Source file src/net/mail/message.go

     1	// Copyright 2011 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	/*
     6	Package mail implements parsing of mail messages.
     7	
     8	For the most part, this package follows the syntax as specified by RFC 5322 and
     9	extended by RFC 6532.
    10	Notable divergences:
    11		* Obsolete address formats are not parsed, including addresses with
    12		  embedded route information.
    13		* The full range of spacing (the CFWS syntax element) is not supported,
    14		  such as breaking addresses across lines.
    15		* No unicode normalization is performed.
    16		* The special characters ()[]:;@\, are allowed to appear unquoted in names.
    17	*/
    18	package mail
    19	
    20	import (
    21		"bufio"
    22		"errors"
    23		"fmt"
    24		"io"
    25		"log"
    26		"mime"
    27		"net/textproto"
    28		"strings"
    29		"sync"
    30		"time"
    31		"unicode/utf8"
    32	)
    33	
    34	var debug = debugT(false)
    35	
    36	type debugT bool
    37	
    38	func (d debugT) Printf(format string, args ...interface{}) {
    39		if d {
    40			log.Printf(format, args...)
    41		}
    42	}
    43	
    44	// A Message represents a parsed mail message.
    45	type Message struct {
    46		Header Header
    47		Body   io.Reader
    48	}
    49	
    50	// ReadMessage reads a message from r.
    51	// The headers are parsed, and the body of the message will be available
    52	// for reading from msg.Body.
    53	func ReadMessage(r io.Reader) (msg *Message, err error) {
    54		tp := textproto.NewReader(bufio.NewReader(r))
    55	
    56		hdr, err := tp.ReadMIMEHeader()
    57		if err != nil {
    58			return nil, err
    59		}
    60	
    61		return &Message{
    62			Header: Header(hdr),
    63			Body:   tp.R,
    64		}, nil
    65	}
    66	
    67	// Layouts suitable for passing to time.Parse.
    68	// These are tried in order.
    69	var (
    70		dateLayoutsBuildOnce sync.Once
    71		dateLayouts          []string
    72	)
    73	
    74	func buildDateLayouts() {
    75		// Generate layouts based on RFC 5322, section 3.3.
    76	
    77		dows := [...]string{"", "Mon, "}   // day-of-week
    78		days := [...]string{"2", "02"}     // day = 1*2DIGIT
    79		years := [...]string{"2006", "06"} // year = 4*DIGIT / 2*DIGIT
    80		seconds := [...]string{":05", ""}  // second
    81		// "-0700 (MST)" is not in RFC 5322, but is common.
    82		zones := [...]string{"-0700", "MST", "-0700 (MST)"} // zone = (("+" / "-") 4DIGIT) / "GMT" / ...
    83	
    84		for _, dow := range dows {
    85			for _, day := range days {
    86				for _, year := range years {
    87					for _, second := range seconds {
    88						for _, zone := range zones {
    89							s := dow + day + " Jan " + year + " 15:04" + second + " " + zone
    90							dateLayouts = append(dateLayouts, s)
    91						}
    92					}
    93				}
    94			}
    95		}
    96	}
    97	
    98	// ParseDate parses an RFC 5322 date string.
    99	func ParseDate(date string) (time.Time, error) {
   100		dateLayoutsBuildOnce.Do(buildDateLayouts)
   101		for _, layout := range dateLayouts {
   102			t, err := time.Parse(layout, date)
   103			if err == nil {
   104				return t, nil
   105			}
   106		}
   107		return time.Time{}, errors.New("mail: header could not be parsed")
   108	}
   109	
   110	// A Header represents the key-value pairs in a mail message header.
   111	type Header map[string][]string
   112	
   113	// Get gets the first value associated with the given key.
   114	// It is case insensitive; CanonicalMIMEHeaderKey is used
   115	// to canonicalize the provided key.
   116	// If there are no values associated with the key, Get returns "".
   117	// To access multiple values of a key, or to use non-canonical keys,
   118	// access the map directly.
   119	func (h Header) Get(key string) string {
   120		return textproto.MIMEHeader(h).Get(key)
   121	}
   122	
   123	var ErrHeaderNotPresent = errors.New("mail: header not in message")
   124	
   125	// Date parses the Date header field.
   126	func (h Header) Date() (time.Time, error) {
   127		hdr := h.Get("Date")
   128		if hdr == "" {
   129			return time.Time{}, ErrHeaderNotPresent
   130		}
   131		return ParseDate(hdr)
   132	}
   133	
   134	// AddressList parses the named header field as a list of addresses.
   135	func (h Header) AddressList(key string) ([]*Address, error) {
   136		hdr := h.Get(key)
   137		if hdr == "" {
   138			return nil, ErrHeaderNotPresent
   139		}
   140		return ParseAddressList(hdr)
   141	}
   142	
   143	// Address represents a single mail address.
   144	// An address such as "Barry Gibbs <bg@example.com>" is represented
   145	// as Address{Name: "Barry Gibbs", Address: "bg@example.com"}.
   146	type Address struct {
   147		Name    string // Proper name; may be empty.
   148		Address string // user@domain
   149	}
   150	
   151	// ParseAddress parses a single RFC 5322 address, e.g. "Barry Gibbs <bg@example.com>"
   152	func ParseAddress(address string) (*Address, error) {
   153		return (&addrParser{s: address}).parseSingleAddress()
   154	}
   155	
   156	// ParseAddressList parses the given string as a list of addresses.
   157	func ParseAddressList(list string) ([]*Address, error) {
   158		return (&addrParser{s: list}).parseAddressList()
   159	}
   160	
   161	// An AddressParser is an RFC 5322 address parser.
   162	type AddressParser struct {
   163		// WordDecoder optionally specifies a decoder for RFC 2047 encoded-words.
   164		WordDecoder *mime.WordDecoder
   165	}
   166	
   167	// Parse parses a single RFC 5322 address of the
   168	// form "Gogh Fir <gf@example.com>" or "foo@example.com".
   169	func (p *AddressParser) Parse(address string) (*Address, error) {
   170		return (&addrParser{s: address, dec: p.WordDecoder}).parseSingleAddress()
   171	}
   172	
   173	// ParseList parses the given string as a list of comma-separated addresses
   174	// of the form "Gogh Fir <gf@example.com>" or "foo@example.com".
   175	func (p *AddressParser) ParseList(list string) ([]*Address, error) {
   176		return (&addrParser{s: list, dec: p.WordDecoder}).parseAddressList()
   177	}
   178	
   179	// String formats the address as a valid RFC 5322 address.
   180	// If the address's name contains non-ASCII characters
   181	// the name will be rendered according to RFC 2047.
   182	func (a *Address) String() string {
   183		// Format address local@domain
   184		at := strings.LastIndex(a.Address, "@")
   185		var local, domain string
   186		if at < 0 {
   187			// This is a malformed address ("@" is required in addr-spec);
   188			// treat the whole address as local-part.
   189			local = a.Address
   190		} else {
   191			local, domain = a.Address[:at], a.Address[at+1:]
   192		}
   193	
   194		// Add quotes if needed
   195		quoteLocal := false
   196		for i, r := range local {
   197			if isAtext(r, false, false) {
   198				continue
   199			}
   200			if r == '.' {
   201				// Dots are okay if they are surrounded by atext.
   202				// We only need to check that the previous byte is
   203				// not a dot, and this isn't the end of the string.
   204				if i > 0 && local[i-1] != '.' && i < len(local)-1 {
   205					continue
   206				}
   207			}
   208			quoteLocal = true
   209			break
   210		}
   211		if quoteLocal {
   212			local = quoteString(local)
   213	
   214		}
   215	
   216		s := "<" + local + "@" + domain + ">"
   217	
   218		if a.Name == "" {
   219			return s
   220		}
   221	
   222		// If every character is printable ASCII, quoting is simple.
   223		allPrintable := true
   224		for _, r := range a.Name {
   225			// isWSP here should actually be isFWS,
   226			// but we don't support folding yet.
   227			if !isVchar(r) && !isWSP(r) || isMultibyte(r) {
   228				allPrintable = false
   229				break
   230			}
   231		}
   232		if allPrintable {
   233			return quoteString(a.Name) + " " + s
   234		}
   235	
   236		// Text in an encoded-word in a display-name must not contain certain
   237		// characters like quotes or parentheses (see RFC 2047 section 5.3).
   238		// When this is the case encode the name using base64 encoding.
   239		if strings.ContainsAny(a.Name, "\"#$%&'(),.:;<>@[]^`{|}~") {
   240			return mime.BEncoding.Encode("utf-8", a.Name) + " " + s
   241		}
   242		return mime.QEncoding.Encode("utf-8", a.Name) + " " + s
   243	}
   244	
   245	type addrParser struct {
   246		s   string
   247		dec *mime.WordDecoder // may be nil
   248	}
   249	
   250	func (p *addrParser) parseAddressList() ([]*Address, error) {
   251		var list []*Address
   252		for {
   253			p.skipSpace()
   254			addrs, err := p.parseAddress(true)
   255			if err != nil {
   256				return nil, err
   257			}
   258			list = append(list, addrs...)
   259	
   260			if !p.skipCFWS() {
   261				return nil, errors.New("mail: misformatted parenthetical comment")
   262			}
   263			if p.empty() {
   264				break
   265			}
   266			if !p.consume(',') {
   267				return nil, errors.New("mail: expected comma")
   268			}
   269		}
   270		return list, nil
   271	}
   272	
   273	func (p *addrParser) parseSingleAddress() (*Address, error) {
   274		addrs, err := p.parseAddress(true)
   275		if err != nil {
   276			return nil, err
   277		}
   278		if !p.skipCFWS() {
   279			return nil, errors.New("mail: misformatted parenthetical comment")
   280		}
   281		if !p.empty() {
   282			return nil, fmt.Errorf("mail: expected single address, got %q", p.s)
   283		}
   284		if len(addrs) == 0 {
   285			return nil, errors.New("mail: empty group")
   286		}
   287		if len(addrs) > 1 {
   288			return nil, errors.New("mail: group with multiple addresses")
   289		}
   290		return addrs[0], nil
   291	}
   292	
   293	// parseAddress parses a single RFC 5322 address at the start of p.
   294	func (p *addrParser) parseAddress(handleGroup bool) ([]*Address, error) {
   295		debug.Printf("parseAddress: %q", p.s)
   296		p.skipSpace()
   297		if p.empty() {
   298			return nil, errors.New("mail: no address")
   299		}
   300	
   301		// address = mailbox / group
   302		// mailbox = name-addr / addr-spec
   303		// group = display-name ":" [group-list] ";" [CFWS]
   304	
   305		// addr-spec has a more restricted grammar than name-addr,
   306		// so try parsing it first, and fallback to name-addr.
   307		// TODO(dsymonds): Is this really correct?
   308		spec, err := p.consumeAddrSpec()
   309		if err == nil {
   310			var displayName string
   311			p.skipSpace()
   312			if !p.empty() && p.peek() == '(' {
   313				displayName, err = p.consumeDisplayNameComment()
   314				if err != nil {
   315					return nil, err
   316				}
   317			}
   318	
   319			return []*Address{{
   320				Name:    displayName,
   321				Address: spec,
   322			}}, err
   323		}
   324		debug.Printf("parseAddress: not an addr-spec: %v", err)
   325		debug.Printf("parseAddress: state is now %q", p.s)
   326	
   327		// display-name
   328		var displayName string
   329		if p.peek() != '<' {
   330			displayName, err = p.consumePhrase()
   331			if err != nil {
   332				return nil, err
   333			}
   334		}
   335		debug.Printf("parseAddress: displayName=%q", displayName)
   336	
   337		p.skipSpace()
   338		if handleGroup {
   339			if p.consume(':') {
   340				return p.consumeGroupList()
   341			}
   342		}
   343		// angle-addr = "<" addr-spec ">"
   344		if !p.consume('<') {
   345			atext := true
   346			for _, r := range displayName {
   347				if !isAtext(r, true, false) {
   348					atext = false
   349					break
   350				}
   351			}
   352			if atext {
   353				// The input is like "foo.bar"; it's possible the input
   354				// meant to be "foo.bar@domain", or "foo.bar <...>".
   355				return nil, errors.New("mail: missing '@' or angle-addr")
   356			}
   357			// The input is like "Full Name", which couldn't possibly be a
   358			// valid email address if followed by "@domain"; the input
   359			// likely meant to be "Full Name <...>".
   360			return nil, errors.New("mail: no angle-addr")
   361		}
   362		spec, err = p.consumeAddrSpec()
   363		if err != nil {
   364			return nil, err
   365		}
   366		if !p.consume('>') {
   367			return nil, errors.New("mail: unclosed angle-addr")
   368		}
   369		debug.Printf("parseAddress: spec=%q", spec)
   370	
   371		return []*Address{{
   372			Name:    displayName,
   373			Address: spec,
   374		}}, nil
   375	}
   376	
   377	func (p *addrParser) consumeGroupList() ([]*Address, error) {
   378		var group []*Address
   379		// handle empty group.
   380		p.skipSpace()
   381		if p.consume(';') {
   382			p.skipCFWS()
   383			return group, nil
   384		}
   385	
   386		for {
   387			p.skipSpace()
   388			// embedded groups not allowed.
   389			addrs, err := p.parseAddress(false)
   390			if err != nil {
   391				return nil, err
   392			}
   393			group = append(group, addrs...)
   394	
   395			if !p.skipCFWS() {
   396				return nil, errors.New("mail: misformatted parenthetical comment")
   397			}
   398			if p.consume(';') {
   399				p.skipCFWS()
   400				break
   401			}
   402			if !p.consume(',') {
   403				return nil, errors.New("mail: expected comma")
   404			}
   405		}
   406		return group, nil
   407	}
   408	
   409	// consumeAddrSpec parses a single RFC 5322 addr-spec at the start of p.
   410	func (p *addrParser) consumeAddrSpec() (spec string, err error) {
   411		debug.Printf("consumeAddrSpec: %q", p.s)
   412	
   413		orig := *p
   414		defer func() {
   415			if err != nil {
   416				*p = orig
   417			}
   418		}()
   419	
   420		// local-part = dot-atom / quoted-string
   421		var localPart string
   422		p.skipSpace()
   423		if p.empty() {
   424			return "", errors.New("mail: no addr-spec")
   425		}
   426		if p.peek() == '"' {
   427			// quoted-string
   428			debug.Printf("consumeAddrSpec: parsing quoted-string")
   429			localPart, err = p.consumeQuotedString()
   430			if localPart == "" {
   431				err = errors.New("mail: empty quoted string in addr-spec")
   432			}
   433		} else {
   434			// dot-atom
   435			debug.Printf("consumeAddrSpec: parsing dot-atom")
   436			localPart, err = p.consumeAtom(true, false)
   437		}
   438		if err != nil {
   439			debug.Printf("consumeAddrSpec: failed: %v", err)
   440			return "", err
   441		}
   442	
   443		if !p.consume('@') {
   444			return "", errors.New("mail: missing @ in addr-spec")
   445		}
   446	
   447		// domain = dot-atom / domain-literal
   448		var domain string
   449		p.skipSpace()
   450		if p.empty() {
   451			return "", errors.New("mail: no domain in addr-spec")
   452		}
   453		// TODO(dsymonds): Handle domain-literal
   454		domain, err = p.consumeAtom(true, false)
   455		if err != nil {
   456			return "", err
   457		}
   458	
   459		return localPart + "@" + domain, nil
   460	}
   461	
   462	// consumePhrase parses the RFC 5322 phrase at the start of p.
   463	func (p *addrParser) consumePhrase() (phrase string, err error) {
   464		debug.Printf("consumePhrase: [%s]", p.s)
   465		// phrase = 1*word
   466		var words []string
   467		var isPrevEncoded bool
   468		for {
   469			// word = atom / quoted-string
   470			var word string
   471			p.skipSpace()
   472			if p.empty() {
   473				break
   474			}
   475			isEncoded := false
   476			if p.peek() == '"' {
   477				// quoted-string
   478				word, err = p.consumeQuotedString()
   479			} else {
   480				// atom
   481				// We actually parse dot-atom here to be more permissive
   482				// than what RFC 5322 specifies.
   483				word, err = p.consumeAtom(true, true)
   484				if err == nil {
   485					word, isEncoded, err = p.decodeRFC2047Word(word)
   486				}
   487			}
   488	
   489			if err != nil {
   490				break
   491			}
   492			debug.Printf("consumePhrase: consumed %q", word)
   493			if isPrevEncoded && isEncoded {
   494				words[len(words)-1] += word
   495			} else {
   496				words = append(words, word)
   497			}
   498			isPrevEncoded = isEncoded
   499		}
   500		// Ignore any error if we got at least one word.
   501		if err != nil && len(words) == 0 {
   502			debug.Printf("consumePhrase: hit err: %v", err)
   503			return "", fmt.Errorf("mail: missing word in phrase: %v", err)
   504		}
   505		phrase = strings.Join(words, " ")
   506		return phrase, nil
   507	}
   508	
   509	// consumeQuotedString parses the quoted string at the start of p.
   510	func (p *addrParser) consumeQuotedString() (qs string, err error) {
   511		// Assume first byte is '"'.
   512		i := 1
   513		qsb := make([]rune, 0, 10)
   514	
   515		escaped := false
   516	
   517	Loop:
   518		for {
   519			r, size := utf8.DecodeRuneInString(p.s[i:])
   520	
   521			switch {
   522			case size == 0:
   523				return "", errors.New("mail: unclosed quoted-string")
   524	
   525			case size == 1 && r == utf8.RuneError:
   526				return "", fmt.Errorf("mail: invalid utf-8 in quoted-string: %q", p.s)
   527	
   528			case escaped:
   529				//  quoted-pair = ("\" (VCHAR / WSP))
   530	
   531				if !isVchar(r) && !isWSP(r) {
   532					return "", fmt.Errorf("mail: bad character in quoted-string: %q", r)
   533				}
   534	
   535				qsb = append(qsb, r)
   536				escaped = false
   537	
   538			case isQtext(r) || isWSP(r):
   539				// qtext (printable US-ASCII excluding " and \), or
   540				// FWS (almost; we're ignoring CRLF)
   541				qsb = append(qsb, r)
   542	
   543			case r == '"':
   544				break Loop
   545	
   546			case r == '\\':
   547				escaped = true
   548	
   549			default:
   550				return "", fmt.Errorf("mail: bad character in quoted-string: %q", r)
   551	
   552			}
   553	
   554			i += size
   555		}
   556		p.s = p.s[i+1:]
   557		return string(qsb), nil
   558	}
   559	
   560	// consumeAtom parses an RFC 5322 atom at the start of p.
   561	// If dot is true, consumeAtom parses an RFC 5322 dot-atom instead.
   562	// If permissive is true, consumeAtom will not fail on:
   563	// - leading/trailing/double dots in the atom (see golang.org/issue/4938)
   564	// - special characters (RFC 5322 3.2.3) except '<', '>', ':' and '"' (see golang.org/issue/21018)
   565	func (p *addrParser) consumeAtom(dot bool, permissive bool) (atom string, err error) {
   566		i := 0
   567	
   568	Loop:
   569		for {
   570			r, size := utf8.DecodeRuneInString(p.s[i:])
   571			switch {
   572			case size == 1 && r == utf8.RuneError:
   573				return "", fmt.Errorf("mail: invalid utf-8 in address: %q", p.s)
   574	
   575			case size == 0 || !isAtext(r, dot, permissive):
   576				break Loop
   577	
   578			default:
   579				i += size
   580	
   581			}
   582		}
   583	
   584		if i == 0 {
   585			return "", errors.New("mail: invalid string")
   586		}
   587		atom, p.s = p.s[:i], p.s[i:]
   588		if !permissive {
   589			if strings.HasPrefix(atom, ".") {
   590				return "", errors.New("mail: leading dot in atom")
   591			}
   592			if strings.Contains(atom, "..") {
   593				return "", errors.New("mail: double dot in atom")
   594			}
   595			if strings.HasSuffix(atom, ".") {
   596				return "", errors.New("mail: trailing dot in atom")
   597			}
   598		}
   599		return atom, nil
   600	}
   601	
   602	func (p *addrParser) consumeDisplayNameComment() (string, error) {
   603		if !p.consume('(') {
   604			return "", errors.New("mail: comment does not start with (")
   605		}
   606		comment, ok := p.consumeComment()
   607		if !ok {
   608			return "", errors.New("mail: misformatted parenthetical comment")
   609		}
   610	
   611		// TODO(stapelberg): parse quoted-string within comment
   612		words := strings.FieldsFunc(comment, func(r rune) bool { return r == ' ' || r == '\t' })
   613		for idx, word := range words {
   614			decoded, isEncoded, err := p.decodeRFC2047Word(word)
   615			if err != nil {
   616				return "", err
   617			}
   618			if isEncoded {
   619				words[idx] = decoded
   620			}
   621		}
   622	
   623		return strings.Join(words, " "), nil
   624	}
   625	
   626	func (p *addrParser) consume(c byte) bool {
   627		if p.empty() || p.peek() != c {
   628			return false
   629		}
   630		p.s = p.s[1:]
   631		return true
   632	}
   633	
   634	// skipSpace skips the leading space and tab characters.
   635	func (p *addrParser) skipSpace() {
   636		p.s = strings.TrimLeft(p.s, " \t")
   637	}
   638	
   639	func (p *addrParser) peek() byte {
   640		return p.s[0]
   641	}
   642	
   643	func (p *addrParser) empty() bool {
   644		return p.len() == 0
   645	}
   646	
   647	func (p *addrParser) len() int {
   648		return len(p.s)
   649	}
   650	
   651	// skipCFWS skips CFWS as defined in RFC5322.
   652	func (p *addrParser) skipCFWS() bool {
   653		p.skipSpace()
   654	
   655		for {
   656			if !p.consume('(') {
   657				break
   658			}
   659	
   660			if _, ok := p.consumeComment(); !ok {
   661				return false
   662			}
   663	
   664			p.skipSpace()
   665		}
   666	
   667		return true
   668	}
   669	
   670	func (p *addrParser) consumeComment() (string, bool) {
   671		// '(' already consumed.
   672		depth := 1
   673	
   674		var comment string
   675		for {
   676			if p.empty() || depth == 0 {
   677				break
   678			}
   679	
   680			if p.peek() == '\\' && p.len() > 1 {
   681				p.s = p.s[1:]
   682			} else if p.peek() == '(' {
   683				depth++
   684			} else if p.peek() == ')' {
   685				depth--
   686			}
   687			if depth > 0 {
   688				comment += p.s[:1]
   689			}
   690			p.s = p.s[1:]
   691		}
   692	
   693		return comment, depth == 0
   694	}
   695	
   696	func (p *addrParser) decodeRFC2047Word(s string) (word string, isEncoded bool, err error) {
   697		if p.dec != nil {
   698			word, err = p.dec.Decode(s)
   699		} else {
   700			word, err = rfc2047Decoder.Decode(s)
   701		}
   702	
   703		if err == nil {
   704			return word, true, nil
   705		}
   706	
   707		if _, ok := err.(charsetError); ok {
   708			return s, true, err
   709		}
   710	
   711		// Ignore invalid RFC 2047 encoded-word errors.
   712		return s, false, nil
   713	}
   714	
   715	var rfc2047Decoder = mime.WordDecoder{
   716		CharsetReader: func(charset string, input io.Reader) (io.Reader, error) {
   717			return nil, charsetError(charset)
   718		},
   719	}
   720	
   721	type charsetError string
   722	
   723	func (e charsetError) Error() string {
   724		return fmt.Sprintf("charset not supported: %q", string(e))
   725	}
   726	
   727	// isAtext reports whether r is an RFC 5322 atext character.
   728	// If dot is true, period is included.
   729	// If permissive is true, RFC 5322 3.2.3 specials is included,
   730	// except '<', '>', ':' and '"'.
   731	func isAtext(r rune, dot, permissive bool) bool {
   732		switch r {
   733		case '.':
   734			return dot
   735	
   736		// RFC 5322 3.2.3. specials
   737		case '(', ')', '[', ']', ';', '@', '\\', ',':
   738			return permissive
   739	
   740		case '<', '>', '"', ':':
   741			return false
   742		}
   743		return isVchar(r)
   744	}
   745	
   746	// isQtext reports whether r is an RFC 5322 qtext character.
   747	func isQtext(r rune) bool {
   748		// Printable US-ASCII, excluding backslash or quote.
   749		if r == '\\' || r == '"' {
   750			return false
   751		}
   752		return isVchar(r)
   753	}
   754	
   755	// quoteString renders a string as an RFC 5322 quoted-string.
   756	func quoteString(s string) string {
   757		var buf strings.Builder
   758		buf.WriteByte('"')
   759		for _, r := range s {
   760			if isQtext(r) || isWSP(r) {
   761				buf.WriteRune(r)
   762			} else if isVchar(r) {
   763				buf.WriteByte('\\')
   764				buf.WriteRune(r)
   765			}
   766		}
   767		buf.WriteByte('"')
   768		return buf.String()
   769	}
   770	
   771	// isVchar reports whether r is an RFC 5322 VCHAR character.
   772	func isVchar(r rune) bool {
   773		// Visible (printing) characters.
   774		return '!' <= r && r <= '~' || isMultibyte(r)
   775	}
   776	
   777	// isMultibyte reports whether r is a multi-byte UTF-8 character
   778	// as supported by RFC 6532
   779	func isMultibyte(r rune) bool {
   780		return r >= utf8.RuneSelf
   781	}
   782	
   783	// isWSP reports whether r is a WSP (white space).
   784	// WSP is a space or horizontal tab (RFC 5234 Appendix B).
   785	func isWSP(r rune) bool {
   786		return r == ' ' || r == '\t'
   787	}
   788	

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