syncthing/lib/connections/registry/registry.go

// 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 (
	"strings"

	"github.com/syncthing/syncthing/lib/sync"
)

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 {
			candidates[i] = candidates[len(candidates)-1]
			candidates[len(candidates)-1] = nil
			r.available[scheme] = candidates[:len(candidates)-1]
			break
		}
	}
}

// Get returns an item for a schema compatible with the given scheme.
// If any item satisfies preferred, that has precedence over other items.
func (r *Registry) Get(scheme string, preferred func(interface{}) bool) interface{} {
	r.mut.Lock()
	defer r.mut.Unlock()

	var (
		best       interface{}
		bestPref   bool
		bestScheme string
	)
	for availableScheme, items := range r.available {
		// quic:// should be considered ok for both quic4:// and quic6://
		if !strings.HasPrefix(scheme, availableScheme) {
			continue
		}
		for _, item := range items {
			better := best == nil
			pref := preferred(item)
			if !better {
				// In case of a tie, prefer "quic" to "quic[46]" etc.
				better = pref &&
					(!bestPref || len(availableScheme) < len(bestScheme))
			}
			if !better {
				continue
			}
			best, bestPref, bestScheme = item, pref, availableScheme
		}
	}
	return best
}