// Copyright (c) 2012-2014 Jeremy Latt
// Copyright (c) 2014-2015 Edmund Huber
// Copyright (c) 2016-2017 Daniel Oaks <daniel@danieloaks.net>
// released under the MIT license

package irc

import (
	"crypto/x509"
	"fmt"
	"net"
	"runtime/debug"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/goshuirc/irc-go/ircfmt"
	"github.com/goshuirc/irc-go/ircmsg"
	ident "github.com/oragono/go-ident"
	"github.com/oragono/oragono/irc/caps"
	"github.com/oragono/oragono/irc/connection_limits"
	"github.com/oragono/oragono/irc/history"
	"github.com/oragono/oragono/irc/modes"
	"github.com/oragono/oragono/irc/sno"
	"github.com/oragono/oragono/irc/utils"
)

const (
	// maximum line length not including tags; don't change this for a public server
	MaxLineLen = 512

	// IdentTimeout is how long before our ident (username) check times out.
	IdentTimeout         = time.Second + 500*time.Millisecond
	IRCv3TimestampFormat = utils.IRCv3TimestampFormat
	// limit the number of device IDs a client can use, as a DoS mitigation
	maxDeviceIDsPerClient = 64
	// controls how often often we write an autoreplay-missed client's
	// deviceid->lastseentime mapping to the database
	lastSeenWriteInterval = time.Hour
)

const (
	// RegisterTimeout is how long clients have to register before we disconnect them
	RegisterTimeout = time.Minute
	// DefaultIdleTimeout is how long without traffic before we send the client a PING
	DefaultIdleTimeout = time.Minute + 30*time.Second
	// For Tor clients, we send a PING at least every 30 seconds, as a workaround for this bug
	// (single-onion circuits will close unless the client sends data once every 60 seconds):
	// https://bugs.torproject.org/29665
	TorIdleTimeout = time.Second * 30
	// This is how long a client gets without sending any message, including the PONG to our
	// PING, before we disconnect them:
	DefaultTotalTimeout = 2*time.Minute + 30*time.Second
	// Resumeable clients (clients who have negotiated caps.Resume) get longer:
	ResumeableTotalTimeout = 3*time.Minute + 30*time.Second

	// round off the ping interval by this much, see below:
	PingCoalesceThreshold = time.Second
)

// ResumeDetails is a place to stash data at various stages of
// the resume process: when handling the RESUME command itself,
// when completing the registration, and when rejoining channels.
type ResumeDetails struct {
	PresentedToken    string
	Timestamp         time.Time
	HistoryIncomplete bool
}

// Client is an IRC client.
type Client struct {
	account            string
	accountName        string // display name of the account: uncasefolded, '*' if not logged in
	accountRegDate     time.Time
	accountSettings    AccountSettings
	away               bool
	autoAway           bool
	awayMessage        string
	brbTimer           BrbTimer
	channels           ChannelSet
	ctime              time.Time
	destroyed          bool
	modes              modes.ModeSet
	hostname           string
	invitedTo          utils.StringSet
	isSTSOnly          bool
	languages          []string
	lastActive         time.Time            // last time they sent a command that wasn't PONG or similar
	lastSeen           map[string]time.Time // maps device ID (including "") to time of last received command
	lastSeenLastWrite  time.Time            // last time `lastSeen` was written to the datastore
	loginThrottle      connection_limits.GenericThrottle
	nextSessionID      int64 // Incremented when a new session is established
	nick               string
	nickCasefolded     string
	nickMaskCasefolded string
	nickMaskString     string // cache for nickmask string since it's used with lots of replies
	oper               *Oper
	preregNick         string
	proxiedIP          net.IP // actual remote IP if using the PROXY protocol
	rawHostname        string
	cloakedHostname    string
	realname           string
	realIP             net.IP
	requireSASLMessage string
	requireSASL        bool
	registered         bool
	registerCmdSent    bool // already sent the draft/register command, can't send it again
	registrationTimer  *time.Timer
	resumeID           string
	server             *Server
	skeleton           string
	sessions           []*Session
	stateMutex         sync.RWMutex // tier 1
	alwaysOn           bool
	username           string
	vhost              string
	history            history.Buffer
	dirtyBits          uint
	writerSemaphore    utils.Semaphore // tier 1.5
}

type saslStatus struct {
	mechanism string
	value     string
}

func (s *saslStatus) Clear() {
	*s = saslStatus{}
}

// what stage the client is at w.r.t. the PASS command:
type serverPassStatus uint

const (
	serverPassUnsent serverPassStatus = iota
	serverPassSuccessful
	serverPassFailed
)

// Session is an individual client connection to the server (TCP connection
// and associated per-connection data, such as capabilities). There is a
// many-one relationship between sessions and clients.
type Session struct {
	client *Client

	deviceID string

	ctime      time.Time
	lastActive time.Time // last non-CTCP PRIVMSG sent; updates publicly visible idle time
	lastTouch  time.Time // last line sent; updates timer for idle timeouts
	idleTimer  *time.Timer
	pingSent   bool // we sent PING to a putatively idle connection and we're waiting for PONG

	sessionID   int64
	socket      *Socket
	realIP      net.IP
	proxiedIP   net.IP
	rawHostname string
	isTor       bool

	fakelag              Fakelag
	deferredFakelagCount int
	destroyed            uint32

	certfp     string
	peerCerts  []*x509.Certificate
	sasl       saslStatus
	passStatus serverPassStatus

	batchCounter uint32

	quitMessage string

	capabilities caps.Set
	capState     caps.State
	capVersion   caps.Version

	registrationMessages int

	resumeID              string
	resumeDetails         *ResumeDetails
	zncPlaybackTimes      *zncPlaybackTimes
	autoreplayMissedSince time.Time

	batch MultilineBatch
}

// MultilineBatch tracks the state of a client-to-server multiline batch.
type MultilineBatch struct {
	label         string // this is the first param to BATCH (the "reference tag")
	command       string
	target        string
	responseLabel string // this is the value of the labeled-response tag sent with BATCH
	message       utils.SplitMessage
	lenBytes      int
	tags          map[string]string
}

// Starts a multiline batch, failing if there's one already open
func (s *Session) StartMultilineBatch(label, target, responseLabel string, tags map[string]string) (err error) {
	if s.batch.label != "" {
		return errInvalidMultilineBatch
	}

	s.batch.label, s.batch.target, s.batch.responseLabel, s.batch.tags = label, target, responseLabel, tags
	s.fakelag.Suspend()
	return
}

// Closes a multiline batch unconditionally; returns the batch and whether
// it was validly terminated (pass "" as the label if you don't care about the batch)
func (s *Session) EndMultilineBatch(label string) (batch MultilineBatch, err error) {
	batch = s.batch
	s.batch = MultilineBatch{}
	s.fakelag.Unsuspend()

	// heuristics to estimate how much data they used while fakelag was suspended
	fakelagBill := (batch.lenBytes / MaxLineLen) + 1
	fakelagBillLines := (batch.message.LenLines() * 60) / MaxLineLen
	if fakelagBill < fakelagBillLines {
		fakelagBill = fakelagBillLines
	}
	s.deferredFakelagCount = fakelagBill

	if batch.label == "" || batch.label != label || !batch.message.ValidMultiline() {
		err = errInvalidMultilineBatch
		return
	}

	batch.message.SetTime()

	return
}

// sets the session quit message, if there isn't one already
func (sd *Session) SetQuitMessage(message string) (set bool) {
	if message == "" {
		message = "Connection closed"
	}
	if sd.quitMessage == "" {
		sd.quitMessage = message
		return true
	} else {
		return false
	}
}

func (s *Session) IP() net.IP {
	if s.proxiedIP != nil {
		return s.proxiedIP
	}
	return s.realIP
}

// returns whether the session was actively destroyed (for example, by ping
// timeout or NS GHOST).
// avoids a race condition between asynchronous idle-timing-out of sessions,
// and a condition that allows implicit BRB on connection errors (since
// destroy()'s socket.Close() appears to socket.Read() as a connection error)
func (session *Session) Destroyed() bool {
	return atomic.LoadUint32(&session.destroyed) == 1
}

// sets the timed-out flag
func (session *Session) SetDestroyed() {
	atomic.StoreUint32(&session.destroyed, 1)
}

// returns whether the client supports a smart history replay cap,
// and therefore autoreplay-on-join and similar should be suppressed
func (session *Session) HasHistoryCaps() bool {
	return session.capabilities.Has(caps.Chathistory) || session.capabilities.Has(caps.ZNCPlayback)
}

// generates a batch ID. the uniqueness requirements for this are fairly weak:
// any two batch IDs that are active concurrently (either through interleaving
// or nesting) on an individual session connection need to be unique.
// this allows ~4 billion such batches which should be fine.
func (session *Session) generateBatchID() string {
	id := atomic.AddUint32(&session.batchCounter, 1)
	return strconv.FormatInt(int64(id), 32)
}

// WhoWas is the subset of client details needed to answer a WHOWAS query
type WhoWas struct {
	nick           string
	nickCasefolded string
	username       string
	hostname       string
	realname       string
}

// ClientDetails is a standard set of details about a client
type ClientDetails struct {
	WhoWas

	nickMask           string
	nickMaskCasefolded string
	account            string
	accountName        string
}

// RunClient sets up a new client and runs its goroutine.
func (server *Server) RunClient(conn IRCConn) {
	config := server.Config()
	wConn := conn.UnderlyingConn()
	var isBanned, requireSASL bool
	var banMsg string
	realIP := utils.AddrToIP(wConn.RemoteAddr())
	var proxiedIP net.IP
	if wConn.Config.Tor {
		// cover up details of the tor proxying infrastructure (not a user privacy concern,
		// but a hardening measure):
		proxiedIP = utils.IPv4LoopbackAddress
		isBanned, banMsg = server.checkTorLimits()
	} else {
		ipToCheck := realIP
		if wConn.ProxiedIP != nil {
			proxiedIP = wConn.ProxiedIP
			ipToCheck = proxiedIP
		}
		// XXX only run the check script now if the IP cannot be replaced by PROXY or WEBIRC,
		// otherwise we'll do it in ApplyProxiedIP.
		checkScripts := proxiedIP != nil || !utils.IPInNets(realIP, config.Server.proxyAllowedFromNets)
		isBanned, requireSASL, banMsg = server.checkBans(config, ipToCheck, checkScripts)
	}

	if isBanned {
		// this might not show up properly on some clients,
		// but our objective here is just to close the connection out before it has a load impact on us
		conn.WriteLine([]byte(fmt.Sprintf(errorMsg, banMsg)))
		conn.Close()
		return
	}

	server.logger.Info("connect-ip", fmt.Sprintf("Client connecting: real IP %v, proxied IP %v", realIP, proxiedIP))

	now := time.Now().UTC()
	// give them 1k of grace over the limit:
	socket := NewSocket(conn, config.Server.MaxSendQBytes)
	client := &Client{
		lastActive: now,
		channels:   make(ChannelSet),
		ctime:      now,
		isSTSOnly:  wConn.Config.STSOnly,
		languages:  server.Languages().Default(),
		loginThrottle: connection_limits.GenericThrottle{
			Duration: config.Accounts.LoginThrottling.Duration,
			Limit:    config.Accounts.LoginThrottling.MaxAttempts,
		},
		server:         server,
		accountName:    "*",
		nick:           "*", // * is used until actual nick is given
		nickCasefolded: "*",
		nickMaskString: "*", // * is used until actual nick is given
		realIP:         realIP,
		proxiedIP:      proxiedIP,
		requireSASL:    requireSASL,
		nextSessionID:  1,
	}
	if requireSASL {
		client.requireSASLMessage = banMsg
	}
	client.writerSemaphore.Initialize(1)
	client.history.Initialize(config.History.ClientLength, time.Duration(config.History.AutoresizeWindow))
	client.brbTimer.Initialize(client)
	session := &Session{
		client:     client,
		socket:     socket,
		capVersion: caps.Cap301,
		capState:   caps.NoneState,
		ctime:      now,
		lastActive: now,
		realIP:     realIP,
		proxiedIP:  proxiedIP,
		isTor:      wConn.Config.Tor,
	}
	client.sessions = []*Session{session}

	session.resetFakelag()

	if wConn.Secure {
		client.SetMode(modes.TLS, true)
	}

	if wConn.Config.TLSConfig != nil {
		// error is not useful to us here anyways so we can ignore it
		session.certfp, session.peerCerts, _ = utils.GetCertFP(wConn.Conn, RegisterTimeout)
	}

	if session.isTor {
		session.rawHostname = config.Server.TorListeners.Vhost
		client.rawHostname = session.rawHostname
	} else {
		if config.Server.CheckIdent {
			client.doIdentLookup(wConn.Conn)
		}
	}

	client.registrationTimer = time.AfterFunc(RegisterTimeout, client.handleRegisterTimeout)
	server.stats.Add()
	client.run(session)
}

func (server *Server) AddAlwaysOnClient(account ClientAccount, chnames []string, lastSeen map[string]time.Time, uModes modes.Modes, realname string) {
	now := time.Now().UTC()
	config := server.Config()
	if lastSeen == nil && account.Settings.AutoreplayMissed {
		lastSeen = map[string]time.Time{"": now}
	}

	hostname := server.name
	if config.Server.Cloaks.EnabledForAlwaysOn {
		hostname = config.Server.Cloaks.ComputeAccountCloak(account.Name)
	}

	client := &Client{
		lastSeen:   lastSeen,
		lastActive: now,
		channels:   make(ChannelSet),
		ctime:      now,
		languages:  server.Languages().Default(),
		server:     server,

		username:    "~user",
		rawHostname: hostname,
		realIP:      utils.IPv4LoopbackAddress,

		alwaysOn: true,
		realname: realname,

		nextSessionID: 1,
	}

	client.SetMode(modes.TLS, true)
	for _, m := range uModes {
		client.SetMode(m, true)
	}
	client.writerSemaphore.Initialize(1)
	client.history.Initialize(0, 0)
	client.brbTimer.Initialize(client)

	server.accounts.Login(client, account)

	client.resizeHistory(config)

	_, err, _ := server.clients.SetNick(client, nil, account.Name, false)
	if err != nil {
		server.logger.Error("internal", "could not establish always-on client", account.Name, err.Error())
		return
	} else {
		server.logger.Debug("accounts", "established always-on client", account.Name)
	}

	// XXX set this last to avoid confusing SetNick:
	client.registered = true

	for _, chname := range chnames {
		// XXX we're using isSajoin=true, to make these joins succeed even without channel key
		// this is *probably* ok as long as the persisted memberships are accurate
		server.channels.Join(client, chname, "", true, nil)
	}

	if persistenceEnabled(config.Accounts.Multiclient.AutoAway, client.accountSettings.AutoAway) {
		client.autoAway = true
		client.away = true
		client.awayMessage = client.t("User is currently disconnected")
	}
}

func (client *Client) resizeHistory(config *Config) {
	status, _ := client.historyStatus(config)
	if status == HistoryEphemeral {
		client.history.Resize(config.History.ClientLength, time.Duration(config.History.AutoresizeWindow))
	} else {
		client.history.Resize(0, 0)
	}
}

// resolve an IP to an IRC-ready hostname, using reverse DNS, forward-confirming if necessary,
// and sending appropriate notices to the client
func (client *Client) lookupHostname(session *Session, overwrite bool) {
	if session.isTor {
		return
	} // else: even if cloaking is enabled, look up the real hostname to show to operators

	config := client.server.Config()
	ip := session.realIP
	if session.proxiedIP != nil {
		ip = session.proxiedIP
	}
	ipString := ip.String()

	var hostname, candidate string
	if config.Server.lookupHostnames {
		session.Notice("*** Looking up your hostname...")

		names, err := net.LookupAddr(ipString)
		if err == nil && 0 < len(names) {
			candidate = strings.TrimSuffix(names[0], ".")
		}
		if utils.IsHostname(candidate) {
			if config.Server.ForwardConfirmHostnames {
				addrs, err := net.LookupHost(candidate)
				if err == nil {
					for _, addr := range addrs {
						if addr == ipString {
							hostname = candidate // successful forward confirmation
							break
						}
					}
				}
			} else {
				hostname = candidate
			}
		}
	}

	if hostname != "" {
		session.Notice("*** Found your hostname")
	} else {
		if config.Server.lookupHostnames {
			session.Notice("*** Couldn't look up your hostname")
		}
		hostname = utils.IPStringToHostname(ipString)
	}

	session.rawHostname = hostname
	cloakedHostname := config.Server.Cloaks.ComputeCloak(ip)
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()
	// update the hostname if this is a new connection or a resume, but not if it's a reattach
	if overwrite || client.rawHostname == "" {
		client.rawHostname = hostname
		client.cloakedHostname = cloakedHostname
		client.updateNickMaskNoMutex()
	}
}

func (client *Client) doIdentLookup(conn net.Conn) {
	localTCPAddr, ok := conn.LocalAddr().(*net.TCPAddr)
	if !ok {
		return
	}
	serverPort := localTCPAddr.Port
	remoteTCPAddr, ok := conn.RemoteAddr().(*net.TCPAddr)
	if !ok {
		return
	}
	clientPort := remoteTCPAddr.Port

	client.Notice(client.t("*** Looking up your username"))
	resp, err := ident.Query(remoteTCPAddr.IP.String(), serverPort, clientPort, IdentTimeout)
	if err == nil {
		err := client.SetNames(resp.Identifier, "", true)
		if err == nil {
			client.Notice(client.t("*** Found your username"))
			// we don't need to updateNickMask here since nickMask is not used for anything yet
		} else {
			client.Notice(client.t("*** Got a malformed username, ignoring"))
		}
	} else {
		client.Notice(client.t("*** Could not find your username"))
	}
}

type AuthOutcome uint

const (
	authSuccess AuthOutcome = iota
	authFailPass
	authFailTorSaslRequired
	authFailSaslRequired
)

func (client *Client) isAuthorized(server *Server, config *Config, session *Session, forceRequireSASL bool) AuthOutcome {
	saslSent := client.account != ""
	// PASS requirement
	if (config.Server.passwordBytes != nil) && session.passStatus != serverPassSuccessful && !(config.Accounts.SkipServerPassword && saslSent) {
		return authFailPass
	}
	// Tor connections may be required to authenticate with SASL
	if session.isTor && config.Server.TorListeners.RequireSasl && !saslSent {
		return authFailTorSaslRequired
	}
	// finally, enforce require-sasl
	if !saslSent && (forceRequireSASL || config.Accounts.RequireSasl.Enabled || server.Defcon() <= 2) &&
		!utils.IPInNets(session.IP(), config.Accounts.RequireSasl.exemptedNets) {
		return authFailSaslRequired
	}
	return authSuccess
}

func (session *Session) resetFakelag() {
	var flc FakelagConfig = session.client.server.Config().Fakelag
	flc.Enabled = flc.Enabled && !session.client.HasRoleCapabs("nofakelag")
	session.fakelag.Initialize(flc)
}

// IP returns the IP address of this client.
func (client *Client) IP() net.IP {
	client.stateMutex.RLock()
	defer client.stateMutex.RUnlock()

	if client.proxiedIP != nil {
		return client.proxiedIP
	}
	return client.realIP
}

// IPString returns the IP address of this client as a string.
func (client *Client) IPString() string {
	ip := client.IP().String()
	if 0 < len(ip) && ip[0] == ':' {
		ip = "0" + ip
	}
	return ip
}

// t returns the translated version of the given string, based on the languages configured by the client.
func (client *Client) t(originalString string) string {
	languageManager := client.server.Config().languageManager
	if !languageManager.Enabled() {
		return originalString
	}
	return languageManager.Translate(client.Languages(), originalString)
}

// main client goroutine: read lines and execute the corresponding commands
// `proxyLine` is the PROXY-before-TLS line, if there was one
func (client *Client) run(session *Session) {

	defer func() {
		if r := recover(); r != nil {
			client.server.logger.Error("internal",
				fmt.Sprintf("Client caused panic: %v\n%s", r, debug.Stack()))
			if client.server.Config().Debug.recoverFromErrors {
				client.server.logger.Error("internal", "Disconnecting client and attempting to recover")
			} else {
				panic(r)
			}
		}
		// ensure client connection gets closed
		client.destroy(session)
	}()

	isReattach := client.Registered()
	if isReattach {
		client.Touch(session)
		if session.resumeDetails != nil {
			session.playResume()
			session.resumeDetails = nil
			client.brbTimer.Disable()
			client.SetAway(false, "") // clear BRB message if any
		} else {
			client.playReattachMessages(session)
		}
	}

	firstLine := !isReattach

	for {
		var invalidUtf8 bool
		line, err := session.socket.Read()
		if err == errInvalidUtf8 {
			invalidUtf8 = true // handle as normal, including labeling
		} else if err != nil {
			quitMessage := "connection closed"
			if err == errReadQ {
				quitMessage = "readQ exceeded"
			}
			client.Quit(quitMessage, session)
			// since the client did not actually send us a QUIT,
			// give them a chance to resume if applicable:
			if !session.Destroyed() {
				client.brbTimer.Enable()
			}
			break
		}

		if client.server.logger.IsLoggingRawIO() {
			client.server.logger.Debug("userinput", client.nick, "<- ", line)
		}

		// special-cased handling of PROXY protocol, see `handleProxyCommand` for details:
		if firstLine {
			firstLine = false
			if strings.HasPrefix(line, "PROXY") {
				err = handleProxyCommand(client.server, client, session, line)
				if err != nil {
					break
				} else {
					continue
				}
			}
		}

		if client.registered {
			touches := session.deferredFakelagCount + 1
			session.deferredFakelagCount = 0
			for i := 0; i < touches; i++ {
				session.fakelag.Touch()
			}
		} else {
			// DoS hardening, #505
			session.registrationMessages++
			if client.server.Config().Limits.RegistrationMessages < session.registrationMessages {
				client.Send(nil, client.server.name, ERR_UNKNOWNERROR, "*", client.t("You have sent too many registration messages"))
				break
			}
		}

		msg, err := ircmsg.ParseLineStrict(line, true, MaxLineLen)
		if err == ircmsg.ErrorLineIsEmpty {
			continue
		} else if err == ircmsg.ErrorLineTooLong {
			session.Send(nil, client.server.name, ERR_INPUTTOOLONG, client.Nick(), client.t("Input line too long"))
			continue
		} else if err != nil {
			client.Quit(client.t("Received malformed line"), session)
			break
		}

		cmd, exists := Commands[msg.Command]
		if !exists {
			cmd = unknownCommand
		} else if invalidUtf8 {
			cmd = invalidUtf8Command
		}

		isExiting := cmd.Run(client.server, client, session, msg)
		if isExiting {
			break
		} else if session.client != client {
			// bouncer reattach
			go session.client.run(session)
			break
		}
	}
}

func (client *Client) playReattachMessages(session *Session) {
	client.server.playRegistrationBurst(session)
	hasHistoryCaps := session.HasHistoryCaps()
	for _, channel := range session.client.Channels() {
		channel.playJoinForSession(session)
		// clients should receive autoreplay-on-join lines, if applicable:
		if hasHistoryCaps {
			continue
		}
		// if they negotiated znc.in/playback or chathistory, they will receive nothing,
		// because those caps disable autoreplay-on-join and they haven't sent the relevant
		// *playback PRIVMSG or CHATHISTORY command yet
		rb := NewResponseBuffer(session)
		channel.autoReplayHistory(client, rb, "")
		rb.Send(true)
	}
	if !session.autoreplayMissedSince.IsZero() && !hasHistoryCaps {
		rb := NewResponseBuffer(session)
		zncPlayPrivmsgs(client, rb, "*", time.Now().UTC(), session.autoreplayMissedSince)
		rb.Send(true)
	}
	session.autoreplayMissedSince = time.Time{}
}

//
// idle, quit, timers and timeouts
//

// Touch indicates that we received a line from the client (so the connection is healthy
// at this time, modulo network latency and fakelag).
func (client *Client) Touch(session *Session) {
	var markDirty bool
	now := time.Now().UTC()
	client.stateMutex.Lock()
	if client.accountSettings.AutoreplayMissed || session.deviceID != "" {
		client.setLastSeen(now, session.deviceID)
		if now.Sub(client.lastSeenLastWrite) > lastSeenWriteInterval {
			markDirty = true
			client.lastSeenLastWrite = now
		}
	}
	client.updateIdleTimer(session, now)
	client.stateMutex.Unlock()
	if markDirty {
		client.markDirty(IncludeLastSeen)
	}
}

func (client *Client) setLastSeen(now time.Time, deviceID string) {
	if client.lastSeen == nil {
		client.lastSeen = make(map[string]time.Time)
	}
	client.lastSeen[deviceID] = now
	// evict the least-recently-used entry if necessary
	if maxDeviceIDsPerClient < len(client.lastSeen) {
		var minLastSeen time.Time
		var minClientId string
		for deviceID, lastSeen := range client.lastSeen {
			if minLastSeen.IsZero() || lastSeen.Before(minLastSeen) {
				minClientId, minLastSeen = deviceID, lastSeen
			}
		}
		delete(client.lastSeen, minClientId)
	}
}

func (client *Client) updateIdleTimer(session *Session, now time.Time) {
	session.lastTouch = now
	session.pingSent = false

	if session.idleTimer == nil {
		pingTimeout := DefaultIdleTimeout
		if session.isTor {
			pingTimeout = TorIdleTimeout
		}
		session.idleTimer = time.AfterFunc(pingTimeout, session.handleIdleTimeout)
	}
}

func (session *Session) handleIdleTimeout() {
	totalTimeout := DefaultTotalTimeout
	if session.capabilities.Has(caps.Resume) {
		totalTimeout = ResumeableTotalTimeout
	}
	pingTimeout := DefaultIdleTimeout
	if session.isTor {
		pingTimeout = TorIdleTimeout
	}

	session.client.stateMutex.Lock()
	now := time.Now()
	timeUntilDestroy := session.lastTouch.Add(totalTimeout).Sub(now)
	timeUntilPing := session.lastTouch.Add(pingTimeout).Sub(now)
	shouldDestroy := session.pingSent && timeUntilDestroy <= 0
	// XXX this should really be time <= 0, but let's do some hacky timer coalescing:
	// a typical idling client will do nothing other than respond immediately to our pings,
	// so we'll PING at t=0, they'll respond at t=0.05, then we'll wake up at t=90 and find
	// that we need to PING again at t=90.05. Rather than wake up again, just send it now:
	shouldSendPing := !session.pingSent && timeUntilPing <= PingCoalesceThreshold
	if !shouldDestroy {
		if shouldSendPing {
			session.pingSent = true
		}
		// check in again at the minimum of these 3 possible intervals:
		// 1. the ping timeout (assuming we PING and they reply immediately with PONG)
		// 2. the next time we would send PING (if they don't send any more lines)
		// 3. the next time we would destroy (if they don't send any more lines)
		nextTimeout := pingTimeout
		if PingCoalesceThreshold < timeUntilPing && timeUntilPing < nextTimeout {
			nextTimeout = timeUntilPing
		}
		if 0 < timeUntilDestroy && timeUntilDestroy < nextTimeout {
			nextTimeout = timeUntilDestroy
		}
		session.idleTimer.Stop()
		session.idleTimer.Reset(nextTimeout)
	}
	session.client.stateMutex.Unlock()

	if shouldDestroy {
		session.client.Quit(fmt.Sprintf("Ping timeout: %v", totalTimeout), session)
		session.client.destroy(session)
	} else if shouldSendPing {
		session.Ping()
	}
}

func (session *Session) stopIdleTimer() {
	session.client.stateMutex.Lock()
	defer session.client.stateMutex.Unlock()
	if session.idleTimer != nil {
		session.idleTimer.Stop()
	}
}

// Ping sends the client a PING message.
func (session *Session) Ping() {
	session.Send(nil, "", "PING", session.client.Nick())
}

// tryResume tries to resume if the client asked us to.
func (session *Session) tryResume() (success bool) {
	var oldResumeID string

	defer func() {
		if success {
			// "On a successful request, the server [...] terminates the old client's connection"
			oldSession := session.client.GetSessionByResumeID(oldResumeID)
			if oldSession != nil {
				session.client.destroy(oldSession)
			}
		} else {
			session.resumeDetails = nil
		}
	}()

	client := session.client
	server := client.server
	config := server.Config()

	oldClient, oldResumeID := server.resumeManager.VerifyToken(client, session.resumeDetails.PresentedToken)
	if oldClient == nil {
		session.Send(nil, server.name, "FAIL", "RESUME", "INVALID_TOKEN", client.t("Cannot resume connection, token is not valid"))
		return
	}

	resumeAllowed := config.Server.AllowPlaintextResume || (oldClient.HasMode(modes.TLS) && client.HasMode(modes.TLS))
	if !resumeAllowed {
		session.Send(nil, server.name, "FAIL", "RESUME", "INSECURE_SESSION", client.t("Cannot resume connection, old and new clients must have TLS"))
		return
	}

	err := server.clients.Resume(oldClient, session)
	if err != nil {
		session.Send(nil, server.name, "FAIL", "RESUME", "CANNOT_RESUME", client.t("Cannot resume connection"))
		return
	}

	success = true
	client.server.logger.Debug("quit", fmt.Sprintf("%s is being resumed", oldClient.Nick()))

	return
}

// playResume is called from the session's fresh goroutine after a resume;
// it sends notifications to friends, then plays the registration burst and replays
// stored history to the session
func (session *Session) playResume() {
	client := session.client
	server := client.server
	config := server.Config()

	friends := make(ClientSet)
	var oldestLostMessage time.Time

	// work out how much time, if any, is not covered by history buffers
	// assume that a persistent buffer covers the whole resume period
	for _, channel := range client.Channels() {
		for _, member := range channel.auditoriumFriends(client) {
			friends.Add(member)
		}
		status, _ := channel.historyStatus(config)
		if status == HistoryEphemeral {
			lastDiscarded := channel.history.LastDiscarded()
			if oldestLostMessage.Before(lastDiscarded) {
				oldestLostMessage = lastDiscarded
			}
		}
	}
	cHistoryStatus, _ := client.historyStatus(config)
	if cHistoryStatus == HistoryEphemeral {
		lastDiscarded := client.history.LastDiscarded()
		if oldestLostMessage.Before(lastDiscarded) {
			oldestLostMessage = lastDiscarded
		}
	}
	_, privmsgSeq, _ := server.GetHistorySequence(nil, client, "*")
	if privmsgSeq != nil {
		privmsgs, _, _ := privmsgSeq.Between(history.Selector{}, history.Selector{}, config.History.ClientLength)
		for _, item := range privmsgs {
			sender := server.clients.Get(stripMaskFromNick(item.Nick))
			if sender != nil {
				friends.Add(sender)
			}
		}
	}

	timestamp := session.resumeDetails.Timestamp
	gap := oldestLostMessage.Sub(timestamp)
	session.resumeDetails.HistoryIncomplete = gap > 0 || timestamp.IsZero()
	gapSeconds := int(gap.Seconds()) + 1 // round up to avoid confusion

	details := client.Details()
	oldNickmask := details.nickMask
	client.lookupHostname(session, true)
	hostname := client.Hostname() // may be a vhost
	timestampString := timestamp.Format(IRCv3TimestampFormat)

	// send quit/resume messages to friends
	for friend := range friends {
		if friend == client {
			continue
		}
		for _, fSession := range friend.Sessions() {
			if fSession.capabilities.Has(caps.Resume) {
				if !session.resumeDetails.HistoryIncomplete {
					fSession.Send(nil, oldNickmask, "RESUMED", hostname, "ok")
				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
					fSession.Send(nil, oldNickmask, "RESUMED", hostname, timestampString)
				} else {
					fSession.Send(nil, oldNickmask, "RESUMED", hostname)
				}
			} else {
				if !session.resumeDetails.HistoryIncomplete {
					fSession.Send(nil, oldNickmask, "QUIT", friend.t("Client reconnected"))
				} else if session.resumeDetails.HistoryIncomplete && !timestamp.IsZero() {
					fSession.Send(nil, oldNickmask, "QUIT", fmt.Sprintf(friend.t("Client reconnected (up to %d seconds of message history lost)"), gapSeconds))
				} else {
					fSession.Send(nil, oldNickmask, "QUIT", friend.t("Client reconnected (message history may have been lost)"))
				}
			}
		}
	}

	if session.resumeDetails.HistoryIncomplete {
		if !timestamp.IsZero() {
			session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", fmt.Sprintf(client.t("Resume may have lost up to %d seconds of history"), gapSeconds))
		} else {
			session.Send(nil, client.server.name, "WARN", "RESUME", "HISTORY_LOST", client.t("Resume may have lost some message history"))
		}
	}

	session.Send(nil, client.server.name, "RESUME", "SUCCESS", details.nick)

	server.playRegistrationBurst(session)

	for _, channel := range client.Channels() {
		channel.Resume(session, timestamp)
	}

	// replay direct PRIVSMG history
	if !timestamp.IsZero() && privmsgSeq != nil {
		after := history.Selector{Time: timestamp}
		items, complete, _ := privmsgSeq.Between(after, history.Selector{}, config.History.ZNCMax)
		if len(items) != 0 {
			rb := NewResponseBuffer(session)
			client.replayPrivmsgHistory(rb, items, "", complete)
			rb.Send(true)
		}
	}

	session.resumeDetails = nil
}

func (client *Client) replayPrivmsgHistory(rb *ResponseBuffer, items []history.Item, target string, complete bool) {
	var batchID string
	details := client.Details()
	nick := details.nick
	if target == "" {
		target = nick
	}
	batchID = rb.StartNestedHistoryBatch(target)

	allowTags := rb.session.capabilities.Has(caps.EventPlayback)
	for _, item := range items {
		var command string
		switch item.Type {
		case history.Privmsg:
			command = "PRIVMSG"
		case history.Notice:
			command = "NOTICE"
		case history.Tagmsg:
			if allowTags {
				command = "TAGMSG"
			} else {
				continue
			}
		default:
			continue
		}
		var tags map[string]string
		if allowTags {
			tags = item.Tags
		}
		// XXX: Params[0] is the message target. if the source of this message is an in-memory
		// buffer, then it's "" for an incoming message and the recipient's nick for an outgoing
		// message. if the source of the message is mysql, then mysql only sees one copy of the
		// message, and it's the version with the recipient's nick filled in. so this is an
		// incoming message if Params[0] (the recipient's nick) equals the client's nick:
		if item.Params[0] == "" || item.Params[0] == nick {
			rb.AddSplitMessageFromClient(item.Nick, item.AccountName, tags, command, nick, item.Message)
		} else {
			// this message was sent *from* the client to another nick; the target is item.Params[0]
			// substitute client's current nickmask in case client changed nick
			rb.AddSplitMessageFromClient(details.nickMask, item.AccountName, tags, command, item.Params[0], item.Message)
		}
	}

	rb.EndNestedBatch(batchID)
	if !complete {
		rb.Add(nil, "HistServ", "NOTICE", nick, client.t("Some additional message history may have been lost"))
	}
}

// IdleTime returns how long this client's been idle.
func (client *Client) IdleTime() time.Duration {
	client.stateMutex.RLock()
	defer client.stateMutex.RUnlock()
	return time.Since(client.lastActive)
}

// SignonTime returns this client's signon time as a unix timestamp.
func (client *Client) SignonTime() int64 {
	return client.ctime.Unix()
}

// IdleSeconds returns the number of seconds this client's been idle.
func (client *Client) IdleSeconds() uint64 {
	return uint64(client.IdleTime().Seconds())
}

// SetNames sets the client's ident and realname.
func (client *Client) SetNames(username, realname string, fromIdent bool) error {
	config := client.server.Config()
	limit := config.Limits.IdentLen
	if !fromIdent {
		limit -= 1 // leave room for the prepended ~
	}
	if limit < len(username) {
		username = username[:limit]
	}

	if !isIdent(username) {
		return errInvalidUsername
	}

	if config.Server.SuppressIdent {
		username = "~user"
	} else if !fromIdent {
		username = "~" + username
	}

	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()

	if client.username == "" {
		client.username = username
	}

	if client.realname == "" {
		client.realname = realname
	}

	return nil
}

// HasRoleCapabs returns true if client has the given (role) capabilities.
func (client *Client) HasRoleCapabs(capabs ...string) bool {
	oper := client.Oper()
	if oper == nil {
		return false
	}

	for _, capab := range capabs {
		if !oper.Class.Capabilities.Has(capab) {
			return false
		}
	}

	return true
}

// ModeString returns the mode string for this client.
func (client *Client) ModeString() (str string) {
	return "+" + client.modes.String()
}

// Friends refers to clients that share a channel with this client.
func (client *Client) Friends(capabs ...caps.Capability) (result map[*Session]empty) {
	result = make(map[*Session]empty)

	// look at the client's own sessions
	addFriendsToSet(result, client, capabs...)

	for _, channel := range client.Channels() {
		for _, member := range channel.auditoriumFriends(client) {
			addFriendsToSet(result, member, capabs...)
		}
	}

	return
}

// helper for Friends
func addFriendsToSet(set map[*Session]empty, client *Client, capabs ...caps.Capability) {
	client.stateMutex.RLock()
	defer client.stateMutex.RUnlock()
	for _, session := range client.sessions {
		if session.capabilities.HasAll(capabs...) {
			set[session] = empty{}
		}
	}
}

func (client *Client) SetOper(oper *Oper) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()
	client.oper = oper
	// operators typically get a vhost, update the nickmask
	client.updateNickMaskNoMutex()
}

// XXX: CHGHOST requires prefix nickmask to have original hostname,
// this is annoying to do correctly
func (client *Client) sendChghost(oldNickMask string, vhost string) {
	details := client.Details()
	for fClient := range client.Friends(caps.ChgHost) {
		fClient.sendFromClientInternal(false, time.Time{}, "", oldNickMask, details.accountName, nil, "CHGHOST", details.username, vhost)
	}
}

// choose the correct vhost to display
func (client *Client) getVHostNoMutex() string {
	// hostserv vhost OR operclass vhost OR nothing (i.e., normal rdns hostmask)
	if client.vhost != "" {
		return client.vhost
	} else if client.oper != nil {
		return client.oper.Vhost
	} else {
		return ""
	}
}

// SetVHost updates the client's hostserv-based vhost
func (client *Client) SetVHost(vhost string) (updated bool) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()
	updated = (client.vhost != vhost)
	client.vhost = vhost
	if updated {
		client.updateNickMaskNoMutex()
	}
	return
}

// SetNick gives the client a nickname and marks it as registered, if necessary
func (client *Client) SetNick(nick, nickCasefolded, skeleton string) (success bool) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()
	if client.destroyed {
		return false
	} else if !client.registered {
		// XXX test this before setting it to avoid annoying the race detector
		client.registered = true
		if client.registrationTimer != nil {
			client.registrationTimer.Stop()
			client.registrationTimer = nil
		}
	}
	client.nick = nick
	client.nickCasefolded = nickCasefolded
	client.skeleton = skeleton
	client.updateNickMaskNoMutex()
	return true
}

// updateNickMaskNoMutex updates the casefolded nickname and nickmask, not acquiring any mutexes.
func (client *Client) updateNickMaskNoMutex() {
	if client.nick == "*" {
		return // pre-registration, don't bother generating the hostname
	}

	client.hostname = client.getVHostNoMutex()
	if client.hostname == "" {
		client.hostname = client.cloakedHostname
		if client.hostname == "" {
			client.hostname = client.rawHostname
		}
	}

	cfhostname := strings.ToLower(client.hostname)
	client.nickMaskString = fmt.Sprintf("%s!%s@%s", client.nick, client.username, client.hostname)
	client.nickMaskCasefolded = fmt.Sprintf("%s!%s@%s", client.nickCasefolded, strings.ToLower(client.username), cfhostname)
}

// AllNickmasks returns all the possible nickmasks for the client.
func (client *Client) AllNickmasks() (masks []string) {
	client.stateMutex.RLock()
	nick := client.nickCasefolded
	username := client.username
	rawHostname := client.rawHostname
	cloakedHostname := client.cloakedHostname
	vhost := client.getVHostNoMutex()
	client.stateMutex.RUnlock()
	username = strings.ToLower(username)

	if len(vhost) > 0 {
		cfvhost := strings.ToLower(vhost)
		masks = append(masks, fmt.Sprintf("%s!%s@%s", nick, username, cfvhost))
	}

	var rawhostmask string
	cfrawhost := strings.ToLower(rawHostname)
	rawhostmask = fmt.Sprintf("%s!%s@%s", nick, username, cfrawhost)
	masks = append(masks, rawhostmask)

	if cloakedHostname != "" {
		masks = append(masks, fmt.Sprintf("%s!%s@%s", nick, username, cloakedHostname))
	}

	ipmask := fmt.Sprintf("%s!%s@%s", nick, username, client.IPString())
	if ipmask != rawhostmask {
		masks = append(masks, ipmask)
	}

	return
}

// LoggedIntoAccount returns true if this client is logged into an account.
func (client *Client) LoggedIntoAccount() bool {
	return client.Account() != ""
}

// Quit sets the given quit message for the client.
// (You must ensure separately that destroy() is called, e.g., by returning `true` from
// the command handler or calling it yourself.)
func (client *Client) Quit(message string, session *Session) {
	setFinalData := func(sess *Session) {
		message := sess.quitMessage
		var finalData []byte
		// #364: don't send QUIT lines to unregistered clients
		if client.registered {
			quitMsg := ircmsg.MakeMessage(nil, client.nickMaskString, "QUIT", message)
			finalData, _ = quitMsg.LineBytesStrict(false, MaxLineLen)
		}

		errorMsg := ircmsg.MakeMessage(nil, "", "ERROR", message)
		errorMsgBytes, _ := errorMsg.LineBytesStrict(false, MaxLineLen)
		finalData = append(finalData, errorMsgBytes...)

		sess.socket.SetFinalData(finalData)
	}

	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()

	var sessions []*Session
	if session != nil {
		sessions = []*Session{session}
	} else {
		sessions = client.sessions
	}

	for _, session := range sessions {
		if session.SetQuitMessage(message) {
			setFinalData(session)
		}
	}
}

// destroy gets rid of a client, removes them from server lists etc.
// if `session` is nil, destroys the client unconditionally, removing all sessions;
// otherwise, destroys one specific session, only destroying the client if it
// has no more sessions.
func (client *Client) destroy(session *Session) {
	config := client.server.Config()
	var sessionsToDestroy []*Session
	var saveLastSeen bool

	client.stateMutex.Lock()

	details := client.detailsNoMutex()
	brbState := client.brbTimer.state
	brbAt := client.brbTimer.brbAt
	wasReattach := session != nil && session.client != client
	sessionRemoved := false
	registered := client.registered
	// XXX a temporary (reattaching) client can be marked alwaysOn when it logs in,
	// but then the session attaches to another client and we need to clean it up here
	alwaysOn := registered && client.alwaysOn

	var remainingSessions int
	if session == nil {
		sessionsToDestroy = client.sessions
		client.sessions = nil
		remainingSessions = 0
	} else {
		sessionRemoved, remainingSessions = client.removeSession(session)
		if sessionRemoved {
			sessionsToDestroy = []*Session{session}
		}
	}

	// save last seen if applicable:
	if alwaysOn {
		if client.accountSettings.AutoreplayMissed {
			saveLastSeen = true
		} else {
			for _, session := range sessionsToDestroy {
				if session.deviceID != "" {
					saveLastSeen = true
					break
				}
			}
		}
	}

	// should we destroy the whole client this time?
	// BRB is not respected if this is a destroy of the whole client (i.e., session == nil)
	brbEligible := session != nil && brbState == BrbEnabled
	shouldDestroy := !client.destroyed && remainingSessions == 0 && !brbEligible && !alwaysOn
	// decrement stats on a true destroy, or for the removal of the last connected session
	// of an always-on client
	shouldDecrement := shouldDestroy || (alwaysOn && len(sessionsToDestroy) != 0 && len(client.sessions) == 0)
	if shouldDestroy {
		// if it's our job to destroy it, don't let anyone else try
		client.destroyed = true
	}
	if saveLastSeen {
		client.dirtyBits |= IncludeLastSeen
	}

	autoAway := false
	var awayMessage string
	if alwaysOn && !client.away && remainingSessions == 0 &&
		persistenceEnabled(config.Accounts.Multiclient.AutoAway, client.accountSettings.AutoAway) {
		autoAway = true
		client.autoAway = true
		client.away = true
		awayMessage = config.languageManager.Translate(client.languages, `User is currently disconnected`)
		client.awayMessage = awayMessage
	}

	if client.registrationTimer != nil {
		// unconditionally stop; if the client is still unregistered it must be destroyed
		client.registrationTimer.Stop()
	}

	client.stateMutex.Unlock()

	// XXX there is no particular reason to persist this state here rather than
	// any other place: it would be correct to persist it after every `Touch`. However,
	// I'm not comfortable introducing that many database writes, and I don't want to
	// design a throttle.
	if saveLastSeen {
		client.wakeWriter()
	}

	// destroy all applicable sessions:
	var quitMessage string
	for _, session := range sessionsToDestroy {
		if session.client != client {
			// session has been attached to a new client; do not destroy it
			continue
		}
		session.stopIdleTimer()
		// send quit/error message to client if they haven't been sent already
		client.Quit("", session)
		quitMessage = session.quitMessage
		session.SetDestroyed()
		session.socket.Close()

		// clean up monitor state
		client.server.monitorManager.RemoveAll(session)

		// remove from connection limits
		var source string
		if session.isTor {
			client.server.torLimiter.RemoveClient()
			source = "tor"
		} else {
			ip := session.realIP
			if session.proxiedIP != nil {
				ip = session.proxiedIP
			}
			client.server.connectionLimiter.RemoveClient(ip)
			source = ip.String()
		}
		client.server.logger.Info("connect-ip", fmt.Sprintf("disconnecting session of %s from %s", details.nick, source))
	}

	// decrement stats if we have no more sessions, even if the client will not be destroyed
	if shouldDecrement {
		invisible := client.HasMode(modes.Invisible)
		operator := client.HasMode(modes.LocalOperator) || client.HasMode(modes.Operator)
		client.server.stats.Remove(registered, invisible, operator)
	}

	if autoAway {
		dispatchAwayNotify(client, true, awayMessage)
	}

	if !shouldDestroy {
		return
	}

	splitQuitMessage := utils.MakeMessage(quitMessage)
	quitItem := history.Item{
		Type:        history.Quit,
		Nick:        details.nickMask,
		AccountName: details.accountName,
		Message:     splitQuitMessage,
	}
	var channels []*Channel
	// use a defer here to avoid writing to mysql while holding the destroy semaphore:
	defer func() {
		for _, channel := range channels {
			channel.AddHistoryItem(quitItem, details.account)
		}
	}()

	// see #235: deduplicating the list of PART recipients uses (comparatively speaking)
	// a lot of RAM, so limit concurrency to avoid thrashing
	client.server.semaphores.ClientDestroy.Acquire()
	defer client.server.semaphores.ClientDestroy.Release()

	if !wasReattach {
		client.server.logger.Debug("quit", fmt.Sprintf("%s is no longer on the server", details.nick))
	}

	if registered {
		client.server.whoWas.Append(client.WhoWas())
	}

	client.server.resumeManager.Delete(client)

	// alert monitors
	if registered {
		client.server.monitorManager.AlertAbout(details.nick, details.nickCasefolded, false)
	}

	// clean up channels
	// (note that if this is a reattach, client has no channels and therefore no friends)
	friends := make(ClientSet)
	channels = client.Channels()
	for _, channel := range channels {
		for _, member := range channel.auditoriumFriends(client) {
			friends.Add(member)
		}
		channel.Quit(client)
	}
	friends.Remove(client)

	// clean up server
	client.server.clients.Remove(client)

	// clean up self
	client.brbTimer.Disable()

	client.server.accounts.Logout(client)

	// this happens under failure to return from BRB
	if quitMessage == "" {
		if brbState == BrbDead && !brbAt.IsZero() {
			awayMessage := client.AwayMessage()
			if awayMessage == "" {
				awayMessage = "Disconnected" // auto-BRB
			}
			quitMessage = fmt.Sprintf("%s [%s ago]", awayMessage, time.Since(brbAt).Truncate(time.Second).String())
		}
	}
	if quitMessage == "" {
		quitMessage = "Exited"
	}
	for friend := range friends {
		friend.sendFromClientInternal(false, splitQuitMessage.Time, splitQuitMessage.Msgid, details.nickMask, details.accountName, nil, "QUIT", quitMessage)
	}

	if registered {
		client.server.snomasks.Send(sno.LocalQuits, fmt.Sprintf(ircfmt.Unescape("%s$r exited the network"), details.nick))
	}
}

// SendSplitMsgFromClient sends an IRC PRIVMSG/NOTICE coming from a specific client.
// Adds account-tag to the line as well.
func (session *Session) sendSplitMsgFromClientInternal(blocking bool, nickmask, accountName string, tags map[string]string, command, target string, message utils.SplitMessage) {
	if message.Is512() {
		session.sendFromClientInternal(blocking, message.Time, message.Msgid, nickmask, accountName, tags, command, target, message.Message)
	} else {
		if session.capabilities.Has(caps.Multiline) {
			for _, msg := range session.composeMultilineBatch(nickmask, accountName, tags, command, target, message) {
				session.SendRawMessage(msg, blocking)
			}
		} else {
			msgidSent := false // send msgid on the first nonblank line
			for _, messagePair := range message.Split {
				if len(messagePair.Message) == 0 {
					continue
				}
				var msgid string
				if !msgidSent {
					msgidSent = true
					msgid = message.Msgid
				}
				session.sendFromClientInternal(blocking, message.Time, msgid, nickmask, accountName, tags, command, target, messagePair.Message)
			}
		}
	}
}

// Sends a line with `nickmask` as the prefix, adding `time` and `account` tags if supported
func (client *Client) sendFromClientInternal(blocking bool, serverTime time.Time, msgid string, nickmask, accountName string, tags map[string]string, command string, params ...string) (err error) {
	for _, session := range client.Sessions() {
		err_ := session.sendFromClientInternal(blocking, serverTime, msgid, nickmask, accountName, tags, command, params...)
		if err_ != nil {
			err = err_
		}
	}
	return
}

func (session *Session) sendFromClientInternal(blocking bool, serverTime time.Time, msgid string, nickmask, accountName string, tags map[string]string, command string, params ...string) (err error) {
	msg := ircmsg.MakeMessage(tags, nickmask, command, params...)
	// attach account-tag
	if session.capabilities.Has(caps.AccountTag) && accountName != "*" {
		msg.SetTag("account", accountName)
	}
	// attach message-id
	if msgid != "" && session.capabilities.Has(caps.MessageTags) {
		msg.SetTag("msgid", msgid)
	}
	// attach server-time
	session.setTimeTag(&msg, serverTime)

	return session.SendRawMessage(msg, blocking)
}

func (session *Session) composeMultilineBatch(fromNickMask, fromAccount string, tags map[string]string, command, target string, message utils.SplitMessage) (result []ircmsg.IrcMessage) {
	batchID := session.generateBatchID()
	batchStart := ircmsg.MakeMessage(tags, fromNickMask, "BATCH", "+"+batchID, caps.MultilineBatchType, target)
	batchStart.SetTag("time", message.Time.Format(IRCv3TimestampFormat))
	batchStart.SetTag("msgid", message.Msgid)
	if session.capabilities.Has(caps.AccountTag) && fromAccount != "*" {
		batchStart.SetTag("account", fromAccount)
	}
	result = append(result, batchStart)

	for _, msg := range message.Split {
		message := ircmsg.MakeMessage(nil, fromNickMask, command, target, msg.Message)
		message.SetTag("batch", batchID)
		if msg.Concat {
			message.SetTag(caps.MultilineConcatTag, "")
		}
		result = append(result, message)
	}

	result = append(result, ircmsg.MakeMessage(nil, fromNickMask, "BATCH", "-"+batchID))
	return
}

var (
	// these are all the output commands that MUST have their last param be a trailing.
	// this is needed because dumb clients like to treat trailing params separately from the
	// other params in messages.
	commandsThatMustUseTrailing = map[string]bool{
		"PRIVMSG": true,
		"NOTICE":  true,

		RPL_WHOISCHANNELS: true,
		RPL_USERHOST:      true,

		// mirc's handling of RPL_NAMREPLY is broken:
		// https://forums.mirc.com/ubbthreads.php/topics/266939/re-nick-list
		RPL_NAMREPLY: true,
	}
)

// SendRawMessage sends a raw message to the client.
func (session *Session) SendRawMessage(message ircmsg.IrcMessage, blocking bool) error {
	// use dumb hack to force the last param to be a trailing param if required
	config := session.client.server.Config()
	if config.Server.Compatibility.forceTrailing && commandsThatMustUseTrailing[message.Command] {
		message.ForceTrailing()
	}

	// assemble message
	line, err := message.LineBytesStrict(false, MaxLineLen)
	if err != nil {
		errorParams := []string{"Error assembling message for sending", err.Error(), message.Command}
		errorParams = append(errorParams, message.Params...)
		session.client.server.logger.Error("internal", errorParams...)

		message = ircmsg.MakeMessage(nil, session.client.server.name, ERR_UNKNOWNERROR, "*", "Error assembling message for sending")
		line, _ := message.LineBytesStrict(false, 0)

		if blocking {
			session.socket.BlockingWrite(line)
		} else {
			session.socket.Write(line)
		}
		return err
	}

	if session.client.server.logger.IsLoggingRawIO() {
		logline := string(line[:len(line)-2]) // strip "\r\n"
		session.client.server.logger.Debug("useroutput", session.client.Nick(), " ->", logline)
	}

	if blocking {
		err = session.socket.BlockingWrite(line)
	} else {
		err = session.socket.Write(line)
	}
	if err != nil {
		session.client.server.logger.Info("quit", "send error to client", fmt.Sprintf("%s [%d]", session.client.Nick(), session.sessionID), err.Error())
	}
	return err
}

// Send sends an IRC line to the client.
func (client *Client) Send(tags map[string]string, prefix string, command string, params ...string) (err error) {
	for _, session := range client.Sessions() {
		err_ := session.Send(tags, prefix, command, params...)
		if err_ != nil {
			err = err_
		}
	}
	return
}

func (session *Session) Send(tags map[string]string, prefix string, command string, params ...string) (err error) {
	msg := ircmsg.MakeMessage(tags, prefix, command, params...)
	session.setTimeTag(&msg, time.Time{})
	return session.SendRawMessage(msg, false)
}

func (session *Session) setTimeTag(msg *ircmsg.IrcMessage, serverTime time.Time) {
	if session.capabilities.Has(caps.ServerTime) && !msg.HasTag("time") {
		if serverTime.IsZero() {
			serverTime = time.Now()
		}
		msg.SetTag("time", serverTime.UTC().Format(IRCv3TimestampFormat))
	}
}

// Notice sends the client a notice from the server.
func (client *Client) Notice(text string) {
	client.Send(nil, client.server.name, "NOTICE", client.Nick(), text)
}

func (session *Session) Notice(text string) {
	session.Send(nil, session.client.server.name, "NOTICE", session.client.Nick(), text)
}

// `simulated` is for the fake join of an always-on client
// (we just read the channel name from the database, there's no need to write it back)
func (client *Client) addChannel(channel *Channel, simulated bool) (err error) {
	config := client.server.Config()

	client.stateMutex.Lock()
	alwaysOn := client.alwaysOn
	if client.destroyed {
		err = errClientDestroyed
	} else if client.oper == nil && len(client.channels) >= config.Channels.MaxChannelsPerClient {
		err = errTooManyChannels
	} else {
		client.channels[channel] = empty{} // success
	}
	client.stateMutex.Unlock()

	if err == nil && alwaysOn && !simulated {
		client.markDirty(IncludeChannels)
	}
	return
}

func (client *Client) removeChannel(channel *Channel) {
	client.stateMutex.Lock()
	delete(client.channels, channel)
	alwaysOn := client.alwaysOn
	client.stateMutex.Unlock()

	if alwaysOn {
		client.markDirty(IncludeChannels)
	}
}

// Records that the client has been invited to join an invite-only channel
func (client *Client) Invite(casefoldedChannel string) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()

	if client.invitedTo == nil {
		client.invitedTo = make(utils.StringSet)
	}

	client.invitedTo.Add(casefoldedChannel)
}

// Checks that the client was invited to join a given channel
func (client *Client) CheckInvited(casefoldedChannel string) (invited bool) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()

	invited = client.invitedTo.Has(casefoldedChannel)
	// joining an invited channel "uses up" your invite, so you can't rejoin on kick
	delete(client.invitedTo, casefoldedChannel)
	return
}

// Implements auto-oper by certfp (scans for an auto-eligible operator block that matches
// the client's cert, then applies it).
func (client *Client) attemptAutoOper(session *Session) {
	if session.certfp == "" || client.HasMode(modes.Operator) {
		return
	}
	for _, oper := range client.server.Config().operators {
		if oper.Auto && oper.Pass == nil && oper.Certfp != "" && oper.Certfp == session.certfp {
			rb := NewResponseBuffer(session)
			applyOper(client, oper, rb)
			rb.Send(true)
			return
		}
	}
}

func (client *Client) checkLoginThrottle() (throttled bool, remainingTime time.Duration) {
	client.stateMutex.Lock()
	defer client.stateMutex.Unlock()
	return client.loginThrottle.Touch()
}

func (client *Client) historyStatus(config *Config) (status HistoryStatus, target string) {
	if !config.History.Enabled {
		return HistoryDisabled, ""
	}

	client.stateMutex.RLock()
	target = client.account
	historyStatus := client.accountSettings.DMHistory
	client.stateMutex.RUnlock()

	if target == "" {
		return HistoryEphemeral, ""
	}
	status = historyEnabled(config.History.Persistent.DirectMessages, historyStatus)
	if status != HistoryPersistent {
		target = ""
	}
	return
}

func (client *Client) handleRegisterTimeout() {
	client.Quit(fmt.Sprintf("Registration timeout: %v", RegisterTimeout), nil)
	client.destroy(nil)
}

func (client *Client) copyLastSeen() (result map[string]time.Time) {
	client.stateMutex.RLock()
	defer client.stateMutex.RUnlock()
	result = make(map[string]time.Time, len(client.lastSeen))
	for id, lastSeen := range client.lastSeen {
		result[id] = lastSeen
	}
	return
}

// these are bit flags indicating what part of the client status is "dirty"
// and needs to be read from memory and written to the db
const (
	IncludeChannels uint = 1 << iota
	IncludeLastSeen
	IncludeUserModes
	IncludeRealname
)

func (client *Client) markDirty(dirtyBits uint) {
	client.stateMutex.Lock()
	alwaysOn := client.alwaysOn
	client.dirtyBits = client.dirtyBits | dirtyBits
	client.stateMutex.Unlock()

	if alwaysOn {
		client.wakeWriter()
	}
}

func (client *Client) wakeWriter() {
	if client.writerSemaphore.TryAcquire() {
		go client.writeLoop()
	}
}

func (client *Client) writeLoop() {
	for {
		client.performWrite(0)
		client.writerSemaphore.Release()

		client.stateMutex.RLock()
		isDirty := client.dirtyBits != 0
		client.stateMutex.RUnlock()

		if !isDirty || !client.writerSemaphore.TryAcquire() {
			return
		}
	}
}

func (client *Client) performWrite(additionalDirtyBits uint) {
	client.stateMutex.Lock()
	dirtyBits := client.dirtyBits | additionalDirtyBits
	client.dirtyBits = 0
	account := client.account
	client.stateMutex.Unlock()

	if account == "" {
		client.server.logger.Error("internal", "attempting to persist logged-out client", client.Nick())
		return
	}

	if (dirtyBits & IncludeChannels) != 0 {
		channels := client.Channels()
		channelNames := make([]string, len(channels))
		for i, channel := range channels {
			channelNames[i] = channel.Name()
		}
		client.server.accounts.saveChannels(account, channelNames)
	}
	if (dirtyBits & IncludeLastSeen) != 0 {
		client.server.accounts.saveLastSeen(account, client.copyLastSeen())
	}
	if (dirtyBits & IncludeUserModes) != 0 {
		uModes := make(modes.Modes, 0, len(modes.SupportedUserModes))
		for _, m := range modes.SupportedUserModes {
			switch m {
			case modes.Operator, modes.ServerNotice:
				// these can't be persisted because they depend on the operator block
			default:
				if client.HasMode(m) {
					uModes = append(uModes, m)
				}
			}
		}
		client.server.accounts.saveModes(account, uModes)
	}
	if (dirtyBits & IncludeRealname) != 0 {
		client.server.accounts.saveRealname(account, client.realname)
	}
}

// Blocking store; see Channel.Store and Socket.BlockingWrite
func (client *Client) Store(dirtyBits uint) (err error) {
	defer func() {
		client.stateMutex.Lock()
		isDirty := client.dirtyBits != 0
		client.stateMutex.Unlock()

		if isDirty {
			client.wakeWriter()
		}
	}()

	client.writerSemaphore.Acquire()
	defer client.writerSemaphore.Release()
	client.performWrite(dirtyBits)
	return nil
}