client.go 22.6 KB
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781
// 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
}

// 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)
}