1package message

2

3import (

4 "bytes"

5 "fmt"

6 "net/mail"

7 "net/textproto"

8)

9

10// ParseHeaderFields parses only the header fields in "fields" from the complete

11// header buffer "header". It uses "scratch" as temporary space, which can be

12// reused across calls, potentially saving lots of unneeded allocations when only a

13// few headers are needed and/or many messages are parsed.

14func ParseHeaderFields(header []byte, scratch []byte, fields [][]byte) (textproto.MIMEHeader, error) {

15 // todo: should not use mail.ReadMessage, it allocates a bufio.Reader. should implement header parsing ourselves.

16

17 // Gather the raw lines for the fields, with continuations, without the other

18 // headers. Put them in a byte slice and only parse those headers. For now, use

19 // mail.ReadMessage without letting it do allocations for all headers.

20 scratch = scratch[:0]

21 var keepcontinuation bool

22 for len(header) > 0 {

23 if header[0] == ' ' || header[0] == '\t' {

24 // Continuation.

25 i := bytes.IndexByte(header, '\n')

26 if i < 0 {

27 i = len(header)

28 } else {

29 i++

30 }

31 if keepcontinuation {

32 scratch = append(scratch, header[:i]...)

33 }

34 header = header[i:]

35 continue

36 }

37 i := bytes.IndexByte(header, ':')

38 if i < 0 || i > 0 && (header[i-1] == ' ' || header[i-1] == '\t') {

39 i = bytes.IndexByte(header, '\n')

40 if i < 0 {

41 break

42 }

43 header = header[i+1:]

44 keepcontinuation = false

45 continue

46 }

47 k := header[:i]

48 keepcontinuation = false

49 for _, f := range fields {

50 if bytes.EqualFold(k, f) {

51 keepcontinuation = true

52 break

53 }

54 }

55 i = bytes.IndexByte(header, '\n')

56 if i < 0 {

57 i = len(header)

58 } else {

59 i++

60 }

61 if keepcontinuation {

62 scratch = append(scratch, header[:i]...)

63 }

64 header = header[i:]

65 }

66

67 if len(scratch) == 0 {

68 return nil, nil

69 }

70

71 scratch = append(scratch, "\r\n"...)

72

73 msg, err := mail.ReadMessage(bytes.NewReader(scratch))

74 if err != nil {

75 return nil, fmt.Errorf("reading message header")

76 }

77 return textproto.MIMEHeader(msg.Header), nil

78}

79