path: root/p2p/message.go

package p2p

import (
    "bytes"
    "encoding/binary"
    "errors"
    "io"
    "io/ioutil"
    "math/big"
    "sync/atomic"

    "github.com/ethereum/go-ethereum/ethutil"
    "github.com/ethereum/go-ethereum/rlp"
)

// Msg defines the structure of a p2p message.
//
// Note that a Msg can only be sent once since the Payload reader is
// consumed during sending. It is not possible to create a Msg and
// send it any number of times. If you want to reuse an encoded
// structure, encode the payload into a byte array and create a
// separate Msg with a bytes.Reader as Payload for each send.
type Msg struct {
    Code    uint64
    Size    uint32 // size of the paylod
    Payload io.Reader
}

// NewMsg creates an RLP-encoded message with the given code.
func NewMsg(code uint64, params ...interface{}) Msg {
    buf := new(bytes.Buffer)
    for _, p := range params {
        buf.Write(ethutil.Encode(p))
    }
    return Msg{Code: code, Size: uint32(buf.Len()), Payload: buf}
}

func encodePayload(params ...interface{}) []byte {
    buf := new(bytes.Buffer)
    for _, p := range params {
        buf.Write(ethutil.Encode(p))
    }
    return buf.Bytes()
}

// Decode parse the RLP content of a message into
// the given value, which must be a pointer.
//
// For the decoding rules, please see package rlp.
func (msg Msg) Decode(val interface{}) error {
    s := rlp.NewListStream(msg.Payload, uint64(msg.Size))
    return s.Decode(val)
}

// Discard reads any remaining payload data into a black hole.
func (msg Msg) Discard() error {
    _, err := io.Copy(ioutil.Discard, msg.Payload)
    return err
}

type MsgReader interface {
    ReadMsg() (Msg, error)
}

type MsgWriter interface {
    // WriteMsg sends an existing message.
    // The Payload reader of the message is consumed.
    // Note that messages can be sent only once.
    WriteMsg(Msg) error

    // EncodeMsg writes an RLP-encoded message with the given
    // code and data elements.
    EncodeMsg(code uint64, data ...interface{}) error
}

// MsgReadWriter provides reading and writing of encoded messages.
type MsgReadWriter interface {
    MsgReader
    MsgWriter
}

var magicToken = []byte{34, 64, 8, 145}

func writeMsg(w io.Writer, msg Msg) error {
    // TODO: handle case when Size + len(code) + len(listhdr) overflows uint32
    code := ethutil.Encode(uint32(msg.Code))
    listhdr := makeListHeader(msg.Size + uint32(len(code)))
    payloadLen := uint32(len(listhdr)) + uint32(len(code)) + msg.Size

    start := make([]byte, 8)
    copy(start, magicToken)
    binary.BigEndian.PutUint32(start[4:], payloadLen)

    for _, b := range [][]byte{start, listhdr, code} {
        if _, err := w.Write(b); err != nil {
            return err
        }
    }
    _, err := io.CopyN(w, msg.Payload, int64(msg.Size))
    return err
}

func makeListHeader(length uint32) []byte {
    if length < 56 {
        return []byte{byte(length + 0xc0)}
    }
    enc := big.NewInt(int64(length)).Bytes()
    lenb := byte(len(enc)) + 0xf7
    return append([]byte{lenb}, enc...)
}

// readMsg reads a message header from r.
// It takes an rlp.ByteReader to ensure that the decoding doesn't buffer.
func readMsg(r rlp.ByteReader) (msg Msg, err error) {
    // read magic and payload size
    start := make([]byte, 8)
    if _, err = io.ReadFull(r, start); err != nil {
        return msg, newPeerError(errRead, "%v", err)
    }
    if !bytes.HasPrefix(start, magicToken) {
        return msg, newPeerError(errMagicTokenMismatch, "got %x, want %x", start[:4], magicToken)
    }
    size := binary.BigEndian.Uint32(start[4:])

    // decode start of RLP message to get the message code
    posr := &postrack{r, 0}
    s := rlp.NewStream(posr)
    if _, err := s.List(); err != nil {
        return msg, err
    }
    code, err := s.Uint()
    if err != nil {
        return msg, err
    }
    payloadsize := size - posr.p
    return Msg{code, payloadsize, io.LimitReader(r, int64(payloadsize))}, nil
}

// postrack wraps an rlp.ByteReader with a position counter.
type postrack struct {
    r rlp.ByteReader
    p uint32
}

func (r *postrack) Read(buf []byte) (int, error) {
    n, err := r.r.Read(buf)
    r.p += uint32(n)
    return n, err
}

func (r *postrack) ReadByte() (byte, error) {
    b, err := r.r.ReadByte()
    if err == nil {
        r.p++
    }
    return b, err
}

// MsgPipe creates a message pipe. Reads on one end are matched
// with writes on the other. The pipe is full-duplex, both ends
// implement MsgReadWriter.
func MsgPipe() (*MsgPipeRW, *MsgPipeRW) {
    var (
        c1, c2  = make(chan Msg), make(chan Msg)
        closing = make(chan struct{})
        closed  = new(int32)
        rw1     = &MsgPipeRW{c1, c2, closing, closed}
        rw2     = &MsgPipeRW{c2, c1, closing, closed}
    )
    return rw1, rw2
}

// ErrPipeClosed is returned from pipe operations after the
// pipe has been closed.
var ErrPipeClosed = errors.New("p2p: read or write on closed message pipe")

// MsgPipeRW is an endpoint of a MsgReadWriter pipe.
type MsgPipeRW struct {
    w       chan<- Msg
    r       <-chan Msg
    closing chan struct{}
    closed  *int32
}

// WriteMsg sends a messsage on the pipe.
// It blocks until the receiver has consumed the message payload.
func (p *MsgPipeRW) WriteMsg(msg Msg) error {
    if atomic.LoadInt32(p.closed) == 0 {
        consumed := make(chan struct{}, 1)
        msg.Payload = &eofSignal{msg.Payload, int64(msg.Size), consumed}
        select {
        case p.w <- msg:
            if msg.Size > 0 {
                // wait for payload read or discard
                <-consumed
            }
            return nil
        case <-p.closing:
        }
    }
    return ErrPipeClosed
}

// EncodeMsg is a convenient shorthand for sending an RLP-encoded message.
func (p *MsgPipeRW) EncodeMsg(code uint64, data ...interface{}) error {
    return p.WriteMsg(NewMsg(code, data...))
}

// ReadMsg returns a message sent on the other end of the pipe.
func (p *MsgPipeRW) ReadMsg() (Msg, error) {
    if atomic.LoadInt32(p.closed) == 0 {
        select {
        case msg := <-p.r:
            return msg, nil
        case <-p.closing:
        }
    }
    return Msg{}, ErrPipeClosed
}

// Close unblocks any pending ReadMsg and WriteMsg calls on both ends
// of the pipe. They will return ErrPipeClosed. Note that Close does
// not interrupt any reads from a message payload.
func (p *MsgPipeRW) Close() error {
    if atomic.AddInt32(p.closed, 1) != 1 {
        // someone else is already closing
        atomic.StoreInt32(p.closed, 1) // avoid overflow
        return nil
    }
    close(p.closing)
    return nil
}