Source: formdata.go in package mime/multipart


package multipart

import (
	"bytes"
	"errors"
	"io"
	"math"
	"net/textproto"
	"os"
)

ErrMessageTooLarge is returned by ReadForm if the message form data is too large to be processed.

var ErrMessageTooLarge = errors.New("multipart: message too large")

TODO(adg,bradfitz): find a way to unify the DoS-prevention strategy here with that of the http package's ParseForm.

ReadForm parses an entire multipart message whose parts have a Content-Disposition of "form-data". It stores up to maxMemory bytes + 10MB (reserved for non-file parts) in memory. File parts which can't be stored in memory will be stored on disk in temporary files. It returns ErrMessageTooLarge if all non-file parts can't be stored in memory.

func (r *Reader) ReadForm(maxMemory int64) (*Form, error) {
	return r.readForm(maxMemory)
}

func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
	form := &Form{make(map[string][]string), make(map[string][]*FileHeader)}
	defer func() {
		if err != nil {
			form.RemoveAll()
		}
	}()

Reserve an additional 10 MB for non-file parts.

	maxValueBytes := maxMemory + int64(10<<20)
	if maxValueBytes <= 0 {
		if maxMemory < 0 {
			maxValueBytes = 0
		} else {
			maxValueBytes = math.MaxInt64
		}
	}
	for {
		p, err := r.NextPart()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, err
		}

		name := p.FormName()
		if name == "" {
			continue
		}
		filename := p.FileName()

		var b bytes.Buffer

value, store as string in memory

			n, err := io.CopyN(&b, p, maxValueBytes+1)
			if err != nil && err != io.EOF {
				return nil, err
			}
			maxValueBytes -= n
			if maxValueBytes < 0 {
				return nil, ErrMessageTooLarge
			}
			form.Value[name] = append(form.Value[name], b.String())
			continue
		}

file, store in memory or on disk

		fh := &FileHeader{
			Filename: filename,
			Header:   p.Header,
		}
		n, err := io.CopyN(&b, p, maxMemory+1)
		if err != nil && err != io.EOF {
			return nil, err
		}

too big, write to disk and flush buffer

			file, err := os.CreateTemp("", "multipart-")
			if err != nil {
				return nil, err
			}
			size, err := io.Copy(file, io.MultiReader(&b, p))
			if cerr := file.Close(); err == nil {
				err = cerr
			}
			if err != nil {
				os.Remove(file.Name())
				return nil, err
			}
			fh.tmpfile = file.Name()
			fh.Size = size
		} else {
			fh.content = b.Bytes()
			fh.Size = int64(len(fh.content))
			maxMemory -= n
			maxValueBytes -= n
		}
		form.File[name] = append(form.File[name], fh)
	}

	return form, nil
}

Form is a parsed multipart form. Its File parts are stored either in memory or on disk, and are accessible via the *FileHeader's Open method. Its Value parts are stored as strings. Both are keyed by field name.

type Form struct {
	Value map[string][]string
	File  map[string][]*FileHeader
}

RemoveAll removes any temporary files associated with a Form.

func (f *Form) RemoveAll() error {
	var err error
	for _, fhs := range f.File {
		for _, fh := range fhs {
			if fh.tmpfile != "" {
				e := os.Remove(fh.tmpfile)
				if e != nil && err == nil {
					err = e
				}
			}
		}
	}
	return err
}

A FileHeader describes a file part of a multipart request.

type FileHeader struct {
	Filename string
	Header   textproto.MIMEHeader
	Size     int64

	content []byte
	tmpfile string
}

Open opens and returns the FileHeader's associated File.

func (fh *FileHeader) Open() (File, error) {
	if b := fh.content; b != nil {
		r := io.NewSectionReader(bytes.NewReader(b), 0, int64(len(b)))
		return sectionReadCloser{r}, nil
	}
	return os.Open(fh.tmpfile)
}

File is an interface to access the file part of a multipart message. Its contents may be either stored in memory or on disk. If stored on disk, the File's underlying concrete type will be an *os.File.

type File interface {
	io.Reader
	io.ReaderAt
	io.Seeker
	io.Closer
}

helper types to turn a []byte into a File


type sectionReadCloser struct {
	*io.SectionReader
}

func (rc sectionReadCloser) Close() error {
	return nil