards, l.parityShards, l, l.conn, from, l.block)
 							s.kcpInput(data)
 							l.sessions[key] = s