Source: writer.go in package mime/multipart


package multipart

import (
	"bytes"
	"crypto/rand"
	"errors"
	"fmt"
	"io"
	"net/textproto"
	"sort"
	"strings"
)

A Writer generates multipart messages.

type Writer struct {
	w        io.Writer
	boundary string
	lastpart *part
}

NewWriter returns a new multipart Writer with a random boundary, writing to w.

func NewWriter(w io.Writer) *Writer {
	return &Writer{
		w:        w,
		boundary: randomBoundary(),
	}
}

Boundary returns the Writer's boundary.

func (w *Writer) Boundary() string {
	return w.boundary
}

SetBoundary overrides the Writer's default randomly-generated boundary separator with an explicit value. SetBoundary must be called before any parts are created, may only contain certain ASCII characters, and must be non-empty and at most 70 bytes long.

func (w *Writer) SetBoundary(boundary string) error {
	if w.lastpart != nil {
		return errors.New("mime: SetBoundary called after write")

rfc2046#section-5.1.1

	if len(boundary) < 1 || len(boundary) > 70 {
		return errors.New("mime: invalid boundary length")
	}
	end := len(boundary) - 1
	for i, b := range boundary {
		if 'A' <= b && b <= 'Z' || 'a' <= b && b <= 'z' || '0' <= b && b <= '9' {
			continue
		}
		switch b {
		case '\'', '(', ')', '+', '_', ',', '-', '.', '/', ':', '=', '?':
			continue
		case ' ':
			if i != end {
				continue
			}
		}
		return errors.New("mime: invalid boundary character")
	}
	w.boundary = boundary
	return nil
}

FormDataContentType returns the Content-Type for an HTTP multipart/form-data with this Writer's Boundary.

func (w *Writer) FormDataContentType() string {

We must quote the boundary if it contains any of the tspecials characters defined by RFC 2045, or space.

	if strings.ContainsAny(b, `()<>@,;:\"/[]?= `) {
		b = `"` + b + `"`
	}
	return "multipart/form-data; boundary=" + b
}

func randomBoundary() string {
	var buf [30]byte
	_, err := io.ReadFull(rand.Reader, buf[:])
	if err != nil {
		panic(err)
	}
	return fmt.Sprintf("%x", buf[:])
}

CreatePart creates a new multipart section with the provided header. The body of the part should be written to the returned Writer. After calling CreatePart, any previous part may no longer be written to.

func (w *Writer) CreatePart(header textproto.MIMEHeader) (io.Writer, error) {
	if w.lastpart != nil {
		if err := w.lastpart.close(); err != nil {
			return nil, err
		}
	}
	var b bytes.Buffer
	if w.lastpart != nil {
		fmt.Fprintf(&b, "\r\n--%s\r\n", w.boundary)
	} else {
		fmt.Fprintf(&b, "--%s\r\n", w.boundary)
	}

	keys := make([]string, 0, len(header))
	for k := range header {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	for _, k := range keys {
		for _, v := range header[k] {
			fmt.Fprintf(&b, "%s: %s\r\n", k, v)
		}
	}
	fmt.Fprintf(&b, "\r\n")
	_, err := io.Copy(w.w, &b)
	if err != nil {
		return nil, err
	}
	p := &part{
		mw: w,
	}
	w.lastpart = p
	return p, nil
}

var quoteEscaper = strings.NewReplacer("\\", "\\\\", `"`, "\\\"")

func escapeQuotes(s string) string {
	return quoteEscaper.Replace(s)
}

CreateFormFile is a convenience wrapper around CreatePart. It creates a new form-data header with the provided field name and file name.

func (w *Writer) CreateFormFile(fieldname, filename string) (io.Writer, error) {
	h := make(textproto.MIMEHeader)
	h.Set("Content-Disposition",
		fmt.Sprintf(`form-data; name="%s"; filename="%s"`,
			escapeQuotes(fieldname), escapeQuotes(filename)))
	h.Set("Content-Type", "application/octet-stream")
	return w.CreatePart(h)
}

CreateFormField calls CreatePart with a header using the given field name.

func (w *Writer) CreateFormField(fieldname string) (io.Writer, error) {
	h := make(textproto.MIMEHeader)
	h.Set("Content-Disposition",
		fmt.Sprintf(`form-data; name="%s"`, escapeQuotes(fieldname)))
	return w.CreatePart(h)
}

WriteField calls CreateFormField and then writes the given value.

func (w *Writer) WriteField(fieldname, value string) error {
	p, err := w.CreateFormField(fieldname)
	if err != nil {
		return err
	}
	_, err = p.Write([]byte(value))
	return err
}

Close finishes the multipart message and writes the trailing boundary end line to the output.

func (w *Writer) Close() error {
	if w.lastpart != nil {
		if err := w.lastpart.close(); err != nil {
			return err
		}
		w.lastpart = nil
	}
	_, err := fmt.Fprintf(w.w, "\r\n--%s--\r\n", w.boundary)
	return err
}

type part struct {
	mw     *Writer
	closed bool
	we     error // last error that occurred writing
}

func (p *part) close() error {
	p.closed = true
	return p.we
}

func (p *part) Write(d []byte) (n int, err error) {
	if p.closed {
		return 0, errors.New("multipart: can't write to finished part")
	}
	n, err = p.mw.w.Write(d)
	if err != nil {
		p.we = err
	}
	return