message.go

Documentation: net/mail

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