rpcwebsocket.go

package main

import (
	"bytes"
	"container/list"
	"crypto/sha256"
	"crypto/subtle"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"github.com/btcsuite/websocket"
	"github.com/parallelcointeam/pod/blockchain"
	"github.com/parallelcointeam/pod/btcjson"
	"github.com/parallelcointeam/pod/btcutil"
	"github.com/parallelcointeam/pod/chaincfg"
	"github.com/parallelcointeam/pod/chaincfg/chainhash"
	"github.com/parallelcointeam/pod/database"
	"github.com/parallelcointeam/pod/txscript"
	"github.com/parallelcointeam/pod/wire"
	"golang.org/x/crypto/ripemd160"
	"io"
	"math"
	"sync"
	"time"
)

const (
	// websocketSendBufferSize is the number of elements the send channel can queue before blocking.  Note that this only applies to requests handled directly in the websocket client input handler or the async handler since notifications have their own queuing mechanism independent of the send channel buffer.
	websocketSendBufferSize = 50
)

type semaphore chan struct{}

func makeSemaphore(n int) semaphore {
	return make(chan struct{}, n)
}
func (s semaphore) acquire() { s <- struct{}{} }
func (s semaphore) release() { <-s }

// timeZeroVal is simply the zero value for a time.Time and is used to avoid creating multiple instances.
var timeZeroVal time.Time

// wsCommandHandler describes a callback function used to handle a specific command.
type wsCommandHandler func(*wsClient, interface{}) (interface{}, error)

// wsHandlers maps RPC command strings to appropriate websocket handler functions.  This is set by init because help references wsHandlers and thus causes a dependency loop.
var wsHandlers map[string]wsCommandHandler
var wsHandlersBeforeInit = map[string]wsCommandHandler{
	"loadtxfilter":              handleLoadTxFilter,
	"help":                      handleWebsocketHelp,
	"notifyblocks":              handleNotifyBlocks,
	"notifynewtransactions":     handleNotifyNewTransactions,
	"notifyreceived":            handleNotifyReceived,
	"notifyspent":               handleNotifySpent,
	"session":                   handleSession,
	"stopnotifyblocks":          handleStopNotifyBlocks,
	"stopnotifynewtransactions": handleStopNotifyNewTransactions,
	"stopnotifyspent":           handleStopNotifySpent,
	"stopnotifyreceived":        handleStopNotifyReceived,
	"rescan":                    handleRescan,
	"rescanblocks":              handleRescanBlocks,
}

// WebsocketHandler handles a new websocket client by creating a new wsClient, starting it, and blocking until the connection closes.  Since it blocks, it must be run in a separate goroutine.  It should be invoked from the websocket server handler which runs each new connection in a new goroutine thereby satisfying the requirement.
func (s *rpcServer) WebsocketHandler(conn *websocket.Conn, remoteAddr string,
	authenticated bool, isAdmin bool) {
	// Clear the read deadline that was set before the websocket hijacked the connection.
	conn.SetReadDeadline(timeZeroVal)
	// Limit max number of websocket clients.
	rpcsLog.Infof("New websocket client %s", remoteAddr)
	if s.ntfnMgr.NumClients()+1 > cfg.RPCMaxWebsockets {
		rpcsLog.Infof("Max websocket clients exceeded [%d] - "+
			"disconnecting client %s", cfg.RPCMaxWebsockets,
			remoteAddr)
		conn.Close()
		return
	}
	// Create a new websocket client to handle the new websocket connection and wait for it to shutdown.  Once it has shutdown (and hence disconnected), remove it and any notifications it registered for.
	client, err := newWebsocketClient(s, conn, remoteAddr, authenticated, isAdmin)
	if err != nil {
		rpcsLog.Errorf("Failed to serve client %s: %v", remoteAddr, err)
		conn.Close()
		return
	}
	s.ntfnMgr.AddClient(client)
	client.Start()
	client.WaitForShutdown()
	s.ntfnMgr.RemoveClient(client)
	rpcsLog.Infof("Disconnected websocket client %s", remoteAddr)
}

// wsNotificationManager is a connection and notification manager used for websockets.  It allows websocket clients to register for notifications they are interested in.  When an event happens elsewhere in the code such as transactions being added to the memory pool or block connects/disconnects, the notification manager is provided with the relevant details needed to figure out which websocket clients need to be notified based on what they have registered for and notifies them accordingly.  It is also used to keep track of all connected websocket clients.
type wsNotificationManager struct {
	// server is the RPC server the notification manager is associated with.
	server *rpcServer
	// queueNotification queues a notification for handling.
	queueNotification chan interface{}
	// notificationMsgs feeds notificationHandler with notifications and client (un)registeration requests from a queue as well as registeration and unregisteration requests from clients.
	notificationMsgs chan interface{}
	// Access channel for current number of connected clients.
	numClients chan int
	// Shutdown handling
	wg   sync.WaitGroup
	quit chan struct{}
}

// queueHandler manages a queue of empty interfaces, reading from in and sending the oldest unsent to out.  This handler stops when either of the in or quit channels are closed, and closes out before returning, without waiting to send any variables still remaining in the queue.
func queueHandler(in <-chan interface{}, out chan<- interface{}, quit <-chan struct{}) {
	var q []interface{}
	var dequeue chan<- interface{}
	skipQueue := out
	var next interface{}
out:
	for {
		select {
		case n, ok := <-in:
			if !ok {
				// Sender closed input channel.
				break out
			}
			// Either send to out immediately if skipQueue is non-nil (queue is empty) and reader is ready, or append to the queue and send later.
			select {
			case skipQueue <- n:
			default:
				q = append(q, n)
				dequeue = out
				skipQueue = nil
				next = q[0]
			}
		case dequeue <- next:
			copy(q, q[1:])
			q[len(q)-1] = nil // avoid leak
			q = q[:len(q)-1]
			if len(q) == 0 {
				dequeue = nil
				skipQueue = out
			} else {
				next = q[0]
			}
		case <-quit:
			break out
		}
	}
	close(out)
}

// queueHandler maintains a queue of notifications and notification handler control messages.
func (m *wsNotificationManager) queueHandler() {
	queueHandler(m.queueNotification, m.notificationMsgs, m.quit)
	m.wg.Done()
}

// NotifyBlockConnected passes a block newly-connected to the best chain to the notification manager for block and transaction notification processing.
func (m *wsNotificationManager) NotifyBlockConnected(block *btcutil.Block) {
	// As NotifyBlockConnected will be called by the block manager and the RPC server may no longer be running, use a select statement to unblock enqueuing the notification once the RPC server has begun shutting down.
	select {
	case m.queueNotification <- (*notificationBlockConnected)(block):
	case <-m.quit:
	}
}

// NotifyBlockDisconnected passes a block disconnected from the best chain to the notification manager for block notification processing.
func (m *wsNotificationManager) NotifyBlockDisconnected(block *btcutil.Block) {
	// As NotifyBlockDisconnected will be called by the block manager and the RPC server may no longer be running, use a select statement to unblock enqueuing the notification once the RPC server has begun shutting down.
	select {
	case m.queueNotification <- (*notificationBlockDisconnected)(block):
	case <-m.quit:
	}
}

// NotifyMempoolTx passes a transaction accepted by mempool to the notification manager for transaction notification processing.  If isNew is true, the tx is is a new transaction, rather than one added to the mempool during a reorg.
func (m *wsNotificationManager) NotifyMempoolTx(tx *btcutil.Tx, isNew bool) {
	n := &notificationTxAcceptedByMempool{
		isNew: isNew,
		tx:    tx,
	}
	// As NotifyMempoolTx will be called by mempool and the RPC server may no longer be running, use a select statement to unblock enqueuing the notification once the RPC server has begun shutting down.
	select {
	case m.queueNotification <- n:
	case <-m.quit:
	}
}

// wsClientFilter tracks relevant addresses for each websocket client for the `rescanblocks` extension. It is modified by the `loadtxfilter` command. NOTE: This extension was ported from github.com/decred/dcrd
type wsClientFilter struct {
	mu sync.Mutex
	// Implemented fast paths for address lookup.
	pubKeyHashes        map[[ripemd160.Size]byte]struct{}
	scriptHashes        map[[ripemd160.Size]byte]struct{}
	compressedPubKeys   map[[33]byte]struct{}
	uncompressedPubKeys map[[65]byte]struct{}
	// A fallback address lookup map in case a fast path doesn't exist. Only exists for completeness.  If using this shows up in a profile, there's a good chance a fast path should be added.
	otherAddresses map[string]struct{}
	// Outpoints of unspent outputs.
	unspent map[wire.OutPoint]struct{}
}

// newWSClientFilter creates a new, empty wsClientFilter struct to be used for a websocket client. NOTE: This extension was ported from github.com/decred/dcrd
func newWSClientFilter(addresses []string, unspentOutPoints []wire.OutPoint, params *chaincfg.Params) *wsClientFilter {
	filter := &wsClientFilter{
		pubKeyHashes:        map[[ripemd160.Size]byte]struct{}{},
		scriptHashes:        map[[ripemd160.Size]byte]struct{}{},
		compressedPubKeys:   map[[33]byte]struct{}{},
		uncompressedPubKeys: map[[65]byte]struct{}{},
		otherAddresses:      map[string]struct{}{},
		unspent:             make(map[wire.OutPoint]struct{}, len(unspentOutPoints)),
	}
	for _, s := range addresses {
		filter.addAddressStr(s, params)
	}
	for i := range unspentOutPoints {
		filter.addUnspentOutPoint(&unspentOutPoints[i])
	}
	return filter
}

// addAddress adds an address to a wsClientFilter, treating it correctly based on the type of address passed as an argument. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) addAddress(a btcutil.Address) {
	switch a := a.(type) {
	case *btcutil.AddressPubKeyHash:
		f.pubKeyHashes[*a.Hash160()] = struct{}{}
		return
	case *btcutil.AddressScriptHash:
		f.scriptHashes[*a.Hash160()] = struct{}{}
		return
	case *btcutil.AddressPubKey:
		serializedPubKey := a.ScriptAddress()
		switch len(serializedPubKey) {
		case 33: // compressed
			var compressedPubKey [33]byte
			copy(compressedPubKey[:], serializedPubKey)
			f.compressedPubKeys[compressedPubKey] = struct{}{}
			return
		case 65: // uncompressed
			var uncompressedPubKey [65]byte
			copy(uncompressedPubKey[:], serializedPubKey)
			f.uncompressedPubKeys[uncompressedPubKey] = struct{}{}
			return
		}
	}
	f.otherAddresses[a.EncodeAddress()] = struct{}{}
}

// addAddressStr parses an address from a string and then adds it to the wsClientFilter using addAddress. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) addAddressStr(s string, params *chaincfg.Params) {
	// If address can't be decoded, no point in saving it since it should also impossible to create the address from an inspected transaction output script.
	a, err := btcutil.DecodeAddress(s, params)
	if err != nil {
		return
	}
	f.addAddress(a)
}

// existsAddress returns true if the passed address has been added to the wsClientFilter. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) existsAddress(a btcutil.Address) bool {
	switch a := a.(type) {
	case *btcutil.AddressPubKeyHash:
		_, ok := f.pubKeyHashes[*a.Hash160()]
		return ok
	case *btcutil.AddressScriptHash:
		_, ok := f.scriptHashes[*a.Hash160()]
		return ok
	case *btcutil.AddressPubKey:
		serializedPubKey := a.ScriptAddress()
		switch len(serializedPubKey) {
		case 33: // compressed
			var compressedPubKey [33]byte
			copy(compressedPubKey[:], serializedPubKey)
			_, ok := f.compressedPubKeys[compressedPubKey]
			if !ok {
				_, ok = f.pubKeyHashes[*a.AddressPubKeyHash().Hash160()]
			}
			return ok
		case 65: // uncompressed
			var uncompressedPubKey [65]byte
			copy(uncompressedPubKey[:], serializedPubKey)
			_, ok := f.uncompressedPubKeys[uncompressedPubKey]
			if !ok {
				_, ok = f.pubKeyHashes[*a.AddressPubKeyHash().Hash160()]
			}
			return ok
		}
	}
	_, ok := f.otherAddresses[a.EncodeAddress()]
	return ok
}

// removeAddress removes the passed address, if it exists, from the wsClientFilter. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) removeAddress(a btcutil.Address) {
	switch a := a.(type) {
	case *btcutil.AddressPubKeyHash:
		delete(f.pubKeyHashes, *a.Hash160())
		return
	case *btcutil.AddressScriptHash:
		delete(f.scriptHashes, *a.Hash160())
		return
	case *btcutil.AddressPubKey:
		serializedPubKey := a.ScriptAddress()
		switch len(serializedPubKey) {
		case 33: // compressed
			var compressedPubKey [33]byte
			copy(compressedPubKey[:], serializedPubKey)
			delete(f.compressedPubKeys, compressedPubKey)
			return
		case 65: // uncompressed
			var uncompressedPubKey [65]byte
			copy(uncompressedPubKey[:], serializedPubKey)
			delete(f.uncompressedPubKeys, uncompressedPubKey)
			return
		}
	}
	delete(f.otherAddresses, a.EncodeAddress())
}

// removeAddressStr parses an address from a string and then removes it from the wsClientFilter using removeAddress. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) removeAddressStr(s string, params *chaincfg.Params) {
	a, err := btcutil.DecodeAddress(s, params)
	if err == nil {
		f.removeAddress(a)
	} else {
		delete(f.otherAddresses, s)
	}
}

// addUnspentOutPoint adds an outpoint to the wsClientFilter. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) addUnspentOutPoint(op *wire.OutPoint) {
	f.unspent[*op] = struct{}{}
}

// existsUnspentOutPoint returns true if the passed outpoint has been added to the wsClientFilter. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) existsUnspentOutPoint(op *wire.OutPoint) bool {
	_, ok := f.unspent[*op]
	return ok
}

// removeUnspentOutPoint removes the passed outpoint, if it exists, from the wsClientFilter. NOTE: This extension was ported from github.com/decred/dcrd
func (f *wsClientFilter) removeUnspentOutPoint(op *wire.OutPoint) {
	delete(f.unspent, *op)
}

// Notification types
type notificationBlockConnected btcutil.Block
type notificationBlockDisconnected btcutil.Block
type notificationTxAcceptedByMempool struct {
	isNew bool
	tx    *btcutil.Tx
}

// Notification control requests
type notificationRegisterClient wsClient
type notificationUnregisterClient wsClient
type notificationRegisterBlocks wsClient
type notificationUnregisterBlocks wsClient
type notificationRegisterNewMempoolTxs wsClient
type notificationUnregisterNewMempoolTxs wsClient
type notificationRegisterSpent struct {
	wsc *wsClient
	ops []*wire.OutPoint
}
type notificationUnregisterSpent struct {
	wsc *wsClient
	op  *wire.OutPoint
}
type notificationRegisterAddr struct {
	wsc   *wsClient
	addrs []string
}
type notificationUnregisterAddr struct {
	wsc  *wsClient
	addr string
}

// notificationHandler reads notifications and control messages from the queue handler and processes one at a time.
func (m *wsNotificationManager) notificationHandler() {
	// clients is a map of all currently connected websocket clients.
	clients := make(map[chan struct{}]*wsClient)
	// Maps used to hold lists of websocket clients to be notified on certain events.  Each websocket client also keeps maps for the events which have multiple triggers to make removal from these lists on connection close less horrendously. Where possible, the quit channel is used as the unique id for a client since it is quite a bit more efficient than using the entire struct.
	blockNotifications := make(map[chan struct{}]*wsClient)
	txNotifications := make(map[chan struct{}]*wsClient)
	watchedOutPoints := make(map[wire.OutPoint]map[chan struct{}]*wsClient)
	watchedAddrs := make(map[string]map[chan struct{}]*wsClient)
out:
	for {
		select {
		case n, ok := <-m.notificationMsgs:
			if !ok {
				// queueHandler quit.
				break out
			}
			switch n := n.(type) {
			case *notificationBlockConnected:
				block := (*btcutil.Block)(n)
				// Skip iterating through all txs if no tx notification requests exist.
				if len(watchedOutPoints) != 0 || len(watchedAddrs) != 0 {
					for _, tx := range block.Transactions() {
						m.notifyForTx(watchedOutPoints,
							watchedAddrs, tx, block)
					}
				}
				if len(blockNotifications) != 0 {
					m.notifyBlockConnected(blockNotifications,
						block)
					m.notifyFilteredBlockConnected(blockNotifications,
						block)
				}
			case *notificationBlockDisconnected:
				block := (*btcutil.Block)(n)
				if len(blockNotifications) != 0 {
					m.notifyBlockDisconnected(blockNotifications,
						block)
					m.notifyFilteredBlockDisconnected(blockNotifications,
						block)
				}
			case *notificationTxAcceptedByMempool:
				if n.isNew && len(txNotifications) != 0 {
					m.notifyForNewTx(txNotifications, n.tx)
				}
				m.notifyForTx(watchedOutPoints, watchedAddrs, n.tx, nil)
				m.notifyRelevantTxAccepted(n.tx, clients)
			case *notificationRegisterBlocks:
				wsc := (*wsClient)(n)
				blockNotifications[wsc.quit] = wsc
			case *notificationUnregisterBlocks:
				wsc := (*wsClient)(n)
				delete(blockNotifications, wsc.quit)
			case *notificationRegisterClient:
				wsc := (*wsClient)(n)
				clients[wsc.quit] = wsc
			case *notificationUnregisterClient:
				wsc := (*wsClient)(n)
				// Remove any requests made by the client as well as the client itself.
				delete(blockNotifications, wsc.quit)
				delete(txNotifications, wsc.quit)
				for k := range wsc.spentRequests {
					op := k
					m.removeSpentRequest(watchedOutPoints, wsc, &op)
				}
				for addr := range wsc.addrRequests {
					m.removeAddrRequest(watchedAddrs, wsc, addr)
				}
				delete(clients, wsc.quit)
			case *notificationRegisterSpent:
				m.addSpentRequests(watchedOutPoints, n.wsc, n.ops)
			case *notificationUnregisterSpent:
				m.removeSpentRequest(watchedOutPoints, n.wsc, n.op)
			case *notificationRegisterAddr:
				m.addAddrRequests(watchedAddrs, n.wsc, n.addrs)
			case *notificationUnregisterAddr:
				m.removeAddrRequest(watchedAddrs, n.wsc, n.addr)
			case *notificationRegisterNewMempoolTxs:
				wsc := (*wsClient)(n)
				txNotifications[wsc.quit] = wsc
			case *notificationUnregisterNewMempoolTxs:
				wsc := (*wsClient)(n)
				delete(txNotifications, wsc.quit)
			default:
				rpcsLog.Warn("Unhandled notification type")
			}
		case m.numClients <- len(clients):
		case <-m.quit:
			// RPC server shutting down.
			break out
		}
	}
	for _, c := range clients {
		c.Disconnect()
	}
	m.wg.Done()
}

// NumClients returns the number of clients actively being served.
func (m *wsNotificationManager) NumClients() (n int) {
	select {
	case n = <-m.numClients:
	case <-m.quit: // Use default n (0) if server has shut down.
	}
	return
}

// RegisterBlockUpdates requests block update notifications to the passed websocket client.
func (m *wsNotificationManager) RegisterBlockUpdates(wsc *wsClient) {
	m.queueNotification <- (*notificationRegisterBlocks)(wsc)
}

// UnregisterBlockUpdates removes block update notifications for the passed websocket client.
func (m *wsNotificationManager) UnregisterBlockUpdates(wsc *wsClient) {
	m.queueNotification <- (*notificationUnregisterBlocks)(wsc)
}

// subscribedClients returns the set of all websocket client quit channels that are registered to receive notifications regarding tx, either due to tx spending a watched output or outputting to a watched address.  Matching client's filters are updated based on this transaction's outputs and output addresses that may be relevant for a client.
func (m *wsNotificationManager) subscribedClients(tx *btcutil.Tx,
	clients map[chan struct{}]*wsClient) map[chan struct{}]struct{} {
	// Use a map of client quit channels as keys to prevent duplicates when multiple inputs and/or outputs are relevant to the client.
	subscribed := make(map[chan struct{}]struct{})
	msgTx := tx.MsgTx()
	for _, input := range msgTx.TxIn {
		for quitChan, wsc := range clients {
			wsc.Lock()
			filter := wsc.filterData
			wsc.Unlock()
			if filter == nil {
				continue
			}
			filter.mu.Lock()
			if filter.existsUnspentOutPoint(&input.PreviousOutPoint) {
				subscribed[quitChan] = struct{}{}
			}
			filter.mu.Unlock()
		}
	}
	for i, output := range msgTx.TxOut {
		_, addrs, _, err := txscript.ExtractPkScriptAddrs(
			output.PkScript, m.server.cfg.ChainParams)
		if err != nil {
			// Clients are not able to subscribe to nonstandard or non-address outputs.
			continue
		}
		for quitChan, wsc := range clients {
			wsc.Lock()
			filter := wsc.filterData
			wsc.Unlock()
			if filter == nil {
				continue
			}
			filter.mu.Lock()
			for _, a := range addrs {
				if filter.existsAddress(a) {
					subscribed[quitChan] = struct{}{}
					op := wire.OutPoint{
						Hash:  *tx.Hash(),
						Index: uint32(i),
					}
					filter.addUnspentOutPoint(&op)
				}
			}
			filter.mu.Unlock()
		}
	}
	return subscribed
}

// notifyBlockConnected notifies websocket clients that have registered for block updates when a block is connected to the main chain.
func (*wsNotificationManager) notifyBlockConnected(clients map[chan struct{}]*wsClient,
	block *btcutil.Block) {
	// Notify interested websocket clients about the connected block.
	ntfn := btcjson.NewBlockConnectedNtfn(block.Hash().String(), block.Height(),
		block.MsgBlock().Header.Timestamp.Unix())
	marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
	if err != nil {
		rpcsLog.Errorf("Failed to marshal block connected notification: "+
			"%v", err)
		return
	}
	for _, wsc := range clients {
		wsc.QueueNotification(marshalledJSON)
	}
}

// notifyBlockDisconnected notifies websocket clients that have registered for block updates when a block is disconnected from the main chain (due to a reorganize).
func (*wsNotificationManager) notifyBlockDisconnected(clients map[chan struct{}]*wsClient, block *btcutil.Block) {
	// Skip notification creation if no clients have requested block connected/disconnected notifications.
	if len(clients) == 0 {
		return
	}
	// Notify interested websocket clients about the disconnected block.
	ntfn := btcjson.NewBlockDisconnectedNtfn(block.Hash().String(),
		block.Height(), block.MsgBlock().Header.Timestamp.Unix())
	marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
	if err != nil {
		rpcsLog.Errorf("Failed to marshal block disconnected "+
			"notification: %v", err)
		return
	}
	for _, wsc := range clients {
		wsc.QueueNotification(marshalledJSON)
	}
}

// notifyFilteredBlockConnected notifies websocket clients that have registered for block updates when a block is connected to the main chain.
func (m *wsNotificationManager) notifyFilteredBlockConnected(clients map[chan struct{}]*wsClient,
	block *btcutil.Block) {
	// Create the common portion of the notification that is the same for every client.
	var w bytes.Buffer
	err := block.MsgBlock().Header.Serialize(&w)
	if err != nil {
		rpcsLog.Errorf("Failed to serialize header for filtered block "+
			"connected notification: %v", err)
		return
	}
	ntfn := btcjson.NewFilteredBlockConnectedNtfn(block.Height(),
		hex.EncodeToString(w.Bytes()), nil)
	// Search for relevant transactions for each client and save them serialized in hex encoding for the notification.
	subscribedTxs := make(map[chan struct{}][]string)
	for _, tx := range block.Transactions() {
		var txHex string
		for quitChan := range m.subscribedClients(tx, clients) {
			if txHex == "" {
				txHex = txHexString(tx.MsgTx())
			}
			subscribedTxs[quitChan] = append(subscribedTxs[quitChan], txHex)
		}
	}
	for quitChan, wsc := range clients {
		// Add all discovered transactions for this client. For clients that have no new-style filter, add the empty string slice.
		ntfn.SubscribedTxs = subscribedTxs[quitChan]
		// Marshal and queue notification.
		marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
		if err != nil {
			rpcsLog.Errorf("Failed to marshal filtered block "+
				"connected notification: %v", err)
			return
		}
		wsc.QueueNotification(marshalledJSON)
	}
}

// notifyFilteredBlockDisconnected notifies websocket clients that have registered for block updates when a block is disconnected from the main chain (due to a reorganize).
func (*wsNotificationManager) notifyFilteredBlockDisconnected(clients map[chan struct{}]*wsClient,
	block *btcutil.Block) {
	// Skip notification creation if no clients have requested block connected/disconnected notifications.
	if len(clients) == 0 {
		return
	}
	// Notify interested websocket clients about the disconnected block.
	var w bytes.Buffer
	err := block.MsgBlock().Header.Serialize(&w)
	if err != nil {
		rpcsLog.Errorf("Failed to serialize header for filtered block "+
			"disconnected notification: %v", err)
		return
	}
	ntfn := btcjson.NewFilteredBlockDisconnectedNtfn(block.Height(),
		hex.EncodeToString(w.Bytes()))
	marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
	if err != nil {
		rpcsLog.Errorf("Failed to marshal filtered block disconnected "+
			"notification: %v", err)
		return
	}
	for _, wsc := range clients {
		wsc.QueueNotification(marshalledJSON)
	}
}

// RegisterNewMempoolTxsUpdates requests notifications to the passed websocket client when new transactions are added to the memory pool.
func (m *wsNotificationManager) RegisterNewMempoolTxsUpdates(wsc *wsClient) {
	m.queueNotification <- (*notificationRegisterNewMempoolTxs)(wsc)
}

// UnregisterNewMempoolTxsUpdates removes notifications to the passed websocket client when new transaction are added to the memory pool.
func (m *wsNotificationManager) UnregisterNewMempoolTxsUpdates(wsc *wsClient) {
	m.queueNotification <- (*notificationUnregisterNewMempoolTxs)(wsc)
}

// notifyForNewTx notifies websocket clients that have registered for updates when a new transaction is added to the memory pool.
func (m *wsNotificationManager) notifyForNewTx(clients map[chan struct{}]*wsClient, tx *btcutil.Tx) {
	txHashStr := tx.Hash().String()
	mtx := tx.MsgTx()
	var amount int64
	for _, txOut := range mtx.TxOut {
		amount += txOut.Value
	}
	ntfn := btcjson.NewTxAcceptedNtfn(txHashStr, btcutil.Amount(amount).ToDUO())
	marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
	if err != nil {
		rpcsLog.Errorf("Failed to marshal tx notification: %s", err.Error())
		return
	}
	var verboseNtfn *btcjson.TxAcceptedVerboseNtfn
	var marshalledJSONVerbose []byte
	for _, wsc := range clients {
		if wsc.verboseTxUpdates {
			if marshalledJSONVerbose != nil {
				wsc.QueueNotification(marshalledJSONVerbose)
				continue
			}
			net := m.server.cfg.ChainParams
			rawTx, err := createTxRawResult(net, mtx, txHashStr, nil,
				"", 0, 0)
			if err != nil {
				return
			}
			verboseNtfn = btcjson.NewTxAcceptedVerboseNtfn(*rawTx)
			marshalledJSONVerbose, err = btcjson.MarshalCmd(nil,
				verboseNtfn)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal verbose tx "+
					"notification: %s", err.Error())
				return
			}
			wsc.QueueNotification(marshalledJSONVerbose)
		} else {
			wsc.QueueNotification(marshalledJSON)
		}
	}
}

// RegisterSpentRequests requests a notification when each of the passed outpoints is confirmed spent (contained in a block connected to the main chain) for the passed websocket client.  The request is automatically removed once the notification has been sent.
func (m *wsNotificationManager) RegisterSpentRequests(wsc *wsClient, ops []*wire.OutPoint) {
	m.queueNotification <- &notificationRegisterSpent{
		wsc: wsc,
		ops: ops,
	}
}

// addSpentRequests modifies a map of watched outpoints to sets of websocket clients to add a new request watch all of the outpoints in ops and create and send a notification when spent to the websocket client wsc.
func (m *wsNotificationManager) addSpentRequests(opMap map[wire.OutPoint]map[chan struct{}]*wsClient,
	wsc *wsClient, ops []*wire.OutPoint) {
	for _, op := range ops {
		// Track the request in the client as well so it can be quickly be removed on disconnect.
		wsc.spentRequests[*op] = struct{}{}
		// Add the client to the list to notify when the outpoint is seen. Create the list as needed.
		cmap, ok := opMap[*op]
		if !ok {
			cmap = make(map[chan struct{}]*wsClient)
			opMap[*op] = cmap
		}
		cmap[wsc.quit] = wsc
	}
	// Check if any transactions spending these outputs already exists in the mempool, if so send the notification immediately.
	spends := make(map[chainhash.Hash]*btcutil.Tx)
	for _, op := range ops {
		spend := m.server.cfg.TxMemPool.CheckSpend(*op)
		if spend != nil {
			rpcsLog.Debugf("Found existing mempool spend for "+
				"outpoint<%v>: %v", op, spend.Hash())
			spends[*spend.Hash()] = spend
		}
	}
	for _, spend := range spends {
		m.notifyForTx(opMap, nil, spend, nil)
	}
}

// UnregisterSpentRequest removes a request from the passed websocket client to be notified when the passed outpoint is confirmed spent (contained in a block connected to the main chain).
func (m *wsNotificationManager) UnregisterSpentRequest(wsc *wsClient, op *wire.OutPoint) {
	m.queueNotification <- &notificationUnregisterSpent{
		wsc: wsc,
		op:  op,
	}
}

// removeSpentRequest modifies a map of watched outpoints to remove the websocket client wsc from the set of clients to be notified when a watched outpoint is spent.  If wsc is the last client, the outpoint key is removed from the map.
func (*wsNotificationManager) removeSpentRequest(ops map[wire.OutPoint]map[chan struct{}]*wsClient,
	wsc *wsClient, op *wire.OutPoint) {
	// Remove the request tracking from the client.
	delete(wsc.spentRequests, *op)
	// Remove the client from the list to notify.
	notifyMap, ok := ops[*op]
	if !ok {
		rpcsLog.Warnf("Attempt to remove nonexistent spent request "+
			"for websocket client %s", wsc.addr)
		return
	}
	delete(notifyMap, wsc.quit)
	// Remove the map entry altogether if there are no more clients interested in it.
	if len(notifyMap) == 0 {
		delete(ops, *op)
	}
}

// txHexString returns the serialized transaction encoded in hexadecimal.
func txHexString(tx *wire.MsgTx) string {
	buf := bytes.NewBuffer(make([]byte, 0, tx.SerializeSize()))
	// Ignore Serialize's error, as writing to a bytes.buffer cannot fail.
	tx.Serialize(buf)
	return hex.EncodeToString(buf.Bytes())
}

// blockDetails creates a BlockDetails struct to include in btcws notifications from a block and a transaction's block index.
func blockDetails(block *btcutil.Block, txIndex int) *btcjson.BlockDetails {
	if block == nil {
		return nil
	}
	return &btcjson.BlockDetails{
		Height: block.Height(),
		Hash:   block.Hash().String(),
		Index:  txIndex,
		Time:   block.MsgBlock().Header.Timestamp.Unix(),
	}
}

// newRedeemingTxNotification returns a new marshalled redeemingtx notification with the passed parameters.
func newRedeemingTxNotification(txHex string, index int, block *btcutil.Block) ([]byte, error) {
	// Create and marshal the notification.
	ntfn := btcjson.NewRedeemingTxNtfn(txHex, blockDetails(block, index))
	return btcjson.MarshalCmd(nil, ntfn)
}

// notifyForTxOuts examines each transaction output, notifying interested websocket clients of the transaction if an output spends to a watched address.  A spent notification request is automatically registered for the client for each matching output.
func (m *wsNotificationManager) notifyForTxOuts(ops map[wire.OutPoint]map[chan struct{}]*wsClient,
	addrs map[string]map[chan struct{}]*wsClient, tx *btcutil.Tx, block *btcutil.Block) {
	// Nothing to do if nobody is listening for address notifications.
	if len(addrs) == 0 {
		return
	}
	txHex := ""
	wscNotified := make(map[chan struct{}]struct{})
	for i, txOut := range tx.MsgTx().TxOut {
		_, txAddrs, _, err := txscript.ExtractPkScriptAddrs(
			txOut.PkScript, m.server.cfg.ChainParams)
		if err != nil {
			continue
		}
		for _, txAddr := range txAddrs {
			cmap, ok := addrs[txAddr.EncodeAddress()]
			if !ok {
				continue
			}
			if txHex == "" {
				txHex = txHexString(tx.MsgTx())
			}
			ntfn := btcjson.NewRecvTxNtfn(txHex, blockDetails(block,
				tx.Index()))
			marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal processedtx notification: %v", err)
				continue
			}
			op := []*wire.OutPoint{wire.NewOutPoint(tx.Hash(), uint32(i))}
			for wscQuit, wsc := range cmap {
				m.addSpentRequests(ops, wsc, op)
				if _, ok := wscNotified[wscQuit]; !ok {
					wscNotified[wscQuit] = struct{}{}
					wsc.QueueNotification(marshalledJSON)
				}
			}
		}
	}
}

// notifyRelevantTxAccepted examines the inputs and outputs of the passed transaction, notifying websocket clients of outputs spending to a watched address and inputs spending a watched outpoint.  Any outputs paying to a watched address result in the output being watched as well for future notifications.
func (m *wsNotificationManager) notifyRelevantTxAccepted(tx *btcutil.Tx,
	clients map[chan struct{}]*wsClient) {
	clientsToNotify := m.subscribedClients(tx, clients)
	if len(clientsToNotify) != 0 {
		n := btcjson.NewRelevantTxAcceptedNtfn(txHexString(tx.MsgTx()))
		marshalled, err := btcjson.MarshalCmd(nil, n)
		if err != nil {
			rpcsLog.Errorf("Failed to marshal notification: %v", err)
			return
		}
		for quitChan := range clientsToNotify {
			clients[quitChan].QueueNotification(marshalled)
		}
	}
}

// notifyForTx examines the inputs and outputs of the passed transaction, notifying websocket clients of outputs spending to a watched address and inputs spending a watched outpoint.
func (m *wsNotificationManager) notifyForTx(ops map[wire.OutPoint]map[chan struct{}]*wsClient,
	addrs map[string]map[chan struct{}]*wsClient, tx *btcutil.Tx, block *btcutil.Block) {
	if len(ops) != 0 {
		m.notifyForTxIns(ops, tx, block)
	}
	if len(addrs) != 0 {
		m.notifyForTxOuts(ops, addrs, tx, block)
	}
}

// notifyForTxIns examines the inputs of the passed transaction and sends interested websocket clients a redeemingtx notification if any inputs spend a watched output.  If block is non-nil, any matching spent requests are removed.
func (m *wsNotificationManager) notifyForTxIns(ops map[wire.OutPoint]map[chan struct{}]*wsClient,
	tx *btcutil.Tx, block *btcutil.Block) {
	// Nothing to do if nobody is watching outpoints.
	if len(ops) == 0 {
		return
	}
	txHex := ""
	wscNotified := make(map[chan struct{}]struct{})
	for _, txIn := range tx.MsgTx().TxIn {
		prevOut := &txIn.PreviousOutPoint
		if cmap, ok := ops[*prevOut]; ok {
			if txHex == "" {
				txHex = txHexString(tx.MsgTx())
			}
			marshalledJSON, err := newRedeemingTxNotification(txHex, tx.Index(), block)
			if err != nil {
				rpcsLog.Warnf("Failed to marshal redeemingtx notification: %v", err)
				continue
			}
			for wscQuit, wsc := range cmap {
				if block != nil {
					m.removeSpentRequest(ops, wsc, prevOut)
				}
				if _, ok := wscNotified[wscQuit]; !ok {
					wscNotified[wscQuit] = struct{}{}
					wsc.QueueNotification(marshalledJSON)
				}
			}
		}
	}
}

// RegisterTxOutAddressRequests requests notifications to the passed websocket client when a transaction output spends to the passed address.
func (m *wsNotificationManager) RegisterTxOutAddressRequests(wsc *wsClient, addrs []string) {
	m.queueNotification <- &notificationRegisterAddr{
		wsc:   wsc,
		addrs: addrs,
	}
}

// addAddrRequests adds the websocket client wsc to the address to client set addrMap so wsc will be notified for any mempool or block transaction outputs spending to any of the addresses in addrs.
func (*wsNotificationManager) addAddrRequests(addrMap map[string]map[chan struct{}]*wsClient,
	wsc *wsClient, addrs []string) {
	for _, addr := range addrs {
		// Track the request in the client as well so it can be quickly be removed on disconnect.
		wsc.addrRequests[addr] = struct{}{}
		// Add the client to the set of clients to notify when the outpoint is seen.  Create map as needed.
		cmap, ok := addrMap[addr]
		if !ok {
			cmap = make(map[chan struct{}]*wsClient)
			addrMap[addr] = cmap
		}
		cmap[wsc.quit] = wsc
	}
}

// UnregisterTxOutAddressRequest removes a request from the passed websocket client to be notified when a transaction spends to the passed address.
func (m *wsNotificationManager) UnregisterTxOutAddressRequest(wsc *wsClient, addr string) {
	m.queueNotification <- &notificationUnregisterAddr{
		wsc:  wsc,
		addr: addr,
	}
}

// removeAddrRequest removes the websocket client wsc from the address to client set addrs so it will no longer receive notification updates for any transaction outputs send to addr.
func (*wsNotificationManager) removeAddrRequest(addrs map[string]map[chan struct{}]*wsClient,
	wsc *wsClient, addr string) {
	// Remove the request tracking from the client.
	delete(wsc.addrRequests, addr)
	// Remove the client from the list to notify.
	cmap, ok := addrs[addr]
	if !ok {
		rpcsLog.Warnf("Attempt to remove nonexistent addr request "+
			"<%s> for websocket client %s", addr, wsc.addr)
		return
	}
	delete(cmap, wsc.quit)
	// Remove the map entry altogether if there are no more clients interested in it.
	if len(cmap) == 0 {
		delete(addrs, addr)
	}
}

// AddClient adds the passed websocket client to the notification manager.
func (m *wsNotificationManager) AddClient(wsc *wsClient) {
	m.queueNotification <- (*notificationRegisterClient)(wsc)
}

// RemoveClient removes the passed websocket client and all notifications registered for it.
func (m *wsNotificationManager) RemoveClient(wsc *wsClient) {
	select {
	case m.queueNotification <- (*notificationUnregisterClient)(wsc):
	case <-m.quit:
	}
}

// Start starts the goroutines required for the manager to queue and process websocket client notifications.
func (m *wsNotificationManager) Start() {
	m.wg.Add(2)
	go m.queueHandler()
	go m.notificationHandler()
}

// WaitForShutdown blocks until all notification manager goroutines have finished.
func (m *wsNotificationManager) WaitForShutdown() {
	m.wg.Wait()
}

// Shutdown shuts down the manager, stopping the notification queue and notification handler goroutines.
func (m *wsNotificationManager) Shutdown() {
	close(m.quit)
}

// newWsNotificationManager returns a new notification manager ready for use. See wsNotificationManager for more details.
func newWsNotificationManager(server *rpcServer) *wsNotificationManager {
	return &wsNotificationManager{
		server:            server,
		queueNotification: make(chan interface{}),
		notificationMsgs:  make(chan interface{}),
		numClients:        make(chan int),
		quit:              make(chan struct{}),
	}
}

// wsResponse houses a message to send to a connected websocket client as well as a channel to reply on when the message is sent.
type wsResponse struct {
	msg      []byte
	doneChan chan bool
}

// wsClient provides an abstraction for handling a websocket client.  The overall data flow is split into 3 main goroutines, a possible 4th goroutine for long-running operations (only started if request is made), and a websocket manager which is used to allow things such as broadcasting requested notifications to all connected websocket clients.   Inbound messages are read via the inHandler goroutine and generally dispatched to their own handler.  However, certain potentially long-running operations such as rescans, are sent to the asyncHander goroutine and are limited to one at a time.  There are two outbound message types - one for responding to client requests and another for async notifications.  Responses to client requests use SendMessage which employs a buffered channel thereby limiting the number of outstanding requests that can be made.  Notifications are sent via QueueNotification which implements a queue via notificationQueueHandler to ensure sending notifications from other subsystems can't block.  Ultimately, all messages are sent via the outHandler.
type wsClient struct {
	sync.Mutex
	// server is the RPC server that is servicing the client.
	server *rpcServer
	// conn is the underlying websocket connection.
	conn *websocket.Conn
	// disconnected indicated whether or not the websocket client is disconnected.
	disconnected bool
	// addr is the remote address of the client.
	addr string
	// authenticated specifies whether a client has been authenticated and therefore is allowed to communicated over the websocket.
	authenticated bool
	// isAdmin specifies whether a client may change the state of the server; false means its access is only to the limited set of RPC calls.
	isAdmin bool
	// sessionID is a random ID generated for each client when connected. These IDs may be queried by a client using the session RPC.  A change to the session ID indicates that the client reconnected.
	sessionID uint64
	// verboseTxUpdates specifies whether a client has requested verbose information about all new transactions.
	verboseTxUpdates bool
	// addrRequests is a set of addresses the caller has requested to be notified about.  It is maintained here so all requests can be removed when a wallet disconnects.  Owned by the notification manager.
	addrRequests map[string]struct{}
	// spentRequests is a set of unspent Outpoints a wallet has requested notifications for when they are spent by a processed transaction. Owned by the notification manager.
	spentRequests map[wire.OutPoint]struct{}
	// filterData is the new generation transaction filter backported from github.com/decred/dcrd for the new backported `loadtxfilter` and `rescanblocks` methods.
	filterData *wsClientFilter
	// Networking infrastructure.
	serviceRequestSem semaphore
	ntfnChan          chan []byte
	sendChan          chan wsResponse
	quit              chan struct{}
	wg                sync.WaitGroup
}

// inHandler handles all incoming messages for the websocket connection.  It must be run as a goroutine.
func (c *wsClient) inHandler() {
out:
	for {
		// Break out of the loop once the quit channel has been closed. Use a non-blocking select here so we fall through otherwise.
		select {
		case <-c.quit:
			break out
		default:
		}
		_, msg, err := c.conn.ReadMessage()
		if err != nil {
			// Log the error if it's not due to disconnecting.
			if err != io.EOF {
				rpcsLog.Errorf("Websocket receive error from "+
					"%s: %v", c.addr, err)
			}
			break out
		}
		var request btcjson.Request
		err = json.Unmarshal(msg, &request)
		if err != nil {
			if !c.authenticated {
				break out
			}
			jsonErr := &btcjson.RPCError{
				Code:    btcjson.ErrRPCParse.Code,
				Message: "Failed to parse request: " + err.Error(),
			}
			reply, err := createMarshalledReply(nil, nil, jsonErr)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal parse failure "+
					"reply: %v", err)
				continue
			}
			c.SendMessage(reply, nil)
			continue
		}
		// The JSON-RPC 1.0 spec defines that notifications must have their "id" set to null and states that notifications do not have a response.
		// A JSON-RPC 2.0 notification is a request with "json-rpc":"2.0", and without an "id" member. The specification states that notifications must not be responded to. JSON-RPC 2.0 permits the null value as a valid request id, therefore such requests are not notifications.
		// Bitcoin Core serves requests with "id":null or even an absent "id", and responds to such requests with "id":null in the response.
		// Pod does not respond to any request without and "id" or "id":null, regardless the indicated JSON-RPC protocol version unless RPC quirks are enabled. With RPC quirks enabled, such requests will be responded to if the reqeust does not indicate JSON-RPC version.
		// RPC quirks can be enabled by the user to avoid compatibility issues with software relying on Core's behavior.
		if request.ID == nil && !(cfg.RPCQuirks && request.Jsonrpc == "") {
			if !c.authenticated {
				break out
			}
			continue
		}
		cmd := parseCmd(&request)
		if cmd.err != nil {
			if !c.authenticated {
				break out
			}
			reply, err := createMarshalledReply(cmd.id, nil, cmd.err)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal parse failure "+
					"reply: %v", err)
				continue
			}
			c.SendMessage(reply, nil)
			continue
		}
		rpcsLog.Debugf("Received command <%s> from %s", cmd.method, c.addr)
		// Check auth.  The client is immediately disconnected if the first request of an unauthentiated websocket client is not the authenticate request, an authenticate request is received when the client is already authenticated, or incorrect authentication credentials are provided in the request.
		switch authCmd, ok := cmd.cmd.(*btcjson.AuthenticateCmd); {
		case c.authenticated && ok:
			rpcsLog.Warnf("Websocket client %s is already authenticated",
				c.addr)
			break out
		case !c.authenticated && !ok:
			rpcsLog.Warnf("Unauthenticated websocket message " +
				"received")
			break out
		case !c.authenticated:
			// Check credentials.
			login := authCmd.Username + ":" + authCmd.Passphrase
			auth := "Basic " + base64.StdEncoding.EncodeToString([]byte(login))
			authSha := sha256.Sum256([]byte(auth))
			cmp := subtle.ConstantTimeCompare(authSha[:], c.server.authsha[:])
			limitcmp := subtle.ConstantTimeCompare(authSha[:], c.server.limitauthsha[:])
			if cmp != 1 && limitcmp != 1 {
				rpcsLog.Warnf("Auth failure.")
				break out
			}
			c.authenticated = true
			c.isAdmin = cmp == 1
			// Marshal and send response.
			reply, err := createMarshalledReply(cmd.id, nil, nil)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal authenticate reply: "+
					"%v", err.Error())
				continue
			}
			c.SendMessage(reply, nil)
			continue
		}
		// Check if the client is using limited RPC credentials and error when not authorized to call this RPC.
		if !c.isAdmin {
			if _, ok := rpcLimited[request.Method]; !ok {
				jsonErr := &btcjson.RPCError{
					Code:    btcjson.ErrRPCInvalidParams.Code,
					Message: "limited user not authorized for this method",
				}
				// Marshal and send response.
				reply, err := createMarshalledReply(request.ID, nil, jsonErr)
				if err != nil {
					rpcsLog.Errorf("Failed to marshal parse failure "+
						"reply: %v", err)
					continue
				}
				c.SendMessage(reply, nil)
				continue
			}
		}
		// Asynchronously handle the request.  A semaphore is used to limit the number of concurrent requests currently being serviced.  If the semaphore can not be acquired, simply wait until a request finished before reading the next RPC request from the websocket client.
		// This could be a little fancier by timing out and erroring when it takes too long to service the request, but if that is done, the read of the next request should not be blocked by this semaphore, otherwise the next request will be read and will probably sit here for another few seconds before timing out as well.  This will cause the total timeout duration for later requests to be much longer than the check here would imply.
		// If a timeout is added, the semaphore acquiring should be moved inside of the new goroutine with a select statement that also reads a time.After channel.  This will unblock the read of the next request from the websocket client and allow many requests to be waited on concurrently.
		c.serviceRequestSem.acquire()
		go func() {
			c.serviceRequest(cmd)
			c.serviceRequestSem.release()
		}()
	}
	// Ensure the connection is closed.
	c.Disconnect()
	c.wg.Done()
	rpcsLog.Tracef("Websocket client input handler done for %s", c.addr)
}

// serviceRequest services a parsed RPC request by looking up and executing the appropriate RPC handler.  The response is marshalled and sent to the websocket client.
func (c *wsClient) serviceRequest(r *parsedRPCCmd) {
	var (
		result interface{}
		err    error
	)
	// Lookup the websocket extension for the command and if it doesn't exist fallback to handling the command as a standard command.
	wsHandler, ok := wsHandlers[r.method]
	if ok {
		result, err = wsHandler(c, r.cmd)
	} else {
		result, err = c.server.standardCmdResult(r, nil)
	}
	reply, err := createMarshalledReply(r.id, result, err)
	if err != nil {
		rpcsLog.Errorf("Failed to marshal reply for <%s> "+
			"command: %v", r.method, err)
		return
	}
	c.SendMessage(reply, nil)
}

// notificationQueueHandler handles the queuing of outgoing notifications for the websocket client.  This runs as a muxer for various sources of input to ensure that queuing up notifications to be sent will not block.  Otherwise, slow clients could bog down the other systems (such as the mempool or block manager) which are queuing the data.  The data is passed on to outHandler to actually be written.  It must be run as a goroutine.
func (c *wsClient) notificationQueueHandler() {
	ntfnSentChan := make(chan bool, 1) // nonblocking sync
	// pendingNtfns is used as a queue for notifications that are ready to be sent once there are no outstanding notifications currently being sent.  The waiting flag is used over simply checking for items in the pending list to ensure cleanup knows what has and hasn't been sent to the outHandler.  Currently no special cleanup is needed, however if something like a done channel is added to notifications in the future, not knowing what has and hasn't been sent to the outHandler (and thus who should respond to the done channel) would be problematic without using this approach.
	pendingNtfns := list.New()
	waiting := false
out:
	for {
		select {
		// This channel is notified when a message is being queued to be sent across the network socket.  It will either send the message immediately if a send is not already in progress, or queue the message to be sent once the other pending messages are sent.
		case msg := <-c.ntfnChan:
			if !waiting {
				c.SendMessage(msg, ntfnSentChan)
			} else {
				pendingNtfns.PushBack(msg)
			}
			waiting = true
		// This channel is notified when a notification has been sent across the network socket.
		case <-ntfnSentChan:
			// No longer waiting if there are no more messages in the pending messages queue.
			next := pendingNtfns.Front()
			if next == nil {
				waiting = false
				continue
			}
			// Notify the outHandler about the next item to asynchronously send.
			msg := pendingNtfns.Remove(next).([]byte)
			c.SendMessage(msg, ntfnSentChan)
		case <-c.quit:
			break out
		}
	}
	// Drain any wait channels before exiting so nothing is left waiting around to send.
cleanup:
	for {
		select {
		case <-c.ntfnChan:
		case <-ntfnSentChan:
		default:
			break cleanup
		}
	}
	c.wg.Done()
	rpcsLog.Tracef("Websocket client notification queue handler done "+
		"for %s", c.addr)
}

// outHandler handles all outgoing messages for the websocket connection.  It must be run as a goroutine.  It uses a buffered channel to serialize output messages while allowing the sender to continue running asynchronously.  It must be run as a goroutine.
func (c *wsClient) outHandler() {
out:
	for {
		// Send any messages ready for send until the quit channel is closed.
		select {
		case r := <-c.sendChan:
			err := c.conn.WriteMessage(websocket.TextMessage, r.msg)
			if err != nil {
				c.Disconnect()
				break out
			}
			if r.doneChan != nil {
				r.doneChan <- true
			}
		case <-c.quit:
			break out
		}
	}
	// Drain any wait channels before exiting so nothing is left waiting around to send.
cleanup:
	for {
		select {
		case r := <-c.sendChan:
			if r.doneChan != nil {
				r.doneChan <- false
			}
		default:
			break cleanup
		}
	}
	c.wg.Done()
	rpcsLog.Tracef("Websocket client output handler done for %s", c.addr)
}

// SendMessage sends the passed json to the websocket client.  It is backed by a buffered channel, so it will not block until the send channel is full. Note however that QueueNotification must be used for sending async notifications instead of the this function.  This approach allows a limit to the number of outstanding requests a client can make without preventing or blocking on async notifications.
func (c *wsClient) SendMessage(marshalledJSON []byte, doneChan chan bool) {
	// Don't send the message if disconnected.
	if c.Disconnected() {
		if doneChan != nil {
			doneChan <- false
		}
		return
	}
	c.sendChan <- wsResponse{msg: marshalledJSON, doneChan: doneChan}
}

// ErrClientQuit describes the error where a client send is not processed due to the client having already been disconnected or dropped.
var ErrClientQuit = errors.New("client quit")

// QueueNotification queues the passed notification to be sent to the websocket client.  This function, as the name implies, is only intended for notifications since it has additional logic to prevent other subsystems, such as the memory pool and block manager, from blocking even when the send channel is full.
// If the client is in the process of shutting down, this function returns ErrClientQuit.  This is intended to be checked by long-running notification handlers to stop processing if there is no more work needed to be done.
func (c *wsClient) QueueNotification(marshalledJSON []byte) error {
	// Don't queue the message if disconnected.
	if c.Disconnected() {
		return ErrClientQuit
	}
	c.ntfnChan <- marshalledJSON
	return nil
}

// Disconnected returns whether or not the websocket client is disconnected.
func (c *wsClient) Disconnected() bool {
	c.Lock()
	isDisconnected := c.disconnected
	c.Unlock()
	return isDisconnected
}

// Disconnect disconnects the websocket client.
func (c *wsClient) Disconnect() {
	c.Lock()
	defer c.Unlock()
	// Nothing to do if already disconnected.
	if c.disconnected {
		return
	}
	rpcsLog.Tracef("Disconnecting websocket client %s", c.addr)
	close(c.quit)
	c.conn.Close()
	c.disconnected = true
}

// Start begins processing input and output messages.
func (c *wsClient) Start() {
	rpcsLog.Tracef("Starting websocket client %s", c.addr)
	// Start processing input and output.
	c.wg.Add(3)
	go c.inHandler()
	go c.notificationQueueHandler()
	go c.outHandler()
}

// WaitForShutdown blocks until the websocket client goroutines are stopped and the connection is closed.
func (c *wsClient) WaitForShutdown() {
	c.wg.Wait()
}

// newWebsocketClient returns a new websocket client given the notification manager, websocket connection, remote address, and whether or not the client has already been authenticated (via HTTP Basic access authentication).  The returned client is ready to start.  Once started, the client will process incoming and outgoing messages in separate goroutines complete with queuing and asynchrous handling for long-running operations.
func newWebsocketClient(server *rpcServer, conn *websocket.Conn,
	remoteAddr string, authenticated bool, isAdmin bool) (*wsClient, error) {
	sessionID, err := wire.RandomUint64()
	if err != nil {
		return nil, err
	}
	client := &wsClient{
		conn:              conn,
		addr:              remoteAddr,
		authenticated:     authenticated,
		isAdmin:           isAdmin,
		sessionID:         sessionID,
		server:            server,
		addrRequests:      make(map[string]struct{}),
		spentRequests:     make(map[wire.OutPoint]struct{}),
		serviceRequestSem: makeSemaphore(cfg.RPCMaxConcurrentReqs),
		ntfnChan:          make(chan []byte, 1), // nonblocking sync
		sendChan:          make(chan wsResponse, websocketSendBufferSize),
		quit:              make(chan struct{}),
	}
	return client, nil
}

// handleWebsocketHelp implements the help command for websocket connections.
func handleWebsocketHelp(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.HelpCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	// Provide a usage overview of all commands when no specific command was specified.
	var command string
	if cmd.Command != nil {
		command = *cmd.Command
	}
	if command == "" {
		usage, err := wsc.server.helpCacher.rpcUsage(true)
		if err != nil {
			context := "Failed to generate RPC usage"
			return nil, internalRPCError(err.Error(), context)
		}
		return usage, nil
	}
	// Check that the command asked for is supported and implemented. Search the list of websocket handlers as well as the main list of handlers since help should only be provided for those cases.
	valid := true
	if _, ok := rpcHandlers[command]; !ok {
		if _, ok := wsHandlers[command]; !ok {
			valid = false
		}
	}
	if !valid {
		return nil, &btcjson.RPCError{
			Code:    btcjson.ErrRPCInvalidParameter,
			Message: "Unknown command: " + command,
		}
	}
	// Get the help for the command.
	help, err := wsc.server.helpCacher.rpcMethodHelp(command)
	if err != nil {
		context := "Failed to generate help"
		return nil, internalRPCError(err.Error(), context)
	}
	return help, nil
}

// handleLoadTxFilter implements the loadtxfilter command extension for websocket connections. NOTE: This extension is ported from github.com/decred/dcrd
func handleLoadTxFilter(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd := icmd.(*btcjson.LoadTxFilterCmd)
	outPoints := make([]wire.OutPoint, len(cmd.OutPoints))
	for i := range cmd.OutPoints {
		hash, err := chainhash.NewHashFromStr(cmd.OutPoints[i].Hash)
		if err != nil {
			return nil, &btcjson.RPCError{
				Code:    btcjson.ErrRPCInvalidParameter,
				Message: err.Error(),
			}
		}
		outPoints[i] = wire.OutPoint{
			Hash:  *hash,
			Index: cmd.OutPoints[i].Index,
		}
	}
	params := wsc.server.cfg.ChainParams
	wsc.Lock()
	if cmd.Reload || wsc.filterData == nil {
		wsc.filterData = newWSClientFilter(cmd.Addresses, outPoints,
			params)
		wsc.Unlock()
	} else {
		wsc.Unlock()
		wsc.filterData.mu.Lock()
		for _, a := range cmd.Addresses {
			wsc.filterData.addAddressStr(a, params)
		}
		for i := range outPoints {
			wsc.filterData.addUnspentOutPoint(&outPoints[i])
		}
		wsc.filterData.mu.Unlock()
	}
	return nil, nil
}

// handleNotifyBlocks implements the notifyblocks command extension for websocket connections.
func handleNotifyBlocks(wsc *wsClient, icmd interface{}) (interface{}, error) {
	wsc.server.ntfnMgr.RegisterBlockUpdates(wsc)
	return nil, nil
}

// handleSession implements the session command extension for websocket connections.
func handleSession(wsc *wsClient, icmd interface{}) (interface{}, error) {
	return &btcjson.SessionResult{SessionID: wsc.sessionID}, nil
}

// handleStopNotifyBlocks implements the stopnotifyblocks command extension for websocket connections.
func handleStopNotifyBlocks(wsc *wsClient, icmd interface{}) (interface{}, error) {
	wsc.server.ntfnMgr.UnregisterBlockUpdates(wsc)
	return nil, nil
}

// handleNotifySpent implements the notifyspent command extension for websocket connections.
func handleNotifySpent(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.NotifySpentCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	outpoints, err := deserializeOutpoints(cmd.OutPoints)
	if err != nil {
		return nil, err
	}
	wsc.server.ntfnMgr.RegisterSpentRequests(wsc, outpoints)
	return nil, nil
}

// handleNotifyNewTransations implements the notifynewtransactions command extension for websocket connections.
func handleNotifyNewTransactions(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.NotifyNewTransactionsCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	wsc.verboseTxUpdates = cmd.Verbose != nil && *cmd.Verbose
	wsc.server.ntfnMgr.RegisterNewMempoolTxsUpdates(wsc)
	return nil, nil
}

// handleStopNotifyNewTransations implements the stopnotifynewtransactions command extension for websocket connections.
func handleStopNotifyNewTransactions(wsc *wsClient, icmd interface{}) (interface{}, error) {
	wsc.server.ntfnMgr.UnregisterNewMempoolTxsUpdates(wsc)
	return nil, nil
}

// handleNotifyReceived implements the notifyreceived command extension for websocket connections.
func handleNotifyReceived(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.NotifyReceivedCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	// Decode addresses to validate input, but the strings slice is used directly if these are all ok.
	err := checkAddressValidity(cmd.Addresses, wsc.server.cfg.ChainParams)
	if err != nil {
		return nil, err
	}
	wsc.server.ntfnMgr.RegisterTxOutAddressRequests(wsc, cmd.Addresses)
	return nil, nil
}

// handleStopNotifySpent implements the stopnotifyspent command extension for websocket connections.
func handleStopNotifySpent(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.StopNotifySpentCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	outpoints, err := deserializeOutpoints(cmd.OutPoints)
	if err != nil {
		return nil, err
	}
	for _, outpoint := range outpoints {
		wsc.server.ntfnMgr.UnregisterSpentRequest(wsc, outpoint)
	}
	return nil, nil
}

// handleStopNotifyReceived implements the stopnotifyreceived command extension for websocket connections.
func handleStopNotifyReceived(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.StopNotifyReceivedCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	// Decode addresses to validate input, but the strings slice is used directly if these are all ok.
	err := checkAddressValidity(cmd.Addresses, wsc.server.cfg.ChainParams)
	if err != nil {
		return nil, err
	}
	for _, addr := range cmd.Addresses {
		wsc.server.ntfnMgr.UnregisterTxOutAddressRequest(wsc, addr)
	}
	return nil, nil
}

// checkAddressValidity checks the validity of each address in the passed string slice. It does this by attempting to decode each address using the current active network parameters. If any single address fails to decode properly, the function returns an error. Otherwise, nil is returned.
func checkAddressValidity(addrs []string, params *chaincfg.Params) error {
	for _, addr := range addrs {
		_, err := btcutil.DecodeAddress(addr, params)
		if err != nil {
			return &btcjson.RPCError{
				Code: btcjson.ErrRPCInvalidAddressOrKey,
				Message: fmt.Sprintf("Invalid address or key: %v",
					addr),
			}
		}
	}
	return nil
}

// deserializeOutpoints deserializes each serialized outpoint.
func deserializeOutpoints(serializedOuts []btcjson.OutPoint) ([]*wire.OutPoint, error) {
	outpoints := make([]*wire.OutPoint, 0, len(serializedOuts))
	for i := range serializedOuts {
		blockHash, err := chainhash.NewHashFromStr(serializedOuts[i].Hash)
		if err != nil {
			return nil, rpcDecodeHexError(serializedOuts[i].Hash)
		}
		index := serializedOuts[i].Index
		outpoints = append(outpoints, wire.NewOutPoint(blockHash, index))
	}
	return outpoints, nil
}

type rescanKeys struct {
	fallbacks           map[string]struct{}
	pubKeyHashes        map[[ripemd160.Size]byte]struct{}
	scriptHashes        map[[ripemd160.Size]byte]struct{}
	compressedPubKeys   map[[33]byte]struct{}
	uncompressedPubKeys map[[65]byte]struct{}
	unspent             map[wire.OutPoint]struct{}
}

// unspentSlice returns a slice of currently-unspent outpoints for the rescan lookup keys.  This is primarily intended to be used to register outpoints for continuous notifications after a rescan has completed.
func (r *rescanKeys) unspentSlice() []*wire.OutPoint {
	ops := make([]*wire.OutPoint, 0, len(r.unspent))
	for op := range r.unspent {
		opCopy := op
		ops = append(ops, &opCopy)
	}
	return ops
}

// ErrRescanReorg defines the error that is returned when an unrecoverable reorganize is detected during a rescan.
var ErrRescanReorg = btcjson.RPCError{
	Code:    btcjson.ErrRPCDatabase,
	Message: "Reorganize",
}

// rescanBlock rescans all transactions in a single block.  This is a helper function for handleRescan.
func rescanBlock(wsc *wsClient, lookups *rescanKeys, blk *btcutil.Block) {
	for _, tx := range blk.Transactions() {
		// Hexadecimal representation of this tx.  Only created if needed, and reused for later notifications if already made.
		var txHex string
		// All inputs and outputs must be iterated through to correctly modify the unspent map, however, just a single notification for any matching transaction inputs or outputs should be created and sent.
		spentNotified := false
		recvNotified := false
		for _, txin := range tx.MsgTx().TxIn {
			if _, ok := lookups.unspent[txin.PreviousOutPoint]; ok {
				delete(lookups.unspent, txin.PreviousOutPoint)
				if spentNotified {
					continue
				}
				if txHex == "" {
					txHex = txHexString(tx.MsgTx())
				}
				marshalledJSON, err := newRedeemingTxNotification(txHex, tx.Index(), blk)
				if err != nil {
					rpcsLog.Errorf("Failed to marshal redeemingtx notification: %v", err)
					continue
				}
				err = wsc.QueueNotification(marshalledJSON)
				// Stop the rescan early if the websocket client disconnected.
				if err == ErrClientQuit {
					return
				}
				spentNotified = true
			}
		}
		for txOutIdx, txout := range tx.MsgTx().TxOut {
			_, addrs, _, _ := txscript.ExtractPkScriptAddrs(
				txout.PkScript, wsc.server.cfg.ChainParams)
			for _, addr := range addrs {
				switch a := addr.(type) {
				case *btcutil.AddressPubKeyHash:
					if _, ok := lookups.pubKeyHashes[*a.Hash160()]; !ok {
						continue
					}
				case *btcutil.AddressScriptHash:
					if _, ok := lookups.scriptHashes[*a.Hash160()]; !ok {
						continue
					}
				case *btcutil.AddressPubKey:
					found := false
					switch sa := a.ScriptAddress(); len(sa) {
					case 33: // Compressed
						var key [33]byte
						copy(key[:], sa)
						if _, ok := lookups.compressedPubKeys[key]; ok {
							found = true
						}
					case 65: // Uncompressed
						var key [65]byte
						copy(key[:], sa)
						if _, ok := lookups.uncompressedPubKeys[key]; ok {
							found = true
						}
					default:
						rpcsLog.Warnf("Skipping rescanned pubkey of unknown "+
							"serialized length %d", len(sa))
						continue
					}
					// If the transaction output pays to the pubkey of a rescanned P2PKH address, include it as well.
					if !found {
						pkh := a.AddressPubKeyHash()
						if _, ok := lookups.pubKeyHashes[*pkh.Hash160()]; !ok {
							continue
						}
					}
				default:
					// A new address type must have been added.  Encode as a payment address string and check the fallback map.
					addrStr := addr.EncodeAddress()
					_, ok := lookups.fallbacks[addrStr]
					if !ok {
						continue
					}
				}
				outpoint := wire.OutPoint{
					Hash:  *tx.Hash(),
					Index: uint32(txOutIdx),
				}
				lookups.unspent[outpoint] = struct{}{}
				if recvNotified {
					continue
				}
				if txHex == "" {
					txHex = txHexString(tx.MsgTx())
				}
				ntfn := btcjson.NewRecvTxNtfn(txHex,
					blockDetails(blk, tx.Index()))
				marshalledJSON, err := btcjson.MarshalCmd(nil, ntfn)
				if err != nil {
					rpcsLog.Errorf("Failed to marshal recvtx notification: %v", err)
					return
				}
				err = wsc.QueueNotification(marshalledJSON)
				// Stop the rescan early if the websocket client disconnected.
				if err == ErrClientQuit {
					return
				}
				recvNotified = true
			}
		}
	}
}

// rescanBlockFilter rescans a block for any relevant transactions for the passed lookup keys. Any discovered transactions are returned hex encoded as a string slice. NOTE: This extension is ported from github.com/decred/dcrd
func rescanBlockFilter(filter *wsClientFilter, block *btcutil.Block, params *chaincfg.Params) []string {
	var transactions []string
	filter.mu.Lock()
	for _, tx := range block.Transactions() {
		msgTx := tx.MsgTx()
		// Keep track of whether the transaction has already been added to the result.  It shouldn't be added twice.
		added := false
		// Scan inputs if not a coinbase transaction.
		if !blockchain.IsCoinBaseTx(msgTx) {
			for _, input := range msgTx.TxIn {
				if !filter.existsUnspentOutPoint(&input.PreviousOutPoint) {
					continue
				}
				if !added {
					transactions = append(
						transactions,
						txHexString(msgTx))
					added = true
				}
			}
		}
		// Scan outputs.
		for i, output := range msgTx.TxOut {
			_, addrs, _, err := txscript.ExtractPkScriptAddrs(
				output.PkScript, params)
			if err != nil {
				continue
			}
			for _, a := range addrs {
				if !filter.existsAddress(a) {
					continue
				}
				op := wire.OutPoint{
					Hash:  *tx.Hash(),
					Index: uint32(i),
				}
				filter.addUnspentOutPoint(&op)
				if !added {
					transactions = append(
						transactions,
						txHexString(msgTx))
					added = true
				}
			}
		}
	}
	filter.mu.Unlock()
	return transactions
}

// handleRescanBlocks implements the rescanblocks command extension for websocket connections. NOTE: This extension is ported from github.com/decred/dcrd
func handleRescanBlocks(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.RescanBlocksCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	// Load client's transaction filter.  Must exist in order to continue.
	wsc.Lock()
	filter := wsc.filterData
	wsc.Unlock()
	if filter == nil {
		return nil, &btcjson.RPCError{
			Code:    btcjson.ErrRPCMisc,
			Message: "Transaction filter must be loaded before rescanning",
		}
	}
	blockHashes := make([]*chainhash.Hash, len(cmd.BlockHashes))
	for i := range cmd.BlockHashes {
		hash, err := chainhash.NewHashFromStr(cmd.BlockHashes[i])
		if err != nil {
			return nil, err
		}
		blockHashes[i] = hash
	}
	discoveredData := make([]btcjson.RescannedBlock, 0, len(blockHashes))
	// Iterate over each block in the request and rescan.  When a block contains relevant transactions, add it to the response.
	bc := wsc.server.cfg.Chain
	params := wsc.server.cfg.ChainParams
	var lastBlockHash *chainhash.Hash
	for i := range blockHashes {
		block, err := bc.BlockByHash(blockHashes[i])
		if err != nil {
			return nil, &btcjson.RPCError{
				Code:    btcjson.ErrRPCBlockNotFound,
				Message: "Failed to fetch block: " + err.Error(),
			}
		}
		if lastBlockHash != nil && block.MsgBlock().Header.PrevBlock != *lastBlockHash {
			return nil, &btcjson.RPCError{
				Code: btcjson.ErrRPCInvalidParameter,
				Message: fmt.Sprintf("Block %v is not a child of %v",
					blockHashes[i], lastBlockHash),
			}
		}
		lastBlockHash = blockHashes[i]
		transactions := rescanBlockFilter(filter, block, params)
		if len(transactions) != 0 {
			discoveredData = append(discoveredData, btcjson.RescannedBlock{
				Hash:         cmd.BlockHashes[i],
				Transactions: transactions,
			})
		}
	}
	return &discoveredData, nil
}

// recoverFromReorg attempts to recover from a detected reorganize during a rescan.  It fetches a new range of block shas from the database and verifies that the new range of blocks is on the same fork as a previous range of blocks.  If this condition does not hold true, the JSON-RPC error for an unrecoverable reorganize is returned.
func recoverFromReorg(chain *blockchain.BlockChain, minBlock, maxBlock int32,
	lastBlock *chainhash.Hash) ([]chainhash.Hash, error) {
	hashList, err := chain.HeightRange(minBlock, maxBlock)
	if err != nil {
		rpcsLog.Errorf("Error looking up block range: %v", err)
		return nil, &btcjson.RPCError{
			Code:    btcjson.ErrRPCDatabase,
			Message: "Database error: " + err.Error(),
		}
	}
	if lastBlock == nil || len(hashList) == 0 {
		return hashList, nil
	}
	blk, err := chain.BlockByHash(&hashList[0])
	if err != nil {
		rpcsLog.Errorf("Error looking up possibly reorged block: %v",
			err)
		return nil, &btcjson.RPCError{
			Code:    btcjson.ErrRPCDatabase,
			Message: "Database error: " + err.Error(),
		}
	}
	jsonErr := descendantBlock(lastBlock, blk)
	if jsonErr != nil {
		return nil, jsonErr
	}
	return hashList, nil
}

// descendantBlock returns the appropriate JSON-RPC error if a current block fetched during a reorganize is not a direct child of the parent block hash.
func descendantBlock(prevHash *chainhash.Hash, curBlock *btcutil.Block) error {
	curHash := &curBlock.MsgBlock().Header.PrevBlock
	if !prevHash.IsEqual(curHash) {
		rpcsLog.Errorf("Stopping rescan for reorged block %v "+
			"(replaced by block %v)", prevHash, curHash)
		return &ErrRescanReorg
	}
	return nil
}

// handleRescan implements the rescan command extension for websocket connections.
// NOTE: This does not smartly handle reorgs, and fixing requires database changes (for safe, concurrent access to full block ranges, and support for other chains than the best chain).  It will, however, detect whether a reorg removed a block that was previously processed, and result in the handler erroring.  Clients must handle this by finding a block still in the chain (perhaps from a rescanprogress notification) to resume their rescan.
func handleRescan(wsc *wsClient, icmd interface{}) (interface{}, error) {
	cmd, ok := icmd.(*btcjson.RescanCmd)
	if !ok {
		return nil, btcjson.ErrRPCInternal
	}
	outpoints := make([]*wire.OutPoint, 0, len(cmd.OutPoints))
	for i := range cmd.OutPoints {
		cmdOutpoint := &cmd.OutPoints[i]
		blockHash, err := chainhash.NewHashFromStr(cmdOutpoint.Hash)
		if err != nil {
			return nil, rpcDecodeHexError(cmdOutpoint.Hash)
		}
		outpoint := wire.NewOutPoint(blockHash, cmdOutpoint.Index)
		outpoints = append(outpoints, outpoint)
	}
	numAddrs := len(cmd.Addresses)
	if numAddrs == 1 {
		rpcsLog.Info("Beginning rescan for 1 address")
	} else {
		rpcsLog.Infof("Beginning rescan for %d addresses", numAddrs)
	}
	// Build lookup maps.
	lookups := rescanKeys{
		fallbacks:           map[string]struct{}{},
		pubKeyHashes:        map[[ripemd160.Size]byte]struct{}{},
		scriptHashes:        map[[ripemd160.Size]byte]struct{}{},
		compressedPubKeys:   map[[33]byte]struct{}{},
		uncompressedPubKeys: map[[65]byte]struct{}{},
		unspent:             map[wire.OutPoint]struct{}{},
	}
	var compressedPubkey [33]byte
	var uncompressedPubkey [65]byte
	params := wsc.server.cfg.ChainParams
	for _, addrStr := range cmd.Addresses {
		addr, err := btcutil.DecodeAddress(addrStr, params)
		if err != nil {
			jsonErr := btcjson.RPCError{
				Code: btcjson.ErrRPCInvalidAddressOrKey,
				Message: "Rescan address " + addrStr + ": " +
					err.Error(),
			}
			return nil, &jsonErr
		}
		switch a := addr.(type) {
		case *btcutil.AddressPubKeyHash:
			lookups.pubKeyHashes[*a.Hash160()] = struct{}{}
		case *btcutil.AddressScriptHash:
			lookups.scriptHashes[*a.Hash160()] = struct{}{}
		case *btcutil.AddressPubKey:
			pubkeyBytes := a.ScriptAddress()
			switch len(pubkeyBytes) {
			case 33: // Compressed
				copy(compressedPubkey[:], pubkeyBytes)
				lookups.compressedPubKeys[compressedPubkey] = struct{}{}
			case 65: // Uncompressed
				copy(uncompressedPubkey[:], pubkeyBytes)
				lookups.uncompressedPubKeys[uncompressedPubkey] = struct{}{}
			default:
				jsonErr := btcjson.RPCError{
					Code:    btcjson.ErrRPCInvalidAddressOrKey,
					Message: "Pubkey " + addrStr + " is of unknown length",
				}
				return nil, &jsonErr
			}
		default:
			// A new address type must have been added.  Use encoded payment address string as a fallback until a fast path is added.
			lookups.fallbacks[addrStr] = struct{}{}
		}
	}
	for _, outpoint := range outpoints {
		lookups.unspent[*outpoint] = struct{}{}
	}
	chain := wsc.server.cfg.Chain
	minBlockHash, err := chainhash.NewHashFromStr(cmd.BeginBlock)
	if err != nil {
		return nil, rpcDecodeHexError(cmd.BeginBlock)
	}
	minBlock, err := chain.BlockHeightByHash(minBlockHash)
	if err != nil {
		return nil, &btcjson.RPCError{
			Code:    btcjson.ErrRPCBlockNotFound,
			Message: "Error getting block: " + err.Error(),
		}
	}
	maxBlock := int32(math.MaxInt32)
	if cmd.EndBlock != nil {
		maxBlockHash, err := chainhash.NewHashFromStr(*cmd.EndBlock)
		if err != nil {
			return nil, rpcDecodeHexError(*cmd.EndBlock)
		}
		maxBlock, err = chain.BlockHeightByHash(maxBlockHash)
		if err != nil {
			return nil, &btcjson.RPCError{
				Code:    btcjson.ErrRPCBlockNotFound,
				Message: "Error getting block: " + err.Error(),
			}
		}
	}
	// lastBlock and lastBlockHash track the previously-rescanned block. They equal nil when no previous blocks have been rescanned.
	var lastBlock *btcutil.Block
	var lastBlockHash *chainhash.Hash
	// A ticker is created to wait at least 10 seconds before notifying the websocket client of the current progress completed by the rescan.
	ticker := time.NewTicker(10 * time.Second)
	defer ticker.Stop()
	// Instead of fetching all block shas at once, fetch in smaller chunks to ensure large rescans consume a limited amount of memory.
fetchRange:
	for minBlock < maxBlock {
		// Limit the max number of hashes to fetch at once to the maximum number of items allowed in a single inventory. This value could be higher since it's not creating inventory messages, but this mirrors the limiting logic used in the peer-to-peer protocol.
		maxLoopBlock := maxBlock
		if maxLoopBlock-minBlock > wire.MaxInvPerMsg {
			maxLoopBlock = minBlock + wire.MaxInvPerMsg
		}
		hashList, err := chain.HeightRange(minBlock, maxLoopBlock)
		if err != nil {
			rpcsLog.Errorf("Error looking up block range: %v", err)
			return nil, &btcjson.RPCError{
				Code:    btcjson.ErrRPCDatabase,
				Message: "Database error: " + err.Error(),
			}
		}
		if len(hashList) == 0 {
			// The rescan is finished if no blocks hashes for this range were successfully fetched and a stop block was provided.
			if maxBlock != math.MaxInt32 {
				break
			}
			// If the rescan is through the current block, set up the client to continue to receive notifications regarding all rescanned addresses and the current set of unspent outputs.
			// This is done safely by temporarily grabbing exclusive access of the block manager.  If no more blocks have been attached between this pause and the fetch above, then it is safe to register the websocket client for continuous notifications if necessary.  Otherwise, continue the fetch loop again to rescan the new blocks (or error due to an irrecoverable reorganize).
			pauseGuard := wsc.server.cfg.SyncMgr.Pause()
			best := wsc.server.cfg.Chain.BestSnapshot()
			curHash := &best.Hash
			again := true
			if lastBlockHash == nil || *lastBlockHash == *curHash {
				again = false
				n := wsc.server.ntfnMgr
				n.RegisterSpentRequests(wsc, lookups.unspentSlice())
				n.RegisterTxOutAddressRequests(wsc, cmd.Addresses)
			}
			close(pauseGuard)
			if err != nil {
				rpcsLog.Errorf("Error fetching best block "+
					"hash: %v", err)
				return nil, &btcjson.RPCError{
					Code: btcjson.ErrRPCDatabase,
					Message: "Database error: " +
						err.Error(),
				}
			}
			if again {
				continue
			}
			break
		}
	loopHashList:
		for i := range hashList {
			blk, err := chain.BlockByHash(&hashList[i])
			if err != nil {
				// Only handle reorgs if a block could not be found for the hash.
				if dbErr, ok := err.(database.Error); !ok ||
					dbErr.ErrorCode != database.ErrBlockNotFound {
					rpcsLog.Errorf("Error looking up "+
						"block: %v", err)
					return nil, &btcjson.RPCError{
						Code: btcjson.ErrRPCDatabase,
						Message: "Database error: " +
							err.Error(),
					}
				}
				// If an absolute max block was specified, don't attempt to handle the reorg.
				if maxBlock != math.MaxInt32 {
					rpcsLog.Errorf("Stopping rescan for "+
						"reorged block %v",
						cmd.EndBlock)
					return nil, &ErrRescanReorg
				}
				// If the lookup for the previously valid block hash failed, there may have been a reorg. Fetch a new range of block hashes and verify that the previously processed block (if there was any) still exists in the database.  If it doesn't, we error.
				// A goto is used to branch executation back to before the range was evaluated, as it must be reevaluated for the new hashList.
				minBlock += int32(i)
				hashList, err = recoverFromReorg(chain,
					minBlock, maxBlock, lastBlockHash)
				if err != nil {
					return nil, err
				}
				if len(hashList) == 0 {
					break fetchRange
				}
				goto loopHashList
			}
			if i == 0 && lastBlockHash != nil {
				// Ensure the new hashList is on the same fork as the last block from the old hashList.
				jsonErr := descendantBlock(lastBlockHash, blk)
				if jsonErr != nil {
					return nil, jsonErr
				}
			}
			// A select statement is used to stop rescans if the client requesting the rescan has disconnected.
			select {
			case <-wsc.quit:
				rpcsLog.Debugf("Stopped rescan at height %v "+
					"for disconnected client", blk.Height())
				return nil, nil
			default:
				rescanBlock(wsc, &lookups, blk)
				lastBlock = blk
				lastBlockHash = blk.Hash()
			}
			// Periodically notify the client of the progress completed.  Continue with next block if no progress notification is needed yet.
			select {
			case <-ticker.C: // fallthrough
			default:
				continue
			}
			n := btcjson.NewRescanProgressNtfn(hashList[i].String(),
				blk.Height(), blk.MsgBlock().Header.Timestamp.Unix())
			mn, err := btcjson.MarshalCmd(nil, n)
			if err != nil {
				rpcsLog.Errorf("Failed to marshal rescan "+
					"progress notification: %v", err)
				continue
			}
			if err = wsc.QueueNotification(mn); err == ErrClientQuit {
				// Finished if the client disconnected.
				rpcsLog.Debugf("Stopped rescan at height %v "+
					"for disconnected client", blk.Height())
				return nil, nil
			}
		}
		minBlock += int32(len(hashList))
	}
	// Notify websocket client of the finished rescan.  Due to how pod asynchronously queues notifications to not block calling code, there is no guarantee that any of the notifications created during rescan (such as rescanprogress, recvtx and redeemingtx) will be received before the rescan RPC returns.  Therefore, another method is needed to safely inform clients that all rescan notifications have been sent.
	n := btcjson.NewRescanFinishedNtfn(lastBlockHash.String(),
		lastBlock.Height(),
		lastBlock.MsgBlock().Header.Timestamp.Unix())
	if mn, err := btcjson.MarshalCmd(nil, n); err != nil {
		rpcsLog.Errorf("Failed to marshal rescan finished "+
			"notification: %v", err)
	} else {
		// The rescan is finished, so we don't care whether the client has disconnected at this point, so discard error.
		_ = wsc.QueueNotification(mn)
	}
	rpcsLog.Info("Finished rescan")
	return nil, nil
}
func init() {
	wsHandlers = wsHandlersBeforeInit
}