path: root/p2p/discover/udp_test.go

// Copyright 2015 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 discover

import (
    "bytes"
    "crypto/ecdsa"
    "encoding/binary"
    "encoding/hex"
    "errors"
    "fmt"
    "io"
    "math/rand"
    "net"
    "path/filepath"
    "reflect"
    "runtime"
    "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
}

// 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{10, 0, 1, 99}, Port: 30303},
    }
    test.table, test.udp, _ = newUDP(test.pipe, Config{PrivateKey: test.localkey})
    // Wait for initial refresh so the table doesn't send unexpected findnode.
    <-test.table.initDone
    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{}) ([]byte, error) {
    dgram := test.pipe.waitPacketOut()
    p, _, hash, err := decodePacket(dgram)
    if err != nil {
        return hash, test.errorf("sent packet decode error: %v", err)
    }
    fn := reflect.ValueOf(validate)
    exptype := fn.Type().In(0)
    if reflect.TypeOf(p) != exptype {
        return hash, test.errorf("sent packet type mismatch, got: %v, want: %v", reflect.TypeOf(p), exptype)
    }
    fn.Call([]reflect.Value{reflect.ValueOf(p)})
    return hash, 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_pingTimeout(t *testing.T) {
    t.Parallel()
    test := newUDPTest(t)
    defer test.table.Close()

    toaddr := &net.UDPAddr{IP: net.ParseIP("1.2.3.4"), Port: 2222}
    toid := NodeID{1, 2, 3, 4}
    if err := test.udp.ping(toid, toaddr); err != errTimeout {
        t.Error("expected timeout error, got", err)
    }
}

func TestUDP_responseTimeouts(t *testing.T) {
    t.Parallel()
    test := newUDPTest(t)
    defer test.table.Close()

    rand.Seed(time.Now().UnixNano())
    randomDuration := func(max time.Duration) time.Duration {
        return time.Duration(rand.Int63n(int64(max)))
    }

    var (
        nReqs      = 200
        nTimeouts  = 0                       // number of requests with ptype > 128
        nilErr     = make(chan error, nReqs) // for requests that get a reply
        timeoutErr = make(chan error, nReqs) // for requests that time out
    )
    for i := 0; i < nReqs; i++ {
        // Create a matcher for a random request in udp.loop. Requests
        // with ptype <= 128 will not get a reply and should time out.
        // For all other requests, a reply is scheduled to arrive
        // within the timeout window.
        p := &pending{
            ptype:    byte(rand.Intn(255)),
            callback: func(interface{}) bool { return true },
        }
        binary.BigEndian.PutUint64(p.from[:], uint64(i))
        if p.ptype <= 128 {
            p.errc = timeoutErr
            test.udp.addpending <- p
            nTimeouts++
        } else {
            p.errc = nilErr
            test.udp.addpending <- p
            time.AfterFunc(randomDuration(60*time.Millisecond), func() {
                if !test.udp.handleReply(p.from, p.ptype, nil) {
                    t.Logf("not matched: %v", p)
                }
            })
        }
        time.Sleep(randomDuration(30 * time.Millisecond))
    }

    // Check that all timeouts were delivered and that the rest got nil errors.
    // The replies must be delivered.
    var (
        recvDeadline        = time.After(20 * time.Second)
        nTimeoutsRecv, nNil = 0, 0
    )
    for i := 0; i < nReqs; i++ {
        select {
        case err := <-timeoutErr:
            if err != errTimeout {
                t.Fatalf("got non-timeout error on timeoutErr %d: %v", i, err)
            }
            nTimeoutsRecv++
        case err := <-nilErr:
            if err != nil {
                t.Fatalf("got non-nil error on nilErr %d: %v", i, err)
            }
            nNil++
        case <-recvDeadline:
            t.Fatalf("exceeded recv deadline")
        }
    }
    if nTimeoutsRecv != nTimeouts {
        t.Errorf("wrong number of timeout errors received: got %d, want %d", nTimeoutsRecv, nTimeouts)
    }
    if nNil != nReqs-nTimeouts {
        t.Errorf("wrong number of successful replies: got %d, want %d", nNil, nReqs-nTimeouts)
    }
}

func TestUDP_findnodeTimeout(t *testing.T) {
    t.Parallel()
    test := newUDPTest(t)
    defer test.table.Close()

    toaddr := &net.UDPAddr{IP: net.ParseIP("1.2.3.4"), Port: 2222}
    toid := NodeID{1, 2, 3, 4}
    target := NodeID{4, 5, 6, 7}
    result, err := test.udp.findnode(toid, toaddr, target)
    if err != errTimeout {
        t.Error("expected timeout error, got", err)
    }
    if len(result) > 0 {
        t.Error("expected empty result, got", result)
    }
}

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, although 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.updateBondTime(PubkeyID(&test.remotekey.PublicKey), time.Now())

    // 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:
        want := append(list[:2], list[3:]...)
        if !reflect.DeepEqual(result, want) {
            t.Errorf("neighbors mismatch:\n  got:  %v\n  want: %v", result, want)
        }
    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.
    hash, _ := 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{ReplyTok: hash, 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 !n.IP.Equal(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: "577be4349c4dd26768081f58de4c6f375a7a22f3f7adda654d1428637412c3d7fe917cadc56d4e5e7ffae1dbe3efffb9849feb71b262de37977e7c7a44e677295680e9e38ab26bee2fcbae207fba3ff3d74069a50b902a82c9903ed37cc993c50001f83e82022bd79020010db83c4d001500000000abcdef12820cfa8215a8d79020010db885a308d313198a2e037073488208ae82823a8443b9a355c5010203040531b9019afde696e582a78fa8d95ea13ce3297d4afb8ba6433e4154caa5ac6431af1b80ba76023fa4090c408f6b4bc3701562c031041d4702971d102c9ab7fa5eed4cd6bab8f7af956f7d565ee1917084a95398b6a21eac920fe3dd1345ec0a7ef39367ee69ddf092cbfe5b93e5e568ebc491983c09c76d922dc3",
        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: "c679fc8fe0b8b12f06577f2e802d34f6fa257e6137a995f6f4cbfc9ee50ed3710faf6e66f932c4c8d81d64343f429651328758b47d3dbc02c4042f0fff6946a50f4a49037a72bb550f3a7872363a83e1b9ee6469856c24eb4ef80b7535bcf99c0004f9015bf90150f84d846321163782115c82115db8403155e1427f85f10a5c9a7755877748041af1bcd8d474ec065eb33df57a97babf54bfd2103575fa829115d224c523596b401065a97f74010610fce76382c0bf32f84984010203040101b840312c55512422cf9b8a4097e9a6ad79402e87a15ae909a4bfefa22398f03d20951933beea1e4dfa6f968212385e829f04c2d314fc2d4e255e0d3bc08792b069dbf8599020010db83c4d001500000000abcdef12820d05820d05b84038643200b172dcfef857492156971f0e6aa2c538d8b74010f8e140811d53b98c765dd2d96126051913f44582e8c199ad7c6d6819e9a56483f637feaac9448aacf8599020010db885a308d313198a2e037073488203e78203e8b8408dcab8618c3253b558d459da53bd8fa68935a719aff8b811197101a4b2b47dd2d47295286fc00cc081bb542d760717d1bdd6bec2c37cd72eca367d6dd3b9df738443b9a355010203b525a138aa34383fec3d2719a0",
        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) {
    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)
        }
        packet, nodeid, _, err := decodePacket(input)
        if err != nil {
            t.Errorf("did not accept packet %s\n%v", test.input, err)
            continue
        }
        if !reflect.DeepEqual(packet, test.wantPacket) {
            t.Errorf("got %s\nwant %s", spew.Sdump(packet), spew.Sdump(test.wantPacket))
        }
        if nodeid != wantNodeID {
            t.Errorf("got id %v\nwant id %v", nodeid, 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
}