formdata.go

     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  package multipart
     6  
     7  import (
     8  	"bytes"
     9  	"errors"
    10  	"internal/godebug"
    11  	"io"
    12  	"math"
    13  	"net/textproto"
    14  	"os"
    15  	"strconv"
    16  )
    17  
    18  // ErrMessageTooLarge is returned by ReadForm if the message form
    19  // data is too large to be processed.
    20  var ErrMessageTooLarge = errors.New("multipart: message too large")
    21  
    22  // TODO(adg,bradfitz): find a way to unify the DoS-prevention strategy here
    23  // with that of the http package's ParseForm.
    24  
    25  // ReadForm parses an entire multipart message whose parts have
    26  // a Content-Disposition of "form-data".
    27  // It stores up to maxMemory bytes + 10MB (reserved for non-file parts)
    28  // in memory. File parts which can't be stored in memory will be stored on
    29  // disk in temporary files.
    30  // It returns [ErrMessageTooLarge] if all non-file parts can't be stored in
    31  // memory.
    32  func (r *Reader) ReadForm(maxMemory int64) (*Form, error) {
    33  	return r.readForm(maxMemory)
    34  }
    35  
    36  var (
    37  	multipartfiles    = godebug.New("#multipartfiles") // TODO: document and remove #
    38  	multipartmaxparts = godebug.New("multipartmaxparts")
    39  )
    40  
    41  func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
    42  	form := &Form{make(map[string][]string), make(map[string][]*FileHeader)}
    43  	var (
    44  		file    *os.File
    45  		fileOff int64
    46  	)
    47  	numDiskFiles := 0
    48  	combineFiles := true
    49  	if multipartfiles.Value() == "distinct" {
    50  		combineFiles = false
    51  		// multipartfiles.IncNonDefault() // TODO: uncomment after documenting
    52  	}
    53  	maxParts := 1000
    54  	if s := multipartmaxparts.Value(); s != "" {
    55  		if v, err := strconv.Atoi(s); err == nil && v >= 0 {
    56  			maxParts = v
    57  			multipartmaxparts.IncNonDefault()
    58  		}
    59  	}
    60  	maxHeaders := maxMIMEHeaders()
    61  
    62  	defer func() {
    63  		if file != nil {
    64  			if cerr := file.Close(); err == nil {
    65  				err = cerr
    66  			}
    67  		}
    68  		if combineFiles && numDiskFiles > 1 {
    69  			for _, fhs := range form.File {
    70  				for _, fh := range fhs {
    71  					fh.tmpshared = true
    72  				}
    73  			}
    74  		}
    75  		if err != nil {
    76  			form.RemoveAll()
    77  			if file != nil {
    78  				os.Remove(file.Name())
    79  			}
    80  		}
    81  	}()
    82  
    83  	// maxFileMemoryBytes is the maximum bytes of file data we will store in memory.
    84  	// Data past this limit is written to disk.
    85  	// This limit strictly applies to content, not metadata (filenames, MIME headers, etc.),
    86  	// since metadata is always stored in memory, not disk.
    87  	//
    88  	// maxMemoryBytes is the maximum bytes we will store in memory, including file content,
    89  	// non-file part values, metadata, and map entry overhead.
    90  	//
    91  	// We reserve an additional 10 MB in maxMemoryBytes for non-file data.
    92  	//
    93  	// The relationship between these parameters, as well as the overly-large and
    94  	// unconfigurable 10 MB added on to maxMemory, is unfortunate but difficult to change
    95  	// within the constraints of the API as documented.
    96  	if maxMemory < 0 {
    97  		maxMemory = 0
    98  	}
    99  	maxFileMemoryBytes := maxMemory
   100  	if maxFileMemoryBytes == math.MaxInt64 {
   101  		maxFileMemoryBytes--
   102  	}
   103  	maxMemoryBytes := maxMemory + int64(10<<20)
   104  	if maxMemoryBytes <= 0 {
   105  		maxMemoryBytes = math.MaxInt64
   106  	}
   107  	var copyBuf []byte
   108  	for {
   109  		p, err := r.nextPart(false, maxMemoryBytes, maxHeaders)
   110  		if err == io.EOF {
   111  			break
   112  		}
   113  		if err != nil {
   114  			return nil, err
   115  		}
   116  		if maxParts <= 0 {
   117  			return nil, ErrMessageTooLarge
   118  		}
   119  		maxParts--
   120  
   121  		name := p.FormName()
   122  		if name == "" {
   123  			continue
   124  		}
   125  		filename := p.FileName()
   126  
   127  		// Multiple values for the same key (one map entry, longer slice) are cheaper
   128  		// than the same number of values for different keys (many map entries), but
   129  		// using a consistent per-value cost for overhead is simpler.
   130  		const mapEntryOverhead = 200
   131  		maxMemoryBytes -= int64(len(name))
   132  		maxMemoryBytes -= mapEntryOverhead
   133  		if maxMemoryBytes < 0 {
   134  			// We can't actually take this path, since nextPart would already have
   135  			// rejected the MIME headers for being too large. Check anyway.
   136  			return nil, ErrMessageTooLarge
   137  		}
   138  
   139  		var b bytes.Buffer
   140  
   141  		if filename == "" {
   142  			// value, store as string in memory
   143  			n, err := io.CopyN(&b, p, maxMemoryBytes+1)
   144  			if err != nil && err != io.EOF {
   145  				return nil, err
   146  			}
   147  			maxMemoryBytes -= n
   148  			if maxMemoryBytes < 0 {
   149  				return nil, ErrMessageTooLarge
   150  			}
   151  			form.Value[name] = append(form.Value[name], b.String())
   152  			continue
   153  		}
   154  
   155  		// file, store in memory or on disk
   156  		const fileHeaderSize = 100
   157  		maxMemoryBytes -= mimeHeaderSize(p.Header)
   158  		maxMemoryBytes -= mapEntryOverhead
   159  		maxMemoryBytes -= fileHeaderSize
   160  		if maxMemoryBytes < 0 {
   161  			return nil, ErrMessageTooLarge
   162  		}
   163  		for _, v := range p.Header {
   164  			maxHeaders -= int64(len(v))
   165  		}
   166  		fh := &FileHeader{
   167  			Filename: filename,
   168  			Header:   p.Header,
   169  		}
   170  		n, err := io.CopyN(&b, p, maxFileMemoryBytes+1)
   171  		if err != nil && err != io.EOF {
   172  			return nil, err
   173  		}
   174  		if n > maxFileMemoryBytes {
   175  			if file == nil {
   176  				file, err = os.CreateTemp(r.tempDir, "multipart-")
   177  				if err != nil {
   178  					return nil, err
   179  				}
   180  			}
   181  			numDiskFiles++
   182  			if _, err := file.Write(b.Bytes()); err != nil {
   183  				return nil, err
   184  			}
   185  			if copyBuf == nil {
   186  				copyBuf = make([]byte, 32*1024) // same buffer size as io.Copy uses
   187  			}
   188  			// os.File.ReadFrom will allocate its own copy buffer if we let io.Copy use it.
   189  			type writerOnly struct{ io.Writer }
   190  			remainingSize, err := io.CopyBuffer(writerOnly{file}, p, copyBuf)
   191  			if err != nil {
   192  				return nil, err
   193  			}
   194  			fh.tmpfile = file.Name()
   195  			fh.Size = int64(b.Len()) + remainingSize
   196  			fh.tmpoff = fileOff
   197  			fileOff += fh.Size
   198  			if !combineFiles {
   199  				if err := file.Close(); err != nil {
   200  					return nil, err
   201  				}
   202  				file = nil
   203  			}
   204  		} else {
   205  			fh.content = b.Bytes()
   206  			fh.Size = int64(len(fh.content))
   207  			maxFileMemoryBytes -= n
   208  			maxMemoryBytes -= n
   209  		}
   210  		form.File[name] = append(form.File[name], fh)
   211  	}
   212  
   213  	return form, nil
   214  }
   215  
   216  func mimeHeaderSize(h textproto.MIMEHeader) (size int64) {
   217  	size = 400
   218  	for k, vs := range h {
   219  		size += int64(len(k))
   220  		size += 200 // map entry overhead
   221  		for _, v := range vs {
   222  			size += int64(len(v))
   223  		}
   224  	}
   225  	return size
   226  }
   227  
   228  // Form is a parsed multipart form.
   229  // Its File parts are stored either in memory or on disk,
   230  // and are accessible via the [*FileHeader]'s Open method.
   231  // Its Value parts are stored as strings.
   232  // Both are keyed by field name.
   233  type Form struct {
   234  	Value map[string][]string
   235  	File  map[string][]*FileHeader
   236  }
   237  
   238  // RemoveAll removes any temporary files associated with a [Form].
   239  func (f *Form) RemoveAll() error {
   240  	var err error
   241  	for _, fhs := range f.File {
   242  		for _, fh := range fhs {
   243  			if fh.tmpfile != "" {
   244  				e := os.Remove(fh.tmpfile)
   245  				if e != nil && !errors.Is(e, os.ErrNotExist) && err == nil {
   246  					err = e
   247  				}
   248  			}
   249  		}
   250  	}
   251  	return err
   252  }
   253  
   254  // A FileHeader describes a file part of a multipart request.
   255  type FileHeader struct {
   256  	Filename string
   257  	Header   textproto.MIMEHeader
   258  	Size     int64
   259  
   260  	content   []byte
   261  	tmpfile   string
   262  	tmpoff    int64
   263  	tmpshared bool
   264  }
   265  
   266  // Open opens and returns the [FileHeader]'s associated File.
   267  func (fh *FileHeader) Open() (File, error) {
   268  	if b := fh.content; b != nil {
   269  		r := io.NewSectionReader(bytes.NewReader(b), 0, int64(len(b)))
   270  		return sectionReadCloser{r, nil}, nil
   271  	}
   272  	if fh.tmpshared {
   273  		f, err := os.Open(fh.tmpfile)
   274  		if err != nil {
   275  			return nil, err
   276  		}
   277  		r := io.NewSectionReader(f, fh.tmpoff, fh.Size)
   278  		return sectionReadCloser{r, f}, nil
   279  	}
   280  	return os.Open(fh.tmpfile)
   281  }
   282  
   283  // File is an interface to access the file part of a multipart message.
   284  // Its contents may be either stored in memory or on disk.
   285  // If stored on disk, the File's underlying concrete type will be an *os.File.
   286  type File interface {
   287  	io.Reader
   288  	io.ReaderAt
   289  	io.Seeker
   290  	io.Closer
   291  }
   292  
   293  // helper types to turn a []byte into a File
   294  
   295  type sectionReadCloser struct {
   296  	*io.SectionReader
   297  	io.Closer
   298  }
   299  
   300  func (rc sectionReadCloser) Close() error {
   301  	if rc.Closer != nil {
   302  		return rc.Closer.Close()
   303  	}
   304  	return nil
   305  }
   306
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