Added files.

internal/imapwire/decoder.go

@@ -0,0 +1,654 @@
+package imapwire
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+	"unicode"
+
+	"github.com/emersion/go-imap/v2"
+	"github.com/emersion/go-imap/v2/internal/imapnum"
+	"github.com/emersion/go-imap/v2/internal/utf7"
+)
+
+// This limits the max list nesting depth to prevent stack overflow.
+const maxListDepth = 1000
+
+// IsAtomChar returns true if ch is an ATOM-CHAR.
+func IsAtomChar(ch byte) bool {
+	switch ch {
+	case '(', ')', '{', ' ', '%', '*', '"', '\\', ']':
+		return false
+	default:
+		return !unicode.IsControl(rune(ch))
+	}
+}
+
+// Is non-empty char
+func isAStringChar(ch byte) bool {
+	return IsAtomChar(ch) || ch == ']'
+}
+
+// DecoderExpectError is an error due to the Decoder.Expect family of methods.
+type DecoderExpectError struct {
+	Message string
+}
+
+func (err *DecoderExpectError) Error() string {
+	return fmt.Sprintf("imapwire: %v", err.Message)
+}
+
+// A Decoder reads IMAP data.
+//
+// There are multiple families of methods:
+//
+//   - Methods directly named after IMAP grammar elements attempt to decode
+//     said element, and return false if it's another element.
+//   - "Expect" methods do the same, but set the decoder error (see Err) on
+//     failure.
+type Decoder struct {
+	// CheckBufferedLiteralFunc is called when a literal is about to be decoded
+	// and needs to be fully buffered in memory.
+	CheckBufferedLiteralFunc func(size int64, nonSync bool) error
+	// MaxSize defines a maximum number of bytes to be read from the input.
+	// Literals are ignored.
+	MaxSize int64
+
+	r         *bufio.Reader
+	side      ConnSide
+	err       error
+	literal   bool
+	crlf      bool
+	listDepth int
+	readBytes int64
+}
+
+// NewDecoder creates a new decoder.
+func NewDecoder(r *bufio.Reader, side ConnSide) *Decoder {
+	return &Decoder{r: r, side: side}
+}
+
+func (dec *Decoder) mustUnreadByte() {
+	if err := dec.r.UnreadByte(); err != nil {
+		panic(fmt.Errorf("imapwire: failed to unread byte: %v", err))
+	}
+	dec.readBytes--
+}
+
+// Err returns the decoder error, if any.
+func (dec *Decoder) Err() error {
+	return dec.err
+}
+
+func (dec *Decoder) returnErr(err error) bool {
+	if err == nil {
+		return true
+	}
+	if dec.err == nil {
+		dec.err = err
+	}
+	return false
+}
+
+func (dec *Decoder) readByte() (byte, bool) {
+	if dec.MaxSize > 0 && dec.readBytes > dec.MaxSize {
+		return 0, dec.returnErr(fmt.Errorf("imapwire: max size exceeded"))
+	}
+	dec.crlf = false
+	if dec.literal {
+		return 0, dec.returnErr(fmt.Errorf("imapwire: cannot decode while a literal is open"))
+	}
+	b, err := dec.r.ReadByte()
+	if err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		return b, dec.returnErr(err)
+	}
+	dec.readBytes++
+	return b, true
+}
+
+func (dec *Decoder) acceptByte(want byte) bool {
+	got, ok := dec.readByte()
+	if !ok {
+		return false
+	} else if got != want {
+		dec.mustUnreadByte()
+		return false
+	}
+	return true
+}
+
+// EOF returns true if end-of-file is reached.
+func (dec *Decoder) EOF() bool {
+	_, err := dec.r.ReadByte()
+	if err == io.EOF {
+		return true
+	} else if err != nil {
+		return dec.returnErr(err)
+	}
+	dec.mustUnreadByte()
+	return false
+}
+
+// Expect sets the decoder error if ok is false.
+func (dec *Decoder) Expect(ok bool, name string) bool {
+	if !ok {
+		msg := fmt.Sprintf("expected %v", name)
+		if dec.r.Buffered() > 0 {
+			b, _ := dec.r.Peek(1)
+			msg += fmt.Sprintf(", got %q", b)
+		}
+		return dec.returnErr(&DecoderExpectError{Message: msg})
+	}
+	return true
+}
+
+func (dec *Decoder) SP() bool {
+	if dec.acceptByte(' ') {
+		// https://github.com/emersion/go-imap/issues/571
+		b, ok := dec.readByte()
+		if !ok {
+			return false
+		}
+		dec.mustUnreadByte()
+		return b != '\r' && b != '\n'
+	}
+
+	// Special case: SP is optional if the next field is a parenthesized list
+	b, ok := dec.readByte()
+	if !ok {
+		return false
+	}
+	dec.mustUnreadByte()
+	return b == '('
+}
+
+func (dec *Decoder) ExpectSP() bool {
+	return dec.Expect(dec.SP(), "SP")
+}
+
+func (dec *Decoder) CRLF() bool {
+	dec.acceptByte(' ')  // https://github.com/emersion/go-imap/issues/540
+	dec.acceptByte('\r') // be liberal in what we receive and accept lone LF
+	if !dec.acceptByte('\n') {
+		return false
+	}
+	dec.crlf = true
+	return true
+}
+
+func (dec *Decoder) ExpectCRLF() bool {
+	return dec.Expect(dec.CRLF(), "CRLF")
+}
+
+func (dec *Decoder) Func(ptr *string, valid func(ch byte) bool) bool {
+	var sb strings.Builder
+	for {
+		b, ok := dec.readByte()
+		if !ok {
+			return false
+		}
+
+		if !valid(b) {
+			dec.mustUnreadByte()
+			break
+		}
+
+		sb.WriteByte(b)
+	}
+	if sb.Len() == 0 {
+		return false
+	}
+	*ptr = sb.String()
+	return true
+}
+
+func (dec *Decoder) Atom(ptr *string) bool {
+	return dec.Func(ptr, IsAtomChar)
+}
+
+func (dec *Decoder) ExpectAtom(ptr *string) bool {
+	return dec.Expect(dec.Atom(ptr), "atom")
+}
+
+func (dec *Decoder) ExpectNIL() bool {
+	var s string
+	return dec.ExpectAtom(&s) && dec.Expect(s == "NIL", "NIL")
+}
+
+func (dec *Decoder) Special(b byte) bool {
+	return dec.acceptByte(b)
+}
+
+func (dec *Decoder) ExpectSpecial(b byte) bool {
+	return dec.Expect(dec.Special(b), fmt.Sprintf("'%v'", string(b)))
+}
+
+func (dec *Decoder) Text(ptr *string) bool {
+	var sb strings.Builder
+	for {
+		b, ok := dec.readByte()
+		if !ok {
+			return false
+		} else if b == '\r' || b == '\n' {
+			dec.mustUnreadByte()
+			break
+		}
+		sb.WriteByte(b)
+	}
+	if sb.Len() == 0 {
+		return false
+	}
+	*ptr = sb.String()
+	return true
+}
+
+func (dec *Decoder) ExpectText(ptr *string) bool {
+	return dec.Expect(dec.Text(ptr), "text")
+}
+
+func (dec *Decoder) DiscardUntilByte(untilCh byte) {
+	for {
+		ch, ok := dec.readByte()
+		if !ok {
+			return
+		} else if ch == untilCh {
+			dec.mustUnreadByte()
+			return
+		}
+	}
+}
+
+func (dec *Decoder) DiscardLine() {
+	if dec.crlf {
+		return
+	}
+	var text string
+	dec.Text(&text)
+	dec.CRLF()
+}
+
+func (dec *Decoder) DiscardValue() bool {
+	var s string
+	if dec.String(&s) {
+		return true
+	}
+
+	isList, err := dec.List(func() error {
+		if !dec.DiscardValue() {
+			return dec.Err()
+		}
+		return nil
+	})
+	if err != nil {
+		return false
+	} else if isList {
+		return true
+	}
+
+	if dec.Atom(&s) {
+		return true
+	}
+
+	dec.Expect(false, "value")
+	return false
+}
+
+func (dec *Decoder) numberStr() (s string, ok bool) {
+	var sb strings.Builder
+	for {
+		ch, ok := dec.readByte()
+		if !ok {
+			return "", false
+		} else if ch < '0' || ch > '9' {
+			dec.mustUnreadByte()
+			break
+		}
+		sb.WriteByte(ch)
+	}
+	if sb.Len() == 0 {
+		return "", false
+	}
+	return sb.String(), true
+}
+
+func (dec *Decoder) Number(ptr *uint32) bool {
+	s, ok := dec.numberStr()
+	if !ok {
+		return false
+	}
+	v64, err := strconv.ParseUint(s, 10, 32)
+	if err != nil {
+		return false // can happen on overflow
+	}
+	*ptr = uint32(v64)
+	return true
+}
+
+func (dec *Decoder) ExpectNumber(ptr *uint32) bool {
+	return dec.Expect(dec.Number(ptr), "number")
+}
+
+func (dec *Decoder) ExpectBodyFldOctets(ptr *uint32) bool {
+	// Workaround: some servers incorrectly return "-1" for the body structure
+	// size. See:
+	// https://github.com/emersion/go-imap/issues/534
+	if dec.acceptByte('-') {
+		*ptr = 0
+		return dec.Expect(dec.acceptByte('1'), "-1 (body-fld-octets workaround)")
+	}
+	return dec.ExpectNumber(ptr)
+}
+
+func (dec *Decoder) Number64(ptr *int64) bool {
+	s, ok := dec.numberStr()
+	if !ok {
+		return false
+	}
+	v, err := strconv.ParseInt(s, 10, 64)
+	if err != nil {
+		return false // can happen on overflow
+	}
+	*ptr = v
+	return true
+}
+
+func (dec *Decoder) ExpectNumber64(ptr *int64) bool {
+	return dec.Expect(dec.Number64(ptr), "number64")
+}
+
+func (dec *Decoder) ModSeq(ptr *uint64) bool {
+	s, ok := dec.numberStr()
+	if !ok {
+		return false
+	}
+	v, err := strconv.ParseUint(s, 10, 64)
+	if err != nil {
+		return false // can happen on overflow
+	}
+	*ptr = v
+	return true
+}
+
+func (dec *Decoder) ExpectModSeq(ptr *uint64) bool {
+	return dec.Expect(dec.ModSeq(ptr), "mod-sequence-value")
+}
+
+func (dec *Decoder) Quoted(ptr *string) bool {
+	if !dec.Special('"') {
+		return false
+	}
+	var sb strings.Builder
+	for {
+		ch, ok := dec.readByte()
+		if !ok {
+			return false
+		}
+
+		if ch == '"' {
+			break
+		}
+
+		if ch == '\\' {
+			ch, ok = dec.readByte()
+			if !ok {
+				return false
+			}
+		}
+
+		sb.WriteByte(ch)
+	}
+	*ptr = sb.String()
+	return true
+}
+
+func (dec *Decoder) ExpectAString(ptr *string) bool {
+	if dec.Quoted(ptr) {
+		return true
+	}
+	if dec.Literal(ptr) {
+		return true
+	}
+	// We cannot do dec.Atom(ptr) here because sometimes mailbox names are unquoted,
+	// and they can contain special characters like `]`.
+	return dec.Expect(dec.Func(ptr, isAStringChar), "ASTRING-CHAR")
+}
+
+func (dec *Decoder) String(ptr *string) bool {
+	return dec.Quoted(ptr) || dec.Literal(ptr)
+}
+
+func (dec *Decoder) ExpectString(ptr *string) bool {
+	return dec.Expect(dec.String(ptr), "string")
+}
+
+func (dec *Decoder) ExpectNString(ptr *string) bool {
+	var s string
+	if dec.Atom(&s) {
+		if !dec.Expect(s == "NIL", "nstring") {
+			return false
+		}
+		*ptr = ""
+		return true
+	}
+	return dec.ExpectString(ptr)
+}
+
+func (dec *Decoder) ExpectNStringReader() (lit *LiteralReader, nonSync, ok bool) {
+	var s string
+	if dec.Atom(&s) {
+		if !dec.Expect(s == "NIL", "nstring") {
+			return nil, false, false
+		}
+		return nil, true, true
+	}
+	// TODO: read quoted string as a string instead of buffering
+	if dec.Quoted(&s) {
+		return newLiteralReaderFromString(s), true, true
+	}
+	if lit, nonSync, ok = dec.LiteralReader(); ok {
+		return lit, nonSync, true
+	} else {
+		return nil, false, dec.Expect(false, "nstring")
+	}
+}
+
+func (dec *Decoder) List(f func() error) (isList bool, err error) {
+	if !dec.Special('(') {
+		return false, nil
+	}
+	if dec.Special(')') {
+		return true, nil
+	}
+
+	dec.listDepth++
+	defer func() {
+		dec.listDepth--
+	}()
+
+	if dec.listDepth >= maxListDepth {
+		return false, fmt.Errorf("imapwire: exceeded max depth")
+	}
+
+	for {
+		if err := f(); err != nil {
+			return true, err
+		}
+
+		if dec.Special(')') {
+			return true, nil
+		} else if !dec.ExpectSP() {
+			return true, dec.Err()
+		}
+	}
+}
+
+func (dec *Decoder) ExpectList(f func() error) error {
+	isList, err := dec.List(f)
+	if err != nil {
+		return err
+	} else if !dec.Expect(isList, "(") {
+		return dec.Err()
+	}
+	return nil
+}
+
+func (dec *Decoder) ExpectNList(f func() error) error {
+	var s string
+	if dec.Atom(&s) {
+		if !dec.Expect(s == "NIL", "NIL") {
+			return dec.Err()
+		}
+		return nil
+	}
+	return dec.ExpectList(f)
+}
+
+func (dec *Decoder) ExpectMailbox(ptr *string) bool {
+	var name string
+	if !dec.ExpectAString(&name) {
+		return false
+	}
+	if strings.EqualFold(name, "INBOX") {
+		*ptr = "INBOX"
+		return true
+	}
+	name, err := utf7.Decode(name)
+	if err == nil {
+		*ptr = name
+	}
+	return dec.returnErr(err)
+}
+
+func (dec *Decoder) ExpectUID(ptr *imap.UID) bool {
+	var num uint32
+	if !dec.ExpectNumber(&num) {
+		return false
+	}
+	*ptr = imap.UID(num)
+	return true
+}
+
+func (dec *Decoder) ExpectNumSet(kind NumKind, ptr *imap.NumSet) bool {
+	if dec.Special('$') {
+		*ptr = imap.SearchRes()
+		return true
+	}
+
+	var s string
+	if !dec.Expect(dec.Func(&s, isNumSetChar), "sequence-set") {
+		return false
+	}
+	numSet, err := imapnum.ParseSet(s)
+	if err != nil {
+		return dec.returnErr(err)
+	}
+
+	switch kind {
+	case NumKindSeq:
+		*ptr = seqSetFromNumSet(numSet)
+	case NumKindUID:
+		*ptr = uidSetFromNumSet(numSet)
+	}
+	return true
+}
+
+func (dec *Decoder) ExpectUIDSet(ptr *imap.UIDSet) bool {
+	var numSet imap.NumSet
+	ok := dec.ExpectNumSet(NumKindUID, &numSet)
+	if ok {
+		*ptr = numSet.(imap.UIDSet)
+	}
+	return ok
+}
+
+func isNumSetChar(ch byte) bool {
+	return ch == '*' || IsAtomChar(ch)
+}
+
+func (dec *Decoder) Literal(ptr *string) bool {
+	lit, nonSync, ok := dec.LiteralReader()
+	if !ok {
+		return false
+	}
+	if dec.CheckBufferedLiteralFunc != nil {
+		if err := dec.CheckBufferedLiteralFunc(lit.Size(), nonSync); err != nil {
+			lit.cancel()
+			return false
+		}
+	}
+	var sb strings.Builder
+	_, err := io.Copy(&sb, lit)
+	if err == nil {
+		*ptr = sb.String()
+	}
+	return dec.returnErr(err)
+}
+
+func (dec *Decoder) LiteralReader() (lit *LiteralReader, nonSync, ok bool) {
+	if !dec.Special('{') {
+		return nil, false, false
+	}
+	var size int64
+	if !dec.ExpectNumber64(&size) {
+		return nil, false, false
+	}
+	if dec.side == ConnSideServer {
+		nonSync = dec.acceptByte('+')
+	}
+	if !dec.ExpectSpecial('}') || !dec.ExpectCRLF() {
+		return nil, false, false
+	}
+	dec.literal = true
+	lit = &LiteralReader{
+		dec:  dec,
+		size: size,
+		r:    io.LimitReader(dec.r, size),
+	}
+	return lit, nonSync, true
+}
+
+func (dec *Decoder) ExpectLiteralReader() (lit *LiteralReader, nonSync bool, err error) {
+	lit, nonSync, ok := dec.LiteralReader()
+	if !dec.Expect(ok, "literal") {
+		return nil, false, dec.Err()
+	}
+	return lit, nonSync, nil
+}
+
+type LiteralReader struct {
+	dec  *Decoder
+	size int64
+	r    io.Reader
+}
+
+func newLiteralReaderFromString(s string) *LiteralReader {
+	return &LiteralReader{
+		size: int64(len(s)),
+		r:    strings.NewReader(s),
+	}
+}
+
+func (lit *LiteralReader) Size() int64 {
+	return lit.size
+}
+
+func (lit *LiteralReader) Read(b []byte) (int, error) {
+	n, err := lit.r.Read(b)
+	if err == io.EOF {
+		lit.cancel()
+	}
+	return n, err
+}
+
+func (lit *LiteralReader) cancel() {
+	if lit.dec == nil {
+		return
+	}
+	lit.dec.literal = false
+	lit.dec = nil
+}

internal/imapwire/encoder.go

@@ -0,0 +1,341 @@
+package imapwire
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+	"unicode"
+
+	"github.com/emersion/go-imap/v2"
+	"github.com/emersion/go-imap/v2/internal/utf7"
+)
+
+// An Encoder writes IMAP data.
+//
+// Most methods don't return an error, instead they defer error handling until
+// CRLF is called. These methods return the Encoder so that calls can be
+// chained.
+type Encoder struct {
+	// QuotedUTF8 allows raw UTF-8 in quoted strings. This requires IMAP4rev2
+	// to be available, or UTF8=ACCEPT to be enabled.
+	QuotedUTF8 bool
+	// LiteralMinus enables non-synchronizing literals for short payloads.
+	// This requires IMAP4rev2 or LITERAL-. This is only meaningful for
+	// clients.
+	LiteralMinus bool
+	// LiteralPlus enables non-synchronizing literals for all payloads. This
+	// requires LITERAL+. This is only meaningful for clients.
+	LiteralPlus bool
+	// NewContinuationRequest creates a new continuation request. This is only
+	// meaningful for clients.
+	NewContinuationRequest func() *ContinuationRequest
+
+	w       *bufio.Writer
+	side    ConnSide
+	err     error
+	literal bool
+}
+
+// NewEncoder creates a new encoder.
+func NewEncoder(w *bufio.Writer, side ConnSide) *Encoder {
+	return &Encoder{w: w, side: side}
+}
+
+func (enc *Encoder) setErr(err error) {
+	if enc.err == nil {
+		enc.err = err
+	}
+}
+
+func (enc *Encoder) writeString(s string) *Encoder {
+	if enc.err != nil {
+		return enc
+	}
+	if enc.literal {
+		enc.err = fmt.Errorf("imapwire: cannot encode while a literal is open")
+		return enc
+	}
+	if _, err := enc.w.WriteString(s); err != nil {
+		enc.err = err
+	}
+	return enc
+}
+
+// CRLF writes a "\r\n" sequence and flushes the buffered writer.
+func (enc *Encoder) CRLF() error {
+	enc.writeString("\r\n")
+	if enc.err != nil {
+		return enc.err
+	}
+	return enc.w.Flush()
+}
+
+func (enc *Encoder) Atom(s string) *Encoder {
+	return enc.writeString(s)
+}
+
+func (enc *Encoder) SP() *Encoder {
+	return enc.writeString(" ")
+}
+
+func (enc *Encoder) Special(ch byte) *Encoder {
+	return enc.writeString(string(ch))
+}
+
+func (enc *Encoder) Quoted(s string) *Encoder {
+	var sb strings.Builder
+	sb.Grow(2 + len(s))
+	sb.WriteByte('"')
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
+		if ch == '"' || ch == '\\' {
+			sb.WriteByte('\\')
+		}
+		sb.WriteByte(ch)
+	}
+	sb.WriteByte('"')
+	return enc.writeString(sb.String())
+}
+
+func (enc *Encoder) String(s string) *Encoder {
+	if !enc.validQuoted(s) {
+		enc.stringLiteral(s)
+		return enc
+	}
+	return enc.Quoted(s)
+}
+
+func (enc *Encoder) validQuoted(s string) bool {
+	if len(s) > 4096 {
+		return false
+	}
+
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
+
+		// NUL, CR and LF are never valid
+		switch ch {
+		case 0, '\r', '\n':
+			return false
+		}
+
+		if !enc.QuotedUTF8 && ch > unicode.MaxASCII {
+			return false
+		}
+	}
+	return true
+}
+
+func (enc *Encoder) stringLiteral(s string) {
+	var sync *ContinuationRequest
+	if enc.side == ConnSideClient && (!enc.LiteralMinus || len(s) > 4096) && !enc.LiteralPlus {
+		if enc.NewContinuationRequest != nil {
+			sync = enc.NewContinuationRequest()
+		}
+		if sync == nil {
+			enc.setErr(fmt.Errorf("imapwire: cannot send synchronizing literal"))
+			return
+		}
+	}
+	wc := enc.Literal(int64(len(s)), sync)
+	_, writeErr := io.WriteString(wc, s)
+	closeErr := wc.Close()
+	if writeErr != nil {
+		enc.setErr(writeErr)
+	} else if closeErr != nil {
+		enc.setErr(closeErr)
+	}
+}
+
+func (enc *Encoder) Mailbox(name string) *Encoder {
+	if strings.EqualFold(name, "INBOX") {
+		return enc.Atom("INBOX")
+	} else {
+		if enc.QuotedUTF8 {
+			name = utf7.Escape(name)
+		} else {
+			name = utf7.Encode(name)
+		}
+		return enc.String(name)
+	}
+}
+
+func (enc *Encoder) NumSet(numSet imap.NumSet) *Encoder {
+	s := numSet.String()
+	if s == "" {
+		enc.setErr(fmt.Errorf("imapwire: cannot encode empty sequence set"))
+		return enc
+	}
+	return enc.writeString(s)
+}
+
+func (enc *Encoder) Flag(flag imap.Flag) *Encoder {
+	if flag != "\\*" && !isValidFlag(string(flag)) {
+		enc.setErr(fmt.Errorf("imapwire: invalid flag %q", flag))
+		return enc
+	}
+	return enc.writeString(string(flag))
+}
+
+func (enc *Encoder) MailboxAttr(attr imap.MailboxAttr) *Encoder {
+	if !strings.HasPrefix(string(attr), "\\") || !isValidFlag(string(attr)) {
+		enc.setErr(fmt.Errorf("imapwire: invalid mailbox attribute %q", attr))
+		return enc
+	}
+	return enc.writeString(string(attr))
+}
+
+// isValidFlag checks whether the provided string satisfies
+// flag-keyword / flag-extension.
+func isValidFlag(s string) bool {
+	for i := 0; i < len(s); i++ {
+		ch := s[i]
+		if ch == '\\' {
+			if i != 0 {
+				return false
+			}
+		} else {
+			if !IsAtomChar(ch) {
+				return false
+			}
+		}
+	}
+	return len(s) > 0
+}
+
+func (enc *Encoder) Number(v uint32) *Encoder {
+	return enc.writeString(strconv.FormatUint(uint64(v), 10))
+}
+
+func (enc *Encoder) Number64(v int64) *Encoder {
+	// TODO: disallow negative values
+	return enc.writeString(strconv.FormatInt(v, 10))
+}
+
+func (enc *Encoder) ModSeq(v uint64) *Encoder {
+	// TODO: disallow zero values
+	return enc.writeString(strconv.FormatUint(v, 10))
+}
+
+// List writes a parenthesized list.
+func (enc *Encoder) List(n int, f func(i int)) *Encoder {
+	enc.Special('(')
+	for i := 0; i < n; i++ {
+		if i > 0 {
+			enc.SP()
+		}
+		f(i)
+	}
+	enc.Special(')')
+	return enc
+}
+
+func (enc *Encoder) BeginList() *ListEncoder {
+	enc.Special('(')
+	return &ListEncoder{enc: enc}
+}
+
+func (enc *Encoder) NIL() *Encoder {
+	return enc.Atom("NIL")
+}
+
+func (enc *Encoder) Text(s string) *Encoder {
+	return enc.writeString(s)
+}
+
+func (enc *Encoder) UID(uid imap.UID) *Encoder {
+	return enc.Number(uint32(uid))
+}
+
+// Literal writes a literal.
+//
+// The caller must write exactly size bytes to the returned writer.
+//
+// If sync is non-nil, the literal is synchronizing: the encoder will wait for
+// nil to be sent to the channel before writing the literal data. If an error
+// is sent to the channel, the literal will be cancelled.
+func (enc *Encoder) Literal(size int64, sync *ContinuationRequest) io.WriteCloser {
+	if sync != nil && enc.side == ConnSideServer {
+		panic("imapwire: sync must be nil on a server-side Encoder.Literal")
+	}
+
+	// TODO: literal8
+	enc.writeString("{")
+	enc.Number64(size)
+	if sync == nil && enc.side == ConnSideClient {
+		enc.writeString("+")
+	}
+	enc.writeString("}")
+
+	if sync == nil {
+		enc.writeString("\r\n")
+	} else {
+		if err := enc.CRLF(); err != nil {
+			return errorWriter{err}
+		}
+		if _, err := sync.Wait(); err != nil {
+			enc.setErr(err)
+			return errorWriter{err}
+		}
+	}
+
+	enc.literal = true
+	return &literalWriter{
+		enc: enc,
+		n:   size,
+	}
+}
+
+type errorWriter struct {
+	err error
+}
+
+func (ew errorWriter) Write(b []byte) (int, error) {
+	return 0, ew.err
+}
+
+func (ew errorWriter) Close() error {
+	return ew.err
+}
+
+type literalWriter struct {
+	enc *Encoder
+	n   int64
+}
+
+func (lw *literalWriter) Write(b []byte) (int, error) {
+	if lw.n-int64(len(b)) < 0 {
+		return 0, fmt.Errorf("wrote too many bytes in literal")
+	}
+	n, err := lw.enc.w.Write(b)
+	lw.n -= int64(n)
+	return n, err
+}
+
+func (lw *literalWriter) Close() error {
+	lw.enc.literal = false
+	if lw.n != 0 {
+		return fmt.Errorf("wrote too few bytes in literal (%v remaining)", lw.n)
+	}
+	return nil
+}
+
+type ListEncoder struct {
+	enc *Encoder
+	n   int
+}
+
+func (le *ListEncoder) Item() *Encoder {
+	if le.n > 0 {
+		le.enc.SP()
+	}
+	le.n++
+	return le.enc
+}
+
+func (le *ListEncoder) End() {
+	le.enc.Special(')')
+	le.enc = nil
+}

internal/imapwire/imapwire.go

@@ -0,0 +1,47 @@
+// Package imapwire implements the IMAP wire protocol.
+//
+// The IMAP wire protocol is defined in RFC 9051 section 4.
+package imapwire
+
+import (
+	"fmt"
+)
+
+// ConnSide describes the side of a connection: client or server.
+type ConnSide int
+
+const (
+	ConnSideClient ConnSide = 1 + iota
+	ConnSideServer
+)
+
+// ContinuationRequest is a continuation request.
+//
+// The sender must call either Done or Cancel. The receiver must call Wait.
+type ContinuationRequest struct {
+	done chan struct{}
+	err  error
+	text string
+}
+
+func NewContinuationRequest() *ContinuationRequest {
+	return &ContinuationRequest{done: make(chan struct{})}
+}
+
+func (cont *ContinuationRequest) Cancel(err error) {
+	if err == nil {
+		err = fmt.Errorf("imapwire: continuation request cancelled")
+	}
+	cont.err = err
+	close(cont.done)
+}
+
+func (cont *ContinuationRequest) Done(text string) {
+	cont.text = text
+	close(cont.done)
+}
+
+func (cont *ContinuationRequest) Wait() (string, error) {
+	<-cont.done
+	return cont.text, cont.err
+}

internal/imapwire/num.go

@@ -0,0 +1,39 @@
+package imapwire
+
+import (
+	"unsafe"
+
+	"github.com/emersion/go-imap/v2"
+	"github.com/emersion/go-imap/v2/internal/imapnum"
+)
+
+type NumKind int
+
+const (
+	NumKindSeq NumKind = iota + 1
+	NumKindUID
+)
+
+func seqSetFromNumSet(s imapnum.Set) imap.SeqSet {
+	return *(*imap.SeqSet)(unsafe.Pointer(&s))
+}
+
+func uidSetFromNumSet(s imapnum.Set) imap.UIDSet {
+	return *(*imap.UIDSet)(unsafe.Pointer(&s))
+}
+
+func NumSetKind(numSet imap.NumSet) NumKind {
+	switch numSet.(type) {
+	case imap.SeqSet:
+		return NumKindSeq
+	case imap.UIDSet:
+		return NumKindUID
+	default:
+		panic("imap: invalid NumSet type")
+	}
+}
+
+func ParseSeqSet(s string) (imap.SeqSet, error) {
+	numSet, err := imapnum.ParseSet(s)
+	return seqSetFromNumSet(numSet), err
+}