/*
 * Copyright (c) 2013 Conformal Systems LLC <info@conformal.com>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

package main

import (
	"code.google.com/p/go.net/websocket"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/sha256"
	_ "crypto/sha512" // for cert generation
	"crypto/subtle"
	"crypto/tls"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/base64"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"github.com/conformal/btcjson"
	"github.com/conformal/btcwallet/wallet"
	"github.com/conformal/btcwire"
	"github.com/conformal/btcws"
	"github.com/conformal/go-socks"
	"math/big"
	"net"
	"net/http"
	"os"
	"runtime"
	"sync"
	"time"
)

var (
	// ErrConnRefused represents an error where a connection to another
	// process cannot be established.
	ErrConnRefused = errors.New("connection refused")

	// ErrConnLost represents an error where a connection to another
	// process cannot be established.
	ErrConnLost = errors.New("connection lost")

	// Channel for updates and boolean with the most recent update of
	// whether the connection to btcd is active or not.
	btcdConnected = struct {
		b bool
		c chan bool
	}{
		c: make(chan bool),
	}

	// Channel to send messages btcwallet does not understand and requests
	// from btcwallet to btcd.
	btcdMsgs = make(chan []byte)

	// Adds a frontend listener channel
	addFrontendListener = make(chan (chan []byte))

	// Removes a frontend listener channel
	deleteFrontendListener = make(chan (chan []byte))

	// Messages sent to this channel are sent to each connected frontend.
	frontendNotificationMaster = make(chan []byte, 100)

	// replyHandlers maps between a unique number (passed as part of
	// the JSON Id field) and a function to handle a reply or notification
	// from btcd.  As requests are received, this map is checked for a
	// handler function to route the reply to.  If the function returns
	// true, the handler is removed from the map.
	replyHandlers = struct {
		sync.Mutex
		m map[uint64]func(interface{}, *btcjson.Error) bool
	}{
		m: make(map[uint64]func(interface{}, *btcjson.Error) bool),
	}

	// replyRouter maps unique uint64 ids to reply channels, so btcd
	// replies can be routed to the correct frontend.
	replyRouter = struct {
		sync.Mutex
		m map[uint64]chan []byte
	}{
		m: make(map[uint64]chan []byte),
	}
)

// server holds the items the RPC server may need to access (auth,
// config, shutdown, etc.)
type server struct {
	wg        sync.WaitGroup
	listeners []net.Listener
	authsha   [sha256.Size]byte
}

// parseListeners splits the list of listen addresses passed in addrs into
// IPv4 and IPv6 slices and returns them.  This allows easy creation of the
// listeners on the correct interface "tcp4" and "tcp6".  It also properly
// detects addresses which apply to "all interfaces" and adds the address to
// both slices.
func parseListeners(addrs []string) ([]string, []string, error) {
	ipv4ListenAddrs := make([]string, 0, len(addrs)*2)
	ipv6ListenAddrs := make([]string, 0, len(addrs)*2)
	for _, addr := range addrs {
		host, _, err := net.SplitHostPort(addr)
		if err != nil {
			// Shouldn't happen due to already being normalized.
			return nil, nil, err
		}

		// Empty host or host of * on plan9 is both IPv4 and IPv6.
		if host == "" || (host == "*" && runtime.GOOS == "plan9") {
			ipv4ListenAddrs = append(ipv4ListenAddrs, addr)
			ipv6ListenAddrs = append(ipv6ListenAddrs, addr)
			continue
		}

		// Parse the IP.
		ip := net.ParseIP(host)
		if ip == nil {
			return nil, nil, fmt.Errorf("'%s' is not a valid IP "+
				"address", host)
		}

		// To4 returns nil when the IP is not an IPv4 address, so use
		// this determine the address type.
		if ip.To4() == nil {
			ipv6ListenAddrs = append(ipv6ListenAddrs, addr)
		} else {
			ipv4ListenAddrs = append(ipv4ListenAddrs, addr)
		}
	}
	return ipv4ListenAddrs, ipv6ListenAddrs, nil
}

// newServer returns a new instance of the server struct.
func newServer(listenAddrs []string) (*server, error) {
	login := cfg.Username + ":" + cfg.Password
	auth := "Basic " + base64.StdEncoding.EncodeToString([]byte(login))
	s := server{
		authsha: sha256.Sum256([]byte(auth)),
	}

	// Check for existence of cert file and key file
	if !fileExists(cfg.RPCKey) && !fileExists(cfg.RPCCert) {
		// if both files do not exist, we generate them.
		err := genKey(cfg.RPCKey, cfg.RPCCert)
		if err != nil {
			return nil, err
		}
	}
	keypair, err := tls.LoadX509KeyPair(cfg.RPCCert, cfg.RPCKey)
	if err != nil {
		return nil, err
	}

	tlsConfig := tls.Config{
		Certificates: []tls.Certificate{keypair},
	}

	ipv4ListenAddrs, ipv6ListenAddrs, err := parseListeners(listenAddrs)
	listeners := make([]net.Listener, 0,
		len(ipv6ListenAddrs)+len(ipv4ListenAddrs))
	for _, addr := range ipv4ListenAddrs {
		listener, err := tls.Listen("tcp4", addr, &tlsConfig)
		if err != nil {
			log.Warnf("RPCS: Can't listen on %s: %v", addr,
				err)
			continue
		}
		listeners = append(listeners, listener)
	}

	for _, addr := range ipv6ListenAddrs {
		listener, err := tls.Listen("tcp6", addr, &tlsConfig)
		if err != nil {
			log.Warnf("RPCS: Can't listen on %s: %v", addr,
				err)
			continue
		}
		listeners = append(listeners, listener)
	}
	if len(listeners) == 0 {
		return nil, errors.New("RPCS: No valid listen address")
	}

	s.listeners = listeners

	return &s, nil
}

// genkey generates a key/cert pair to the paths provided.
// TODO(oga) wrap errors with fmt.Errorf for more context?
func genKey(key, cert string) error {
	log.Infof("Generating TLS certificates...")
	priv, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
	if err != nil {
		return err
	}

	notBefore := time.Now()
	notAfter := notBefore.Add(10 * 365 * 24 * time.Hour)

	// end of ASN.1 time
	endOfTime := time.Date(2049, 12, 31, 23, 59, 59, 0, time.UTC)
	if notAfter.After(endOfTime) {
		notAfter = endOfTime
	}

	template := x509.Certificate{
		SerialNumber: new(big.Int).SetInt64(0),
		Subject: pkix.Name{
			Organization: []string{"btcwallet autogenerated cert"},
		},
		NotBefore: notBefore,
		NotAfter:  notAfter,

		KeyUsage:    x509.KeyUsageKeyEncipherment | x509.KeyUsageCertSign,
		ExtKeyUsage: []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
		IsCA:        true, // so can sign self.
		BasicConstraintsValid: true,
	}

	host, err := os.Hostname()
	if err != nil {
		return err
	}
	template.DNSNames = append(template.DNSNames, host, "localhost")

	needLocalhost := true
	addrs, err := net.InterfaceAddrs()
	if err != nil {
		return err
	}
	for _, a := range addrs {
		ip, _, err := net.ParseCIDR(a.String())
		if err == nil {
			if ip.String() == "127.0.0.1" {
				needLocalhost = false
			}
			template.IPAddresses = append(template.IPAddresses, ip)
		}
	}
	if needLocalhost {
		localHost := net.ParseIP("127.0.0.1")
		template.IPAddresses = append(template.IPAddresses, localHost)
	}

	derBytes, err := x509.CreateCertificate(rand.Reader, &template,
		&template, &priv.PublicKey, priv)
	if err != nil {
		fmt.Fprintf(os.Stderr, "Failed to create certificate: %v\n", err)
		os.Exit(-1)
	}

	certOut, err := os.Create(cert)
	if err != nil {
		return err
	}
	pem.Encode(certOut, &pem.Block{Type: "CERTIFICATE", Bytes: derBytes})
	certOut.Close()

	keyOut, err := os.OpenFile(key, os.O_WRONLY|os.O_CREATE|os.O_TRUNC,
		0600)
	if err != nil {
		os.Remove(cert)
		return err
	}
	keybytes, err := x509.MarshalECPrivateKey(priv)
	if err != nil {
		os.Remove(key)
		os.Remove(cert)
		return err
	}
	pem.Encode(keyOut, &pem.Block{Type: "EC PRIVATE KEY", Bytes: keybytes})
	keyOut.Close()

	log.Info("Done generating TLS certificates")

	return nil
}

// handleRPCRequest processes a JSON-RPC request from a frontend.
func (s *server) handleRPCRequest(w http.ResponseWriter, r *http.Request) {
	frontend := make(chan []byte)

	body, err := btcjson.GetRaw(r.Body)
	if err != nil {
		log.Errorf("RPCS: Error getting JSON message: %v", err)
	}

	done := make(chan struct{})
	go func() {
		if _, err := w.Write(<-frontend); err != nil {
			log.Warnf("RPCS: could not respond to RPC request: %v",
				err)
		}
		close(done)
	}()

	ProcessRequest(frontend, body, false)
	<-done
}

// frontendListenerDuplicator listens for new wallet listener channels
// and duplicates messages sent to frontendNotificationMaster to all
// connected listeners.
func frontendListenerDuplicator() {
	// frontendListeners is a map holding each currently connected frontend
	// listener as the key.  The value is ignored, as this is only used as
	// a set.
	frontendListeners := make(map[chan []byte]bool)

	// Don't want to add or delete a wallet listener while iterating
	// through each to propigate to every attached wallet.  Use a mutex to
	// prevent this.
	var mtx sync.Mutex

	// Check for listener channels to add or remove from set.
	go func() {
		for {
			select {
			case c := <-addFrontendListener:
				mtx.Lock()
				frontendListeners[c] = true
				mtx.Unlock()

				// TODO(jrick): these notifications belong somewhere better.
				// Probably want to copy AddWalletListener from btcd, and
				// place these notifications in that function.
				NotifyBtcdConnected(frontendNotificationMaster,
					btcdConnected.b)
				if bs, err := GetCurBlock(); err == nil {
					NotifyNewBlockChainHeight(c, bs)
					NotifyBalances(c)
				}

			case c := <-deleteFrontendListener:
				mtx.Lock()
				delete(frontendListeners, c)
				mtx.Unlock()
			}
		}
	}()

	// Duplicate all messages sent across frontendNotificationMaster, as
	// well as internal btcwallet notifications, to each listening wallet.
	for {
		var ntfn []byte

		select {
		case conn := <-btcdConnected.c:
			NotifyBtcdConnected(frontendNotificationMaster, conn)
			continue

		case ntfn = <-frontendNotificationMaster:
		}

		mtx.Lock()
		for c := range frontendListeners {
			c <- ntfn
		}
		mtx.Unlock()
	}
}

// NotifyBtcdConnected notifies all frontends of a new btcd connection.
func NotifyBtcdConnected(reply chan []byte, conn bool) {
	btcdConnected.b = conn

	ntfn := btcws.NewBtcdConnectedNtfn(conn)
	mntfn, _ := ntfn.MarshalJSON()
	frontendNotificationMaster <- mntfn
}

// frontendSendRecv is the handler function for websocket connections from
// a btcwallet instance.  It reads requests and sends responses to a
// frontend, as well as notififying wallets of chain updates.  There can
// possibly be many of these running, one for each currently connected
// frontend.
func frontendSendRecv(ws *websocket.Conn) {
	// Add frontend notification channel to set so this handler receives
	// updates.
	frontendNotification := make(chan []byte)
	addFrontendListener <- frontendNotification
	defer func() {
		deleteFrontendListener <- frontendNotification
	}()

	// jsonMsgs receives JSON messages from the currently connected frontend.
	jsonMsgs := make(chan []byte)

	// Receive messages from websocket and send across jsonMsgs until
	// connection is lost
	go func() {
		for {
			var m []byte
			if err := websocket.Message.Receive(ws, &m); err != nil {
				close(jsonMsgs)
				return
			}
			jsonMsgs <- m
		}
	}()

	for {
		select {
		case m, ok := <-jsonMsgs:
			if !ok {
				// frontend disconnected.
				return
			}
			// Handle request here.
			go ProcessRequest(frontendNotification, m, true)
		case ntfn, _ := <-frontendNotification:
			if err := websocket.Message.Send(ws, ntfn); err != nil {
				// Frontend disconnected.
				return
			}
		}
	}
}

// BtcdHandler listens for replies and notifications from btcd over a
// websocket and sends messages that btcwallet does not understand to
// btcd.  Unlike FrontendHandler, exactly one BtcdHandler goroutine runs.
// BtcdHandler spawns goroutines to perform these tasks, and closes the
// done channel once they are finished.
func BtcdHandler(ws *websocket.Conn, done chan struct{}) {
	// Listen for replies/notifications from btcd, and decide how to handle them.
	replies := make(chan []byte)
	go func() {
		for {
			var m []byte
			if err := websocket.Message.Receive(ws, &m); err != nil {
				log.Debugf("cannot recevie btcd message: %v", err)
				close(replies)
				return
			}
			replies <- m
		}
	}()

	go func() {
		defer close(done)
		for {
			select {
			case rply, ok := <-replies:
				if !ok {
					// btcd disconnected
					return
				}
				// Handle message here.
				go ProcessBtcdNotificationReply(rply)

			case r := <-btcdMsgs:
				if err := websocket.Message.Send(ws, r); err != nil {
					// btcd disconnected.
					log.Errorf("Unable to send message to btcd: %v", err)
					return
				}
			}
		}
	}()
}

type notificationHandler func(btcjson.Cmd, []byte)

var notificationHandlers = map[string]notificationHandler{
	btcws.BlockConnectedNtfnMethod:    NtfnBlockConnected,
	btcws.BlockDisconnectedNtfnMethod: NtfnBlockDisconnected,
	btcws.TxMinedNtfnMethod:           NtfnTxMined,
}

// ProcessBtcdNotificationReply unmarshalls the JSON notification or
// reply received from btcd and decides how to handle it.  Replies are
// routed back to the frontend who sent the message, and wallet
// notifications are processed by btcwallet, and frontend notifications
// are sent to every connected frontend.
func ProcessBtcdNotificationReply(b []byte) {
	// Idea: instead of reading btcd messages from just one websocket
	// connection, maybe use two so the same connection isn't used
	// for both notifications and responses?  Should make handling
	// must faster as unnecessary unmarshal attempts could be avoided.

	// Check for notifications first.
	if req, err := btcjson.ParseMarshaledCmd(b); err == nil {
		// btcd should not be sending Requests except for
		// notifications.  Check for a nil id.
		if req.Id() != nil {
			// Invalid response
			log.Warnf("btcd sent a non-notification JSON-RPC Request (ID: %v)",
				req.Id())
			return
		}

		// Message is a btcd notification.  Check the method and dispatch
		// correct handler, or if no handler, pass up to each wallet.
		if ntfnHandler, ok := notificationHandlers[req.Method()]; ok {
			ntfnHandler(req, b)
		} else {
			// No handler; send to all wallets.
			frontendNotificationMaster <- b
		}
		return
	}

	// b is not a Request notification, so it must be a Response.
	// Attempt to parse it as one and handle.
	var r btcjson.Reply
	if err := json.Unmarshal(b, &r); err != nil {
		log.Warn("Unable to process btcd message as notification or response")
		return
	}

	// Check for a valid ID.
	//
	// TODO(jrick): Remove this terrible ID overloading.  Each
	// passed-through request should be given a new unique ID number
	// (reading from the NewJSONID channel) and a reply route with the
	// frontend's incoming ID should be set.
	if r.Id == nil {
		// Responses with no IDs cannot be handled.
		log.Warn("Unable to process btcd response without ID")
		return
	}
	idStr, ok := (*r.Id).(string)
	if !ok {
		// btcd's responses to btcwallet should (currently, see TODO above)
		// only ever be sending string IDs.  If ID is not a string, log the
		// error and drop the message.
		log.Error("Incorrect btcd notification id type.")
		return
	}

	var routeID uint64
	var origID string
	n, _ := fmt.Sscanf(idStr, "btcwallet(%d)-%s", &routeID, &origID)
	if n == 1 {
		// Request originated from btcwallet. Run and remove correct
		// handler.
		replyHandlers.Lock()
		f := replyHandlers.m[routeID]
		replyHandlers.Unlock()
		if f != nil {
			go func() {
				if f(r.Result, r.Error) {
					replyHandlers.Lock()
					delete(replyHandlers.m, routeID)
					replyHandlers.Unlock()
				}
			}()
		}
	} else if n == 2 {
		// Attempt to route btcd reply to correct frontend.
		replyRouter.Lock()
		c := replyRouter.m[routeID]
		if c != nil {
			delete(replyRouter.m, routeID)
		} else {
			// Can't route to a frontend, drop reply.
			replyRouter.Unlock()
			log.Info("Unable to route btcd reply to frontend. Dropping.")
			return
		}
		replyRouter.Unlock()

		// Convert string back to number if possible.
		var origIDNum float64
		n, _ := fmt.Sscanf(origID, "%f", &origIDNum)
		var id interface{}
		if n == 1 {
			id = origIDNum
		} else {
			id = origID
		}
		r.Id = &id

		b, err := json.Marshal(r)
		if err != nil {
			log.Error("Error marshalling btcd reply. Dropping.")
			return
		}
		c <- b
	}
}

// NotifyNewBlockChainHeight notifies all frontends of a new
// blockchain height.  This sends the same notification as
// btcd, so this can probably be removed.
func NotifyNewBlockChainHeight(reply chan []byte, bs wallet.BlockStamp) {
	ntfn := btcws.NewBlockConnectedNtfn(bs.Hash.String(), bs.Height)
	mntfn, _ := ntfn.MarshalJSON()
	reply <- mntfn
}

// NtfnBlockConnected handles btcd notifications resulting from newly
// connected blocks to the main blockchain.
//
// TODO(jrick): Send block time with notification.  This will be used
// to mark wallet files with a possibly-better earliest block height,
// and will greatly reduce rescan times for wallets created with an
// out of sync btcd.
func NtfnBlockConnected(n btcjson.Cmd, marshaled []byte) {
	bcn, ok := n.(*btcws.BlockConnectedNtfn)
	if !ok {
		log.Errorf("%v handler: unexpected type", n.Method())
		return
	}
	hash, err := btcwire.NewShaHashFromStr(bcn.Hash)
	if err != nil {
		log.Errorf("%v handler: invalid hash string", n.Method())
		return
	}

	// Update the blockstamp for the newly-connected block.
	bs := &wallet.BlockStamp{
		Height: bcn.Height,
		Hash:   *hash,
	}
	curBlock.Lock()
	curBlock.BlockStamp = *bs
	curBlock.Unlock()

	// btcd notifies btcwallet about transactions first, and then sends
	// the new block notification.  New balance notifications for txs
	// in blocks are therefore sent here after all tx notifications
	// have arrived and finished being processed by the handlers.
	workers := NotifyBalanceRequest{
		block: *hash,
		wg:    make(chan *sync.WaitGroup),
	}
	NotifyBalanceSyncerChans.access <- workers
	if wg := <-workers.wg; wg != nil {
		wg.Wait()
		NotifyBalanceSyncerChans.remove <- *hash
	}
	accountstore.BlockNotify(bs)

	// Pass notification to frontends too.
	frontendNotificationMaster <- marshaled
}

// NtfnBlockDisconnected handles btcd notifications resulting from
// blocks disconnected from the main chain in the event of a chain
// switch and notifies frontends of the new blockchain height.
func NtfnBlockDisconnected(n btcjson.Cmd, marshaled []byte) {
	bdn, ok := n.(*btcws.BlockDisconnectedNtfn)
	if !ok {
		log.Errorf("%v handler: unexpected type", n.Method())
		return
	}
	hash, err := btcwire.NewShaHashFromStr(bdn.Hash)
	if err != nil {
		log.Errorf("%v handler: invalid hash string", n.Method())
		return
	}

	// Rollback Utxo and Tx data stores.
	go func() {
		accountstore.Rollback(bdn.Height, hash)
	}()

	// Pass notification to frontends too.
	frontendNotificationMaster <- marshaled
}

// NtfnTxMined handles btcd notifications resulting from newly
// mined transactions that originated from this wallet.
func NtfnTxMined(n btcjson.Cmd, marshaled []byte) {
	tmn, ok := n.(*btcws.TxMinedNtfn)
	if !ok {
		log.Errorf("%v handler: unexpected type", n.Method())
		return
	}

	txid, err := btcwire.NewShaHashFromStr(tmn.TxID)
	if err != nil {
		log.Errorf("%v handler: invalid hash string", n.Method())
		return
	}
	blockhash, err := btcwire.NewShaHashFromStr(tmn.BlockHash)
	if err != nil {
		log.Errorf("%v handler: invalid block hash string", n.Method())
		return
	}

	err = accountstore.RecordMinedTx(txid, blockhash,
		tmn.BlockHeight, tmn.Index, tmn.BlockTime)
	if err != nil {
		log.Errorf("%v handler: %v", n.Method(), err)
		return
	}

	// Remove mined transaction from pool.
	UnminedTxs.Lock()
	delete(UnminedTxs.m, TXID(*txid))
	UnminedTxs.Unlock()
}

var duplicateOnce sync.Once

// Start starts a HTTP server to provide standard RPC and extension
// websocket connections for any number of btcwallet frontends.
func (s *server) Start() {
	// We'll need to duplicate replies to frontends to each frontend.
	// Replies are sent to frontendReplyMaster, and duplicated to each valid
	// channel in frontendReplySet.  This runs a goroutine to duplicate
	// requests for each channel in the set.
	//
	// Use a sync.Once to insure no extra duplicators run.
	go duplicateOnce.Do(frontendListenerDuplicator)

	log.Trace("Starting RPC server")

	serveMux := http.NewServeMux()
	httpServer := &http.Server{Handler: serveMux}
	serveMux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		if err := s.checkAuth(r); err != nil {
			http.Error(w, "401 Unauthorized.", http.StatusUnauthorized)
			return
		}
		s.handleRPCRequest(w, r)
	})
	serveMux.HandleFunc("/frontend", func(w http.ResponseWriter, r *http.Request) {
		if err := s.checkAuth(r); err != nil {
			http.Error(w, "401 Unauthorized.", http.StatusUnauthorized)
			return
		}
		websocket.Handler(frontendSendRecv).ServeHTTP(w, r)
	})
	for _, listener := range s.listeners {
		s.wg.Add(1)
		go func(listener net.Listener) {
			log.Infof("RPCS: RPC server listening on %s", listener.Addr())
			httpServer.Serve(listener)
			log.Tracef("RPCS: RPC listener done for %s", listener.Addr())
			s.wg.Done()
		}(listener)
	}
}

// checkAuth checks the HTTP Basic authentication supplied by a frontend
// in the HTTP request r.  If the frontend's supplied authentication does
// not match the username and password expected, a non-nil error is
// returned.
//
// This check is time-constant.
func (s *server) checkAuth(r *http.Request) error {
	authhdr := r.Header["Authorization"]
	if len(authhdr) <= 0 {
		log.Infof("Frontend did not supply authentication.")
		return errors.New("auth failure")
	}

	authsha := sha256.Sum256([]byte(authhdr[0]))
	cmp := subtle.ConstantTimeCompare(authsha[:], s.authsha[:])
	if cmp != 1 {
		log.Infof("Frontend did not supply correct authentication.")
		return errors.New("auth failure")
	}
	return nil
}

// BtcdConnect connects to a running btcd instance over a websocket
// for sending and receiving chain-related messages, failing if the
// connection cannot be established or is lost.
func BtcdConnect(certificates []byte, reply chan error) {
	url := fmt.Sprintf("wss://%s/wallet", cfg.Connect)
	config, err := websocket.NewConfig(url, "https://localhost/")
	if err != nil {
		reply <- ErrConnRefused
		return
	}

	pool := x509.NewCertPool()
	pool.AppendCertsFromPEM(certificates)
	config.TlsConfig = &tls.Config{
		RootCAs:    pool,
		MinVersion: tls.VersionTLS12,
	}

	// btcd requires basic authorization, so we use a custom config with
	// the Authorization header set.
	login := cfg.Username + ":" + cfg.Password
	auth := "Basic " + base64.StdEncoding.EncodeToString([]byte(login))
	config.Header.Add("Authorization", auth)

	// Attempt to connect to running btcd instance. Bail if it fails.
	var btcdws *websocket.Conn
	var cerr error
	if cfg.Proxy != "" {
		proxy := &socks.Proxy{
			Addr:     cfg.Proxy,
			Username: cfg.ProxyUser,
			Password: cfg.ProxyPass,
		}
		conn, err := proxy.Dial("tcp", cfg.Connect)
		if err != nil {
			log.Warnf("Error connecting to proxy: %v", err)
			reply <- ErrConnRefused
			return
		}

		tlsConn := tls.Client(conn, config.TlsConfig)
		btcdws, cerr = websocket.NewClient(config, tlsConn)
	} else {
		btcdws, cerr = websocket.DialConfig(config)
	}
	if cerr != nil {
		log.Errorf("%s", cerr)
		reply <- ErrConnRefused
		return
	}
	reply <- nil

	// Remove all reply handlers (if any exist from an old connection).
	replyHandlers.Lock()
	for k := range replyHandlers.m {
		delete(replyHandlers.m, k)
	}
	replyHandlers.Unlock()

	done := make(chan struct{})
	BtcdHandler(btcdws, done)

	if err := BtcdHandshake(btcdws); err != nil {
		log.Errorf("%v", err)
		reply <- ErrConnRefused
		return
	}

	// done is closed when BtcdHandler's goroutines are finished.
	<-done
	reply <- ErrConnLost
}

// resendUnminedTxs resends any transactions in the unmined
// transaction pool to btcd using the 'sendrawtransaction' RPC
// command.
func resendUnminedTxs() {
	for _, createdTx := range UnminedTxs.m {
		n := <-NewJSONID
		var id interface{} = fmt.Sprintf("btcwallet(%v)", n)
		m, err := btcjson.CreateMessageWithId("sendrawtransaction", id, string(createdTx.rawTx))
		if err != nil {
			log.Errorf("cannot create resend request: %v", err)
			continue
		}
		replyHandlers.Lock()
		replyHandlers.m[n] = func(result interface{}, err *btcjson.Error) bool {
			// Do nothing, just remove the handler.
			return true
		}
		replyHandlers.Unlock()
		btcdMsgs <- m
	}
}

// BtcdHandshake first checks that the websocket connection between
// btcwallet and btcd is valid, that is, that there are no mismatching
// settings between the two processes (such as running on different
// Bitcoin networks).  If the sanity checks pass, all wallets are set to
// be tracked against chain notifications from this btcd connection.
//
// TODO(jrick): Track and Rescan commands should be replaced with a
// single TrackSince function (or similar) which requests address
// notifications and performs the rescan since some block height.
func BtcdHandshake(ws *websocket.Conn) error {
	n := <-NewJSONID
	var cmd btcjson.Cmd
	cmd = btcws.NewGetCurrentNetCmd(fmt.Sprintf("btcwallet(%v)", n))
	mcmd, err := cmd.MarshalJSON()
	if err != nil {
		return fmt.Errorf("cannot complete btcd handshake: %v", err)
	}

	correctNetwork := make(chan bool)

	replyHandlers.Lock()
	replyHandlers.m[n] = func(result interface{}, err *btcjson.Error) bool {
		fnet, ok := result.(float64)
		if !ok {
			log.Error("btcd handshake: result is not a number")
			correctNetwork <- false
			return true
		}

		correctNetwork <- btcwire.BitcoinNet(fnet) == cfg.Net()

		// No additional replies expected, remove handler.
		return true
	}
	replyHandlers.Unlock()

	btcdMsgs <- mcmd

	if !<-correctNetwork {
		return errors.New("btcd and btcwallet running on different Bitcoin networks")
	}

	// Get current best block.  If this is before than the oldest
	// saved block hash, assume that this btcd instance is not yet
	// synced up to a previous btcd that was last used with this
	// wallet.
	bs, err := GetCurBlock()
	if err != nil {
		return fmt.Errorf("cannot get best block: %v", err)
	}
	NotifyNewBlockChainHeight(frontendNotificationMaster, bs)
	NotifyBalances(frontendNotificationMaster)

	// Get default account.  Only the default account is used to
	// track recently-seen blocks.
	a, err := accountstore.Account("")
	if err != nil {
		return nil
	}

	// TODO(jrick): if height is less than the earliest-saved block
	// height, should probably wait for btcd to catch up.

	// Check that there was not any reorgs done since last connection.
	// If so, rollback and rescan to catch up.
	it := a.Wallet.NewIterateRecentBlocks()
	for cont := it != nil; cont; cont = it.Prev() {
		bs := it.BlockStamp()
		log.Debugf("Checking for previous saved block with height %v hash %v",
			bs.Height, bs.Hash)

		n = <-NewJSONID
		// NewGetBlockCmd can't fail, so don't check error.
		// TODO(jrick): probably want to remove the error return value.
		cmd, _ = btcjson.NewGetBlockCmd(fmt.Sprintf("btcwallet(%v)", n),
			bs.Hash.String())
		mcmd, _ = cmd.MarshalJSON()

		blockMissing := make(chan bool)

		replyHandlers.Lock()
		replyHandlers.m[n] = func(result interface{}, err *btcjson.Error) bool {
			blockMissing <- err != nil && err.Code == btcjson.ErrBlockNotFound.Code

			// No additional replies expected, remove handler.
			return true
		}
		replyHandlers.Unlock()

		btcdMsgs <- mcmd

		if <-blockMissing {
			continue
		}

		log.Debug("Found matching block.")

		// If we had to go back to any previous blocks (it.Next
		// returns true), then rollback the next and all child blocks.
		// This rollback is done here instead of in the blockMissing
		// check above for each removed block because Rollback will
		// try to write new tx and utxo files on each rollback.
		if it.Next() {
			bs := it.BlockStamp()
			accountstore.Rollback(bs.Height, &bs.Hash)
		}

		// Set default account to be marked in sync with the current
		// blockstamp.  This invalidates the iterator.
		a.Wallet.SetSyncedWith(bs)

		// Begin tracking wallets against this btcd instance.
		accountstore.Track()
		accountstore.RescanActiveAddresses()

		// (Re)send any unmined transactions to btcd in case of a btcd restart.
		resendUnminedTxs()

		// Get current blockchain height and best block hash.
		return nil
	}

	log.Warnf("None of the previous saved blocks in btcd chain.  Must perform full rescan.")

	// Iterator was invalid (wallet has never been synced) or there was a
	// huge chain fork + reorg (more than 20 blocks).  Since we don't know
	// what block (if any) this wallet is synced to, roll back everything
	// and start a new rescan since the earliest block wallet must know
	// about.
	a.fullRescan = true
	accountstore.Track()
	accountstore.RescanActiveAddresses()
	resendUnminedTxs()
	return nil
}