Source: mediatype.go in package mime


package mime

import (
	"errors"
	"fmt"
	"sort"
	"strings"
	"unicode"
)

FormatMediaType serializes mediatype t and the parameters param as a media type conforming to RFC 2045 and RFC 2616. The type and parameter names are written in lower-case. When any of the arguments result in a standard violation then FormatMediaType returns the empty string.

func FormatMediaType(t string, param map[string]string) string {
	var b strings.Builder
	if slash := strings.IndexByte(t, '/'); slash == -1 {
		if !isToken(t) {
			return ""
		}
		b.WriteString(strings.ToLower(t))
	} else {
		major, sub := t[:slash], t[slash+1:]
		if !isToken(major) || !isToken(sub) {
			return ""
		}
		b.WriteString(strings.ToLower(major))
		b.WriteByte('/')
		b.WriteString(strings.ToLower(sub))
	}

	attrs := make([]string, 0, len(param))
	for a := range param {
		attrs = append(attrs, a)
	}
	sort.Strings(attrs)

	for _, attribute := range attrs {
		value := param[attribute]
		b.WriteByte(';')
		b.WriteByte(' ')
		if !isToken(attribute) {
			return ""
		}
		b.WriteString(strings.ToLower(attribute))

		needEnc := needsEncoding(value)

RFC 2231 section 4

			b.WriteByte('*')
		}
		b.WriteByte('=')

		if needEnc {
			b.WriteString("utf-8''")

			offset := 0
			for index := 0; index < len(value); index++ {

{RFC 2231 section 7} attribute-char := <any (US-ASCII) CHAR except SPACE, CTLs, "*", "'", "%", or tspecials>

				if ch <= ' ' || ch >= 0x7F ||
					ch == '*' || ch == '\'' || ch == '%' ||
					isTSpecial(rune(ch)) {

					b.WriteString(value[offset:index])
					offset = index + 1

					b.WriteByte('%')
					b.WriteByte(upperhex[ch>>4])
					b.WriteByte(upperhex[ch&0x0F])
				}
			}
			b.WriteString(value[offset:])
			continue
		}

		if isToken(value) {
			b.WriteString(value)
			continue
		}

		b.WriteByte('"')
		offset := 0
		for index := 0; index < len(value); index++ {
			character := value[index]
			if character == '"' || character == '\\' {
				b.WriteString(value[offset:index])
				offset = index
				b.WriteByte('\\')
			}
		}
		b.WriteString(value[offset:])
		b.WriteByte('"')
	}
	return b.String()
}

func checkMediaTypeDisposition(s string) error {
	typ, rest := consumeToken(s)
	if typ == "" {
		return errors.New("mime: no media type")
	}
	if rest == "" {
		return nil
	}
	if !strings.HasPrefix(rest, "/") {
		return errors.New("mime: expected slash after first token")
	}
	subtype, rest := consumeToken(rest[1:])
	if subtype == "" {
		return errors.New("mime: expected token after slash")
	}
	if rest != "" {
		return errors.New("mime: unexpected content after media subtype")
	}
	return nil
}

ErrInvalidMediaParameter is returned by ParseMediaType if the media type value was found but there was an error parsing the optional parameters

var ErrInvalidMediaParameter = errors.New("mime: invalid media parameter")

ParseMediaType parses a media type value and any optional parameters, per RFC 1521. Media types are the values in Content-Type and Content-Disposition headers (RFC 2183). On success, ParseMediaType returns the media type converted to lowercase and trimmed of white space and a non-nil map. If there is an error parsing the optional parameter, the media type will be returned along with the error ErrInvalidMediaParameter. The returned map, params, maps from the lowercase attribute to the attribute value with its case preserved.

func ParseMediaType(v string) (mediatype string, params map[string]string, err error) {
	i := strings.Index(v, ";")
	if i == -1 {
		i = len(v)
	}
	mediatype = strings.TrimSpace(strings.ToLower(v[0:i]))

	err = checkMediaTypeDisposition(mediatype)
	if err != nil {
		return "", nil, err
	}

	params = make(map[string]string)

Map of base parameter name -> parameter name -> value for parameters containing a '*' character. Lazily initialized.

	var continuation map[string]map[string]string

	v = v[i:]
	for len(v) > 0 {
		v = strings.TrimLeftFunc(v, unicode.IsSpace)
		if len(v) == 0 {
			break
		}
		key, value, rest := consumeMediaParam(v)
		if key == "" {

Ignore trailing semicolons. Not an error.

				return

Parse error.

			return mediatype, nil, ErrInvalidMediaParameter
		}

		pmap := params
		if idx := strings.Index(key, "*"); idx != -1 {
			baseName := key[:idx]
			if continuation == nil {
				continuation = make(map[string]map[string]string)
			}
			var ok bool
			if pmap, ok = continuation[baseName]; !ok {
				continuation[baseName] = make(map[string]string)
				pmap = continuation[baseName]
			}
		}

Duplicate parameter name is bogus.

			return "", nil, errors.New("mime: duplicate parameter name")
		}
		pmap[key] = value
		v = rest
	}

Stitch together any continuations or things with stars (i.e. RFC 2231 things with stars: "foo*0" or "foo*")

	var buf strings.Builder
	for key, pieceMap := range continuation {
		singlePartKey := key + "*"
		if v, ok := pieceMap[singlePartKey]; ok {
			if decv, ok := decode2231Enc(v); ok {
				params[key] = decv
			}
			continue
		}

		buf.Reset()
		valid := false
		for n := 0; ; n++ {
			simplePart := fmt.Sprintf("%s*%d", key, n)
			if v, ok := pieceMap[simplePart]; ok {
				valid = true
				buf.WriteString(v)
				continue
			}
			encodedPart := simplePart + "*"
			v, ok := pieceMap[encodedPart]
			if !ok {
				break
			}
			valid = true
			if n == 0 {
				if decv, ok := decode2231Enc(v); ok {
					buf.WriteString(decv)
				}
			} else {
				decv, _ := percentHexUnescape(v)
				buf.WriteString(decv)
			}
		}
		if valid {
			params[key] = buf.String()
		}
	}

	return
}

func decode2231Enc(v string) (string, bool) {
	sv := strings.SplitN(v, "'", 3)
	if len(sv) != 3 {
		return "", false

TODO: ignoring lang in sv[1] for now. If anybody needs it we'll need to decide how to expose it in the API. But I'm not sure anybody uses it in practice.

	charset := strings.ToLower(sv[0])
	if len(charset) == 0 {
		return "", false
	}

TODO: unsupported encoding

		return "", false
	}
	encv, err := percentHexUnescape(sv[2])
	if err != nil {
		return "", false
	}
	return encv, true
}

func isNotTokenChar(r rune) bool {
	return !isTokenChar(r)
}

consumeToken consumes a token from the beginning of provided string, per RFC 2045 section 5.1 (referenced from 2183), and return the token consumed and the rest of the string. Returns ("", v) on failure to consume at least one character.

func consumeToken(v string) (token, rest string) {
	notPos := strings.IndexFunc(v, isNotTokenChar)
	if notPos == -1 {
		return v, ""
	}
	if notPos == 0 {
		return "", v
	}
	return v[0:notPos], v[notPos:]
}

consumeValue consumes a "value" per RFC 2045, where a value is either a 'token' or a 'quoted-string'. On success, consumeValue returns the value consumed (and de-quoted/escaped, if a quoted-string) and the rest of the string. On failure, returns ("", v).

func consumeValue(v string) (value, rest string) {
	if v == "" {
		return
	}
	if v[0] != '"' {
		return consumeToken(v)
	}

parse a quoted-string

	buffer := new(strings.Builder)
	for i := 1; i < len(v); i++ {
		r := v[i]
		if r == '"' {
			return buffer.String(), v[i+1:]

When MSIE sends a full file path (in "intranet mode"), it does not escape backslashes: "C:\dev\go\foo.txt", not "C:\\dev\\go\\foo.txt". No known MIME generators emit unnecessary backslash escapes for simple token characters like numbers and letters. If we see an unnecessary backslash escape, assume it is from MSIE and intended as a literal backslash. This makes Go servers deal better with MSIE without affecting the way they handle conforming MIME generators.

		if r == '\\' && i+1 < len(v) && isTSpecial(rune(v[i+1])) {
			buffer.WriteByte(v[i+1])
			i++
			continue
		}
		if r == '\r' || r == '\n' {
			return "", v
		}
		buffer.WriteByte(v[i])

Did not find end quote.

	return "", v
}

func consumeMediaParam(v string) (param, value, rest string) {
	rest = strings.TrimLeftFunc(v, unicode.IsSpace)
	if !strings.HasPrefix(rest, ";") {
		return "", "", v
	}

	rest = rest[1:] // consume semicolon
	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
	param, rest = consumeToken(rest)
	param = strings.ToLower(param)
	if param == "" {
		return "", "", v
	}

	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
	if !strings.HasPrefix(rest, "=") {
		return "", "", v
	}
	rest = rest[1:] // consume equals sign
	rest = strings.TrimLeftFunc(rest, unicode.IsSpace)
	value, rest2 := consumeValue(rest)
	if value == "" && rest2 == rest {
		return "", "", v
	}
	rest = rest2
	return param, value, rest
}

Count %, check that they're well-formed.

	percents := 0
	for i := 0; i < len(s); {
		if s[i] != '%' {
			i++
			continue
		}
		percents++
		if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
			s = s[i:]
			if len(s) > 3 {
				s = s[0:3]
			}
			return "", fmt.Errorf("mime: bogus characters after %%: %q", s)
		}
		i += 3
	}
	if percents == 0 {
		return s, nil
	}

	t := make([]byte, len(s)-2*percents)
	j := 0
	for i := 0; i < len(s); {
		switch s[i] {
		case '%':
			t[j] = unhex(s[i+1])<<4 | unhex(s[i+2])
			j++
			i += 3
		default:
			t[j] = s[i]
			j++
			i++
		}
	}
	return string(t), nil
}

func ishex(c byte) bool {
	switch {
	case '0' <= c && c <= '9':
		return true
	case 'a' <= c && c <= 'f':
		return true
	case 'A' <= c && c <= 'F':
		return true
	}
	return false
}

func unhex(c byte) byte {
	switch {
	case '0' <= c && c <= '9':
		return c - '0'
	case 'a' <= c && c <= 'f':
		return c - 'a' + 10
	case 'A' <= c && c <= 'F':
		return c - 'A' + 10
	}
	return 0