mox/imapserver/server.go

// Package imapserver implements an IMAPv4 server, rev2 (RFC 9051) and rev1 with extensions (RFC 3501 and more).
package imapserver

/*
Implementation notes

IMAP4rev2 includes functionality that was in extensions for IMAP4rev1. The
extensions sometimes include features not in IMAP4rev2. We want IMAP4rev1-only
implementations to use extensions, so we implement the full feature set of the
extension and announce it as capability. The extensions: LITERAL+, IDLE,
NAMESPACE, BINARY, UNSELECT, UIDPLUS, ESEARCH, SEARCHRES, SASL-IR, ENABLE,
LIST-EXTENDED, SPECIAL-USE, MOVE, UTF8=ONLY.

We take a liberty with UTF8=ONLY. We are supposed to wait for ENABLE of
UTF8=ACCEPT or IMAP4rev2 before we respond with quoted strings that contain
non-ASCII UTF-8. But we will unconditionally accept UTF-8 at the moment. See
../rfc/6855:251

We always respond with utf8 mailbox names. We do parse utf7 (only in IMAP4rev1,
not in IMAP4rev2). ../rfc/3501:964

- We never execute multiple commands at the same time for a connection. We expect a client to open multiple connections instead. ../rfc/9051:1110
- Do not write output on a connection with an account lock held. Writing can block, a slow client could block account operations.
- When handling commands that modify the selected mailbox, always check that the mailbox is not opened readonly. And always revalidate the selected mailbox, another session may have deleted the mailbox.
- After making changes to an account/mailbox/message, you must broadcast changes. You must do this with the account lock held. Otherwise, other later changes (e.g. message deliveries) may be made and broadcast before changes that were made earlier. Make sure to commit changes in the database first, because the commit may fail.
- Mailbox hierarchies are slash separated, no leading slash. We keep the case, except INBOX is renamed to Inbox, also for submailboxes in INBOX. We don't allow existence of a child where its parent does not exist. We have no \NoInferiors or \NoSelect. Newly created mailboxes are automatically subscribed.
*/

/*
- todo: do not return binary data for a fetch body. at least not for imap4rev1. we should be encoding it as base64?
- todo: on expunge we currently remove the message even if other sessions still have a reference to the uid. if they try to query the uid, they'll get an error. we could be nicer and only actually remove the message when the last reference has gone. we could add a new flag to store.Message marking the message as expunged, not give new session access to such messages, and make store remove them at startup, and clean them when the last session referencing the session goes. however, it will get much more complicated. renaming messages would need special handling. and should we do the same for removed mailboxes?
- todo: CONDSTORE, QRESYNC. Add fields modseq on mailbox and each message. Keep (log of) deleted messages and their modseqs.
- todo: try to recover from syntax errors when the last command line ends with a }, i.e. a literal. we currently abort the entire connection. we may want to read some amount of literal data and continue with a next command.
- future: more extensions: STATUS=SIZE, OBJECTID, MULTISEARCH, REPLACE, NOTIFY, CATENATE, MULTIAPPEND, SORT, THREAD, CREATE-SPECIAL-USE.
- future: implement user-defined keyword flags? ../rfc/9051:566
*/

import (
	"bufio"
	"bytes"
	"context"
	"crypto/tls"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"net"
	"os"
	"path/filepath"
	"regexp"
	"runtime/debug"
	"sort"
	"strings"
	"time"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"golang.org/x/text/unicode/norm"

	"github.com/mjl-/bstore"

	"github.com/mjl-/mox/config"
	"github.com/mjl-/mox/message"
	"github.com/mjl-/mox/metrics"
	"github.com/mjl-/mox/mlog"
	"github.com/mjl-/mox/mox-"
	"github.com/mjl-/mox/moxio"
	"github.com/mjl-/mox/moxvar"
	"github.com/mjl-/mox/scram"
	"github.com/mjl-/mox/store"
)

// Most logging should be done through conn.log* functions.
// Only use imaplog in contexts without connection.
var xlog = mlog.New("imapserver")

var (
	metricIMAPConnection = promauto.NewCounterVec(
		prometheus.CounterOpts{
			Name: "mox_imap_connection_total",
			Help: "Incoming IMAP connections.",
		},
		[]string{
			"service", // imap, imaps
		},
	)
	metricIMAPCommands = promauto.NewHistogramVec(
		prometheus.HistogramOpts{
			Name:    "mox_imap_command_duration_seconds",
			Help:    "IMAP command duration and result codes in seconds.",
			Buckets: []float64{0.001, 0.005, 0.01, 0.05, 0.100, 0.5, 1, 5, 10, 20},
		},
		[]string{
			"cmd",
			"result", // ok, panic, ioerror, badsyntax, servererror, usererror, error
		},
	)
)

// Capabilities (extensions) the server supports. Connections will add a few more, e.g. STARTTLS, LOGINDISABLED, AUTH=PLAIN.
// ENABLE: ../rfc/5161
// LITERAL+: ../rfc/7888
// IDLE: ../rfc/2177
// SASL-IR: ../rfc/4959
// BINARY: ../rfc/3516
// UNSELECT: ../rfc/3691
// UIDPLUS: ../rfc/4315
// ESEARCH: ../rfc/4731
// SEARCHRES: ../rfc/5182
// MOVE: ../rfc/6851
// UTF8=ONLY: ../rfc/6855
// LIST-EXTENDED: ../rfc/5258
// SPECIAL-USE: ../rfc/6154
// LIST-STATUS: ../rfc/5819
// ID: ../rfc/2971
// AUTH=SCRAM-SHA-256: ../rfc/7677 ../rfc/5802
// APPENDLIMIT, we support the max possible size, 1<<63 - 1: ../rfc/7889:129
const serverCapabilities = "IMAP4rev2 IMAP4rev1 ENABLE LITERAL+ IDLE SASL-IR BINARY UNSELECT UIDPLUS ESEARCH SEARCHRES MOVE UTF8=ONLY LIST-EXTENDED SPECIAL-USE LIST-STATUS AUTH=SCRAM-SHA-256 ID APPENDLIMIT=9223372036854775807"

type conn struct {
	cid               int64
	state             state
	conn              net.Conn
	tls               bool          // Whether TLS has been initialized.
	br                *bufio.Reader // From remote, with TLS unwrapped in case of TLS.
	line              chan lineErr  // If set, instead of reading from br, a line is read from this channel. For reading a line in IDLE while also waiting for mailbox/account updates.
	lastLine          string        // For detecting if syntax error is fatal, i.e. if this ends with a literal. Without crlf.
	bw                *bufio.Writer // To remote, with TLS added in case of TLS.
	lastlog           time.Time     // For printing time since previous log line.
	tlsConfig         *tls.Config   // TLS config to use for handshake.
	noRequireSTARTTLS bool
	cmd               string // Currently executing, for deciding to applyChanges and logging.
	cmdMetric         string // Currently executing, for metrics.
	cmdStart          time.Time
	log               *mlog.Log
	enabled           map[capability]bool // All upper-case.

	// Set by SEARCH with SAVE. Can be used by commands accepting a sequence-set with
	// value "$". When used, UIDs must be verified to still exist, because they may
	// have been expunged. Cleared by a SELECT or EXAMINE.
	// Nil means no searchResult is present. An empty list is a valid searchResult,
	// just not matching any messages.
	// ../rfc/5182:13 ../rfc/9051:4040
	searchResult []store.UID

	// Only when authenticated.
	username string // Full username as used during login.
	account  *store.Account
	comm     *store.Comm // For sending/receiving changes on mailboxes in account, e.g. from messages incoming on smtp, or another imap client.

	mailboxID int64       // Only for StateSelected.
	readonly  bool        // If opened mailbox is readonly.
	uids      []store.UID // UIDs known in this session, sorted. todo future: store more space-efficiently, as ranges.
}

// capability for use with ENABLED and CAPABILITY. We always keep this upper case,
// e.g. IMAP4REV2. These values are treated case-insensitive, but it's easier for
// comparison to just always have the same case.
type capability string

const (
	capIMAP4rev2  capability = "IMAP4REV2"
	capUTF8Accept capability = "UTF8=ACCEPT"
)

type lineErr struct {
	line string
	err  error
}

type state byte

const (
	stateNotAuthenticated state = iota
	stateAuthenticated
	stateSelected
)

func stateCommands(cmds ...string) map[string]struct{} {
	r := map[string]struct{}{}
	for _, cmd := range cmds {
		r[cmd] = struct{}{}
	}
	return r
}

var (
	commandsStateAny              = stateCommands("capability", "noop", "logout", "id")
	commandsStateNotAuthenticated = stateCommands("starttls", "authenticate", "login")
	commandsStateAuthenticated    = stateCommands("enable", "select", "examine", "create", "delete", "rename", "subscribe", "unsubscribe", "list", "namespace", "status", "append", "idle", "lsub")
	commandsStateSelected         = stateCommands("close", "unselect", "expunge", "search", "fetch", "store", "copy", "move", "uid expunge", "uid search", "uid fetch", "uid store", "uid copy", "uid move")
)

var commands = map[string]func(c *conn, tag, cmd string, p *parser){
	// Any state.
	"capability": (*conn).cmdCapability,
	"noop":       (*conn).cmdNoop,
	"logout":     (*conn).cmdLogout,
	"id":         (*conn).cmdID,

	// Notauthenticated.
	"starttls":     (*conn).cmdStarttls,
	"authenticate": (*conn).cmdAuthenticate,
	"login":        (*conn).cmdLogin,

	// Authenticated and selected.
	"enable":      (*conn).cmdEnable,
	"select":      (*conn).cmdSelect,
	"examine":     (*conn).cmdExamine,
	"create":      (*conn).cmdCreate,
	"delete":      (*conn).cmdDelete,
	"rename":      (*conn).cmdRename,
	"subscribe":   (*conn).cmdSubscribe,
	"unsubscribe": (*conn).cmdUnsubscribe,
	"list":        (*conn).cmdList,
	"lsub":        (*conn).cmdLsub,
	"namespace":   (*conn).cmdNamespace,
	"status":      (*conn).cmdStatus,
	"append":      (*conn).cmdAppend,
	"idle":        (*conn).cmdIdle,

	// Selected.
	"check":       (*conn).cmdCheck,
	"close":       (*conn).cmdClose,
	"unselect":    (*conn).cmdUnselect,
	"expunge":     (*conn).cmdExpunge,
	"uid expunge": (*conn).cmdUIDExpunge,
	"search":      (*conn).cmdSearch,
	"uid search":  (*conn).cmdUIDSearch,
	"fetch":       (*conn).cmdFetch,
	"uid fetch":   (*conn).cmdUIDFetch,
	"store":       (*conn).cmdStore,
	"uid store":   (*conn).cmdUIDStore,
	"copy":        (*conn).cmdCopy,
	"uid copy":    (*conn).cmdUIDCopy,
	"move":        (*conn).cmdMove,
	"uid move":    (*conn).cmdUIDMove,
}

var errIO = errors.New("fatal io error")             // For read/write errors and errors that should close the connection.
var errProtocol = errors.New("fatal protocol error") // For protocol errors for which a stack trace should be printed.

var sanityChecks bool

// check err for sanity.
// if not nil and checkSanity true (set during tests), then panic. if not nil during normal operation, just log.
func (c *conn) xsanity(err error, format string, args ...any) {
	if err == nil {
		return
	}
	if sanityChecks {
		panic(fmt.Errorf("%s: %s", fmt.Sprintf(format, args...), err))
	}
	c.log.Errorx(fmt.Sprintf(format, args...), err)
}

type msgseq uint32

// ListenAndServe starts all imap listeners for the configuration, in new goroutines.
func ListenAndServe() {
	for name, listener := range mox.Conf.Static.Listeners {
		var tlsConfig *tls.Config
		if listener.TLS != nil {
			tlsConfig = listener.TLS.Config
		}

		if listener.IMAP.Enabled {
			port := config.Port(listener.IMAP.Port, 143)
			for _, ip := range listener.IPs {
				go listenServe("imap", name, ip, port, tlsConfig, false, listener.IMAP.NoRequireSTARTTLS)
			}
		}

		if listener.IMAPS.Enabled {
			port := config.Port(listener.IMAPS.Port, 993)
			for _, ip := range listener.IPs {
				go listenServe("imaps", name, ip, port, tlsConfig, true, false)
			}
		}
	}
}

func listenServe(protocol, listenerName, ip string, port int, tlsConfig *tls.Config, xtls, noRequireSTARTTLS bool) {
	addr := net.JoinHostPort(ip, fmt.Sprintf("%d", port))
	xlog.Print("listening for imap", mlog.Field("listener", listenerName), mlog.Field("addr", addr), mlog.Field("protocol", protocol))
	network := mox.Network(ip)
	var ln net.Listener
	var err error
	if xtls {
		ln, err = tls.Listen(network, addr, tlsConfig)
	} else {
		ln, err = net.Listen(network, addr)
	}
	if err != nil {
		xlog.Fatalx("imap: listen for imap"+mox.LinuxSetcapHint(err), err, mlog.Field("protocol", protocol), mlog.Field("listener", listenerName))
	}

	for {
		conn, err := ln.Accept()
		if err != nil {
			xlog.Infox("imap: accept", err, mlog.Field("protocol", protocol), mlog.Field("listener", listenerName))
			continue
		}

		metricIMAPConnection.WithLabelValues(protocol).Inc()
		go serve(listenerName, mox.Cid(), tlsConfig, conn, xtls, noRequireSTARTTLS)
	}
}

// returns whether this connection accepts utf-8 in strings.
func (c *conn) utf8strings() bool {
	return c.enabled[capIMAP4rev2] || c.enabled[capUTF8Accept]
}

func (c *conn) xdbwrite(fn func(tx *bstore.Tx)) {
	err := c.account.DB.Write(func(tx *bstore.Tx) error {
		fn(tx)
		return nil
	})
	xcheckf(err, "transaction")
}

func (c *conn) xdbread(fn func(tx *bstore.Tx)) {
	err := c.account.DB.Read(func(tx *bstore.Tx) error {
		fn(tx)
		return nil
	})
	xcheckf(err, "transaction")
}

// Closes the currently selected/active mailbox, setting state from selected to authenticated.
// Does not remove messages marked for deletion.
func (c *conn) unselect() {
	if c.state == stateSelected {
		c.state = stateAuthenticated
	}
	c.mailboxID = 0
	c.uids = nil
}

// Write makes a connection an io.Writer. It panics for i/o errors. These errors
// are handled in the connection command loop.
func (c *conn) Write(buf []byte) (int, error) {
	if err := c.conn.SetWriteDeadline(time.Now().Add(30 * time.Second)); err != nil {
		c.log.Errorx("setting write deadline", err)
	}

	n, err := c.conn.Write(buf)
	if err != nil {
		panic(fmt.Errorf("write: %s (%w)", err, errIO))
	}
	return n, err
}

// Cache of line buffers for reading commands.
var bufpool = moxio.NewBufpool(8, 16*1024)

// read line from connection, not going through line channel.
func (c *conn) readline0() (string, error) {
	d := 30 * time.Minute
	if c.state == stateNotAuthenticated {
		d = 30 * time.Second
	}
	c.conn.SetReadDeadline(time.Now().Add(d))

	line, err := bufpool.Readline(c.br)
	if err != nil && errors.Is(err, moxio.ErrLineTooLong) {
		return "", fmt.Errorf("%s (%w)", err, errProtocol)
	} else if err != nil {
		return "", fmt.Errorf("%s (%w)", err, errIO)
	}
	return line, nil
}

func (c *conn) lineChan() chan lineErr {
	if c.line == nil {
		c.line = make(chan lineErr, 1)
		go func() {
			line, err := c.readline0()
			c.line <- lineErr{line, err}
		}()
	}
	return c.line
}

// readline from either the c.line channel, or otherwise read from connection.
func (c *conn) readline(readCmd bool) string {
	var line string
	var err error
	if c.line != nil {
		le := <-c.line
		c.line = nil
		line, err = le.line, le.err
	} else {
		line, err = c.readline0()
	}
	if err != nil {
		if readCmd && errors.Is(err, os.ErrDeadlineExceeded) {
			c.conn.SetWriteDeadline(time.Now().Add(10 * time.Second))
			c.writelinef("* BYE inactive")
		}
		if !errors.Is(err, errIO) && !errors.Is(err, errProtocol) {
			err = fmt.Errorf("%s (%w)", err, errIO)
		}
		panic(err)
	}
	c.lastLine = line

	// We typically respond immediately (IDLE is an exception).
	// The client may not be reading, or may have disappeared.
	// Don't wait more than 5 minutes before closing down the connection.
	// The write deadline is managed in IDLE as well.
	// For unauthenticated connections, we require the client to read faster.
	wd := 5 * time.Minute
	if c.state == stateNotAuthenticated {
		wd = 30 * time.Second
	}
	c.conn.SetWriteDeadline(time.Now().Add(wd))

	return line
}

// write tagged command response, but first write pending changes.
func (c *conn) writeresultf(format string, args ...any) {
	c.bwriteresultf(format, args...)
	c.xflush()
}

// write buffered taggedcommand response, but first write pending changes.
func (c *conn) bwriteresultf(format string, args ...any) {
	switch c.cmd {
	case "fetch", "store", "search":
		// ../rfc/9051:5862
	default:
		if c.comm != nil {
			c.applyChanges(c.comm.Get(), false)
		}
	}
	c.bwritelinef(format, args...)
}

func (c *conn) writelinef(format string, args ...any) {
	c.bwritelinef(format, args...)
	c.xflush()
}

// Buffer line for write.
func (c *conn) bwritelinef(format string, args ...any) {
	format += "\r\n"
	fmt.Fprintf(c.bw, format, args...)
}

func (c *conn) xflush() {
	err := c.bw.Flush()
	xcheckf(err, "flush") // Should never happen, the Write caused by the Flush should panic on i/o error.
}

func (c *conn) readCommand(tag *string) (cmd string, p *parser) {
	line := c.readline(true)
	p = newParser(line, c)
	p.context("tag")
	*tag = p.xtag()
	p.context("command")
	p.xspace()
	cmd = p.xcommand()
	return cmd, newParser(p.remainder(), c)
}

func (c *conn) xreadliteral(size int64, sync bool) string {
	if sync {
		c.writelinef("+")
	}
	buf := make([]byte, size)
	if size > 0 {
		if err := c.conn.SetReadDeadline(time.Now().Add(30 * time.Second)); err != nil {
			c.log.Errorx("setting read deadline", err)
		}

		_, err := io.ReadFull(c.br, buf)
		if err != nil {
			// Cannot use xcheckf due to %w handling of errIO.
			panic(fmt.Errorf("reading literal: %s (%w)", err, errIO))
		}
	}
	return string(buf)
}

var cleanClose struct{} // Sentinel value for panic/recover indicating clean close of connection.

func serve(listenerName string, cid int64, tlsConfig *tls.Config, nc net.Conn, xtls, noRequireSTARTTLS bool) {
	c := &conn{
		cid:               cid,
		conn:              nc,
		tls:               xtls,
		lastlog:           time.Now(),
		tlsConfig:         tlsConfig,
		noRequireSTARTTLS: noRequireSTARTTLS,
		enabled:           map[capability]bool{},
		cmd:               "(greeting)",
		cmdStart:          time.Now(),
	}
	c.log = xlog.MoreFields(func() []mlog.Pair {
		now := time.Now()
		l := []mlog.Pair{
			mlog.Field("cid", c.cid),
			mlog.Field("delta", now.Sub(c.lastlog)),
		}
		c.lastlog = now
		if c.username != "" {
			l = append(l, mlog.Field("username", c.username))
		}
		return l
	})
	c.br = bufio.NewReader(moxio.NewTraceReader(c.log, "C: ", c.conn))
	c.bw = bufio.NewWriter(moxio.NewTraceWriter(c.log, "S: ", c))

	// Many IMAP connections use IDLE to wait for new incoming messages. We'll enable
	// keepalive to get a higher chance of the connection staying alive, or otherwise
	// detecting broken connections early.
	xconn := c.conn
	if xtls {
		xconn = c.conn.(*tls.Conn).NetConn()
	}
	if tcpconn, ok := xconn.(*net.TCPConn); ok {
		if err := tcpconn.SetKeepAlivePeriod(5 * time.Minute); err != nil {
			c.log.Errorx("setting keepalive period", err)
		} else if err := tcpconn.SetKeepAlive(true); err != nil {
			c.log.Errorx("enabling keepalive", err)
		}
	}

	c.log.Info("new connection", mlog.Field("remote", c.conn.RemoteAddr()), mlog.Field("local", c.conn.LocalAddr()), mlog.Field("tls", xtls), mlog.Field("listener", listenerName))

	defer func() {
		c.conn.Close()

		if c.account != nil {
			c.comm.Unregister()
			err := c.account.Close()
			c.xsanity(err, "close account")
			c.account = nil
			c.comm = nil
		}

		x := recover()
		if x == nil || x == cleanClose {
			c.log.Info("connection closed")
		} else if err, ok := x.(error); ok || isClosed(err) {
			c.log.Infox("connection closed", err)
		} else {
			c.log.Error("unhandled error", mlog.Field("err", x))
			debug.PrintStack()
			metrics.PanicInc("imapserver")
		}
	}()

	select {
	case <-mox.Shutdown:
		// ../rfc/9051:5381
		c.writelinef("* BYE mox shutting down")
		panic(errIO)
	default:
	}

	// We register and unregister the original connection, in case it c.conn is
	// replaced with a TLS connection later on.
	mox.Connections.Register(nc, "imap", listenerName)
	defer mox.Connections.Unregister(nc)

	c.writelinef("* OK [CAPABILITY %s] mox imap", c.capabilities())

	for {
		c.command()
		c.xflush() // For flushing errors, or possibly commands that did not flush explicitly.
	}
}

// isClosed returns whether i/o failed, typically because the connection is closed.
// For connection errors, we often want to generate fewer logs.
func isClosed(err error) bool {
	return errors.Is(err, errIO) || errors.Is(err, errProtocol) || moxio.IsClosed(err)
}

func (c *conn) command() {
	var tag, cmd, cmdlow string
	var p *parser

	defer func() {
		var result string
		defer func() {
			metricIMAPCommands.WithLabelValues(c.cmdMetric, result).Observe(float64(time.Since(c.cmdStart)) / float64(time.Second))
		}()

		logFields := []mlog.Pair{
			mlog.Field("cmd", c.cmd),
			mlog.Field("duration", time.Since(c.cmdStart)),
		}
		c.cmd = ""

		x := recover()
		if x == nil || x == cleanClose {
			c.log.Debug("imap command done", logFields...)
			result = "ok"
			return
		}
		err, ok := x.(error)
		if !ok {
			c.log.Error("imap command panic", append([]mlog.Pair{mlog.Field("panic", x)}, logFields...)...)
			result = "panic"
			panic(x)
		}

		if isClosed(err) {
			c.log.Infox("imap command ioerror", err, logFields...)
			result = "ioerror"
			if errors.Is(err, errProtocol) {
				debug.PrintStack()
			}
			panic(err)
		}

		var sxerr syntaxError
		var uerr userError
		var serr serverError
		if errors.As(err, &sxerr) {
			result = "badsyntax"
			c.log.Debugx("imap command syntax error", err, logFields...)
			c.log.Info("imap syntax error", mlog.Field("lastline", c.lastLine))
			fatal := strings.HasSuffix(c.lastLine, "+}")
			if fatal {
				c.conn.SetWriteDeadline(time.Now().Add(5 * time.Second))
			}
			c.bwriteresultf("%s BAD %s unrecognized syntax/command: %v", tag, cmd, err)
			if fatal {
				c.xflush()
				panic(fmt.Errorf("aborting connection after syntax error for command with non-sync literal: %w", errProtocol))
			}
		} else if errors.As(err, &serr) {
			result = "servererror"
			c.log.Errorx("imap command server error", err, logFields...)
			debug.PrintStack()
			c.bwriteresultf("%s NO %s %v", tag, cmd, err)
		} else if errors.As(err, &uerr) {
			result = "usererror"
			c.log.Debugx("imap command user error", err, logFields...)
			if uerr.code != "" {
				c.bwriteresultf("%s NO [%s] %s %v", tag, uerr.code, cmd, err)
			} else {
				c.bwriteresultf("%s NO %s %v", tag, cmd, err)
			}
		} else {
			result = "error"
			c.log.Infox("imap command error", err, logFields...)
			// todo: introduce a store.Error, and check for that, don't blindly pass on errors?
			debug.PrintStack()
			c.bwriteresultf("%s NO %s %v", tag, cmd, err)
		}
	}()

	tag = "*"
	cmd, p = c.readCommand(&tag)
	cmdlow = strings.ToLower(cmd)
	c.cmd = cmdlow
	c.cmdStart = time.Now()
	c.cmdMetric = "(unrecognized)"

	select {
	case <-mox.Shutdown:
		// ../rfc/9051:5375
		c.writelinef("* BYE shutting down")
		panic(errIO)
	default:
	}

	fn := commands[cmdlow]
	if fn == nil {
		xsyntaxErrorf("unknown command %q", cmd)
	}
	c.cmdMetric = c.cmd

	// Check if command is allowed in this state.
	if _, ok1 := commandsStateAny[cmdlow]; ok1 {
	} else if _, ok2 := commandsStateNotAuthenticated[cmdlow]; ok2 && c.state == stateNotAuthenticated {
	} else if _, ok3 := commandsStateAuthenticated[cmdlow]; ok3 && c.state == stateAuthenticated || c.state == stateSelected {
	} else if _, ok4 := commandsStateSelected[cmdlow]; ok4 && c.state == stateSelected {
	} else if ok1 || ok2 || ok3 || ok4 {
		xuserErrorf("not allowed in this connection state")
	} else {
		xserverErrorf("unrecognized command")
	}

	fn(c, tag, cmd, p)
}

func (c *conn) broadcast(changes []store.Change) {
	if len(changes) == 0 {
		return
	}
	c.log.Debug("broadcast changes", mlog.Field("changes", changes))
	c.comm.Broadcast(changes)
}

// matchStringer matches a string against reference + mailbox patterns.
type matchStringer interface {
	MatchString(s string) bool
}

type noMatch struct{}

// MatchString for noMatch always returns false.
func (noMatch) MatchString(s string) bool {
	return false
}

// xmailboxPatternMatcher returns a matcher for mailbox names given the reference and patterns.
// Patterns can include "%" and "*", matching any character excluding and including a slash respectively.
func xmailboxPatternMatcher(ref string, patterns []string) matchStringer {
	if strings.HasPrefix(ref, "/") {
		return noMatch{}
	}

	var subs []string
	for _, pat := range patterns {
		if strings.HasPrefix(pat, "/") {
			continue
		}

		s := pat
		if ref != "" {
			s = filepath.Join(ref, pat)
		}

		// Fix casing for all Inbox paths.
		first := strings.SplitN(s, "/", 2)[0]
		if strings.EqualFold(first, "Inbox") {
			s = "Inbox" + s[len("Inbox"):]
		}

		// ../rfc/9051:2361
		var rs string
		for _, c := range s {
			if c == '%' {
				rs += "[^/]*"
			} else if c == '*' {
				rs += ".*"
			} else {
				rs += regexp.QuoteMeta(string(c))
			}
		}
		subs = append(subs, rs)
	}

	if len(subs) == 0 {
		return noMatch{}
	}
	rs := "^(" + strings.Join(subs, "|") + ")$"
	re, err := regexp.Compile(rs)
	xcheckf(err, "compiling regexp for mailbox patterns")
	return re
}

func (c *conn) sequence(uid store.UID) msgseq {
	return uidSearch(c.uids, uid)
}

func uidSearch(uids []store.UID, uid store.UID) msgseq {
	s := 0
	e := len(uids)
	for s < e {
		i := (s + e) / 2
		m := uids[i]
		if uid == m {
			return msgseq(i + 1)
		} else if uid < m {
			e = i
		} else {
			s = i + 1
		}
	}
	return 0
}

func (c *conn) xsequence(uid store.UID) msgseq {
	seq := c.sequence(uid)
	if seq <= 0 {
		xserverErrorf("unknown uid %d (%w)", uid, errProtocol)
	}
	return seq
}

func (c *conn) sequenceRemove(seq msgseq, uid store.UID) {
	i := seq - 1
	if c.uids[i] != uid {
		xserverErrorf(fmt.Sprintf("got uid %d at msgseq %d, expected uid %d", uid, seq, c.uids[i]))
	}
	copy(c.uids[i:], c.uids[i+1:])
	c.uids = c.uids[:len(c.uids)-1]
	if sanityChecks {
		checkUIDs(c.uids)
	}
}

// add uid to the session. care must be taken that pending changes are fetched
// while holding the account wlock, and applied before adding this uid, because
// those pending changes may contain another new uid that has to be added first.
func (c *conn) uidAppend(uid store.UID) {
	if uidSearch(c.uids, uid) > 0 {
		xserverErrorf("uid already present (%w)", errProtocol)
	}
	if len(c.uids) > 0 && uid < c.uids[len(c.uids)-1] {
		xserverErrorf("new uid %d is smaller than last uid %d (%w)", uid, c.uids[len(c.uids)-1], errProtocol)
	}
	c.uids = append(c.uids, uid)
	if sanityChecks {
		checkUIDs(c.uids)
	}
}

// sanity check that uids are in ascending order.
func checkUIDs(uids []store.UID) {
	for i, uid := range uids {
		if uid == 0 || i > 0 && uid <= uids[i-1] {
			xserverErrorf("bad uids %v", uids)
		}
	}
}

func (c *conn) xnumSetUIDs(isUID bool, nums numSet) []store.UID {
	_, uids := c.xnumSetConditionUIDs(false, true, isUID, nums)
	return uids
}

func (c *conn) xnumSetCondition(isUID bool, nums numSet) []any {
	uidargs, _ := c.xnumSetConditionUIDs(true, false, isUID, nums)
	return uidargs
}

func (c *conn) xnumSetConditionUIDs(forDB, returnUIDs bool, isUID bool, nums numSet) ([]any, []store.UID) {
	if nums.searchResult {
		// Update previously stored UIDs. Some may have been deleted.
		// Once deleted a UID will never come back, so we'll just remove those uids.
		o := 0
		for _, uid := range c.searchResult {
			if uidSearch(c.uids, uid) > 0 {
				c.searchResult[o] = uid
				o++
			}
		}
		c.searchResult = c.searchResult[:o]
		uidargs := make([]any, len(c.searchResult))
		for i, uid := range c.searchResult {
			uidargs[i] = uid
		}
		return uidargs, c.searchResult
	}

	var uidargs []any
	var uids []store.UID

	add := func(uid store.UID) {
		if forDB {
			uidargs = append(uidargs, uid)
		}
		if returnUIDs {
			uids = append(uids, uid)
		}
	}

	if !isUID {
		// Sequence numbers that don't exist, or * on an empty mailbox, should result in a BAD response. ../rfc/9051:7018
		for _, r := range nums.ranges {
			var ia, ib int
			if r.first.star {
				if len(c.uids) == 0 {
					xsyntaxErrorf("invalid seqset * on empty mailbox")
				}
				ia = len(c.uids) - 1
			} else {
				ia = int(r.first.number - 1)
				if ia >= len(c.uids) {
					xsyntaxErrorf("msgseq %d not in mailbox", r.first.number)
				}
			}
			if r.last == nil {
				add(c.uids[ia])
				continue
			}

			if r.last.star {
				if len(c.uids) == 0 {
					xsyntaxErrorf("invalid seqset * on empty mailbox")
				}
				ib = len(c.uids) - 1
			} else {
				ib = int(r.last.number - 1)
				if ib >= len(c.uids) {
					xsyntaxErrorf("msgseq %d not in mailbox", r.last.number)
				}
			}
			if ia > ib {
				ia, ib = ib, ia
			}
			for _, uid := range c.uids[ia : ib+1] {
				add(uid)
			}
		}
		return uidargs, uids
	}

	// UIDs that do not exist can be ignored.
	if len(c.uids) == 0 {
		return nil, nil
	}

	for _, r := range nums.ranges {
		last := r.first
		if r.last != nil {
			last = *r.last
		}

		uida := store.UID(r.first.number)
		if r.first.star {
			uida = c.uids[len(c.uids)-1]
		}

		uidb := store.UID(last.number)
		if last.star {
			uidb = c.uids[len(c.uids)-1]
		}

		if uida > uidb {
			uida, uidb = uidb, uida
		}

		// Binary search for uida.
		s := 0
		e := len(c.uids)
		for s < e {
			m := (s + e) / 2
			if uida < c.uids[m] {
				e = m
			} else if uida > c.uids[m] {
				s = m + 1
			} else {
				break
			}
		}

		for _, uid := range c.uids[s:] {
			if uid >= uida && uid <= uidb {
				add(uid)
			} else if uid > uidb {
				break
			}
		}
	}

	return uidargs, uids
}

func (c *conn) ok(tag, cmd string) {
	c.bwriteresultf("%s OK %s done", tag, cmd)
	c.xflush()
}

// xcheckmailboxname checks if name is valid, returning an INBOX-normalized name.
// I.e. it changes various casings of INBOX and INBOX/* to Inbox and Inbox/*.
// Name is invalid if it contains leading/trailing/double slashes, or when it isn't
// unicode-normalized, or when empty or has special characters.
func xcheckmailboxname(name string, allowInbox bool) string {
	first := strings.SplitN(name, "/", 2)[0]
	if strings.EqualFold(first, "inbox") {
		if len(name) == len("inbox") && !allowInbox {
			xuserErrorf("special mailbox name Inbox not allowed")
		}
		name = "Inbox" + name[len("Inbox"):]
	}

	if norm.NFC.String(name) != name {
		xusercodeErrorf("CANNOT", "non-unicode-normalized mailbox names not allowed")
	}

	if name == "" {
		xusercodeErrorf("CANNOT", "empty mailbox name")
	}
	if strings.HasPrefix(name, "/") || strings.HasSuffix(name, "/") || strings.Contains(name, "//") {
		xusercodeErrorf("CANNOT", "bad slashes in mailbox name")
	}
	for _, c := range name {
		switch c {
		case '%', '*', '#', '&':
			xusercodeErrorf("CANNOT", "character %c not allowed in mailbox name", c)
		}
		// ../rfc/6855:192
		if c <= 0x1f || c >= 0x7f && c <= 0x9f || c == 0x2028 || c == 0x2029 {
			xusercodeErrorf("CANNOT", "control characters not allowed in mailbox name")
		}
	}
	return name
}

// Lookup mailbox by name.
// If the mailbox does not exist, panic is called with a user error.
// Must be called with account rlock held.
func (c *conn) xmailbox(tx *bstore.Tx, name string, missingErrCode string) store.Mailbox {
	mb := c.account.MailboxFindX(tx, name)
	if mb == nil {
		// missingErrCode can be empty, or e.g. TRYCREATE or ALREADYEXISTS.
		xusercodeErrorf(missingErrCode, "%w", store.ErrUnknownMailbox)
	}
	return *mb
}

// Lookup mailbox by ID.
// If the mailbox does not exist, panic is called with a user error.
// Must be called with account rlock held.
func (c *conn) xmailboxID(tx *bstore.Tx, id int64) store.Mailbox {
	mb := store.Mailbox{ID: id}
	err := tx.Get(&mb)
	if err == bstore.ErrAbsent {
		xuserErrorf("%w", store.ErrUnknownMailbox)
	}
	return mb
}

// Apply changes to our session state.
// If initial is false, updates like EXISTS and EXPUNGE are written to the client.
// If initial is true, we only apply the changes.
// Should not be called while holding locks, as changes are written to client connections, which can block.
// Does not flush output.
func (c *conn) applyChanges(changes []store.Change, initial bool) {
	if len(changes) == 0 {
		return
	}

	c.conn.SetWriteDeadline(time.Now().Add(5 * time.Minute))

	c.log.Debug("applying changes", mlog.Field("changes", changes))

	// Only keep changes for the selected mailbox, and changes that are always relevant.
	var n []store.Change
	for _, change := range changes {
		var mbID int64
		switch ch := change.(type) {
		case store.ChangeAddUID:
			mbID = ch.MailboxID
		case store.ChangeRemoveUIDs:
			mbID = ch.MailboxID
		case store.ChangeFlags:
			mbID = ch.MailboxID
		case store.ChangeRemoveMailbox, store.ChangeAddMailbox, store.ChangeRenameMailbox, store.ChangeAddSubscription:
			n = append(n, change)
			continue
		default:
			panic(fmt.Errorf("missing case for %#v", change))
		}
		if c.state == stateSelected && mbID == c.mailboxID {
			n = append(n, change)
		}
	}
	changes = n

	i := 0
	for i < len(changes) {
		// First process all new uids. So we only send a single EXISTS.
		var adds []store.ChangeAddUID
		for ; i < len(changes); i++ {
			ch, ok := changes[i].(store.ChangeAddUID)
			if !ok {
				break
			}
			seq := c.sequence(ch.UID)
			if seq > 0 && initial {
				continue
			}
			c.uidAppend(ch.UID)
			adds = append(adds, ch)
		}
		if len(adds) > 0 {
			if initial {
				continue
			}
			// Write the exists, and the UID and flags as well. Hopefully the client waits for
			// long enough after the EXISTS to see these messages, and doesn't request them
			// again with a FETCH.
			c.bwritelinef("* %d EXISTS", len(c.uids))
			for _, add := range adds {
				seq := c.xsequence(add.UID)
				c.bwritelinef("* %d FETCH (UID %d FLAGS %s)", seq, add.UID, flaglist(add.Flags).pack(c))
			}
			continue
		}

		change := changes[i]
		i++

		switch ch := change.(type) {
		case store.ChangeRemoveUIDs:
			for _, uid := range ch.UIDs {
				var seq msgseq
				if initial {
					seq = c.sequence(uid)
					if seq <= 0 {
						continue
					}
				} else {
					seq = c.xsequence(uid)
				}
				c.sequenceRemove(seq, uid)
				if !initial {
					c.bwritelinef("* %d EXPUNGE", seq)
				}
			}
		case store.ChangeFlags:
			// The uid can be unknown if we just expunged it while another session marked it as deleted just before.
			seq := c.sequence(ch.UID)
			if seq <= 0 {
				continue
			}
			if !initial {
				c.bwritelinef("* %d FETCH (UID %d FLAGS %s)", seq, ch.UID, flaglist(ch.Flags).pack(c))
			}
		case store.ChangeRemoveMailbox:
			c.bwritelinef(`* LIST (\NonExistent) "/" %s`, astring(ch.Name).pack(c))
		case store.ChangeAddMailbox:
			c.bwritelinef(`* LIST (%s) "/" %s`, strings.Join(ch.Flags, " "), astring(ch.Name).pack(c))
		case store.ChangeRenameMailbox:
			c.bwritelinef(`* LIST (%s) "/" %s ("OLDNAME" (%s))`, strings.Join(ch.Flags, " "), astring(ch.NewName).pack(c), string0(ch.OldName).pack(c))
		case store.ChangeAddSubscription:
			c.bwritelinef(`* LIST (\Subscribed) "/" %s`, astring(ch.Name).pack(c))
		default:
			panic(fmt.Sprintf("internal error, missing case for %#v", change))
		}
	}
}

// Capability returns the capabilities this server implements and currently has
// available given the connection state.
//
// State: any
func (c *conn) cmdCapability(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1208 ../rfc/3501:1300

	// Request syntax: ../rfc/9051:6464 ../rfc/3501:4669
	p.xempty()

	caps := c.capabilities()

	// Response syntax: ../rfc/9051:6427 ../rfc/3501:4655
	c.bwritelinef("* CAPABILITY %s", caps)
	c.ok(tag, cmd)
}

// capabilities returns non-empty string with available capabilities based on connection state.
// For use in cmdCapability and untagged OK responses on connection start, login and authenticate.
func (c *conn) capabilities() string {
	caps := serverCapabilities
	// ../rfc/9051:1238
	if !c.tls {
		caps += " STARTTLS"
	}
	if c.tls || c.noRequireSTARTTLS {
		caps += " AUTH=PLAIN"
	} else {
		caps += " LOGINDISABLED"
	}
	return caps
}

// No op, but useful for retrieving pending changes as untagged responses, e.g. of
// message delivery.
//
// State: any
func (c *conn) cmdNoop(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1261 ../rfc/3501:1363

	// Request syntax: ../rfc/9051:6464 ../rfc/3501:4669
	p.xempty()
	c.ok(tag, cmd)
}

// Logout, after which server closes the connection.
//
// State: any
func (c *conn) cmdLogout(tag, cmd string, p *parser) {
	// Commands: ../rfc/3501:1407 ../rfc/9051:1290

	// Request syntax: ../rfc/9051:6464 ../rfc/3501:4669
	p.xempty()

	c.unselect()
	c.state = stateNotAuthenticated
	// Response syntax: ../rfc/9051:6886 ../rfc/3501:4935
	c.bwritelinef("* BYE thanks")
	c.ok(tag, cmd)
	panic(cleanClose)
}

// Clients can use ID to tell the server which software they are using. Servers can
// respond with their version. For statistics/logging/debugging purposes.
//
// State: any
func (c *conn) cmdID(tag, cmd string, p *parser) {
	// Command: ../rfc/2971:129

	// Request syntax: ../rfc/2971:241
	p.xspace()
	var params map[string]string
	if p.take("(") {
		params = map[string]string{}
		for !p.take(")") {
			if len(params) > 0 {
				p.xspace()
			}
			k := p.xstring()
			p.xspace()
			v := p.xnilString()
			if _, ok := params[k]; ok {
				xsyntaxErrorf("duplicate key %q", k)
			}
			params[k] = v
		}
	} else {
		p.xnil()
	}
	p.xempty()

	// We just log the client id.
	c.log.Info("client id", mlog.Field("params", params))

	// Response syntax: ../rfc/2971:243
	// We send our name and version. ../rfc/2971:193
	c.bwritelinef(`* ID ("name" "mox" "version" %s)`, string0(moxvar.Version).pack(c))
	c.ok(tag, cmd)
}

// STARTTLS enables TLS on the connection, after a plain text start.
// Only allowed if TLS isn't already enabled, either through connecting to a
// TLS-enabled TCP port, or a previous STARTTLS command.
// After STARTTLS, plain text authentication typically becomes available.
//
// Status: Not authenticated.
func (c *conn) cmdStarttls(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1340 ../rfc/3501:1468

	// Request syntax: ../rfc/9051:6473 ../rfc/3501:4676
	p.xempty()

	if c.tls {
		xsyntaxErrorf("tls already active") // ../rfc/9051:1353
	}

	conn := c.conn
	if n := c.br.Buffered(); n > 0 {
		buf := make([]byte, n)
		_, err := io.ReadFull(c.br, buf)
		xcheckf(err, "reading buffered data for tls handshake")
		conn = &prefixConn{buf, conn}
	}
	c.ok(tag, cmd)

	cidctx := context.WithValue(mox.Context, mlog.CidKey, c.cid)
	ctx, cancel := context.WithTimeout(cidctx, time.Minute)
	defer cancel()
	tlsConn := tls.Server(conn, c.tlsConfig)
	c.log.Debug("starting tls server handshake")
	if err := tlsConn.HandshakeContext(ctx); err != nil {
		panic(fmt.Errorf("starttls handshake: %s (%w)", err, errIO))
	}
	cancel()
	tlsversion, ciphersuite := mox.TLSInfo(tlsConn)
	c.log.Debug("tls server handshake done", mlog.Field("tls", tlsversion), mlog.Field("ciphersuite", ciphersuite))

	c.conn = tlsConn
	c.br = bufio.NewReader(moxio.NewTraceReader(c.log, "C: ", c.conn))
	c.bw = bufio.NewWriter(moxio.NewTraceWriter(c.log, "S: ", c))
	c.tls = true
}

// Authenticate using SASL. Supports multiple back and forths between client and
// server to finish authentication, unlike LOGIN which is just a single
// username/password.
//
// Status: Not authenticated.
func (c *conn) cmdAuthenticate(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1403 ../rfc/3501:1519
	// Examples: ../rfc/9051:1520 ../rfc/3501:1631

	var authVariant string
	authResult := "error"
	defer func() {
		metrics.AuthenticationInc("imap", authVariant, authResult)
	}()

	// Request syntax: ../rfc/9051:6341 ../rfc/3501:4561
	p.xspace()
	authType := p.xatom()

	xreadInitial := func() []byte {
		var line string
		if p.empty() {
			c.writelinef("+ ")
			line = c.readline(false)
		} else {
			// ../rfc/9051:1407 ../rfc/4959:84
			p.xspace()
			line = p.remainder()
			if line == "=" {
				// ../rfc/9051:1450
				line = "" // Base64 decode will result in empty buffer.
			}
		}
		// ../rfc/9051:1442 ../rfc/3501:1553
		if line == "*" {
			authResult = "aborted"
			xsyntaxErrorf("authenticate aborted by client")
		}
		buf, err := base64.StdEncoding.DecodeString(line)
		if err != nil {
			xsyntaxErrorf("parsing base64: %v", err)
		}
		return buf
	}

	xreadContinuation := func() []byte {
		line := c.readline(false)
		if line == "*" {
			authResult = "aborted"
			xsyntaxErrorf("authenticate aborted by client")
		}
		buf, err := base64.StdEncoding.DecodeString(line)
		if err != nil {
			xsyntaxErrorf("parsing base64: %v", err)
		}
		return buf
	}

	switch strings.ToUpper(authType) {
	case "PLAIN":
		authVariant = "plain"

		if !c.noRequireSTARTTLS && !c.tls {
			// ../rfc/9051:5194
			xusercodeErrorf("PRIVACYREQUIRED", "tls required for login")
		}

		buf := xreadInitial()
		plain := bytes.Split(buf, []byte{0})
		if len(plain) != 3 {
			xsyntaxErrorf("bad plain auth data, expected 3 nul-separated tokens, got %d tokens", len(plain))
		}
		authz := string(plain[0])
		authc := string(plain[1])
		password := string(plain[2])
		acc, err := store.OpenEmailAuth(authc, password)
		if err != nil {
			if errors.Is(err, store.ErrUnknownCredentials) {
				authResult = "badcreds"
				xusercodeErrorf("AUTHENTICATIONFAILED", "bad credentials")
			}
			xusercodeErrorf("", "error")
		}
		if authz != "" && authz != authc {
			acc.Close()
			xusercodeErrorf("AUTHORIZATIONFAILED", "cannot assume role")
		}
		c.account = acc
		c.username = authc
		authResult = "ok"

	case "SCRAM-SHA-256":
		// todo: improve handling of errors during scram. e.g. invalid parameters. should we abort the imap command, or continue until the end and respond with a scram-level error?

		authVariant = "scram-sha-256"

		c0 := xreadInitial()
		ss, err := scram.NewServer(c0)
		if err != nil {
			xsyntaxErrorf("starting scram: %w", err)
		}
		c.log.Info("scram auth", mlog.Field("authentication", ss.Authentication))
		acc, _, err := store.OpenEmail(ss.Authentication)
		if err != nil {
			// todo: we could continue scram with a generated salt, deterministically generated
			// from the username. that way we don't have to store anything but attackers cannot
			// learn if an account exists. same for absent scram saltedpassword below.
			xuserErrorf("scram not possible")
		}
		defer func() {
			if acc != nil {
				err := acc.Close()
				c.xsanity(err, "close account")
			}
		}()
		if ss.Authorization != "" && ss.Authorization != ss.Authentication {
			xuserErrorf("authentication with authorization for different user not supported")
		}
		var password store.Password
		acc.WithRLock(func() {
			err := acc.DB.Read(func(tx *bstore.Tx) error {
				password, err = bstore.QueryTx[store.Password](tx).Get()
				xsc := password.SCRAMSHA256
				if err == bstore.ErrAbsent || err == nil && (len(xsc.Salt) == 0 || xsc.Iterations == 0 || len(xsc.SaltedPassword) == 0) {
					xuserErrorf("scram not possible")
				}
				xcheckf(err, "fetching credentials")
				return err
			})
			xcheckf(err, "read tx")
		})
		s1, err := ss.ServerFirst(password.SCRAMSHA256.Iterations, password.SCRAMSHA256.Salt)
		if err != nil {
			xsyntaxErrorf("server first: %w", err)
		}
		xcheckf(err, "scram first server step")
		c.writelinef("+ %s", base64.StdEncoding.EncodeToString([]byte(s1)))
		c2 := xreadContinuation()
		s3, err := ss.Finish(c2, password.SCRAMSHA256.SaltedPassword)
		if len(s3) > 0 {
			c.writelinef("+ %s", base64.StdEncoding.EncodeToString([]byte(s3)))
		}
		if err != nil {
			if errors.Is(err, scram.ErrInvalidProof) {
				authResult = "badcreds"
				xusercodeErrorf("AUTHENTICATIONFAILED", "bad credentials")
			}
			xuserErrorf("server final: %w", err)
		}

		// Client must still respond, but there is nothing to say. See ../rfc/9051:6221
		// The message should be empty. todo: should we require it is empty?
		xreadContinuation()

		c.account = acc
		acc = nil // Cancel cleanup.
		c.username = ss.Authentication
		authResult = "ok"

	default:
		xuserErrorf("method not supported")
	}
	c.comm = store.RegisterComm(c.account)
	c.state = stateAuthenticated
	c.writeresultf("%s OK [CAPABILITY %s] authenticate done", tag, c.capabilities())
}

// Login logs in with username and password.
//
// Status: Not authenticated.
func (c *conn) cmdLogin(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1597 ../rfc/3501:1663

	authResult := "error"
	defer func() {
		metrics.AuthenticationInc("imap", "login", authResult)
	}()

	// Request syntax: ../rfc/9051:6667 ../rfc/3501:4804
	p.xspace()
	userid := p.xastring()
	p.xspace()
	password := p.xastring()
	p.xempty()

	if !c.noRequireSTARTTLS && !c.tls {
		// ../rfc/9051:5194
		xusercodeErrorf("PRIVACYREQUIRED", "tls required for login")
	}

	acc, err := store.OpenEmailAuth(userid, password)
	if err != nil {
		authResult = "badcreds"
		var code string
		if errors.Is(err, store.ErrUnknownCredentials) {
			code = "AUTHENTICATIONFAILED"
		}
		xusercodeErrorf(code, "login failed")
	}
	c.account = acc
	c.username = userid
	c.comm = store.RegisterComm(acc)
	c.state = stateAuthenticated
	authResult = "ok"
	c.writeresultf("%s OK [CAPABILITY %s] login done", tag, c.capabilities())
}

// Enable explicitly opts in to an extension. A server can typically send new kinds
// of responses to a client. Most extensions do not require an ENABLE because a
// client implicitly opts in to new response syntax by making a requests that uses
// new optional extension request syntax.
//
// State: Authenticated and selected.
func (c *conn) cmdEnable(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1652 ../rfc/5161:80
	// Examples: ../rfc/9051:1728 ../rfc/5161:147

	// Request syntax: ../rfc/9051:6518 ../rfc/5161:207
	p.xspace()
	caps := []string{p.xatom()}
	for !p.empty() {
		p.xspace()
		caps = append(caps, p.xatom())
	}

	// Clients should only send capabilities that need enabling.
	// We should only echo that we recognize as needing enabling.
	var enabled string
	for _, s := range caps {
		cap := capability(strings.ToUpper(s))
		switch cap {
		case capIMAP4rev2, capUTF8Accept:
			c.enabled[cap] = true
			enabled += " " + s
		}
	}

	// Response syntax: ../rfc/9051:6520 ../rfc/5161:211
	c.bwritelinef("* ENABLED%s", enabled)
	c.ok(tag, cmd)
}

// State: Authenticated and selected.
func (c *conn) cmdSelect(tag, cmd string, p *parser) {
	c.cmdSelectExamine(true, tag, cmd, p)
}

// State: Authenticated and selected.
func (c *conn) cmdExamine(tag, cmd string, p *parser) {
	c.cmdSelectExamine(false, tag, cmd, p)
}

// Select and examine are almost the same commands. Select just opens a mailbox for
// read/write and examine opens a mailbox readonly.
//
// State: Authenticated and selected.
func (c *conn) cmdSelectExamine(isselect bool, tag, cmd string, p *parser) {
	// Select command: ../rfc/9051:1754 ../rfc/3501:1743
	// Examine command: ../rfc/9051:1868 ../rfc/3501:1855
	// Select examples: ../rfc/9051:1831 ../rfc/3501:1826

	// Select request syntax: ../rfc/9051:7005 ../rfc/3501:4996
	// Examine request syntax: ../rfc/9051:6551 ../rfc/3501:4746
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	// Deselect before attempting the new select. This means we will deselect when an
	// error occurs during select.
	// ../rfc/9051:1809
	if c.state == stateSelected {
		// ../rfc/9051:1812
		c.bwritelinef("* OK [CLOSED] x")
		c.unselect()
	}

	name = xcheckmailboxname(name, true)

	var firstUnseen msgseq = 0
	var mb store.Mailbox
	c.account.WithRLock(func() {
		c.xdbread(func(tx *bstore.Tx) {
			mb = c.xmailbox(tx, name, "")

			q := bstore.QueryTx[store.Message](tx)
			q.FilterNonzero(store.Message{MailboxID: mb.ID})
			q.SortAsc("UID")
			c.uids = []store.UID{}
			var seq msgseq = 1
			err := q.ForEach(func(m store.Message) error {
				c.uids = append(c.uids, m.UID)
				if firstUnseen == 0 && !m.Seen {
					firstUnseen = seq
				}
				seq++
				return nil
			})
			if sanityChecks {
				checkUIDs(c.uids)
			}
			xcheckf(err, "fetching uids")
		})
	})
	c.applyChanges(c.comm.Get(), true)

	c.bwritelinef(`* FLAGS (\Seen \Answered \Flagged \Deleted \Draft $Forwarded $Junk $NotJunk $Phishing $MDNSent)`)
	c.bwritelinef(`* OK [PERMANENTFLAGS (\Seen \Answered \Flagged \Deleted \Draft $Forwarded $Junk $NotJunk $Phishing $MDNSent)] x`)
	if !c.enabled[capIMAP4rev2] {
		c.bwritelinef(`* 0 RECENT`)
	}
	c.bwritelinef(`* %d EXISTS`, len(c.uids))
	if !c.enabled[capIMAP4rev2] && firstUnseen > 0 {
		// ../rfc/9051:8051 ../rfc/3501:1774
		c.bwritelinef(`* OK [UNSEEN %d] x`, firstUnseen)
	}
	c.bwritelinef(`* OK [UIDVALIDITY %d] x`, mb.UIDValidity)
	c.bwritelinef(`* OK [UIDNEXT %d] x`, mb.UIDNext)
	c.bwritelinef(`* LIST () "/" %s`, astring(mb.Name).pack(c))
	if isselect {
		c.bwriteresultf("%s OK [READ-WRITE] x", tag)
		c.readonly = false
	} else {
		c.bwriteresultf("%s OK [READ-ONLY] x", tag)
		c.readonly = true
	}
	c.mailboxID = mb.ID
	c.state = stateSelected
	c.searchResult = nil
	c.xflush()
}

// Create makes a new mailbox, and its parents too if absent.
//
// State: Authenticated and selected.
func (c *conn) cmdCreate(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1900 ../rfc/3501:1888
	// Examples: ../rfc/9051:1951 ../rfc/6154:411 ../rfc/4466:212 ../rfc/3501:1933

	// Request syntax: ../rfc/9051:6484 ../rfc/6154:468 ../rfc/4466:500 ../rfc/3501:4687
	p.xspace()
	name := p.xmailbox()
	// todo: support CREATE-SPECIAL-USE ../rfc/6154:296
	p.xempty()

	origName := name
	name = strings.TrimRight(name, "/") // ../rfc/9051:1930
	name = xcheckmailboxname(name, false)

	var changes []store.Change
	var created []string // Created mailbox names.

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			elems := strings.Split(name, "/")
			var p string
			for i, elem := range elems {
				if i > 0 {
					p += "/"
				}
				p += elem
				if c.account.MailboxExistsX(tx, p) {
					if i == len(elems)-1 {
						// ../rfc/9051:1914
						xuserErrorf("mailbox already exists")
					}
					continue
				}
				_, nchanges := c.account.MailboxEnsureX(tx, p, true)
				changes = append(changes, nchanges...)
				created = append(created, p)
			}
		})

		c.broadcast(changes)
	})

	for _, n := range created {
		var more string
		if n == name && name != origName && !(name == "Inbox" || strings.HasPrefix(name, "Inbox/")) {
			more = fmt.Sprintf(` ("OLDNAME" (%s))`, string0(origName).pack(c))
		}
		c.bwritelinef(`* LIST (\Subscribed) "/" %s%s`, astring(n).pack(c), more)
	}
	c.ok(tag, cmd)
}

// Delete removes a mailbox and all its messages.
// Inbox cannot be removed.
//
// State: Authenticated and selected.
func (c *conn) cmdDelete(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:1972 ../rfc/3501:1946
	// Examples:  ../rfc/9051:2025 ../rfc/3501:1992

	// Request syntax: ../rfc/9051:6505 ../rfc/3501:4716
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	name = xcheckmailboxname(name, false)

	// Messages to remove after having broadcasted the removal of messages.
	var remove []store.Message

	c.account.WithWLock(func() {
		var mb store.Mailbox

		c.xdbwrite(func(tx *bstore.Tx) {
			mb = c.xmailbox(tx, name, "NONEXISTENT")

			// Look for existence of child mailboxes. There is a lot of text in the RFCs about
			// NoInferior and NoSelect. We just require only leaf mailboxes are deleted.
			qmb := bstore.QueryTx[store.Mailbox](tx)
			mbprefix := name + "/"
			qmb.FilterFn(func(mb store.Mailbox) bool {
				return strings.HasPrefix(mb.Name, mbprefix)
			})
			childExists, err := qmb.Exists()
			xcheckf(err, "checking child existence")
			if childExists {
				xusercodeErrorf("HASCHILDREN", "mailbox has a child, only leaf mailboxes can be deleted")
			}

			qm := bstore.QueryTx[store.Message](tx)
			qm.FilterNonzero(store.Message{MailboxID: mb.ID})
			remove, err = qm.List()
			xcheckf(err, "listing messages to remove")

			if len(remove) > 0 {
				removeIDs := make([]any, len(remove))
				for i, m := range remove {
					removeIDs[i] = m.ID
				}
				qmr := bstore.QueryTx[store.Recipient](tx)
				qmr.FilterEqual("MessageID", removeIDs...)
				_, err = qmr.Delete()
				xcheckf(err, "removing message recipients for messages")

				qm = bstore.QueryTx[store.Message](tx)
				qm.FilterNonzero(store.Message{MailboxID: mb.ID})
				_, err = qm.Delete()
				xcheckf(err, "removing messages")

				conf, _ := c.account.Conf()
				if name != conf.RejectsMailbox {
					err = c.account.Untrain(c.log, remove)
					xcheckf(err, "untraining deleted messages")
				}
			}

			err = tx.Delete(&store.Mailbox{ID: mb.ID})
			xcheckf(err, "removing mailbox")
		})

		c.broadcast([]store.Change{store.ChangeRemoveMailbox{Name: name}})
	})

	for _, m := range remove {
		p := c.account.MessagePath(m.ID)
		if err := os.Remove(p); err != nil {
			c.log.Infox("removing message file for mailbox delete", err, mlog.Field("path", p))
		}
	}

	c.ok(tag, cmd)
}

// Rename changes the name of a mailbox.
// Renaming INBOX is special, it moves the inbox messages to a new mailbox, leaving inbox empty.
// Renaming a mailbox with submailboxes also renames all submailboxes.
// Subscriptions stay with the old name, though newly created missing parent
// mailboxes for the destination name are automatically subscribed.
//
// State: Authenticated and selected.
func (c *conn) cmdRename(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:2062 ../rfc/3501:2040
	// Examples: ../rfc/9051:2132 ../rfc/3501:2092

	// Request syntax: ../rfc/9051:6863 ../rfc/3501:4908
	p.xspace()
	src := p.xmailbox()
	p.xspace()
	dst := p.xmailbox()
	p.xempty()

	src = xcheckmailboxname(src, true)
	dst = xcheckmailboxname(dst, false)

	c.account.WithWLock(func() {
		var changes []store.Change

		c.xdbwrite(func(tx *bstore.Tx) {
			uidval, err := c.account.NextUIDValidity(tx)
			xcheckf(err, "next uid validity")

			// Inbox is very special case. Unlike other mailboxes, its children are not moved. And
			// unlike a regular move, its messages are moved to a newly created mailbox.
			// We do indeed create a new destination mailbox and actually move the messages.
			// ../rfc/9051:2101
			if src == "Inbox" {
				if c.account.MailboxExistsX(tx, dst) {
					xusercodeErrorf("ALREADYEXISTS", "destination mailbox %q already exists", dst)
				}
				srcMB := c.account.MailboxFindX(tx, src)
				if srcMB == nil {
					xserverErrorf("inbox not found")
				}
				if dst == src {
					xuserErrorf("cannot move inbox to itself")
				}

				dstMB := store.Mailbox{
					Name:        dst,
					UIDValidity: uidval,
					UIDNext:     1,
				}
				err := tx.Insert(&dstMB)
				xcheckf(err, "create new destination mailbox")

				var messages []store.Message
				q := bstore.QueryTx[store.Message](tx)
				q.FilterNonzero(store.Message{MailboxID: srcMB.ID})
				q.Gather(&messages)
				_, err = q.UpdateNonzero(store.Message{MailboxID: dstMB.ID})
				xcheckf(err, "moving messages from inbox to destination mailbox")

				uids := make([]store.UID, len(messages))
				for i, m := range messages {
					uids[i] = m.UID
				}
				var dstFlags []string
				if tx.Get(&store.Subscription{Name: dstMB.Name}) == nil {
					dstFlags = []string{`\Subscribed`}
				}
				changes = []store.Change{
					store.ChangeRemoveUIDs{MailboxID: srcMB.ID, UIDs: uids},
					store.ChangeAddMailbox{Name: dstMB.Name, Flags: dstFlags},
					// todo: in future, we could announce all messages. no one is listening now though.
				}
				return
			}

			// We gather existing mailboxes that we need for deciding what to create/delete/update.
			q := bstore.QueryTx[store.Mailbox](tx)
			srcPrefix := src + "/"
			dstRoot := strings.SplitN(dst, "/", 2)[0]
			dstRootPrefix := dstRoot + "/"
			q.FilterFn(func(mb store.Mailbox) bool {
				return mb.Name == src || strings.HasPrefix(mb.Name, srcPrefix) || mb.Name == dstRoot || strings.HasPrefix(mb.Name, dstRootPrefix)
			})
			q.SortAsc("Name") // We'll rename the parents before children.
			l, err := q.List()
			xcheckf(err, "listing relevant mailboxes")

			mailboxes := map[string]store.Mailbox{}
			for _, mb := range l {
				mailboxes[mb.Name] = mb
			}

			if _, ok := mailboxes[src]; !ok {
				// ../rfc/9051:5140
				xusercodeErrorf("NONEXISTENT", "mailbox does not exist")
			}

			// Ensure parent mailboxes for the destination paths exist.
			var parent string
			dstElems := strings.Split(dst, "/")
			for i, elem := range dstElems[:len(dstElems)-1] {
				if i > 0 {
					parent += "/"
				}
				parent += elem

				mb, ok := mailboxes[parent]
				if ok {
					continue
				}
				omb := mb
				mb = store.Mailbox{
					ID:          omb.ID,
					Name:        parent,
					UIDValidity: uidval,
					UIDNext:     1,
				}
				err = tx.Insert(&mb)
				xcheckf(err, "creating parent mailbox")
				err = tx.Insert(&store.Subscription{Name: parent})
				if err != nil && !errors.Is(err, bstore.ErrUnique) {
					xcheckf(err, "creating subscription")
				}
				changes = append(changes, store.ChangeAddMailbox{Name: parent, Flags: []string{`\Subscribed`}})
			}

			// Process src mailboxes, renaming them to dst.
			for _, srcmb := range l {
				if srcmb.Name != src && !strings.HasPrefix(srcmb.Name, srcPrefix) {
					continue
				}
				srcName := srcmb.Name
				dstName := dst + srcmb.Name[len(src):]
				if _, ok := mailboxes[dstName]; ok {
					xusercodeErrorf("ALREADYEXISTS", "destination mailbox %q already exists", dstName)
				}

				srcmb.Name = dstName
				srcmb.UIDValidity = uidval
				err = tx.Update(&srcmb)
				xcheckf(err, "renaming mailbox")

				// Renaming Inbox is special, it leaves an empty inbox instead of removing it.
				var dstFlags []string
				if tx.Get(&store.Subscription{Name: dstName}) == nil {
					dstFlags = []string{`\Subscribed`}
				}
				changes = append(changes, store.ChangeRenameMailbox{OldName: srcName, NewName: dstName, Flags: dstFlags})
			}

			// If we renamed e.g. a/b to a/b/c/d, and a/b/c to a/b/c/d/c, we'll have to recreate a/b and a/b/c.
			srcElems := strings.Split(src, "/")
			xsrc := src
			for i := 0; i < len(dstElems) && strings.HasPrefix(dst, xsrc+"/"); i++ {
				mb := store.Mailbox{
					UIDValidity: uidval,
					UIDNext:     1,
					Name:        xsrc,
				}
				err = tx.Insert(&mb)
				xcheckf(err, "creating mailbox at old path")
				xsrc += "/" + dstElems[len(srcElems)+i]
			}
		})
		c.broadcast(changes)
	})

	c.ok(tag, cmd)
}

// Subscribe marks a mailbox path as subscribed. The mailbox does not have to
// exist. Subscribed may mean an email client will show the mailbox in its UI
// and/or periodically fetch new messages for the mailbox.
//
// State: Authenticated and selected.
func (c *conn) cmdSubscribe(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:2172 ../rfc/3501:2135
	// Examples: ../rfc/9051:2198 ../rfc/3501:2162

	// Request syntax: ../rfc/9051:7083 ../rfc/3501:5059
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	name = xcheckmailboxname(name, true)

	c.account.WithWLock(func() {
		var changes []store.Change

		c.xdbwrite(func(tx *bstore.Tx) {
			changes = c.account.SubscriptionEnsureX(tx, name)
		})

		c.broadcast(changes)
	})

	c.ok(tag, cmd)
}

// Unsubscribe marks a mailbox as not subscribed. The mailbox doesn't have to exist.
//
// State: Authenticated and selected.
func (c *conn) cmdUnsubscribe(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:2203 ../rfc/3501:2166
	// Examples: ../rfc/9051:2219 ../rfc/3501:2181

	// Request syntax: ../rfc/9051:7143 ../rfc/3501:5077
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	name = xcheckmailboxname(name, true)

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			// It's OK if not currently subscribed, ../rfc/9051:2215
			err := tx.Delete(&store.Subscription{Name: name})
			if err == bstore.ErrAbsent {
				if !c.account.MailboxExistsX(tx, name) {
					xuserErrorf("mailbox does not exist")
				}
				return
			}
			xcheckf(err, "removing subscription")
		})

		// todo: can we send untagged message about a mailbox no longer being subscribed?
	})

	c.ok(tag, cmd)
}

// LSUB command for listing subscribed mailboxes.
// Removed in IMAP4rev2, only in IMAP4rev1.
//
// State: Authenticated and selected.
func (c *conn) cmdLsub(tag, cmd string, p *parser) {
	// Command: ../rfc/3501:2374
	// Examples: ../rfc/3501:2415

	// Request syntax: ../rfc/3501:4806
	p.xspace()
	ref := p.xmailbox()
	p.xspace()
	pattern := p.xlistMailbox()
	p.xempty()

	re := xmailboxPatternMatcher(ref, []string{pattern})

	var lines []string
	c.xdbread(func(tx *bstore.Tx) {
		q := bstore.QueryTx[store.Subscription](tx)
		q.SortAsc("Name")
		subscriptions, err := q.List()
		xcheckf(err, "querying subscriptions")

		have := map[string]bool{}
		subscribedKids := map[string]bool{}
		ispercent := strings.HasSuffix(pattern, "%")
		for _, sub := range subscriptions {
			name := sub.Name
			if ispercent {
				for p := filepath.Dir(name); p != "."; p = filepath.Dir(p) {
					subscribedKids[p] = true
				}
			}
			if !re.MatchString(name) {
				continue
			}
			have[name] = true
			line := fmt.Sprintf(`* LSUB () "/" %s`, astring(name).pack(c))
			lines = append(lines, line)

		}

		// ../rfc/3501:2394
		if !ispercent {
			return
		}
		qmb := bstore.QueryTx[store.Mailbox](tx)
		qmb.SortAsc("Name")
		err = qmb.ForEach(func(mb store.Mailbox) error {
			if have[mb.Name] || !subscribedKids[mb.Name] || !re.MatchString(mb.Name) {
				return nil
			}
			line := fmt.Sprintf(`* LSUB (\NoSelect) "/" %s`, astring(mb.Name).pack(c))
			lines = append(lines, line)
			return nil
		})
		xcheckf(err, "querying mailboxes")
	})

	// Response syntax: ../rfc/3501:4833 ../rfc/3501:4837
	for _, line := range lines {
		c.bwritelinef("%s", line)
	}
	c.ok(tag, cmd)
}

// The namespace command returns the mailbox path separator. We only implement
// the personal mailbox hierarchy, no shared/other.
//
// In IMAP4rev2, it was an extension before.
//
// State: Authenticated and selected.
func (c *conn) cmdNamespace(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3098 ../rfc/2342:137
	// Examples: ../rfc/9051:3117 ../rfc/2342:155
	// Request syntax: ../rfc/9051:6767 ../rfc/2342:410
	p.xempty()

	// Response syntax: ../rfc/9051:6778 ../rfc/2342:415
	c.bwritelinef(`* NAMESPACE (("" "/")) NIL NIL`)
	c.ok(tag, cmd)
}

// The status command returns information about a mailbox, such as the number of
// messages, "uid validity", etc. Nowadays, the extended LIST command can return
// the same information about many mailboxes for one command.
//
// State: Authenticated and selected.
func (c *conn) cmdStatus(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3328 ../rfc/3501:2424
	// Examples: ../rfc/9051:3400 ../rfc/3501:2501

	// Request syntax: ../rfc/9051:7053 ../rfc/3501:5036
	p.xspace()
	name := p.xmailbox()
	p.xspace()
	p.xtake("(")
	attrs := []string{p.xstatusAtt()}
	for !p.take(")") {
		p.xspace()
		attrs = append(attrs, p.xstatusAtt())
	}
	p.xempty()

	name = xcheckmailboxname(name, true)

	var mb store.Mailbox

	var responseLine string
	c.account.WithRLock(func() {
		c.xdbread(func(tx *bstore.Tx) {
			mb = c.xmailbox(tx, name, "")
			responseLine = c.xstatusLine(tx, mb, attrs)
		})
	})

	c.bwritelinef("%s", responseLine)
	c.ok(tag, cmd)
}

// Response syntax: ../rfc/9051:6681 ../rfc/9051:7070 ../rfc/9051:7059 ../rfc/3501:4834
func (c *conn) xstatusLine(tx *bstore.Tx, mb store.Mailbox, attrs []string) string {
	var count, unseen, deleted int
	var size int64

	q := bstore.QueryTx[store.Message](tx)
	q.FilterNonzero(store.Message{MailboxID: mb.ID})
	err := q.ForEach(func(m store.Message) error {
		count++
		if !m.Seen {
			unseen++
		}
		if m.Deleted {
			deleted++
		}
		size += m.Size
		return nil
	})
	xcheckf(err, "processing mailbox messages")

	status := []string{}
	for _, a := range attrs {
		A := strings.ToUpper(a)
		switch A {
		case "MESSAGES":
			status = append(status, A, fmt.Sprintf("%d", count))
		case "UIDNEXT":
			status = append(status, A, fmt.Sprintf("%d", mb.UIDNext))
		case "UIDVALIDITY":
			status = append(status, A, fmt.Sprintf("%d", mb.UIDValidity))
		case "UNSEEN":
			status = append(status, A, fmt.Sprintf("%d", unseen))
		case "DELETED":
			status = append(status, A, fmt.Sprintf("%d", deleted))
		case "SIZE":
			status = append(status, A, fmt.Sprintf("%d", size))
		case "RECENT":
			status = append(status, A, "0")
		case "APPENDLIMIT":
			// ../rfc/7889:255
			status = append(status, A, "NIL")
		default:
			xsyntaxErrorf("unknown attribute %q", a)
		}
	}
	return fmt.Sprintf("* STATUS %s (%s)", astring(mb.Name).pack(c), strings.Join(status, " "))
}

func xparseStoreFlags(l []string, syntax bool) (flags store.Flags) {
	fields := map[string]*bool{
		`\answered`:  &flags.Answered,
		`\flagged`:   &flags.Flagged,
		`\deleted`:   &flags.Deleted,
		`\seen`:      &flags.Seen,
		`\draft`:     &flags.Draft,
		`$junk`:      &flags.Junk,
		`$notjunk`:   &flags.Notjunk,
		`$forwarded`: &flags.Forwarded,
		`$phishing`:  &flags.Phishing,
		`$mdnsent`:   &flags.MDNSent,
	}
	for _, f := range l {
		if field, ok := fields[strings.ToLower(f)]; !ok {
			if syntax {
				xsyntaxErrorf("unknown flag %q", f)
			}
			xuserErrorf("unknown flag %q", f)
		} else {
			*field = true
		}
	}
	return
}

func flaglist(fl store.Flags) listspace {
	l := listspace{}
	flag := func(v bool, s string) {
		if v {
			l = append(l, bare(s))
		}
	}
	flag(fl.Seen, `\Seen`)
	flag(fl.Answered, `\Answered`)
	flag(fl.Flagged, `\Flagged`)
	flag(fl.Deleted, `\Deleted`)
	flag(fl.Draft, `\Draft`)
	flag(fl.Forwarded, `$Forwarded`)
	flag(fl.Junk, `$Junk`)
	flag(fl.Notjunk, `$NotJunk`)
	flag(fl.Phishing, `$Phishing`)
	flag(fl.MDNSent, `$MDNSent`)
	return l
}

// Append adds a message to a mailbox.
//
// State: Authenticated and selected.
func (c *conn) cmdAppend(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3406 ../rfc/6855:204 ../rfc/3501:2527
	// Examples: ../rfc/9051:3482 ../rfc/3501:2589

	// Request syntax: ../rfc/9051:6325 ../rfc/6855:219 ../rfc/3501:4547
	p.xspace()
	name := p.xmailbox()
	p.xspace()
	var storeFlags store.Flags
	if p.hasPrefix("(") {
		// Error must be a syntax error, to properly abort the connection due to literal.
		storeFlags = xparseStoreFlags(p.xflagList(), true)
		p.xspace()
	}
	var tm time.Time
	if p.hasPrefix(`"`) {
		tm = p.xdateTime()
		p.xspace()
	} else {
		tm = time.Now()
	}
	// todo: only with utf8 should we we accept message headers with utf-8. we currently always accept them.
	// todo: this is only relevant if we also support the CATENATE extension?
	// ../rfc/6855:204
	utf8 := p.take("UTF8 (")
	size, sync := p.xliteralSize(0, utf8)

	name = xcheckmailboxname(name, true)
	c.xdbread(func(tx *bstore.Tx) {
		c.xmailbox(tx, name, "TRYCREATE")
	})
	if sync {
		c.writelinef("+")
	}

	// Read the message into a temporary file.
	msgFile, err := store.CreateMessageTemp("imap-append")
	xcheckf(err, "creating temp file for message")
	defer func() {
		if msgFile != nil {
			err := os.Remove(msgFile.Name())
			c.xsanity(err, "removing APPEND temporary file")
			err = msgFile.Close()
			c.xsanity(err, "closing APPEND temporary file")
		}
	}()
	mw := &message.Writer{Writer: msgFile}
	msize, err := io.Copy(mw, io.LimitReader(c.br, size))
	if err != nil {
		// Cannot use xcheckf due to %w handling of errIO.
		panic(fmt.Errorf("reading literal message: %s (%w)", err, errIO))
	}
	if msize != size {
		xserverErrorf("read %d bytes for message, expected %d (%w)", msize, size, errIO)
	}
	msgPrefix := []byte{}
	// todo: should we treat the message as body? i believe headers are required in messages, and bodies are optional. so would make more sense to treat the data as headers. perhaps only if the headers are valid?
	if !mw.HaveHeaders {
		msgPrefix = []byte("\r\n")
	}

	if utf8 {
		line := c.readline(false)
		np := newParser(line, c)
		np.xtake(")")
		np.xempty()
	} else {
		line := c.readline(false)
		np := newParser(line, c)
		np.xempty()
	}
	p.xempty()
	if !sync {
		name = xcheckmailboxname(name, true)
	}

	var mb store.Mailbox
	var msg store.Message
	var pendingChanges []store.Change

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			mb = c.xmailbox(tx, name, "TRYCREATE")
			msg = store.Message{
				MailboxID:     mb.ID,
				MailboxOrigID: mb.ID,
				Received:      tm,
				Flags:         storeFlags,
				Size:          size,
				MsgPrefix:     msgPrefix,
			}
			isSent := name == "Sent"
			c.account.DeliverX(c.log, tx, &msg, msgFile, true, isSent, true, true)
		})

		// Fetch pending changes, possibly with new UIDs, so we can apply them before adding our own new UID.
		if c.comm != nil {
			pendingChanges = c.comm.Get()
		}

		// Broadcast the change to other connections.
		c.broadcast([]store.Change{store.ChangeAddUID{MailboxID: mb.ID, UID: msg.UID, Flags: msg.Flags}})
	})

	msgFile.Close()
	msgFile = nil

	if c.mailboxID == mb.ID {
		c.applyChanges(pendingChanges, false)
		c.uidAppend(msg.UID)
		c.bwritelinef("* %d EXISTS", len(c.uids))
	}

	c.writeresultf("%s OK [APPENDUID %d %d] appended", tag, mb.UIDValidity, msg.UID)
}

// Idle makes a client wait until the server sends untagged updates, e.g. about
// message delivery or mailbox create/rename/delete/subscription, etc. It allows a
// client to get updates in real-time, not needing the use for NOOP.
//
// State: Authenticated and selected.
func (c *conn) cmdIdle(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3542 ../rfc/2177:49
	// Example: ../rfc/9051:3589 ../rfc/2177:119

	// Request syntax: ../rfc/9051:6594 ../rfc/2177:163
	p.xempty()

	c.writelinef("+ waiting")

	var line string
wait:
	for {
		select {
		case le := <-c.lineChan():
			c.line = nil
			xcheckf(le.err, "get line")
			line = le.line
			break wait
		case changes := <-c.comm.Changes:
			c.applyChanges(changes, false)
			c.xflush()
		case <-mox.Shutdown:
			// ../rfc/9051:5375
			c.writelinef("* BYE shutting down")
			panic(errIO)
		}
	}

	// Reset the write deadline. In case of little activity, with a command timeout of
	// 30 minutes, we have likely passed it.
	c.conn.SetWriteDeadline(time.Now().Add(5 * time.Minute))

	if strings.ToUpper(line) != "DONE" {
		// We just close the connection because our protocols are out of sync.
		panic(fmt.Errorf("%w: in IDLE, expected DONE", errIO))
	}

	c.ok(tag, cmd)
}

// Check is an old deprecated command that is supposed to execute some mailbox consistency checks.
//
// State: Selected
func (c *conn) cmdCheck(tag, cmd string, p *parser) {
	// Command: ../rfc/3501:2618

	// Request syntax: ../rfc/3501:4679
	p.xempty()

	c.account.WithRLock(func() {
		c.xdbread(func(tx *bstore.Tx) {
			c.xmailboxID(tx, c.mailboxID) // Validate.
		})
	})

	c.ok(tag, cmd)
}

// Close undoes select/examine, closing the currently opened mailbox and deleting
// messages that were marked for deletion with the \Deleted flag.
//
// State: Selected
func (c *conn) cmdClose(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3636 ../rfc/3501:2652

	// Request syntax: ../rfc/9051:6476 ../rfc/3501:4679
	p.xempty()

	if c.readonly {
		c.unselect()
		c.ok(tag, cmd)
		return
	}

	remove := c.xexpunge(nil, true)

	defer func() {
		for _, m := range remove {
			p := c.account.MessagePath(m.ID)
			err := os.Remove(p)
			c.xsanity(err, "removing message file for expunge for close")
		}
	}()

	c.unselect()
	c.ok(tag, cmd)
}

// expunge messages marked for deletion in currently selected/active mailbox.
// if uidSet is not nil, only messages matching the set are deleted.
// messages that have been deleted from the database returned, but the corresponding files still have to be removed.
func (c *conn) xexpunge(uidSet *numSet, missingMailboxOK bool) []store.Message {
	var remove []store.Message

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			mb := store.Mailbox{ID: c.mailboxID}
			err := tx.Get(&mb)
			if err == bstore.ErrAbsent {
				if missingMailboxOK {
					return
				}
				xuserErrorf("%w", store.ErrUnknownMailbox)
			}

			qm := bstore.QueryTx[store.Message](tx)
			qm.FilterNonzero(store.Message{MailboxID: c.mailboxID})
			qm.FilterEqual("Deleted", true)
			qm.FilterFn(func(m store.Message) bool {
				// Only remove if this session knows about the message and if present in optional uidSet.
				return uidSearch(c.uids, m.UID) > 0 && (uidSet == nil || uidSet.containsUID(m.UID, c.uids, c.searchResult))
			})
			qm.SortAsc("UID")
			remove, err = qm.List()
			xcheckf(err, "listing messages to delete")

			if len(remove) == 0 {
				return
			}

			removeIDs := make([]int64, len(remove))
			anyIDs := make([]any, len(remove))
			for i, m := range remove {
				removeIDs[i] = m.ID
				anyIDs[i] = m.ID
			}
			qmr := bstore.QueryTx[store.Recipient](tx)
			qmr.FilterEqual("MessageID", anyIDs...)
			_, err = qmr.Delete()
			xcheckf(err, "removing message recipients")

			qm = bstore.QueryTx[store.Message](tx)
			qm.FilterIDs(removeIDs)
			_, err = qm.Delete()
			xcheckf(err, "removing messages marked for deletion")

			conf, _ := c.account.Conf()
			if mb.Name != conf.RejectsMailbox {
				err = c.account.Untrain(c.log, remove)
				xcheckf(err, "untraining deleted messages")
			}
		})

		// Broadcast changes to other connections. We may not have actually removed any
		// messages, so take care not to send an empty update.
		if len(remove) > 0 {
			ouids := make([]store.UID, len(remove))
			for i, m := range remove {
				ouids[i] = m.UID
			}
			changes := []store.Change{store.ChangeRemoveUIDs{MailboxID: c.mailboxID, UIDs: ouids}}
			c.broadcast(changes)
		}
	})
	return remove
}

// Unselect is similar to close in that it closes the currently active mailbox, but
// it does not remove messages marked for deletion.
//
// State: Selected
func (c *conn) cmdUnselect(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3667 ../rfc/3691:89

	// Request syntax: ../rfc/9051:6476 ../rfc/3691:135
	p.xempty()

	c.unselect()
	c.ok(tag, cmd)
}

// Expunge deletes messages marked with \Deleted in the currently selected mailbox.
// Clients are wiser to use UID EXPUNGE because it allows a UID sequence set to
// explicitly opt in to removing specific messages.
//
// State: Selected
func (c *conn) cmdExpunge(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:3687 ../rfc/3501:2695

	// Request syntax: ../rfc/9051:6476 ../rfc/3501:4679
	p.xempty()

	if c.readonly {
		xuserErrorf("mailbox open in read-only mode")
	}

	c.cmdxExpunge(tag, cmd, nil)
}

// UID expunge deletes messages marked with \Deleted in the currently selected
// mailbox if they match a UID sequence set.
//
// State: Selected
func (c *conn) cmdUIDExpunge(tag, cmd string, p *parser) {
	// Command: ../rfc/9051:4775 ../rfc/4315:75

	// Request syntax: ../rfc/9051:7125 ../rfc/9051:7129 ../rfc/4315:298
	p.xspace()
	uidSet := p.xnumSet()
	p.xempty()

	if c.readonly {
		xuserErrorf("mailbox open in read-only mode")
	}

	c.cmdxExpunge(tag, cmd, &uidSet)
}

// Permanently delete messages for the currently selected/active mailbox. If uidset
// is not nil, only those UIDs are removed.
// State: Selected
func (c *conn) cmdxExpunge(tag, cmd string, uidSet *numSet) {
	// Command: ../rfc/9051:3687 ../rfc/3501:2695

	remove := c.xexpunge(uidSet, false)

	defer func() {
		for _, m := range remove {
			p := c.account.MessagePath(m.ID)
			err := os.Remove(p)
			c.xsanity(err, "removing message file for expunge")
		}
	}()

	// Response syntax: ../rfc/9051:6742 ../rfc/3501:4864
	for _, m := range remove {
		seq := c.xsequence(m.UID)
		c.sequenceRemove(seq, m.UID)
		c.bwritelinef("* %d EXPUNGE", seq)
	}

	c.ok(tag, cmd)
}

// State: Selected
func (c *conn) cmdSearch(tag, cmd string, p *parser) {
	c.cmdxSearch(false, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdUIDSearch(tag, cmd string, p *parser) {
	c.cmdxSearch(true, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdFetch(tag, cmd string, p *parser) {
	c.cmdxFetch(false, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdUIDFetch(tag, cmd string, p *parser) {
	c.cmdxFetch(true, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdStore(tag, cmd string, p *parser) {
	c.cmdxStore(false, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdUIDStore(tag, cmd string, p *parser) {
	c.cmdxStore(true, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdCopy(tag, cmd string, p *parser) {
	c.cmdxCopy(false, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdUIDCopy(tag, cmd string, p *parser) {
	c.cmdxCopy(true, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdMove(tag, cmd string, p *parser) {
	c.cmdxMove(false, tag, cmd, p)
}

// State: Selected
func (c *conn) cmdUIDMove(tag, cmd string, p *parser) {
	c.cmdxMove(true, tag, cmd, p)
}

func (c *conn) gatherCopyMoveUIDs(isUID bool, nums numSet) ([]store.UID, []any) {
	// Gather uids, then sort so we can return a consistently simple and hard to
	// misinterpret COPYUID/MOVEUID response. It seems safer to have UIDs in ascending
	// order, because requested uid set of 12:10 is equal to 10:12, so if we would just
	// echo whatever the client sends us without reordering, the client can reorder our
	// response and interpret it differently than we intended.
	// ../rfc/9051:5072
	uids := c.xnumSetUIDs(isUID, nums)
	sort.Slice(uids, func(i, j int) bool {
		return uids[i] < uids[j]
	})
	uidargs := make([]any, len(uids))
	for i, uid := range uids {
		uidargs[i] = uid
	}
	return uids, uidargs
}

// Copy copies messages from the currently selected/active mailbox to another named
// mailbox.
//
// State: Selected
func (c *conn) cmdxCopy(isUID bool, tag, cmd string, p *parser) {
	// Command: ../rfc/9051:4602 ../rfc/3501:3288

	// Request syntax: ../rfc/9051:6482 ../rfc/3501:4685
	p.xspace()
	nums := p.xnumSet()
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	name = xcheckmailboxname(name, true)

	uids, uidargs := c.gatherCopyMoveUIDs(isUID, nums)

	// Files that were created during the copy. Remove them if the operation fails.
	var createdIDs []int64
	defer func() {
		x := recover()
		if x == nil {
			return
		}
		for _, id := range createdIDs {
			p := c.account.MessagePath(id)
			err := os.Remove(p)
			c.xsanity(err, "cleaning up created file")
		}
		panic(x)
	}()

	var mbDst store.Mailbox
	var origUIDs, newUIDs []store.UID
	var flags []store.Flags

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			mbSrc := c.xmailboxID(tx, c.mailboxID) // Validate.
			mbDst = c.xmailbox(tx, name, "TRYCREATE")
			if mbDst.ID == mbSrc.ID {
				xuserErrorf("cannot copy to currently selected mailbox")
			}

			if len(uidargs) == 0 {
				xuserErrorf("no matching messages to copy")
			}

			// Reserve the uids in the destination mailbox.
			uidFirst := mbDst.UIDNext
			mbDst.UIDNext += store.UID(len(uidargs))
			err := tx.Update(&mbDst)
			xcheckf(err, "reserve uid in destination mailbox")

			// Fetch messages from database.
			q := bstore.QueryTx[store.Message](tx)
			q.FilterNonzero(store.Message{MailboxID: c.mailboxID})
			q.FilterEqual("UID", uidargs...)
			xmsgs, err := q.List()
			xcheckf(err, "fetching messages")

			if len(xmsgs) != len(uidargs) {
				xserverErrorf("uid and message mismatch")
			}

			msgs := map[store.UID]store.Message{}
			for _, m := range xmsgs {
				msgs[m.UID] = m
			}

			// Insert new messages into database.
			var origMsgIDs, newMsgIDs []int64
			for i, uid := range uids {
				m, ok := msgs[uid]
				if !ok {
					xuserErrorf("messages changed, could not fetch requested uid")
				}
				origID := m.ID
				origMsgIDs = append(origMsgIDs, origID)
				m.ID = 0
				m.UID = uidFirst + store.UID(i)
				m.MailboxID = mbDst.ID
				m.MailboxOrigID = mbSrc.ID
				err := tx.Insert(&m)
				xcheckf(err, "inserting message")
				msgs[uid] = m
				origUIDs = append(origUIDs, uid)
				newUIDs = append(newUIDs, m.UID)
				newMsgIDs = append(newMsgIDs, m.ID)
				flags = append(flags, m.Flags)

				qmr := bstore.QueryTx[store.Recipient](tx)
				qmr.FilterNonzero(store.Recipient{MessageID: origID})
				mrs, err := qmr.List()
				xcheckf(err, "listing message recipients")
				for _, mr := range mrs {
					mr.ID = 0
					mr.MessageID = m.ID
					err := tx.Insert(&mr)
					xcheckf(err, "inserting message recipient")
				}
			}

			// Copy message files to new message ID's.
			for i := range origMsgIDs {
				src := c.account.MessagePath(origMsgIDs[i])
				dst := c.account.MessagePath(newMsgIDs[i])
				os.MkdirAll(filepath.Dir(dst), 0770) // todo optimization: keep track of dirs we already created, don't create them again
				err := c.linkOrCopyFile(dst, src)
				xcheckf(err, "link or copy file %q to %q", src, dst)
				createdIDs = append(createdIDs, newMsgIDs[i])
			}

			conf, _ := c.account.Conf()
			if mbDst.Name != conf.RejectsMailbox {
				err = c.account.Train(c.log, xmsgs)
				xcheckf(err, "train copied messages")
			}
		})

		// Broadcast changes to other connections.
		if len(newUIDs) > 0 {
			changes := make([]store.Change, len(newUIDs))
			for i, uid := range newUIDs {
				changes[i] = store.ChangeAddUID{MailboxID: mbDst.ID, UID: uid, Flags: flags[i]}
			}
			c.broadcast(changes)
		}
	})

	// All good, prevent defer above from cleaning up copied files.
	createdIDs = nil

	// ../rfc/9051:6881 ../rfc/4315:183
	c.writeresultf("%s OK [COPYUID %d %s %s] copied", tag, mbDst.UIDValidity, compactUIDSet(origUIDs).String(), compactUIDSet(newUIDs).String())
}

func (c *conn) linkOrCopyFile(dst, src string) error {
	// Try hardlink first.
	if err := os.Link(src, dst); err == nil {
		return nil
	}

	// File system may not support hardlinks, or link would cross file systems. Do a regular file copy.
	sf, err := os.Open(src)
	if err != nil {
		return err
	}
	defer func() {
		err := sf.Close()
		c.xsanity(err, "closing copied src file")
	}()

	df, err := os.OpenFile(dst, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0660)
	if err != nil {
		return err
	}
	defer func() {
		if df != nil {
			err = os.Remove(df.Name())
			c.xsanity(err, "removing unfinished dst file")
			df.Close()
			c.xsanity(err, "closing unfinished dst file")
		}
	}()

	if _, err := io.Copy(df, sf); err != nil {
		return err
	}
	if err := df.Close(); err != nil {
		xerr := os.Remove(df.Name())
		c.xsanity(xerr, "removing unfinished dst file")
		df = nil
		return err
	}
	// todo: may need to do a file/dir sync to flush to disk. better to do it once after multiple linkOrCopyFile calls.
	df = nil
	return nil
}

// Move moves messages from the currently selected/active mailbox to a named mailbox.
//
// State: Selected
func (c *conn) cmdxMove(isUID bool, tag, cmd string, p *parser) {
	// Command: ../rfc/9051:4650 ../rfc/6851:119

	// Request syntax: ../rfc/6851:320 ../rfc/9051:6744
	p.xspace()
	nums := p.xnumSet()
	p.xspace()
	name := p.xmailbox()
	p.xempty()

	name = xcheckmailboxname(name, true)

	if c.readonly {
		xuserErrorf("mailbox open in read-only mode")
	}

	uids, uidargs := c.gatherCopyMoveUIDs(isUID, nums)

	var mbDst store.Mailbox
	var changes []store.Change
	var newUIDs []store.UID

	c.account.WithWLock(func() {
		c.xdbwrite(func(tx *bstore.Tx) {
			c.xmailboxID(tx, c.mailboxID) // Validate.
			mbDst = c.xmailbox(tx, name, "TRYCREATE")
			if mbDst.ID == c.mailboxID {
				xuserErrorf("cannot move to currently selected mailbox")
			}

			if len(uidargs) == 0 {
				xuserErrorf("no matching messages to move")
			}

			// Reserve the uids in the destination mailbox.
			uidFirst := mbDst.UIDNext
			uidnext := uidFirst
			mbDst.UIDNext += store.UID(len(uids))
			err := tx.Update(&mbDst)
			xcheckf(err, "reserve uids in destination mailbox")

			// Update UID and MailboxID in database for messages.
			q := bstore.QueryTx[store.Message](tx)
			q.FilterNonzero(store.Message{MailboxID: c.mailboxID})
			q.FilterEqual("UID", uidargs...)
			q.SortAsc("UID")
			msgs, err := q.List()
			xcheckf(err, "listing messages to move")

			if len(msgs) != len(uidargs) {
				xserverErrorf("uid and message mismatch")
			}

			for i := range msgs {
				m := &msgs[i]
				if m.UID != uids[i] {
					xserverErrorf("internal error: got uid %d, expected %d, for index %d", m.UID, uids[i], i)
				}
				m.MailboxID = mbDst.ID
				m.UID = uidnext
				uidnext++
				err := tx.Update(m)
				xcheckf(err, "updating moved message in database")
			}

			// Prepare broadcast changes to other connections.
			changes = make([]store.Change, 0, 1+len(msgs))
			changes = append(changes, store.ChangeRemoveUIDs{MailboxID: c.mailboxID, UIDs: uids})
			for _, m := range msgs {
				newUIDs = append(newUIDs, m.UID)
				changes = append(changes, store.ChangeAddUID{MailboxID: mbDst.ID, UID: m.UID, Flags: m.Flags})
			}
		})

		c.broadcast(changes)
	})

	// ../rfc/9051:4708 ../rfc/6851:254
	// ../rfc/9051:4713
	c.bwritelinef("* OK [COPYUID %d %s %s] moved", mbDst.UIDValidity, compactUIDSet(uids).String(), compactUIDSet(newUIDs).String())
	for i := 0; i < len(uids); i++ {
		seq := c.xsequence(uids[i])
		c.sequenceRemove(seq, uids[i])
		c.bwritelinef("* %d EXPUNGE", seq)
	}

	c.ok(tag, cmd)
}

// Store sets a full set of flags, or adds/removes specific flags.
//
// State: Selected
func (c *conn) cmdxStore(isUID bool, tag, cmd string, p *parser) {
	// Command: ../rfc/9051:4543 ../rfc/3501:3214

	// Request syntax: ../rfc/9051:7076 ../rfc/3501:5052
	p.xspace()
	nums := p.xnumSet()
	p.xspace()
	var plus, minus bool
	if p.take("+") {
		plus = true
	} else if p.take("-") {
		minus = true
	}
	p.xtake("FLAGS")
	silent := p.take(".SILENT")
	p.xspace()
	var flagstrs []string
	if p.hasPrefix("(") {
		flagstrs = p.xflagList()
	} else {
		flagstrs = append(flagstrs, p.xflag())
		for p.space() {
			flagstrs = append(flagstrs, p.xflag())
		}
	}
	p.xempty()

	if c.readonly {
		xuserErrorf("mailbox open in read-only mode")
	}

	var mask, flags store.Flags
	if plus {
		mask = xparseStoreFlags(flagstrs, false)
		flags = store.FlagsAll
	} else if minus {
		mask = xparseStoreFlags(flagstrs, false)
		flags = store.Flags{}
	} else {
		mask = store.FlagsAll
		flags = xparseStoreFlags(flagstrs, false)
	}

	updates := store.FlagsQuerySet(mask, flags)

	var updated []store.Message
	var oflags []store.Flags

	c.account.WithWLock(func() {
		var mb store.Mailbox
		c.xdbwrite(func(tx *bstore.Tx) {
			mb = c.xmailboxID(tx, c.mailboxID) // Validate.

			uidargs := c.xnumSetCondition(isUID, nums)

			if len(uidargs) == 0 {
				return
			}

			q := bstore.QueryTx[store.Message](tx)
			q.FilterNonzero(store.Message{MailboxID: c.mailboxID})
			q.FilterEqual("UID", uidargs...)
			q.FilterFn(func(m store.Message) bool {
				// We use this filter just to get the pre-update flags...
				oflags = append(oflags, m.Flags)
				return true
			})
			if len(updates) == 0 {
				var err error
				updated, err = q.List()
				xcheckf(err, "listing for flags")
			} else {
				q.Gather(&updated)
				_, err := q.UpdateFields(updates)
				xcheckf(err, "updating flags")
			}
		})

		conf, _ := c.account.Conf()
		if mb.Name != conf.RejectsMailbox {
			jf, _, err := c.account.OpenJunkFilter(c.log)
			if err == nil {
				defer func() {
					if jf != nil {
						err := jf.Close()
						c.xsanity(err, "closing junkfilter")
					}
				}()
				for i, m := range updated {
					err := c.account.Retrain(c.log, jf, oflags[i], m)
					xcheckf(err, "retraining message")
				}
				err = jf.Close()
				jf = nil
				xcheckf(err, "closing junkfilter")
			} else if !errors.Is(err, store.ErrNoJunkFilter) {
				xcheckf(err, "open junk filter for retraining")
			}
		}

		// Broadcast changes to other connections.
		changes := make([]store.Change, len(updated))
		for i, m := range updated {
			changes[i] = store.ChangeFlags{MailboxID: m.MailboxID, UID: m.UID, Mask: mask, Flags: m.Flags}
		}
		c.broadcast(changes)
	})

	for _, m := range updated {
		if !silent {
			// ../rfc/9051:6749 ../rfc/3501:4869
			c.bwritelinef("* %d FETCH (UID %d FLAGS %s)", c.xsequence(m.UID), m.UID, flaglist(m.Flags).pack(c))
		}
	}

	c.ok(tag, cmd)
}