lib/connections: Add QUIC protocol support (fixes #5377) (#5737)

lib/connections/registry/registry.go

@@ -0,0 +1,89 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// Registry tracks connections/addresses on which we are listening on, to allow us to pick a connection/address that
+// has a NAT port mapping. This also makes our outgoing port stable and same as incoming port which should allow
+// better probability of punching through.
+package registry
+
+import (
+	"sort"
+	"strings"
+
+	"github.com/syncthing/syncthing/lib/sync"
+)
+
+var (
+	Default = New()
+)
+
+type Registry struct {
+	mut       sync.Mutex
+	available map[string][]interface{}
+}
+
+func New() *Registry {
+	return &Registry{
+		mut:       sync.NewMutex(),
+		available: make(map[string][]interface{}),
+	}
+}
+
+func (r *Registry) Register(scheme string, item interface{}) {
+	r.mut.Lock()
+	defer r.mut.Unlock()
+
+	r.available[scheme] = append(r.available[scheme], item)
+}
+
+func (r *Registry) Unregister(scheme string, item interface{}) {
+	r.mut.Lock()
+	defer r.mut.Unlock()
+
+	candidates := r.available[scheme]
+	for i, existingItem := range candidates {
+		if existingItem == item {
+			copy(candidates[i:], candidates[i+1:])
+			candidates[len(candidates)-1] = nil
+			r.available[scheme] = candidates[:len(candidates)-1]
+			break
+		}
+	}
+}
+
+func (r *Registry) Get(scheme string, less func(i, j interface{}) bool) interface{} {
+	r.mut.Lock()
+	defer r.mut.Unlock()
+
+	candidates := make([]interface{}, 0)
+	for availableScheme, items := range r.available {
+		// quic:// should be considered ok for both quic4:// and quic6://
+		if strings.HasPrefix(scheme, availableScheme) {
+			candidates = append(candidates, items...)
+		}
+	}
+
+	if len(candidates) == 0 {
+		return nil
+	}
+
+	sort.Slice(candidates, func(i, j int) bool {
+		return less(candidates[i], candidates[j])
+	})
+	return candidates[0]
+}
+
+func Register(scheme string, item interface{}) {
+	Default.Register(scheme, item)
+}
+
+func Unregister(scheme string, item interface{}) {
+	Default.Unregister(scheme, item)
+}
+
+func Get(scheme string, less func(i, j interface{}) bool) interface{} {
+	return Default.Get(scheme, less)
+}