path: root/rpc/client.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 rpc

import (
    "bytes"
    "container/list"
    "context"
    "encoding/json"
    "errors"
    "fmt"
    "net"
    "net/url"
    "reflect"
    "strconv"
    "strings"
    "sync"
    "sync/atomic"
    "time"

    "github.com/ethereum/go-ethereum/log"
)

var (
    ErrClientQuit                = errors.New("client is closed")
    ErrNoResult                  = errors.New("no result in JSON-RPC response")
    ErrSubscriptionQueueOverflow = errors.New("subscription queue overflow")
)

const (
    // Timeouts
    tcpKeepAliveInterval = 30 * time.Second
    defaultDialTimeout   = 10 * time.Second // used when dialing if the context has no deadline
    defaultWriteTimeout  = 10 * time.Second // used for calls if the context has no deadline
    subscribeTimeout     = 5 * time.Second  // overall timeout eth_subscribe, rpc_modules calls
)

const (
    // Subscriptions are removed when the subscriber cannot keep up.
    //
    // This can be worked around by supplying a channel with sufficiently sized buffer,
    // but this can be inconvenient and hard to explain in the docs. Another issue with
    // buffered channels is that the buffer is static even though it might not be needed
    // most of the time.
    //
    // The approach taken here is to maintain a per-subscription linked list buffer
    // shrinks on demand. If the buffer reaches the size below, the subscription is
    // dropped.
    maxClientSubscriptionBuffer = 8000
)

// BatchElem is an element in a batch request.
type BatchElem struct {
    Method string
    Args   []interface{}
    // The result is unmarshaled into this field. Result must be set to a
    // non-nil pointer value of the desired type, otherwise the response will be
    // discarded.
    Result interface{}
    // Error is set if the server returns an error for this request, or if
    // unmarshaling into Result fails. It is not set for I/O errors.
    Error error
}

// A value of this type can a JSON-RPC request, notification, successful response or
// error response. Which one it is depends on the fields.
type jsonrpcMessage struct {
    Version string          `json:"jsonrpc"`
    ID      json.RawMessage `json:"id,omitempty"`
    Method  string          `json:"method,omitempty"`
    Params  json.RawMessage `json:"params,omitempty"`
    Error   *jsonError      `json:"error,omitempty"`
    Result  json.RawMessage `json:"result,omitempty"`
}

func (msg *jsonrpcMessage) isNotification() bool {
    return msg.ID == nil && msg.Method != ""
}

func (msg *jsonrpcMessage) isResponse() bool {
    return msg.hasValidID() && msg.Method == "" && len(msg.Params) == 0
}

func (msg *jsonrpcMessage) hasValidID() bool {
    return len(msg.ID) > 0 && msg.ID[0] != '{' && msg.ID[0] != '['
}

func (msg *jsonrpcMessage) String() string {
    b, _ := json.Marshal(msg)
    return string(b)
}

// Client represents a connection to an RPC server.
type Client struct {
    idCounter   uint32
    connectFunc func(ctx context.Context) (net.Conn, error)
    isHTTP      bool

    // writeConn is only safe to access outside dispatch, with the
    // write lock held. The write lock is taken by sending on
    // requestOp and released by sending on sendDone.
    writeConn net.Conn

    // for dispatch
    close       chan struct{}
    didQuit     chan struct{}                  // closed when client quits
    reconnected chan net.Conn                  // where write/reconnect sends the new connection
    readErr     chan error                     // errors from read
    readResp    chan []*jsonrpcMessage         // valid messages from read
    requestOp   chan *requestOp                // for registering response IDs
    sendDone    chan error                     // signals write completion, releases write lock
    respWait    map[string]*requestOp          // active requests
    subs        map[string]*ClientSubscription // active subscriptions
}

type requestOp struct {
    ids  []json.RawMessage
    err  error
    resp chan *jsonrpcMessage // receives up to len(ids) responses
    sub  *ClientSubscription  // only set for EthSubscribe requests
}

func (op *requestOp) wait(ctx context.Context) (*jsonrpcMessage, error) {
    select {
    case <-ctx.Done():
        return nil, ctx.Err()
    case resp := <-op.resp:
        return resp, op.err
    }
}

// Dial creates a new client for the given URL.
//
// The currently supported URL schemes are "http", "https", "ws" and "wss". If rawurl is a
// file name with no URL scheme, a local socket connection is established using UNIX
// domain sockets on supported platforms and named pipes on Windows. If you want to
// configure transport options, use DialHTTP, DialWebsocket or DialIPC instead.
//
// For websocket connections, the origin is set to the local host name.
//
// The client reconnects automatically if the connection is lost.
func Dial(rawurl string) (*Client, error) {
    return DialContext(context.Background(), rawurl)
}

// DialContext creates a new RPC client, just like Dial.
//
// The context is used to cancel or time out the initial connection establishment. It does
// not affect subsequent interactions with the client.
func DialContext(ctx context.Context, rawurl string) (*Client, error) {
    u, err := url.Parse(rawurl)
    if err != nil {
        return nil, err
    }
    switch u.Scheme {
    case "http", "https":
        return DialHTTP(rawurl)
    case "ws", "wss":
        return DialWebsocket(ctx, rawurl, "")
    case "":
        return DialIPC(ctx, rawurl)
    default:
        return nil, fmt.Errorf("no known transport for URL scheme %q", u.Scheme)
    }
}

func newClient(initctx context.Context, connectFunc func(context.Context) (net.Conn, error)) (*Client, error) {
    conn, err := connectFunc(initctx)
    if err != nil {
        return nil, err
    }
    _, isHTTP := conn.(*httpConn)

    c := &Client{
        writeConn:   conn,
        isHTTP:      isHTTP,
        connectFunc: connectFunc,
        close:       make(chan struct{}),
        didQuit:     make(chan struct{}),
        reconnected: make(chan net.Conn),
        readErr:     make(chan error),
        readResp:    make(chan []*jsonrpcMessage),
        requestOp:   make(chan *requestOp),
        sendDone:    make(chan error, 1),
        respWait:    make(map[string]*requestOp),
        subs:        make(map[string]*ClientSubscription),
    }
    if !isHTTP {
        go c.dispatch(conn)
    }
    return c, nil
}

func (c *Client) nextID() json.RawMessage {
    id := atomic.AddUint32(&c.idCounter, 1)
    return []byte(strconv.FormatUint(uint64(id), 10))
}

// SupportedModules calls the rpc_modules method, retrieving the list of
// APIs that are available on the server.
func (c *Client) SupportedModules() (map[string]string, error) {
    var result map[string]string
    ctx, cancel := context.WithTimeout(context.Background(), subscribeTimeout)
    defer cancel()
    err := c.CallContext(ctx, &result, "rpc_modules")
    return result, err
}

// Close closes the client, aborting any in-flight requests.
func (c *Client) Close() {
    if c.isHTTP {
        return
    }
    select {
    case c.close <- struct{}{}:
        <-c.didQuit
    case <-c.didQuit:
    }
}

// Call performs a JSON-RPC call with the given arguments and unmarshals into
// result if no error occurred.
//
// The result must be a pointer so that package json can unmarshal into it. You
// can also pass nil, in which case the result is ignored.
func (c *Client) Call(result interface{}, method string, args ...interface{}) error {
    ctx := context.Background()
    return c.CallContext(ctx, result, method, args...)
}

// CallContext performs a JSON-RPC call with the given arguments. If the context is
// canceled before the call has successfully returned, CallContext returns immediately.
//
// The result must be a pointer so that package json can unmarshal into it. You
// can also pass nil, in which case the result is ignored.
func (c *Client) CallContext(ctx context.Context, result interface{}, method string, args ...interface{}) error {
    msg, err := c.newMessage(method, args...)
    if err != nil {
        return err
    }
    op := &requestOp{ids: []json.RawMessage{msg.ID}, resp: make(chan *jsonrpcMessage, 1)}

    if c.isHTTP {
        err = c.sendHTTP(ctx, op, msg)
    } else {
        err = c.send(ctx, op, msg)
    }
    if err != nil {
        return err
    }

    // dispatch has accepted the request and will close the channel it when it quits.
    switch resp, err := op.wait(ctx); {
    case err != nil:
        return err
    case resp.Error != nil:
        return resp.Error
    case len(resp.Result) == 0:
        return ErrNoResult
    default:
        return json.Unmarshal(resp.Result, &result)
    }
}

// BatchCall sends all given requests as a single batch and waits for the server
// to return a response for all of them.
//
// In contrast to Call, BatchCall only returns I/O errors. Any error specific to
// a request is reported through the Error field of the corresponding BatchElem.
//
// Note that batch calls may not be executed atomically on the server side.
func (c *Client) BatchCall(b []BatchElem) error {
    ctx := context.Background()
    return c.BatchCallContext(ctx, b)
}

// BatchCall sends all given requests as a single batch and waits for the server
// to return a response for all of them. The wait duration is bounded by the
// context's deadline.
//
// In contrast to CallContext, BatchCallContext only returns errors that have occurred
// while sending the request. Any error specific to a request is reported through the
// Error field of the corresponding BatchElem.
//
// Note that batch calls may not be executed atomically on the server side.
func (c *Client) BatchCallContext(ctx context.Context, b []BatchElem) error {
    msgs := make([]*jsonrpcMessage, len(b))
    op := &requestOp{
        ids:  make([]json.RawMessage, len(b)),
        resp: make(chan *jsonrpcMessage, len(b)),
    }
    for i, elem := range b {
        msg, err := c.newMessage(elem.Method, elem.Args...)
        if err != nil {
            return err
        }
        msgs[i] = msg
        op.ids[i] = msg.ID
    }

    var err error
    if c.isHTTP {
        err = c.sendBatchHTTP(ctx, op, msgs)
    } else {
        err = c.send(ctx, op, msgs)
    }

    // Wait for all responses to come back.
    for n := 0; n < len(b) && err == nil; n++ {
        var resp *jsonrpcMessage
        resp, err = op.wait(ctx)
        if err != nil {
            break
        }
        // Find the element corresponding to this response.
        // The element is guaranteed to be present because dispatch
        // only sends valid IDs to our channel.
        var elem *BatchElem
        for i := range msgs {
            if bytes.Equal(msgs[i].ID, resp.ID) {
                elem = &b[i]
                break
            }
        }
        if resp.Error != nil {
            elem.Error = resp.Error
            continue
        }
        if len(resp.Result) == 0 {
            elem.Error = ErrNoResult
            continue
        }
        elem.Error = json.Unmarshal(resp.Result, elem.Result)
    }
    return err
}

// ShhSubscribe calls the "shh_subscribe" method with the given arguments,
// registering a subscription. Server notifications for the subscription are
// sent to the given channel. The element type of the channel must match the
// expected type of content returned by the subscription.
//
// The context argument cancels the RPC request that sets up the subscription but has no
// effect on the subscription after ShhSubscribe has returned.
//
// Slow subscribers will be dropped eventually. Client buffers up to 8000 notifications
// before considering the subscriber dead. The subscription Err channel will receive
// ErrSubscriptionQueueOverflow. Use a sufficiently large buffer on the channel or ensure
// that the channel usually has at least one reader to prevent this issue.
func (c *Client) ShhSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*ClientSubscription, error) {
    // Check type of channel first.
    chanVal := reflect.ValueOf(channel)
    if chanVal.Kind() != reflect.Chan || chanVal.Type().ChanDir()&reflect.SendDir == 0 {
        panic("first argument to ShhSubscribe must be a writable channel")
    }
    if chanVal.IsNil() {
        panic("channel given to ShhSubscribe must not be nil")
    }
    if c.isHTTP {
        return nil, ErrNotificationsUnsupported
    }

    msg, err := c.newMessage("shh"+subscribeMethodSuffix, args...)
    if err != nil {
        return nil, err
    }
    op := &requestOp{
        ids:  []json.RawMessage{msg.ID},
        resp: make(chan *jsonrpcMessage),
        sub:  newClientSubscription(c, "shh", chanVal),
    }

    // Send the subscription request.
    // The arrival and validity of the response is signaled on sub.quit.
    if err := c.send(ctx, op, msg); err != nil {
        return nil, err
    }
    if _, err := op.wait(ctx); err != nil {
        return nil, err
    }
    return op.sub, nil
}

// EthSubscribe calls the "eth_subscribe" method with the given arguments,
// registering a subscription. Server notifications for the subscription are
// sent to the given channel. The element type of the channel must match the
// expected type of content returned by the subscription.
//
// The context argument cancels the RPC request that sets up the subscription but has no
// effect on the subscription after EthSubscribe has returned.
//
// Slow subscribers will be dropped eventually. Client buffers up to 8000 notifications
// before considering the subscriber dead. The subscription Err channel will receive
// ErrSubscriptionQueueOverflow. Use a sufficiently large buffer on the channel or ensure
// that the channel usually has at least one reader to prevent this issue.
func (c *Client) EthSubscribe(ctx context.Context, channel interface{}, args ...interface{}) (*ClientSubscription, error) {
    // Check type of channel first.
    chanVal := reflect.ValueOf(channel)
    if chanVal.Kind() != reflect.Chan || chanVal.Type().ChanDir()&reflect.SendDir == 0 {
        panic("first argument to EthSubscribe must be a writable channel")
    }
    if chanVal.IsNil() {
        panic("channel given to EthSubscribe must not be nil")
    }
    if c.isHTTP {
        return nil, ErrNotificationsUnsupported
    }

    msg, err := c.newMessage("eth"+subscribeMethodSuffix, args...)
    if err != nil {
        return nil, err
    }
    op := &requestOp{
        ids:  []json.RawMessage{msg.ID},
        resp: make(chan *jsonrpcMessage),
        sub:  newClientSubscription(c, "eth", chanVal),
    }

    // Send the subscription request.
    // The arrival and validity of the response is signaled on sub.quit.
    if err := c.send(ctx, op, msg); err != nil {
        return nil, err
    }
    if _, err := op.wait(ctx); err != nil {
        return nil, err
    }
    return op.sub, nil
}

func (c *Client) newMessage(method string, paramsIn ...interface{}) (*jsonrpcMessage, error) {
    params, err := json.Marshal(paramsIn)
    if err != nil {
        return nil, err
    }
    return &jsonrpcMessage{Version: "2.0", ID: c.nextID(), Method: method, Params: params}, nil
}

// send registers op with the dispatch loop, then sends msg on the connection.
// if sending fails, op is deregistered.
func (c *Client) send(ctx context.Context, op *requestOp, msg interface{}) error {
    select {
    case c.requestOp <- op:
        log.Trace("", "msg", log.Lazy{Fn: func() string {
            return fmt.Sprint("sending ", msg)
        }})
        err := c.write(ctx, msg)
        c.sendDone <- err
        return err
    case <-ctx.Done():
        // This can happen if the client is overloaded or unable to keep up with
        // subscription notifications.
        return ctx.Err()
    case <-c.didQuit:
        return ErrClientQuit
    }
}

func (c *Client) write(ctx context.Context, msg interface{}) error {
    deadline, ok := ctx.Deadline()
    if !ok {
        deadline = time.Now().Add(defaultWriteTimeout)
    }
    // The previous write failed. Try to establish a new connection.
    if c.writeConn == nil {
        if err := c.reconnect(ctx); err != nil {
            return err
        }
    }
    c.writeConn.SetWriteDeadline(deadline)
    err := json.NewEncoder(c.writeConn).Encode(msg)
    if err != nil {
        c.writeConn = nil
    }
    return err
}

func (c *Client) reconnect(ctx context.Context) error {
    newconn, err := c.connectFunc(ctx)
    if err != nil {
        log.Trace(fmt.Sprintf("reconnect failed: %v", err))
        return err
    }
    select {
    case c.reconnected <- newconn:
        c.writeConn = newconn
        return nil
    case <-c.didQuit:
        newconn.Close()
        return ErrClientQuit
    }
}

// dispatch is the main loop of the client.
// It sends read messages to waiting calls to Call and BatchCall
// and subscription notifications to registered subscriptions.
func (c *Client) dispatch(conn net.Conn) {
    // Spawn the initial read loop.
    go c.read(conn)

    var (
        lastOp        *requestOp    // tracks last send operation
        requestOpLock = c.requestOp // nil while the send lock is held
        reading       = true        // if true, a read loop is running
    )
    defer close(c.didQuit)
    defer func() {
        c.closeRequestOps(ErrClientQuit)
        conn.Close()
        if reading {
            // Empty read channels until read is dead.
            for {
                select {
                case <-c.readResp:
                case <-c.readErr:
                    return
                }
            }
        }
    }()

    for {
        select {
        case <-c.close:
            return

        // Read path.
        case batch := <-c.readResp:
            for _, msg := range batch {
                switch {
                case msg.isNotification():
                    log.Trace("", "msg", log.Lazy{Fn: func() string {
                        return fmt.Sprint("<-readResp: notification ", msg)
                    }})
                    c.handleNotification(msg)
                case msg.isResponse():
                    log.Trace("", "msg", log.Lazy{Fn: func() string {
                        return fmt.Sprint("<-readResp: response ", msg)
                    }})
                    c.handleResponse(msg)
                default:
                    log.Debug("", "msg", log.Lazy{Fn: func() string {
                        return fmt.Sprint("<-readResp: dropping weird message", msg)
                    }})
                    // TODO: maybe close
                }
            }

        case err := <-c.readErr:
            log.Debug(fmt.Sprintf("<-readErr: %v", err))
            c.closeRequestOps(err)
            conn.Close()
            reading = false

        case newconn := <-c.reconnected:
            log.Debug(fmt.Sprintf("<-reconnected: (reading=%t) %v", reading, conn.RemoteAddr()))
            if reading {
                // Wait for the previous read loop to exit. This is a rare case.
                conn.Close()
                <-c.readErr
            }
            go c.read(newconn)
            reading = true
            conn = newconn

        // Send path.
        case op := <-requestOpLock:
            // Stop listening for further send ops until the current one is done.
            requestOpLock = nil
            lastOp = op
            for _, id := range op.ids {
                c.respWait[string(id)] = op
            }

        case err := <-c.sendDone:
            if err != nil {
                // Remove response handlers for the last send. We remove those here
                // because the error is already handled in Call or BatchCall. When the
                // read loop goes down, it will signal all other current operations.
                for _, id := range lastOp.ids {
                    delete(c.respWait, string(id))
                }
            }
            // Listen for send ops again.
            requestOpLock = c.requestOp
            lastOp = nil
        }
    }
}

// closeRequestOps unblocks pending send ops and active subscriptions.
func (c *Client) closeRequestOps(err error) {
    didClose := make(map[*requestOp]bool)

    for id, op := range c.respWait {
        // Remove the op so that later calls will not close op.resp again.
        delete(c.respWait, id)

        if !didClose[op] {
            op.err = err
            close(op.resp)
            didClose[op] = true
        }
    }
    for id, sub := range c.subs {
        delete(c.subs, id)
        sub.quitWithError(err, false)
    }
}

func (c *Client) handleNotification(msg *jsonrpcMessage) {
    if !strings.HasSuffix(msg.Method, notificationMethodSuffix) {
        log.Debug(fmt.Sprint("dropping non-subscription message: ", msg))
        return
    }
    var subResult struct {
        ID     string          `json:"subscription"`
        Result json.RawMessage `json:"result"`
    }
    if err := json.Unmarshal(msg.Params, &subResult); err != nil {
        log.Debug(fmt.Sprint("dropping invalid subscription message: ", msg))
        return
    }
    if c.subs[subResult.ID] != nil {
        c.subs[subResult.ID].deliver(subResult.Result)
    }
}

func (c *Client) handleResponse(msg *jsonrpcMessage) {
    op := c.respWait[string(msg.ID)]
    if op == nil {
        log.Debug(fmt.Sprintf("unsolicited response %v", msg))
        return
    }
    delete(c.respWait, string(msg.ID))
    // For normal responses, just forward the reply to Call/BatchCall.
    if op.sub == nil {
        op.resp <- msg
        return
    }
    // For subscription responses, start the subscription if the server
    // indicates success. EthSubscribe gets unblocked in either case through
    // the op.resp channel.
    defer close(op.resp)
    if msg.Error != nil {
        op.err = msg.Error
        return
    }
    if op.err = json.Unmarshal(msg.Result, &op.sub.subid); op.err == nil {
        go op.sub.start()
        c.subs[op.sub.subid] = op.sub
    }
}

// Reading happens on a dedicated goroutine.

func (c *Client) read(conn net.Conn) error {
    var (
        buf json.RawMessage
        dec = json.NewDecoder(conn)
    )
    readMessage := func() (rs []*jsonrpcMessage, err error) {
        buf = buf[:0]
        if err = dec.Decode(&buf); err != nil {
            return nil, err
        }
        if isBatch(buf) {
            err = json.Unmarshal(buf, &rs)
        } else {
            rs = make([]*jsonrpcMessage, 1)
            err = json.Unmarshal(buf, &rs[0])
        }
        return rs, err
    }

    for {
        resp, err := readMessage()
        if err != nil {
            c.readErr <- err
            return err
        }
        c.readResp <- resp
    }
}

// Subscriptions.

// A ClientSubscription represents a subscription established through EthSubscribe.
type ClientSubscription struct {
    client    *Client
    etype     reflect.Type
    channel   reflect.Value
    namespace string
    subid     string
    in        chan json.RawMessage

    quitOnce sync.Once     // ensures quit is closed once
    quit     chan struct{} // quit is closed when the subscription exits
    errOnce  sync.Once     // ensures err is closed once
    err      chan error
}

func newClientSubscription(c *Client, namespace string, channel reflect.Value) *ClientSubscription {
    sub := &ClientSubscription{
        client:    c,
        namespace: namespace,
        etype:     channel.Type().Elem(),
        channel:   channel,
        quit:      make(chan struct{}),
        err:       make(chan error, 1),
        in:        make(chan json.RawMessage),
    }
    return sub
}

// Err returns the subscription error channel. The intended use of Err is to schedule
// resubscription when the client connection is closed unexpectedly.
//
// The error channel receives a value when the subscription has ended due
// to an error. The received error is nil if Close has been called
// on the underlying client and no other error has occurred.
//
// The error channel is closed when Unsubscribe is called on the subscription.
func (sub *ClientSubscription) Err() <-chan error {
    return sub.err
}

// Unsubscribe unsubscribes the notification and closes the error channel.
// It can safely be called more than once.
func (sub *ClientSubscription) Unsubscribe() {
    sub.quitWithError(nil, true)
    sub.errOnce.Do(func() { close(sub.err) })
}

func (sub *ClientSubscription) quitWithError(err error, unsubscribeServer bool) {
    sub.quitOnce.Do(func() {
        // The dispatch loop won't be able to execute the unsubscribe call
        // if it is blocked on deliver. Close sub.quit first because it
        // unblocks deliver.
        close(sub.quit)
        if unsubscribeServer {
            sub.requestUnsubscribe()
        }
        if err != nil {
            if err == ErrClientQuit {
                err = nil // Adhere to subscription semantics.
            }
            sub.err <- err
        }
    })
}

func (sub *ClientSubscription) deliver(result json.RawMessage) (ok bool) {
    select {
    case sub.in <- result:
        return true
    case <-sub.quit:
        return false
    }
}

func (sub *ClientSubscription) start() {
    sub.quitWithError(sub.forward())
}

func (sub *ClientSubscription) forward() (err error, unsubscribeServer bool) {
    cases := []reflect.SelectCase{
        {Dir: reflect.SelectRecv, Chan: reflect.ValueOf(sub.quit)},
        {Dir: reflect.SelectRecv, Chan: reflect.ValueOf(sub.in)},
        {Dir: reflect.SelectSend, Chan: sub.channel},
    }
    buffer := list.New()
    defer buffer.Init()
    for {
        var chosen int
        var recv reflect.Value
        if buffer.Len() == 0 {
            // Idle, omit send case.
            chosen, recv, _ = reflect.Select(cases[:2])
        } else {
            // Non-empty buffer, send the first queued item.
            cases[2].Send = reflect.ValueOf(buffer.Front().Value)
            chosen, recv, _ = reflect.Select(cases)
        }

        switch chosen {
        case 0: // <-sub.quit
            return nil, false
        case 1: // <-sub.in
            val, err := sub.unmarshal(recv.Interface().(json.RawMessage))
            if err != nil {
                return err, true
            }
            if buffer.Len() == maxClientSubscriptionBuffer {
                return ErrSubscriptionQueueOverflow, true
            }
            buffer.PushBack(val)
        case 2: // sub.channel<-
            cases[2].Send = reflect.Value{} // Don't hold onto the value.
            buffer.Remove(buffer.Front())
        }
    }
}

func (sub *ClientSubscription) unmarshal(result json.RawMessage) (interface{}, error) {
    val := reflect.New(sub.etype)
    err := json.Unmarshal(result, val.Interface())
    return val.Elem().Interface(), err
}

func (sub *ClientSubscription) requestUnsubscribe() error {
    var result interface{}
    return sub.client.Call(&result, sub.namespace+unsubscribeMethodSuffix, sub.subid)
}