core-chain/p2p/discv5/udp_test.go

// Copyright 2016 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 discv5

import (
	"encoding/hex"
	"errors"
	"io"
	"net"
	"reflect"
	"sync"
	"testing"
	"time"

	"github.com/davecgh/go-spew/spew"
	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/crypto"
	"github.com/ethereum/go-ethereum/rlp"
)

func init() {
	spew.Config.DisableMethods = true
}

// This test checks that isPacketTooBig correctly identifies
// errors that result from receiving a UDP packet larger
// than the supplied receive buffer.
func TestIsPacketTooBig(t *testing.T) {
	listener, err := net.ListenPacket("udp", "127.0.0.1:0")
	if err != nil {
		t.Fatal(err)
	}
	defer listener.Close()
	sender, err := net.Dial("udp", listener.LocalAddr().String())
	if err != nil {
		t.Fatal(err)
	}
	defer sender.Close()

	sendN := 1800
	recvN := 300
	for i := 0; i < 20; i++ {
		go func() {
			buf := make([]byte, sendN)
			for i := range buf {
				buf[i] = byte(i)
			}
			sender.Write(buf)
		}()

		buf := make([]byte, recvN)
		listener.SetDeadline(time.Now().Add(1 * time.Second))
		n, _, err := listener.ReadFrom(buf)
		if err != nil {
			if nerr, ok := err.(net.Error); ok && nerr.Timeout() {
				continue
			}
			if !isPacketTooBig(err) {
				t.Fatal("unexpected read error:", spew.Sdump(err))
			}
			continue
		}
		if n != recvN {
			t.Fatalf("short read: %d, want %d", n, recvN)
		}
		for i := range buf {
			if buf[i] != byte(i) {
				t.Fatalf("error in pattern")
				break
			}
		}
	}
}

// shared test variables
var (
	futureExp          = uint64(time.Now().Add(10 * time.Hour).Unix())
	testTarget         = NodeID{0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1}
	testRemote         = rpcEndpoint{IP: net.ParseIP("1.1.1.1").To4(), UDP: 1, TCP: 2}
	testLocalAnnounced = rpcEndpoint{IP: net.ParseIP("2.2.2.2").To4(), UDP: 3, TCP: 4}
	testLocal          = rpcEndpoint{IP: net.ParseIP("3.3.3.3").To4(), UDP: 5, TCP: 6}
)

// type udpTest struct {
// 	t                   *testing.T
// 	pipe                *dgramPipe
// 	table               *Table
// 	udp                 *udp
// 	sent                [][]byte
// 	localkey, remotekey *ecdsa.PrivateKey
// 	remoteaddr          *net.UDPAddr
// }
//
// func newUDPTest(t *testing.T) *udpTest {
// 	test := &udpTest{
// 		t:          t,
// 		pipe:       newpipe(),
// 		localkey:   newkey(),
// 		remotekey:  newkey(),
// 		remoteaddr: &net.UDPAddr{IP: net.IP{1, 2, 3, 4}, Port: 30303},
// 	}
// 	test.table, test.udp, _ = newUDP(test.localkey, test.pipe, nil, "")
// 	return test
// }
//
// // handles a packet as if it had been sent to the transport.
// func (test *udpTest) packetIn(wantError error, ptype byte, data packet) error {
// 	enc, err := encodePacket(test.remotekey, ptype, data)
// 	if err != nil {
// 		return test.errorf("packet (%d) encode error: %v", ptype, err)
// 	}
// 	test.sent = append(test.sent, enc)
// 	if err = test.udp.handlePacket(test.remoteaddr, enc); err != wantError {
// 		return test.errorf("error mismatch: got %q, want %q", err, wantError)
// 	}
// 	return nil
// }
//
// // waits for a packet to be sent by the transport.
// // validate should have type func(*udpTest, X) error, where X is a packet type.
// func (test *udpTest) waitPacketOut(validate interface{}) error {
// 	dgram := test.pipe.waitPacketOut()
// 	p, _, _, err := decodePacket(dgram)
// 	if err != nil {
// 		return test.errorf("sent packet decode error: %v", err)
// 	}
// 	fn := reflect.ValueOf(validate)
// 	exptype := fn.Type().In(0)
// 	if reflect.TypeOf(p) != exptype {
// 		return test.errorf("sent packet type mismatch, got: %v, want: %v", reflect.TypeOf(p), exptype)
// 	}
// 	fn.Call([]reflect.Value{reflect.ValueOf(p)})
// 	return nil
// }
//
// func (test *udpTest) errorf(format string, args ...interface{}) error {
// 	_, file, line, ok := runtime.Caller(2) // errorf + waitPacketOut
// 	if ok {
// 		file = filepath.Base(file)
// 	} else {
// 		file = "???"
// 		line = 1
// 	}
// 	err := fmt.Errorf(format, args...)
// 	fmt.Printf("\t%s:%d: %v\n", file, line, err)
// 	test.t.Fail()
// 	return err
// }
//
// func TestUDP_packetErrors(t *testing.T) {
// 	test := newUDPTest(t)
// 	defer test.table.Close()
//
// 	test.packetIn(errExpired, pingPacket, &ping{From: testRemote, To: testLocalAnnounced, Version: Version})
// 	test.packetIn(errUnsolicitedReply, pongPacket, &pong{ReplyTok: []byte{}, Expiration: futureExp})
// 	test.packetIn(errUnknownNode, findnodePacket, &findnode{Expiration: futureExp})
// 	test.packetIn(errUnsolicitedReply, neighborsPacket, &neighbors{Expiration: futureExp})
// }
//
// func TestUDP_findnode(t *testing.T) {
// 	test := newUDPTest(t)
// 	defer test.table.Close()
//
// 	// put a few nodes into the table. their exact
// 	// distribution shouldn't matter much, altough we need to
// 	// take care not to overflow any bucket.
// 	targetHash := crypto.Keccak256Hash(testTarget[:])
// 	nodes := &nodesByDistance{target: targetHash}
// 	for i := 0; i < bucketSize; i++ {
// 		nodes.push(nodeAtDistance(test.table.self.sha, i+2), bucketSize)
// 	}
// 	test.table.stuff(nodes.entries)
//
// 	// ensure there's a bond with the test node,
// 	// findnode won't be accepted otherwise.
// 	test.table.db.updateNode(NewNode(
// 		PubkeyID(&test.remotekey.PublicKey),
// 		test.remoteaddr.IP,
// 		uint16(test.remoteaddr.Port),
// 		99,
// 	))
// 	// check that closest neighbors are returned.
// 	test.packetIn(nil, findnodePacket, &findnode{Target: testTarget, Expiration: futureExp})
// 	expected := test.table.closest(targetHash, bucketSize)
//
// 	waitNeighbors := func(want []*Node) {
// 		test.waitPacketOut(func(p *neighbors) {
// 			if len(p.Nodes) != len(want) {
// 				t.Errorf("wrong number of results: got %d, want %d", len(p.Nodes), bucketSize)
// 			}
// 			for i := range p.Nodes {
// 				if p.Nodes[i].ID != want[i].ID {
// 					t.Errorf("result mismatch at %d:\n  got:  %v\n  want: %v", i, p.Nodes[i], expected.entries[i])
// 				}
// 			}
// 		})
// 	}
// 	waitNeighbors(expected.entries[:maxNeighbors])
// 	waitNeighbors(expected.entries[maxNeighbors:])
// }
//
// func TestUDP_findnodeMultiReply(t *testing.T) {
// 	test := newUDPTest(t)
// 	defer test.table.Close()
//
// 	// queue a pending findnode request
// 	resultc, errc := make(chan []*Node), make(chan error)
// 	go func() {
// 		rid := PubkeyID(&test.remotekey.PublicKey)
// 		ns, err := test.udp.findnode(rid, test.remoteaddr, testTarget)
// 		if err != nil && len(ns) == 0 {
// 			errc <- err
// 		} else {
// 			resultc <- ns
// 		}
// 	}()
//
// 	// wait for the findnode to be sent.
// 	// after it is sent, the transport is waiting for a reply
// 	test.waitPacketOut(func(p *findnode) {
// 		if p.Target != testTarget {
// 			t.Errorf("wrong target: got %v, want %v", p.Target, testTarget)
// 		}
// 	})
//
// 	// send the reply as two packets.
// 	list := []*Node{
// 		MustParseNode("enode://ba85011c70bcc5c04d8607d3a0ed29aa6179c092cbdda10d5d32684fb33ed01bd94f588ca8f91ac48318087dcb02eaf36773a7a453f0eedd6742af668097b29c@10.0.1.16:30303?discport=30304"),
// 		MustParseNode("enode://81fa361d25f157cd421c60dcc28d8dac5ef6a89476633339c5df30287474520caca09627da18543d9079b5b288698b542d56167aa5c09111e55acdbbdf2ef799@10.0.1.16:30303"),
// 		MustParseNode("enode://9bffefd833d53fac8e652415f4973bee289e8b1a5c6c4cbe70abf817ce8a64cee11b823b66a987f51aaa9fba0d6a91b3e6bf0d5a5d1042de8e9eeea057b217f8@10.0.1.36:30301?discport=17"),
// 		MustParseNode("enode://1b5b4aa662d7cb44a7221bfba67302590b643028197a7d5214790f3bac7aaa4a3241be9e83c09cf1f6c69d007c634faae3dc1b1221793e8446c0b3a09de65960@10.0.1.16:30303"),
// 	}
// 	rpclist := make([]rpcNode, len(list))
// 	for i := range list {
// 		rpclist[i] = nodeToRPC(list[i])
// 	}
// 	test.packetIn(nil, neighborsPacket, &neighbors{Expiration: futureExp, Nodes: rpclist[:2]})
// 	test.packetIn(nil, neighborsPacket, &neighbors{Expiration: futureExp, Nodes: rpclist[2:]})
//
// 	// check that the sent neighbors are all returned by findnode
// 	select {
// 	case result := <-resultc:
// 		if !reflect.DeepEqual(result, list) {
// 			t.Errorf("neighbors mismatch:\n  got:  %v\n  want: %v", result, list)
// 		}
// 	case err := <-errc:
// 		t.Errorf("findnode error: %v", err)
// 	case <-time.After(5 * time.Second):
// 		t.Error("findnode did not return within 5 seconds")
// 	}
// }
//
// func TestUDP_successfulPing(t *testing.T) {
// 	test := newUDPTest(t)
// 	added := make(chan *Node, 1)
// 	test.table.nodeAddedHook = func(n *Node) { added <- n }
// 	defer test.table.Close()
//
// 	// The remote side sends a ping packet to initiate the exchange.
// 	go test.packetIn(nil, pingPacket, &ping{From: testRemote, To: testLocalAnnounced, Version: Version, Expiration: futureExp})
//
// 	// the ping is replied to.
// 	test.waitPacketOut(func(p *pong) {
// 		pinghash := test.sent[0][:macSize]
// 		if !bytes.Equal(p.ReplyTok, pinghash) {
// 			t.Errorf("got pong.ReplyTok %x, want %x", p.ReplyTok, pinghash)
// 		}
// 		wantTo := rpcEndpoint{
// 			// The mirrored UDP address is the UDP packet sender
// 			IP: test.remoteaddr.IP, UDP: uint16(test.remoteaddr.Port),
// 			// The mirrored TCP port is the one from the ping packet
// 			TCP: testRemote.TCP,
// 		}
// 		if !reflect.DeepEqual(p.To, wantTo) {
// 			t.Errorf("got pong.To %v, want %v", p.To, wantTo)
// 		}
// 	})
//
// 	// remote is unknown, the table pings back.
// 	test.waitPacketOut(func(p *ping) error {
// 		if !reflect.DeepEqual(p.From, test.udp.ourEndpoint) {
// 			t.Errorf("got ping.From %v, want %v", p.From, test.udp.ourEndpoint)
// 		}
// 		wantTo := rpcEndpoint{
// 			// The mirrored UDP address is the UDP packet sender.
// 			IP: test.remoteaddr.IP, UDP: uint16(test.remoteaddr.Port),
// 			TCP: 0,
// 		}
// 		if !reflect.DeepEqual(p.To, wantTo) {
// 			t.Errorf("got ping.To %v, want %v", p.To, wantTo)
// 		}
// 		return nil
// 	})
// 	test.packetIn(nil, pongPacket, &pong{Expiration: futureExp})
//
// 	// the node should be added to the table shortly after getting the
// 	// pong packet.
// 	select {
// 	case n := <-added:
// 		rid := PubkeyID(&test.remotekey.PublicKey)
// 		if n.ID != rid {
// 			t.Errorf("node has wrong ID: got %v, want %v", n.ID, rid)
// 		}
// 		if !bytes.Equal(n.IP, test.remoteaddr.IP) {
// 			t.Errorf("node has wrong IP: got %v, want: %v", n.IP, test.remoteaddr.IP)
// 		}
// 		if int(n.UDP) != test.remoteaddr.Port {
// 			t.Errorf("node has wrong UDP port: got %v, want: %v", n.UDP, test.remoteaddr.Port)
// 		}
// 		if n.TCP != testRemote.TCP {
// 			t.Errorf("node has wrong TCP port: got %v, want: %v", n.TCP, testRemote.TCP)
// 		}
// 	case <-time.After(2 * time.Second):
// 		t.Errorf("node was not added within 2 seconds")
// 	}
// }

var testPackets = []struct {
	input      string
	wantPacket interface{}
}{
	{
		input: "71dbda3a79554728d4f94411e42ee1f8b0d561c10e1e5f5893367948c6a7d70bb87b235fa28a77070271b6c164a2dce8c7e13a5739b53b5e96f2e5acb0e458a02902f5965d55ecbeb2ebb6cabb8b2b232896a36b737666c55265ad0a68412f250001ea04cb847f000001820cfa8215a8d790000000000000000000000000000000018208ae820d058443b9a355",
		wantPacket: &ping{
			Version:    4,
			From:       rpcEndpoint{net.ParseIP("127.0.0.1").To4(), 3322, 5544},
			To:         rpcEndpoint{net.ParseIP("::1"), 2222, 3333},
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{},
		},
	},
	{
		input: "e9614ccfd9fc3e74360018522d30e1419a143407ffcce748de3e22116b7e8dc92ff74788c0b6663aaa3d67d641936511c8f8d6ad8698b820a7cf9e1be7155e9a241f556658c55428ec0563514365799a4be2be5a685a80971ddcfa80cb422cdd0101ec04cb847f000001820cfa8215a8d790000000000000000000000000000000018208ae820d058443b9a3550102",
		wantPacket: &ping{
			Version:    4,
			From:       rpcEndpoint{net.ParseIP("127.0.0.1").To4(), 3322, 5544},
			To:         rpcEndpoint{net.ParseIP("::1"), 2222, 3333},
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{{0x01}, {0x02}},
		},
	},
	{
		input: "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",
		wantPacket: &ping{
			Version:    555,
			From:       rpcEndpoint{net.ParseIP("2001:db8:3c4d:15::abcd:ef12"), 3322, 5544},
			To:         rpcEndpoint{net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"), 2222, 33338},
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{{0xC5, 0x01, 0x02, 0x03, 0x04, 0x05}},
		},
	},
	{
		input: "09b2428d83348d27cdf7064ad9024f526cebc19e4958f0fdad87c15eb598dd61d08423e0bf66b2069869e1724125f820d851c136684082774f870e614d95a2855d000f05d1648b2d5945470bc187c2d2216fbe870f43ed0909009882e176a46b0102f846d79020010db885a308d313198a2e037073488208ae82823aa0fbc914b16819237dcd8801d7e53f69e9719adecb3cc0e790c57e91ca4461c9548443b9a355c6010203c2040506a0c969a58f6f9095004c0177a6b47f451530cab38966a25cca5cb58f055542124e",
		wantPacket: &pong{
			To:         rpcEndpoint{net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"), 2222, 33338},
			ReplyTok:   common.Hex2Bytes("fbc914b16819237dcd8801d7e53f69e9719adecb3cc0e790c57e91ca4461c954"),
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{{0xC6, 0x01, 0x02, 0x03, 0xC2, 0x04, 0x05}, {0x06}},
		},
	},
	{
		input: "c7c44041b9f7c7e41934417ebac9a8e1a4c6298f74553f2fcfdcae6ed6fe53163eb3d2b52e39fe91831b8a927bf4fc222c3902202027e5e9eb812195f95d20061ef5cd31d502e47ecb61183f74a504fe04c51e73df81f25c4d506b26db4517490103f84eb840ca634cae0d49acb401d8a4c6b6fe8c55b70d115bf400769cc1400f3258cd31387574077f301b421bc84df7266c44e9e6d569fc56be00812904767bf5ccd1fc7f8443b9a35582999983999999280dc62cc8255c73471e0a61da0c89acdc0e035e260add7fc0c04ad9ebf3919644c91cb247affc82b69bd2ca235c71eab8e49737c937a2c396",
		wantPacket: &findnode{
			Target:     MustHexID("ca634cae0d49acb401d8a4c6b6fe8c55b70d115bf400769cc1400f3258cd31387574077f301b421bc84df7266c44e9e6d569fc56be00812904767bf5ccd1fc7f"),
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{{0x82, 0x99, 0x99}, {0x83, 0x99, 0x99, 0x99}},
		},
	},
	{
		input: "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",
		wantPacket: &neighbors{
			Nodes: []rpcNode{
				{
					ID:  MustHexID("3155e1427f85f10a5c9a7755877748041af1bcd8d474ec065eb33df57a97babf54bfd2103575fa829115d224c523596b401065a97f74010610fce76382c0bf32"),
					IP:  net.ParseIP("99.33.22.55").To4(),
					UDP: 4444,
					TCP: 4445,
				},
				{
					ID:  MustHexID("312c55512422cf9b8a4097e9a6ad79402e87a15ae909a4bfefa22398f03d20951933beea1e4dfa6f968212385e829f04c2d314fc2d4e255e0d3bc08792b069db"),
					IP:  net.ParseIP("1.2.3.4").To4(),
					UDP: 1,
					TCP: 1,
				},
				{
					ID:  MustHexID("38643200b172dcfef857492156971f0e6aa2c538d8b74010f8e140811d53b98c765dd2d96126051913f44582e8c199ad7c6d6819e9a56483f637feaac9448aac"),
					IP:  net.ParseIP("2001:db8:3c4d:15::abcd:ef12"),
					UDP: 3333,
					TCP: 3333,
				},
				{
					ID:  MustHexID("8dcab8618c3253b558d459da53bd8fa68935a719aff8b811197101a4b2b47dd2d47295286fc00cc081bb542d760717d1bdd6bec2c37cd72eca367d6dd3b9df73"),
					IP:  net.ParseIP("2001:db8:85a3:8d3:1319:8a2e:370:7348"),
					UDP: 999,
					TCP: 1000,
				},
			},
			Expiration: 1136239445,
			Rest:       []rlp.RawValue{{0x01}, {0x02}, {0x03}},
		},
	},
}

func TestForwardCompatibility(t *testing.T) {
	t.Skip("skipped while working on discovery v5")

	testkey, _ := crypto.HexToECDSA("b71c71a67e1177ad4e901695e1b4b9ee17ae16c6668d313eac2f96dbcda3f291")
	wantNodeID := PubkeyID(&testkey.PublicKey)

	for _, test := range testPackets {
		input, err := hex.DecodeString(test.input)
		if err != nil {
			t.Fatalf("invalid hex: %s", test.input)
		}
		var pkt ingressPacket
		if err := decodePacket(input, &pkt); err != nil {
			t.Errorf("did not accept packet %s\n%v", test.input, err)
			continue
		}
		if !reflect.DeepEqual(pkt.data, test.wantPacket) {
			t.Errorf("got %s\nwant %s", spew.Sdump(pkt.data), spew.Sdump(test.wantPacket))
		}
		if pkt.remoteID != wantNodeID {
			t.Errorf("got id %v\nwant id %v", pkt.remoteID, wantNodeID)
		}
	}
}

// dgramPipe is a fake UDP socket. It queues all sent datagrams.
type dgramPipe struct {
	mu      *sync.Mutex
	cond    *sync.Cond
	closing chan struct{}
	closed  bool
	queue   [][]byte
}

func newpipe() *dgramPipe {
	mu := new(sync.Mutex)
	return &dgramPipe{
		closing: make(chan struct{}),
		cond:    &sync.Cond{L: mu},
		mu:      mu,
	}
}

// WriteToUDP queues a datagram.
func (c *dgramPipe) WriteToUDP(b []byte, to *net.UDPAddr) (n int, err error) {
	msg := make([]byte, len(b))
	copy(msg, b)
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.closed {
		return 0, errors.New("closed")
	}
	c.queue = append(c.queue, msg)
	c.cond.Signal()
	return len(b), nil
}

// ReadFromUDP just hangs until the pipe is closed.
func (c *dgramPipe) ReadFromUDP(b []byte) (n int, addr *net.UDPAddr, err error) {
	<-c.closing
	return 0, nil, io.EOF
}

func (c *dgramPipe) Close() error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if !c.closed {
		close(c.closing)
		c.closed = true
	}
	return nil
}

func (c *dgramPipe) LocalAddr() net.Addr {
	return &net.UDPAddr{IP: testLocal.IP, Port: int(testLocal.UDP)}
}

func (c *dgramPipe) waitPacketOut() []byte {
	c.mu.Lock()
	defer c.mu.Unlock()
	for len(c.queue) == 0 {
		c.cond.Wait()
	}
	p := c.queue[0]
	copy(c.queue, c.queue[1:])
	c.queue = c.queue[:len(c.queue)-1]
	return p
}