p2p/dnsdisc: add enode.Iterator API (#20437)

* p2p/dnsdisc: add support for enode.Iterator

This changes the dnsdisc.Client API to support the enode.Iterator
interface.

* p2p/dnsdisc: rate-limit DNS requests

* p2p/dnsdisc: preserve linked trees across root updates

This improves the way links are handled when the link root changes.
Previously, sync would simply remove all links from the current tree and
garbage-collect all unreachable trees before syncing the new list of
links.

This behavior isn't great in certain cases: Consider a structure where
trees A, B, and C reference each other and D links to A. If D's link
root changed, the sync code would first remove trees A, B and C, only to
re-sync them later when the link to A was found again.

The fix for this problem is to track the current set of links in each
clientTree and removing old links only AFTER all links are synced.

* p2p/dnsdisc: deflake iterator test

* cmd/devp2p: adapt dnsClient to new p2p/dnsdisc API

* p2p/dnsdisc: tiny comment fix

cmd/devp2p/dnscmd.go

@@ -214,8 +214,7 @@ func dnsClient(ctx *cli.Context) *dnsdisc.Client {
 	if commandHasFlag(ctx, dnsTimeoutFlag) {
 		cfg.Timeout = ctx.Duration(dnsTimeoutFlag.Name)
 	}
-	c, _ := dnsdisc.NewClient(cfg) // cannot fail because no URLs given
-	return c
+	return dnsdisc.NewClient(cfg)
 }
 
 // There are two file formats for DNS node trees on disk:

go.mod

@@ -60,6 +60,7 @@ require (
 	golang.org/x/sync v0.0.0-20181108010431-42b317875d0f
 	golang.org/x/sys v0.0.0-20190712062909-fae7ac547cb7
 	golang.org/x/text v0.3.2
+	golang.org/x/time v0.0.0-20190308202827-9d24e82272b4
 	gopkg.in/natefinch/npipe.v2 v2.0.0-20160621034901-c1b8fa8bdcce
 	gopkg.in/olebedev/go-duktape.v3 v3.0.0-20190213234257-ec84240a7772
 	gopkg.in/sourcemap.v1 v1.0.5 // indirect

p2p/dnsdisc/client.go

@@ -23,6 +23,7 @@ import (
 	"math/rand"
 	"net"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/ethereum/go-ethereum/common/mclock"
@@ -31,15 +32,13 @@ import (
 	"github.com/ethereum/go-ethereum/p2p/enode"
 	"github.com/ethereum/go-ethereum/p2p/enr"
 	lru "github.com/hashicorp/golang-lru"
+	"golang.org/x/time/rate"
 )
 
 // Client discovers nodes by querying DNS servers.
 type Client struct {
-	cfg       Config
-	clock     mclock.Clock
-	linkCache linkCache
-	trees     map[string]*clientTree
-
+	cfg     Config
+	clock   mclock.Clock
 	entries *lru.Cache
 }
 
@@ -48,6 +47,7 @@ type Config struct {
 	Timeout         time.Duration      // timeout used for DNS lookups (default 5s)
 	RecheckInterval time.Duration      // time between tree root update checks (default 30min)
 	CacheLimit      int                // maximum number of cached records (default 1000)
+	RateLimit       float64            // maximum DNS requests / second (default 3)
 	ValidSchemes    enr.IdentityScheme // acceptable ENR identity schemes (default enode.ValidSchemes)
 	Resolver        Resolver           // the DNS resolver to use (defaults to system DNS)
 	Logger          log.Logger         // destination of client log messages (defaults to root logger)
@@ -60,9 +60,10 @@ type Resolver interface {
 
 func (cfg Config) withDefaults() Config {
 	const (
-		defaultTimeout = 5 * time.Second
-		defaultRecheck = 30 * time.Minute
-		defaultCache   = 1000
+		defaultTimeout   = 5 * time.Second
+		defaultRecheck   = 30 * time.Minute
+		defaultRateLimit = 3
+		defaultCache     = 1000
 	)
 	if cfg.Timeout == 0 {
 		cfg.Timeout = defaultTimeout
@@ -73,6 +74,9 @@ func (cfg Config) withDefaults() Config {
 	if cfg.CacheLimit == 0 {
 		cfg.CacheLimit = defaultCache
 	}
+	if cfg.RateLimit == 0 {
+		cfg.RateLimit = defaultRateLimit
+	}
 	if cfg.ValidSchemes == nil {
 		cfg.ValidSchemes = enode.ValidSchemes
 	}
@@ -86,32 +90,24 @@ func (cfg Config) withDefaults() Config {
 }
 
 // NewClient creates a client.
-func NewClient(cfg Config, urls ...string) (*Client, error) {
-	c := &Client{
-		cfg:   cfg.withDefaults(),
-		clock: mclock.System{},
-		trees: make(map[string]*clientTree),
-	}
-	var err error
-	if c.entries, err = lru.New(c.cfg.CacheLimit); err != nil {
-		return nil, err
-	}
-	for _, url := range urls {
-		if err := c.AddTree(url); err != nil {
-			return nil, err
-		}
+func NewClient(cfg Config) *Client {
+	cfg = cfg.withDefaults()
+	cache, err := lru.New(cfg.CacheLimit)
+	if err != nil {
+		panic(err)
 	}
-	return c, nil
+	rlimit := rate.NewLimiter(rate.Limit(cfg.RateLimit), 10)
+	cfg.Resolver = &rateLimitResolver{cfg.Resolver, rlimit}
+	return &Client{cfg: cfg, entries: cache, clock: mclock.System{}}
 }
 
-// SyncTree downloads the entire node tree at the given URL. This doesn't add the tree for
-// later use, but any previously-synced entries are reused.
+// SyncTree downloads the entire node tree at the given URL.
 func (c *Client) SyncTree(url string) (*Tree, error) {
 	le, err := parseLink(url)
 	if err != nil {
 		return nil, fmt.Errorf("invalid enrtree URL: %v", err)
 	}
-	ct := newClientTree(c, le)
+	ct := newClientTree(c, new(linkCache), le)
 	t := &Tree{entries: make(map[string]entry)}
 	if err := ct.syncAll(t.entries); err != nil {
 		return nil, err
@@ -120,75 +116,16 @@ func (c *Client) SyncTree(url string) (*Tree, error) {
 	return t, nil
 }
 
-// AddTree adds a enrtree:// URL to crawl.
-func (c *Client) AddTree(url string) error {
-	le, err := parseLink(url)
-	if err != nil {
-		return fmt.Errorf("invalid enrtree URL: %v", err)
-	}
-	ct, err := c.ensureTree(le)
-	if err != nil {
-		return err
-	}
-	c.linkCache.add(ct)
-	return nil
-}
-
-func (c *Client) ensureTree(le *linkEntry) (*clientTree, error) {
-	if tree, ok := c.trees[le.domain]; ok {
-		if !tree.matchPubkey(le.pubkey) {
-			return nil, fmt.Errorf("conflicting public keys for domain %q", le.domain)
-		}
-		return tree, nil
-	}
-	ct := newClientTree(c, le)
-	c.trees[le.domain] = ct
-	return ct, nil
-}
-
-// RandomNode retrieves the next random node.
-func (c *Client) RandomNode(ctx context.Context) *enode.Node {
-	for {
-		ct := c.randomTree()
-		if ct == nil {
-			return nil
-		}
-		n, err := ct.syncRandom(ctx)
-		if err != nil {
-			if err == ctx.Err() {
-				return nil // context canceled.
-			}
-			c.cfg.Logger.Debug("Error in DNS random node sync", "tree", ct.loc.domain, "err", err)
-			continue
-		}
-		if n != nil {
-			return n
-		}
-	}
-}
-
-// randomTree returns a random tree.
-func (c *Client) randomTree() *clientTree {
-	if !c.linkCache.valid() {
-		c.gcTrees()
-	}
-	limit := rand.Intn(len(c.trees))
-	for _, ct := range c.trees {
-		if limit == 0 {
-			return ct
+// NewIterator creates an iterator that visits all nodes at the
+// given tree URLs.
+func (c *Client) NewIterator(urls ...string) (enode.Iterator, error) {
+	it := c.newRandomIterator()
+	for _, url := range urls {
+		if err := it.addTree(url); err != nil {
+			return nil, err
 		}
-		limit--
-	}
-	return nil
-}
-
-// gcTrees rebuilds the 'trees' map.
-func (c *Client) gcTrees() {
-	trees := make(map[string]*clientTree)
-	for t := range c.linkCache.all() {
-		trees[t.loc.domain] = t
 	}
-	c.trees = trees
+	return it, nil
 }
 
 // resolveRoot retrieves a root entry via DNS.
@@ -258,3 +195,128 @@ func (c *Client) doResolveEntry(ctx context.Context, domain, hash string) (entry
 	}
 	return nil, nameError{name, errNoEntry}
 }
+
+// rateLimitResolver applies a rate limit to a Resolver.
+type rateLimitResolver struct {
+	r       Resolver
+	limiter *rate.Limiter
+}
+
+func (r *rateLimitResolver) LookupTXT(ctx context.Context, domain string) ([]string, error) {
+	if err := r.limiter.Wait(ctx); err != nil {
+		return nil, err
+	}
+	return r.r.LookupTXT(ctx, domain)
+}
+
+// randomIterator traverses a set of trees and returns nodes found in them.
+type randomIterator struct {
+	cur      *enode.Node
+	ctx      context.Context
+	cancelFn context.CancelFunc
+	c        *Client
+
+	mu    sync.Mutex
+	trees map[string]*clientTree // all trees
+	lc    linkCache              // tracks tree dependencies
+}
+
+func (c *Client) newRandomIterator() *randomIterator {
+	ctx, cancel := context.WithCancel(context.Background())
+	return &randomIterator{
+		c:        c,
+		ctx:      ctx,
+		cancelFn: cancel,
+		trees:    make(map[string]*clientTree),
+	}
+}
+
+// Node returns the current node.
+func (it *randomIterator) Node() *enode.Node {
+	return it.cur
+}
+
+// Close closes the iterator.
+func (it *randomIterator) Close() {
+	it.mu.Lock()
+	defer it.mu.Unlock()
+
+	it.cancelFn()
+	it.trees = nil
+}
+
+// Next moves the iterator to the next node.
+func (it *randomIterator) Next() bool {
+	it.cur = it.nextNode()
+	return it.cur != nil
+}
+
+// addTree adds a enrtree:// URL to the iterator.
+func (it *randomIterator) addTree(url string) error {
+	le, err := parseLink(url)
+	if err != nil {
+		return fmt.Errorf("invalid enrtree URL: %v", err)
+	}
+	it.lc.addLink("", le.str)
+	return nil
+}
+
+// nextNode syncs random tree entries until it finds a node.
+func (it *randomIterator) nextNode() *enode.Node {
+	for {
+		ct := it.nextTree()
+		if ct == nil {
+			return nil
+		}
+		n, err := ct.syncRandom(it.ctx)
+		if err != nil {
+			if err == it.ctx.Err() {
+				return nil // context canceled.
+			}
+			it.c.cfg.Logger.Debug("Error in DNS random node sync", "tree", ct.loc.domain, "err", err)
+			continue
+		}
+		if n != nil {
+			return n
+		}
+	}
+}
+
+// nextTree returns a random tree.
+func (it *randomIterator) nextTree() *clientTree {
+	it.mu.Lock()
+	defer it.mu.Unlock()
+
+	if it.lc.changed {
+		it.rebuildTrees()
+		it.lc.changed = false
+	}
+	if len(it.trees) == 0 {
+		return nil
+	}
+	limit := rand.Intn(len(it.trees))
+	for _, ct := range it.trees {
+		if limit == 0 {
+			return ct
+		}
+		limit--
+	}
+	return nil
+}
+
+// rebuildTrees rebuilds the 'trees' map.
+func (it *randomIterator) rebuildTrees() {
+	// Delete removed trees.
+	for loc := range it.trees {
+		if !it.lc.isReferenced(loc) {
+			delete(it.trees, loc)
+		}
+	}
+	// Add new trees.
+	for loc := range it.lc.backrefs {
+		if it.trees[loc] == nil {
+			link, _ := parseLink(linkPrefix + loc)
+			it.trees[loc] = newClientTree(it.c, &it.lc, link)
+		}
+	}
+}

p2p/dnsdisc/client_test.go

@@ -54,7 +54,7 @@ func TestClientSyncTree(t *testing.T) {
 		wantSeq   = uint(1)
 	)
 
-	c, _ := NewClient(Config{Resolver: r, Logger: testlog.Logger(t, log.LvlTrace)})
+	c := NewClient(Config{Resolver: r, Logger: testlog.Logger(t, log.LvlTrace)})
 	stree, err := c.SyncTree("enrtree://AKPYQIUQIL7PSIACI32J7FGZW56E5FKHEFCCOFHILBIMW3M6LWXS2@n")
 	if err != nil {
 		t.Fatal("sync error:", err)
@@ -68,9 +68,6 @@ func TestClientSyncTree(t *testing.T) {
 	if stree.Seq() != wantSeq {
 		t.Errorf("synced tree has wrong seq: %d", stree.Seq())
 	}
-	if len(c.trees) > 0 {
-		t.Errorf("tree from SyncTree added to client")
-	}
 }
 
 // In this test, syncing the tree fails because it contains an invalid ENR entry.
@@ -91,7 +88,7 @@ func TestClientSyncTreeBadNode(t *testing.T) {
 		"C7HRFPF3BLGF3YR4DY5KX3SMBE.n": "enrtree://AM5FCQLWIZX2QFPNJAP7VUERCCRNGRHWZG3YYHIUV7BVDQ5FDPRT2@morenodes.example.org",
 		"INDMVBZEEQ4ESVYAKGIYU74EAA.n": "enr:-----",
 	}
-	c, _ := NewClient(Config{Resolver: r, Logger: testlog.Logger(t, log.LvlTrace)})
+	c := NewClient(Config{Resolver: r, Logger: testlog.Logger(t, log.LvlTrace)})
 	_, err := c.SyncTree("enrtree://AKPYQIUQIL7PSIACI32J7FGZW56E5FKHEFCCOFHILBIMW3M6LWXS2@n")
 	wantErr := nameError{name: "INDMVBZEEQ4ESVYAKGIYU74EAA.n", err: entryError{typ: "enr", err: errInvalidENR}}
 	if err != wantErr {
@@ -99,57 +96,89 @@ func TestClientSyncTreeBadNode(t *testing.T) {
 	}
 }
 
-// This test checks that RandomNode hits all entries.
-func TestClientRandomNode(t *testing.T) {
+// This test checks that randomIterator finds all entries.
+func TestIterator(t *testing.T) {
 	nodes := testNodes(nodesSeed1, 30)
 	tree, url := makeTestTree("n", nodes, nil)
 	r := mapResolver(tree.ToTXT("n"))
-	c, _ := NewClient(Config{Resolver: r, Logger: testlog.Logger(t, log.LvlTrace)})
-	if err := c.AddTree(url); err != nil {
+	c := NewClient(Config{
+		Resolver:  r,
+		Logger:    testlog.Logger(t, log.LvlTrace),
+		RateLimit: 500,
+	})
+	it, err := c.NewIterator(url)
+	if err != nil {
 		t.Fatal(err)
 	}
 
-	checkRandomNode(t, c, nodes)
+	checkIterator(t, it, nodes)
 }
 
-// This test checks that RandomNode traverses linked trees as well as explicitly added trees.
-func TestClientRandomNodeLinks(t *testing.T) {
+// This test checks if closing randomIterator races.
+func TestIteratorClose(t *testing.T) {
+	nodes := testNodes(nodesSeed1, 500)
+	tree1, url1 := makeTestTree("t1", nodes, nil)
+	c := NewClient(Config{Resolver: newMapResolver(tree1.ToTXT("t1"))})
+	it, err := c.NewIterator(url1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	done := make(chan struct{})
+	go func() {
+		for it.Next() {
+			_ = it.Node()
+		}
+		close(done)
+	}()
+
+	time.Sleep(50 * time.Millisecond)
+	it.Close()
+	<-done
+}
+
+// This test checks that randomIterator traverses linked trees as well as explicitly added trees.
+func TestIteratorLinks(t *testing.T) {
 	nodes := testNodes(nodesSeed1, 40)
 	tree1, url1 := makeTestTree("t1", nodes[:10], nil)
 	tree2, url2 := makeTestTree("t2", nodes[10:], []string{url1})
-	cfg := Config{
-		Resolver: newMapResolver(tree1.ToTXT("t1"), tree2.ToTXT("t2")),
-		Logger:   testlog.Logger(t, log.LvlTrace),
-	}
-	c, _ := NewClient(cfg)
-	if err := c.AddTree(url2); err != nil {
+	c := NewClient(Config{
+		Resolver:  newMapResolver(tree1.ToTXT("t1"), tree2.ToTXT("t2")),
+		Logger:    testlog.Logger(t, log.LvlTrace),
+		RateLimit: 500,
+	})
+	it, err := c.NewIterator(url2)
+	if err != nil {
 		t.Fatal(err)
 	}
 
-	checkRandomNode(t, c, nodes)
+	checkIterator(t, it, nodes)
 }
 
-// This test verifies that RandomNode re-checks the root of the tree to catch
+// This test verifies that randomIterator re-checks the root of the tree to catch
 // updates to nodes.
-func TestClientRandomNodeUpdates(t *testing.T) {
+func TestIteratorNodeUpdates(t *testing.T) {
 	var (
 		clock    = new(mclock.Simulated)
 		nodes    = testNodes(nodesSeed1, 30)
 		resolver = newMapResolver()
-		cfg      = Config{
+		c        = NewClient(Config{
 			Resolver:        resolver,
 			Logger:          testlog.Logger(t, log.LvlTrace),
 			RecheckInterval: 20 * time.Minute,
-		}
-		c, _ = NewClient(cfg)
+			RateLimit:       500,
+		})
 	)
 	c.clock = clock
 	tree1, url := makeTestTree("n", nodes[:25], nil)
+	it, err := c.NewIterator(url)
+	if err != nil {
+		t.Fatal(err)
+	}
 
-	// Sync the original tree.
+	// sync the original tree.
 	resolver.add(tree1.ToTXT("n"))
-	c.AddTree(url)
-	checkRandomNode(t, c, nodes[:25])
+	checkIterator(t, it, nodes[:25])
 
 	// Update some nodes and ensure RandomNode returns the new nodes as well.
 	keys := testKeys(nodesSeed1, len(nodes))
@@ -162,25 +191,25 @@ func TestClientRandomNodeUpdates(t *testing.T) {
 		nodes[i] = n2
 	}
 	tree2, _ := makeTestTree("n", nodes, nil)
-	clock.Run(cfg.RecheckInterval + 1*time.Second)
+	clock.Run(c.cfg.RecheckInterval + 1*time.Second)
 	resolver.clear()
 	resolver.add(tree2.ToTXT("n"))
-	checkRandomNode(t, c, nodes)
+	checkIterator(t, it, nodes)
 }
 
-// This test verifies that RandomNode re-checks the root of the tree to catch
+// This test verifies that randomIterator re-checks the root of the tree to catch
 // updates to links.
-func TestClientRandomNodeLinkUpdates(t *testing.T) {
+func TestIteratorLinkUpdates(t *testing.T) {
 	var (
 		clock    = new(mclock.Simulated)
 		nodes    = testNodes(nodesSeed1, 30)
 		resolver = newMapResolver()
-		cfg      = Config{
+		c        = NewClient(Config{
 			Resolver:        resolver,
 			Logger:          testlog.Logger(t, log.LvlTrace),
 			RecheckInterval: 20 * time.Minute,
-		}
-		c, _ = NewClient(cfg)
+			RateLimit:       500,
+		})
 	)
 	c.clock = clock
 	tree3, url3 := makeTestTree("t3", nodes[20:30], nil)
@@ -190,49 +219,53 @@ func TestClientRandomNodeLinkUpdates(t *testing.T) {
 	resolver.add(tree2.ToTXT("t2"))
 	resolver.add(tree3.ToTXT("t3"))
 
+	it, err := c.NewIterator(url1)
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	// Sync tree1 using RandomNode.
-	c.AddTree(url1)
-	checkRandomNode(t, c, nodes[:20])
+	checkIterator(t, it, nodes[:20])
 
 	// Add link to tree3, remove link to tree2.
 	tree1, _ = makeTestTree("t1", nodes[:10], []string{url3})
 	resolver.add(tree1.ToTXT("t1"))
-	clock.Run(cfg.RecheckInterval + 1*time.Second)
+	clock.Run(c.cfg.RecheckInterval + 1*time.Second)
 	t.Log("tree1 updated")
 
 	var wantNodes []*enode.Node
 	wantNodes = append(wantNodes, tree1.Nodes()...)
 	wantNodes = append(wantNodes, tree3.Nodes()...)
-	checkRandomNode(t, c, wantNodes)
+	checkIterator(t, it, wantNodes)
 
 	// Check that linked trees are GCed when they're no longer referenced.
-	if len(c.trees) != 2 {
-		t.Errorf("client knows %d trees, want 2", len(c.trees))
+	knownTrees := it.(*randomIterator).trees
+	if len(knownTrees) != 2 {
+		t.Errorf("client knows %d trees, want 2", len(knownTrees))
 	}
 }
 
-func checkRandomNode(t *testing.T, c *Client, wantNodes []*enode.Node) {
+func checkIterator(t *testing.T, it enode.Iterator, wantNodes []*enode.Node) {
 	t.Helper()
 
 	var (
 		want     = make(map[enode.ID]*enode.Node)
-		maxCalls = len(wantNodes) * 2
+		maxCalls = len(wantNodes) * 3
 		calls    = 0
-		ctx      = context.Background()
 	)
 	for _, n := range wantNodes {
 		want[n.ID()] = n
 	}
 	for ; len(want) > 0 && calls < maxCalls; calls++ {
-		n := c.RandomNode(ctx)
-		if n == nil {
-			t.Fatalf("RandomNode returned nil (call %d)", calls)
+		if !it.Next() {
+			t.Fatalf("Next returned false (call %d)", calls)
 		}
+		n := it.Node()
 		delete(want, n.ID())
 	}
-	t.Logf("checkRandomNode called RandomNode %d times to find %d nodes", calls, len(wantNodes))
+	t.Logf("checkIterator called Next %d times to find %d nodes", calls, len(wantNodes))
 	for _, n := range want {
-		t.Errorf("RandomNode didn't discover node %v", n.ID())
+		t.Errorf("iterator didn't discover node %v", n.ID())
 	}
 }
 

p2p/dnsdisc/sync.go

@@ -18,7 +18,6 @@ package dnsdisc
 
 import (
 	"context"
-	"crypto/ecdsa"
 	"math/rand"
 	"time"
 
@@ -28,27 +27,21 @@ import (
 
 // clientTree is a full tree being synced.
 type clientTree struct {
-	c             *Client
-	loc           *linkEntry
-	root          *rootEntry
+	c   *Client
+	loc *linkEntry // link to this tree
+
 	lastRootCheck mclock.AbsTime // last revalidation of root
+	root          *rootEntry
 	enrs          *subtreeSync
 	links         *subtreeSync
-	linkCache     linkCache
-}
-
-func newClientTree(c *Client, loc *linkEntry) *clientTree {
-	ct := &clientTree{c: c, loc: loc}
-	ct.linkCache.self = ct
-	return ct
-}
 
-func (ct *clientTree) matchPubkey(key *ecdsa.PublicKey) bool {
-	return keysEqual(ct.loc.pubkey, key)
+	lc         *linkCache          // tracks all links between all trees
+	curLinks   map[string]struct{} // links contained in this tree
+	linkGCRoot string              // root on which last link GC has run
 }
 
-func keysEqual(k1, k2 *ecdsa.PublicKey) bool {
-	return k1.Curve == k2.Curve && k1.X.Cmp(k2.X) == 0 && k1.Y.Cmp(k2.Y) == 0
+func newClientTree(c *Client, lc *linkCache, loc *linkEntry) *clientTree {
+	return &clientTree{c: c, lc: lc, loc: loc}
 }
 
 // syncAll retrieves all entries of the tree.
@@ -78,6 +71,7 @@ func (ct *clientTree) syncRandom(ctx context.Context) (*enode.Node, error) {
 		err := ct.syncNextLink(ctx)
 		return nil, err
 	}
+	ct.gcLinks()
 
 	// Sync next random entry in ENR tree. Once every node has been visited, we simply
 	// start over. This is fine because entries are cached.
@@ -87,6 +81,16 @@ func (ct *clientTree) syncRandom(ctx context.Context) (*enode.Node, error) {
 	return ct.syncNextRandomENR(ctx)
 }
 
+// gcLinks removes outdated links from the global link cache. GC runs once
+// when the link sync finishes.
+func (ct *clientTree) gcLinks() {
+	if !ct.links.done() || ct.root.lroot == ct.linkGCRoot {
+		return
+	}
+	ct.lc.resetLinks(ct.loc.str, ct.curLinks)
+	ct.linkGCRoot = ct.root.lroot
+}
+
 func (ct *clientTree) syncNextLink(ctx context.Context) error {
 	hash := ct.links.missing[0]
 	e, err := ct.links.resolveNext(ctx, hash)
@@ -95,12 +99,9 @@ func (ct *clientTree) syncNextLink(ctx context.Context) error {
 	}
 	ct.links.missing = ct.links.missing[1:]
 
-	if le, ok := e.(*linkEntry); ok {
-		lt, err := ct.c.ensureTree(le)
-		if err != nil {
-			return err
-		}
-		ct.linkCache.add(lt)
+	if dest, ok := e.(*linkEntry); ok {
+		ct.lc.addLink(ct.loc.str, dest.str)
+		ct.curLinks[dest.str] = struct{}{}
 	}
 	return nil
 }
@@ -150,7 +151,7 @@ func (ct *clientTree) updateRoot() error {
 	// Invalidate subtrees if changed.
 	if ct.links == nil || root.lroot != ct.links.root {
 		ct.links = newSubtreeSync(ct.c, ct.loc, root.lroot, true)
-		ct.linkCache.reset()
+		ct.curLinks = make(map[string]struct{})
 	}
 	if ct.enrs == nil || root.eroot != ct.enrs.root {
 		ct.enrs = newSubtreeSync(ct.c, ct.loc, root.eroot, false)
@@ -215,63 +216,51 @@ func (ts *subtreeSync) resolveNext(ctx context.Context, hash string) (entry, err
 	return e, nil
 }
 
-// linkCache tracks the links of a tree.
+// linkCache tracks links between trees.
 type linkCache struct {
-	self    *clientTree
-	directM map[*clientTree]struct{} // direct links
-	allM    map[*clientTree]struct{} // direct & transitive links
+	backrefs map[string]map[string]struct{}
+	changed  bool
 }
 
-// reset clears the cache.
-func (lc *linkCache) reset() {
-	lc.directM = nil
-	lc.allM = nil
+func (lc *linkCache) isReferenced(r string) bool {
+	return len(lc.backrefs[r]) != 0
 }
 
-// add adds a direct link to the cache.
-func (lc *linkCache) add(ct *clientTree) {
-	if lc.directM == nil {
-		lc.directM = make(map[*clientTree]struct{})
-	}
-	if _, ok := lc.directM[ct]; !ok {
-		lc.invalidate()
+func (lc *linkCache) addLink(from, to string) {
+	if _, ok := lc.backrefs[to][from]; ok {
+		return
 	}
-	lc.directM[ct] = struct{}{}
-}
-
-// invalidate resets the cache of transitive links.
-func (lc *linkCache) invalidate() {
-	lc.allM = nil
-}
 
-// valid returns true when the cache of transitive links is up-to-date.
-func (lc *linkCache) valid() bool {
-	// Re-check validity of child caches to catch updates.
-	for ct := range lc.allM {
-		if ct != lc.self && !ct.linkCache.valid() {
-			lc.allM = nil
-			break
-		}
+	if lc.backrefs == nil {
+		lc.backrefs = make(map[string]map[string]struct{})
+	}
+	if _, ok := lc.backrefs[to]; !ok {
+		lc.backrefs[to] = make(map[string]struct{})
 	}
-	return lc.allM != nil
+	lc.backrefs[to][from] = struct{}{}
+	lc.changed = true
 }
 
-// all returns all trees reachable through the cache.
-func (lc *linkCache) all() map[*clientTree]struct{} {
-	if lc.valid() {
-		return lc.allM
-	}
-	// Remake lc.allM it by taking the union of all() across children.
-	m := make(map[*clientTree]struct{})
-	if lc.self != nil {
-		m[lc.self] = struct{}{}
-	}
-	for ct := range lc.directM {
-		m[ct] = struct{}{}
-		for lt := range ct.linkCache.all() {
-			m[lt] = struct{}{}
+// resetLinks clears all links of the given tree.
+func (lc *linkCache) resetLinks(from string, keep map[string]struct{}) {
+	stk := []string{from}
+	for len(stk) > 0 {
+		item := stk[len(stk)-1]
+		stk = stk[:len(stk)-1]
+
+		for r, refs := range lc.backrefs {
+			if _, ok := keep[r]; ok {
+				continue
+			}
+			if _, ok := refs[item]; !ok {
+				continue
+			}
+			lc.changed = true
+			delete(refs, item)
+			if len(refs) == 0 {
+				delete(lc.backrefs, r)
+				stk = append(stk, r)
+			}
 		}
 	}
-	lc.allM = m
-	return m
 }

p2p/dnsdisc/sync_test.go

@@ -0,0 +1,83 @@
+// Copyright 2018 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package dnsdisc
+
+import (
+	"math/rand"
+	"strconv"
+	"testing"
+)
+
+func TestLinkCache(t *testing.T) {
+	var lc linkCache
+
+	// Check adding links.
+	lc.addLink("1", "2")
+	if !lc.changed {
+		t.Error("changed flag not set")
+	}
+	lc.changed = false
+	lc.addLink("1", "2")
+	if lc.changed {
+		t.Error("changed flag set after adding link that's already present")
+	}
+	lc.addLink("2", "3")
+	lc.addLink("3", "1")
+	lc.addLink("2", "4")
+	lc.changed = false
+
+	if !lc.isReferenced("3") {
+		t.Error("3 not referenced")
+	}
+	if lc.isReferenced("6") {
+		t.Error("6 is referenced")
+	}
+
+	lc.resetLinks("1", nil)
+	if !lc.changed {
+		t.Error("changed flag not set")
+	}
+	if len(lc.backrefs) != 0 {
+		t.Logf("%+v", lc)
+		t.Error("reference maps should be empty")
+	}
+}
+
+func TestLinkCacheRandom(t *testing.T) {
+	tags := make([]string, 1000)
+	for i := range tags {
+		tags[i] = strconv.Itoa(i)
+	}
+
+	// Create random links.
+	var lc linkCache
+	var remove []string
+	for i := 0; i < 100; i++ {
+		a, b := tags[rand.Intn(len(tags))], tags[rand.Intn(len(tags))]
+		lc.addLink(a, b)
+		remove = append(remove, a)
+	}
+
+	// Remove all the links.
+	for _, s := range remove {
+		lc.resetLinks(s, nil)
+	}
+	if len(lc.backrefs) != 0 {
+		t.Logf("%+v", lc)
+		t.Error("reference maps should be empty")
+	}
+}

p2p/dnsdisc/tree.go

@@ -48,7 +48,7 @@ func (t *Tree) Sign(key *ecdsa.PrivateKey, domain string) (url string, err error
 	}
 	root.sig = sig
 	t.root = &root
-	link := &linkEntry{domain, &key.PublicKey}
+	link := newLinkEntry(domain, &key.PublicKey)
 	return link.String(), nil
 }
 
@@ -209,6 +209,7 @@ type (
 		node *enode.Node
 	}
 	linkEntry struct {
+		str    string
 		domain string
 		pubkey *ecdsa.PublicKey
 	}
@@ -246,7 +247,8 @@ func (e *rootEntry) sigHash() []byte {
 
 func (e *rootEntry) verifySignature(pubkey *ecdsa.PublicKey) bool {
 	sig := e.sig[:crypto.RecoveryIDOffset] // remove recovery id
-	return crypto.VerifySignature(crypto.FromECDSAPub(pubkey), e.sigHash(), sig)
+	enckey := crypto.FromECDSAPub(pubkey)
+	return crypto.VerifySignature(enckey, e.sigHash(), sig)
 }
 
 func (e *branchEntry) String() string {
@@ -258,8 +260,13 @@ func (e *enrEntry) String() string {
 }
 
 func (e *linkEntry) String() string {
-	pubkey := b32format.EncodeToString(crypto.CompressPubkey(e.pubkey))
-	return fmt.Sprintf("%s%s@%s", linkPrefix, pubkey, e.domain)
+	return linkPrefix + e.str
+}
+
+func newLinkEntry(domain string, pubkey *ecdsa.PublicKey) *linkEntry {
+	key := b32format.EncodeToString(crypto.CompressPubkey(pubkey))
+	str := key + "@" + domain
+	return &linkEntry{str, domain, pubkey}
 }
 
 // Entry Parsing
@@ -319,7 +326,7 @@ func parseLink(e string) (*linkEntry, error) {
 	if err != nil {
 		return nil, entryError{"link", errBadPubkey}
 	}
-	return &linkEntry{domain, key}, nil
+	return &linkEntry{e, domain, key}, nil
 }
 
 func parseBranch(e string) (entry, error) {

p2p/dnsdisc/tree_test.go

@@ -91,7 +91,7 @@ func TestParseEntry(t *testing.T) {
 		// Links
 		{
 			input: "enrtree://AKPYQIUQIL7PSIACI32J7FGZW56E5FKHEFCCOFHILBIMW3M6LWXS2@nodes.example.org",
-			e:     &linkEntry{"nodes.example.org", &testkey.PublicKey},
+			e:     &linkEntry{"AKPYQIUQIL7PSIACI32J7FGZW56E5FKHEFCCOFHILBIMW3M6LWXS2@nodes.example.org", "nodes.example.org", &testkey.PublicKey},
 		},
 		{
 			input: "enrtree://nodes.example.org",