syncthing/lib/discover/cache.go

// Copyright (C) 2015 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/.

package discover

import (
	stdsync "sync"
	"time"

	"github.com/syncthing/syncthing/lib/protocol"
	"github.com/syncthing/syncthing/lib/sync"
	"github.com/syncthing/syncthing/lib/util"
	"github.com/thejerf/suture"
)

// The CachingMux aggregates results from multiple Finders. Each Finder has
// an associated cache time and negative cache time. The cache time sets how
// long we cache and return successful lookup results, the negative cache
// time sets how long we refrain from asking about the same device ID after
// receiving a negative answer. The value of zero disables caching (positive
// or negative).
type CachingMux interface {
	FinderService
	Add(finder Finder, cacheTime, negCacheTime time.Duration)
	ChildErrors() map[string]error
}

type cachingMux struct {
	*suture.Supervisor
	finders []cachedFinder
	caches  []*cache
	mut     sync.RWMutex
}

// A cachedFinder is a Finder with associated cache timeouts.
type cachedFinder struct {
	Finder
	cacheTime    time.Duration
	negCacheTime time.Duration
}

// An error may implement cachedError, in which case it will be interrogated
// to see how long we should cache the error. This overrides the default
// negative cache time.
type cachedError interface {
	CacheFor() time.Duration
}

func NewCachingMux() CachingMux {
	return &cachingMux{
		Supervisor: suture.New("discover.cachingMux", suture.Spec{
			PassThroughPanics: true,
		}),
		mut: sync.NewRWMutex(),
	}
}

// Add registers a new Finder, with associated cache timeouts.
func (m *cachingMux) Add(finder Finder, cacheTime, negCacheTime time.Duration) {
	m.mut.Lock()
	m.finders = append(m.finders, cachedFinder{finder, cacheTime, negCacheTime})
	m.caches = append(m.caches, newCache())
	m.mut.Unlock()

	if service, ok := finder.(suture.Service); ok {
		m.Supervisor.Add(service)
	}
}

// Lookup attempts to resolve the device ID using any of the added Finders,
// while obeying the cache settings.
func (m *cachingMux) Lookup(deviceID protocol.DeviceID) (addresses []string, err error) {
	m.mut.RLock()
	for i, finder := range m.finders {
		if cacheEntry, ok := m.caches[i].Get(deviceID); ok {
			// We have a cache entry. Lets see what it says.

			if cacheEntry.found && time.Since(cacheEntry.when) < finder.cacheTime {
				// It's a positive, valid entry. Use it.
				l.Debugln("cached discovery entry for", deviceID, "at", finder)
				l.Debugln("  cache:", cacheEntry)
				addresses = append(addresses, cacheEntry.Addresses...)
				continue
			}

			valid := time.Now().Before(cacheEntry.validUntil) || time.Since(cacheEntry.when) < finder.negCacheTime
			if !cacheEntry.found && valid {
				// It's a negative, valid entry. We should not make another
				// attempt right now.
				l.Debugln("negative cache entry for", deviceID, "at", finder, "valid until", cacheEntry.when.Add(finder.negCacheTime), "or", cacheEntry.validUntil)
				continue
			}

			// It's expired. Ignore and continue.
		}

		// Perform the actual lookup and cache the result.
		if addrs, err := finder.Lookup(deviceID); err == nil {
			l.Debugln("lookup for", deviceID, "at", finder)
			l.Debugln("  addresses:", addrs)
			addresses = append(addresses, addrs...)
			m.caches[i].Set(deviceID, CacheEntry{
				Addresses: addrs,
				when:      time.Now(),
				found:     len(addrs) > 0,
			})
		} else {
			// Lookup returned error, add a negative cache entry.
			entry := CacheEntry{
				when:  time.Now(),
				found: false,
			}
			if err, ok := err.(cachedError); ok {
				entry.validUntil = time.Now().Add(err.CacheFor())
			}
			m.caches[i].Set(deviceID, entry)
		}
	}
	m.mut.RUnlock()

	l.Debugln("lookup results for", deviceID)
	l.Debugln("  addresses: ", addresses)

	addresses = util.UniqueStrings(addresses)

	return addresses, nil
}

func (m *cachingMux) String() string {
	return "discovery cache"
}

func (m *cachingMux) Error() error {
	return nil
}

func (m *cachingMux) ChildErrors() map[string]error {
	children := make(map[string]error, len(m.finders))
	m.mut.RLock()
	for _, f := range m.finders {
		children[f.String()] = f.Error()
	}
	m.mut.RUnlock()
	return children
}

func (m *cachingMux) Cache() map[protocol.DeviceID]CacheEntry {
	// Res will be the "total" cache, i.e. the union of our cache and all our
	// children's caches.
	res := make(map[protocol.DeviceID]CacheEntry)

	m.mut.RLock()
	for i := range m.finders {
		// Each finder[i] has a corresponding cache at cache[i]. Go through
		// it and populate the total, appending any addresses and keeping
		// the newest "when" time. We skip any negative cache entries.
		for k, v := range m.caches[i].Cache() {
			if v.found {
				cur := res[k]
				if v.when.After(cur.when) {
					cur.when = v.when
				}
				cur.Addresses = append(cur.Addresses, v.Addresses...)
				res[k] = cur
			}
		}

		// Then ask the finder itself for its cache and do the same. If this
		// finder is a global discovery client, it will have no cache. If it's
		// a local discovery client, this will be its current state.
		for k, v := range m.finders[i].Cache() {
			if v.found {
				cur := res[k]
				if v.when.After(cur.when) {
					cur.when = v.when
				}
				cur.Addresses = append(cur.Addresses, v.Addresses...)
				res[k] = cur
			}
		}
	}
	m.mut.RUnlock()

	for k, v := range res {
		v.Addresses = util.UniqueStrings(v.Addresses)
		res[k] = v
	}

	return res
}

// A cache can be embedded wherever useful

type cache struct {
	entries map[protocol.DeviceID]CacheEntry
	mut     stdsync.Mutex
}

func newCache() *cache {
	return &cache{
		entries: make(map[protocol.DeviceID]CacheEntry),
	}
}

func (c *cache) Set(id protocol.DeviceID, ce CacheEntry) {
	c.mut.Lock()
	c.entries[id] = ce
	c.mut.Unlock()
}

func (c *cache) Get(id protocol.DeviceID) (CacheEntry, bool) {
	c.mut.Lock()
	ce, ok := c.entries[id]
	c.mut.Unlock()
	return ce, ok
}

func (c *cache) Cache() map[protocol.DeviceID]CacheEntry {
	c.mut.Lock()
	m := make(map[protocol.DeviceID]CacheEntry, len(c.entries))
	for k, v := range c.entries {
		m[k] = v
	}
	c.mut.Unlock()
	return m
}