archive/zip: add File.OpenRaw, Writer.CreateRaw, Writer.Copy

These new methods provide support for cases where performance is a
primary concern. For example, copying files from an existing zip to a
new zip without incurring the decompression and compression overhead.
Using an optimized, external compression method and writing the output
to a zip archive. And compressing file contents in parallel and then
sequentially writing the compressed bytes to a zip archive.

TestWriterCopy is copied verbatim from https://github.com/rsc/zipmerge

Fixes #34974

Change-Id: Iade5bc245ba34cdbb86364bf59f79f38bb9e2eb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/312310
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Carlos Amedee <carlos@golang.org>

5 files changed, 541 insertions, 61 deletions

diff --git a/src/archive/zip/reader.go b/src/archive/zip/reader.go
index 18f9833db0..808cf274ca 100644
--- a/src/archive/zip/reader.go
+++ b/src/archive/zip/reader.go
@@ -52,12 +52,9 @@ type File struct {
 	FileHeader
 	zip          *Reader
 	zipr         io.ReaderAt
-	zipsize      int64
 	headerOffset int64
-}
-
-func (f *File) hasDataDescriptor() bool {
-	return f.Flags&0x8 != 0
+	zip64        bool  // zip64 extended information extra field presence
+	descErr      error // error reading the data descriptor during init
 }
 
 // OpenReader will open the Zip file specified by name and return a ReadCloser.
@@ -112,7 +109,7 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 	// a bad one, and then only report an ErrFormat or UnexpectedEOF if
 	// the file count modulo 65536 is incorrect.
 	for {
-		f := &File{zip: z, zipr: r, zipsize: size}
+		f := &File{zip: z, zipr: r}
 		err = readDirectoryHeader(f, buf)
 		if err == ErrFormat || err == io.ErrUnexpectedEOF {
 			break
@@ -120,6 +117,7 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 		if err != nil {
 			return err
 		}
+		f.readDataDescriptor()
 		z.File = append(z.File, f)
 	}
 	if uint16(len(z.File)) != uint16(end.directoryRecords) { // only compare 16 bits here
@@ -180,26 +178,68 @@ func (f *File) Open() (io.ReadCloser, error) {
 		return nil, ErrAlgorithm
 	}
 	var rc io.ReadCloser = dcomp(r)
-	var desr io.Reader
-	if f.hasDataDescriptor() {
-		desr = io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset+size, dataDescriptorLen)
-	}
 	rc = &checksumReader{
 		rc:   rc,
 		hash: crc32.NewIEEE(),
 		f:    f,
-		desr: desr,
 	}
 	return rc, nil
 }
 
+// OpenRaw returns a Reader that provides access to the File's contents without
+// decompression.
+func (f *File) OpenRaw() (io.Reader, error) {
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		return nil, err
+	}
+	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset, int64(f.CompressedSize64))
+	return r, nil
+}
+
+func (f *File) readDataDescriptor() {
+	if !f.hasDataDescriptor() {
+		return
+	}
+
+	bodyOffset, err := f.findBodyOffset()
+	if err != nil {
+		f.descErr = err
+		return
+	}
+
+	// In section 4.3.9.2 of the spec: "However ZIP64 format MAY be used
+	// regardless of the size of a file.  When extracting, if the zip64
+	// extended information extra field is present for the file the
+	// compressed and uncompressed sizes will be 8 byte values."
+	//
+	// Historically, this package has used the compressed and uncompressed
+	// sizes from the central directory to determine if the package is
+	// zip64.
+	//
+	// For this case we allow either the extra field or sizes to determine
+	// the data descriptor length.
+	zip64 := f.zip64 || f.isZip64()
+	n := int64(dataDescriptorLen)
+	if zip64 {
+		n = dataDescriptor64Len
+	}
+	size := int64(f.CompressedSize64)
+	r := io.NewSectionReader(f.zipr, f.headerOffset+bodyOffset+size, n)
+	dd, err := readDataDescriptor(r, zip64)
+	if err != nil {
+		f.descErr = err
+		return
+	}
+	f.CRC32 = dd.crc32
+}
+
 type checksumReader struct {
 	rc    io.ReadCloser
 	hash  hash.Hash32
 	nread uint64 // number of bytes read so far
 	f     *File
-	desr  io.Reader // if non-nil, where to read the data descriptor
-	err   error     // sticky error
+	err   error // sticky error
 }
 
 func (r *checksumReader) Stat() (fs.FileInfo, error) {
@@ -220,12 +260,12 @@ func (r *checksumReader) Read(b []byte) (n int, err error) {
 		if r.nread != r.f.UncompressedSize64 {
 			return 0, io.ErrUnexpectedEOF
 		}
-		if r.desr != nil {
-			if err1 := readDataDescriptor(r.desr, r.f); err1 != nil {
-				if err1 == io.EOF {
+		if r.f.hasDataDescriptor() {
+			if r.f.descErr != nil {
+				if r.f.descErr == io.EOF {
 					err = io.ErrUnexpectedEOF
 				} else {
-					err = err1
+					err = r.f.descErr
 				}
 			} else if r.hash.Sum32() != r.f.CRC32 {
 				err = ErrChecksum
@@ -336,6 +376,8 @@ parseExtras:
 
 		switch fieldTag {
 		case zip64ExtraID:
+			f.zip64 = true
+
 			// update directory values from the zip64 extra block.
 			// They should only be consulted if the sizes read earlier
 			// are maxed out.
@@ -435,8 +477,9 @@ parseExtras:
 	return nil
 }
 
-func readDataDescriptor(r io.Reader, f *File) error {
-	var buf [dataDescriptorLen]byte
+func readDataDescriptor(r io.Reader, zip64 bool) (*dataDescriptor, error) {
+	// Create enough space for the largest possible size
+	var buf [dataDescriptor64Len]byte
 
 	// The spec says: "Although not originally assigned a
 	// signature, the value 0x08074b50 has commonly been adopted
@@ -446,10 +489,9 @@ func readDataDescriptor(r io.Reader, f *File) error {
 	// descriptors and should account for either case when reading
 	// ZIP files to ensure compatibility."
 	//
-	// dataDescriptorLen includes the size of the signature but
-	// first read just those 4 bytes to see if it exists.
+	// First read just those 4 bytes to see if the signature exists.
 	if _, err := io.ReadFull(r, buf[:4]); err != nil {
-		return err
+		return nil, err
 	}
 	off := 0
 	maybeSig := readBuf(buf[:4])
@@ -458,21 +500,28 @@ func readDataDescriptor(r io.Reader, f *File) error {
 		// bytes.
 		off += 4
 	}
-	if _, err := io.ReadFull(r, buf[off:12]); err != nil {
-		return err
+
+	end := dataDescriptorLen - 4
+	if zip64 {
+		end = dataDescriptor64Len - 4
 	}
-	b := readBuf(buf[:12])
-	if b.uint32() != f.CRC32 {
-		return ErrChecksum
+	if _, err := io.ReadFull(r, buf[off:end]); err != nil {
+		return nil, err
 	}
+	b := readBuf(buf[:end])
 
-	// The two sizes that follow here can be either 32 bits or 64 bits
-	// but the spec is not very clear on this and different
-	// interpretations has been made causing incompatibilities. We
-	// already have the sizes from the central directory so we can
-	// just ignore these.
+	out := &dataDescriptor{
+		crc32: b.uint32(),
+	}
 
-	return nil
+	if zip64 {
+		out.compressedSize = b.uint64()
+		out.uncompressedSize = b.uint64()
+	} else {
+		out.compressedSize = uint64(b.uint32())
+		out.uncompressedSize = uint64(b.uint32())
+	}
+	return out, nil
 }
 
 func readDirectoryEnd(r io.ReaderAt, size int64) (dir *directoryEnd, err error) {
diff --git a/src/archive/zip/reader_test.go b/src/archive/zip/reader_test.go
index 6ee62ef997..35e681ec69 100644
--- a/src/archive/zip/reader_test.go
+++ b/src/archive/zip/reader_test.go
@@ -499,9 +499,15 @@ func TestReader(t *testing.T) {
 func readTestZip(t *testing.T, zt ZipTest) {
 	var z *Reader
 	var err error
+	var raw []byte
 	if zt.Source != nil {
 		rat, size := zt.Source()
 		z, err = NewReader(rat, size)
+		raw = make([]byte, size)
+		if _, err := rat.ReadAt(raw, 0); err != nil {
+			t.Errorf("ReadAt error=%v", err)
+			return
+		}
 	} else {
 		path := filepath.Join("testdata", zt.Name)
 		if zt.Obscured {
@@ -519,6 +525,12 @@ func readTestZip(t *testing.T, zt ZipTest) {
 			defer rc.Close()
 			z = &rc.Reader
 		}
+		var err2 error
+		raw, err2 = os.ReadFile(path)
+		if err2 != nil {
+			t.Errorf("ReadFile(%s) error=%v", path, err2)
+			return
+		}
 	}
 	if err != zt.Error {
 		t.Errorf("error=%v, want %v", err, zt.Error)
@@ -545,7 +557,7 @@ func readTestZip(t *testing.T, zt ZipTest) {
 
 	// test read of each file
 	for i, ft := range zt.File {
-		readTestFile(t, zt, ft, z.File[i])
+		readTestFile(t, zt, ft, z.File[i], raw)
 	}
 	if t.Failed() {
 		return
@@ -557,7 +569,7 @@ func readTestZip(t *testing.T, zt ZipTest) {
 	for i := 0; i < 5; i++ {
 		for j, ft := range zt.File {
 			go func(j int, ft ZipTestFile) {
-				readTestFile(t, zt, ft, z.File[j])
+				readTestFile(t, zt, ft, z.File[j], raw)
 				done <- true
 			}(j, ft)
 			n++
@@ -574,7 +586,7 @@ func equalTimeAndZone(t1, t2 time.Time) bool {
 	return t1.Equal(t2) && name1 == name2 && offset1 == offset2
 }
 
-func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File) {
+func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File, raw []byte) {
 	if f.Name != ft.Name {
 		t.Errorf("name=%q, want %q", f.Name, ft.Name)
 	}
@@ -594,6 +606,31 @@ func readTestFile(t *testing.T, zt ZipTest, ft ZipTestFile, f *File) {
 		t.Errorf("%v: UncompressedSize=%#x does not match UncompressedSize64=%#x", f.Name, size, f.UncompressedSize64)
 	}
 
+	// Check that OpenRaw returns the correct byte segment
+	rw, err := f.OpenRaw()
+	if err != nil {
+		t.Errorf("%v: OpenRaw error=%v", f.Name, err)
+		return
+	}
+	start, err := f.DataOffset()
+	if err != nil {
+		t.Errorf("%v: DataOffset error=%v", f.Name, err)
+		return
+	}
+	got, err := io.ReadAll(rw)
+	if err != nil {
+		t.Errorf("%v: OpenRaw ReadAll error=%v", f.Name, err)
+		return
+	}
+	end := uint64(start) + f.CompressedSize64
+	want := raw[start:end]
+	if !bytes.Equal(got, want) {
+		t.Logf("got %q", got)
+		t.Logf("want %q", want)
+		t.Errorf("%v: OpenRaw returned unexpected bytes", f.Name)
+		return
+	}
+
 	r, err := f.Open()
 	if err != nil {
 		t.Errorf("%v", err)
@@ -1166,3 +1203,125 @@ func TestCVE202127919(t *testing.T) {
 		t.Errorf("Error reading file: %v", err)
 	}
 }
+
+func TestReadDataDescriptor(t *testing.T) {
+	tests := []struct {
+		desc    string
+		in      []byte
+		zip64   bool
+		want    *dataDescriptor
+		wantErr error
+	}{{
+		desc: "valid 32 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, // compressed size
+			0x08, 0x09, 0x0a, 0x0b, // uncompressed size
+		},
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x07060504,
+			uncompressedSize: 0x0b0a0908,
+		},
+	}, {
+		desc: "valid 32 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, // compressed size
+			0x08, 0x09, 0x0a, 0x0b, // uncompressed size
+		},
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x07060504,
+			uncompressedSize: 0x0b0a0908,
+		},
+	}, {
+		desc: "valid 64 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, // uncompressed size
+		},
+		zip64: true,
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x0b0a090807060504,
+			uncompressedSize: 0x131211100f0e0d0c,
+		},
+	}, {
+		desc: "valid 64 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, // uncompressed size
+		},
+		zip64: true,
+		want: &dataDescriptor{
+			crc32:            0x03020100,
+			compressedSize:   0x0b0a090807060504,
+			uncompressedSize: 0x131211100f0e0d0c,
+		},
+	}, {
+		desc: "invalid 32 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, // unexpected end
+		},
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 32 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, // unexpected end
+		},
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 64 bit with signature",
+		in: []byte{
+			0x50, 0x4b, 0x07, 0x08, // signature
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, // unexpected end
+		},
+		zip64:   true,
+		wantErr: io.ErrUnexpectedEOF,
+	}, {
+		desc: "invalid 64 bit without signature",
+		in: []byte{
+			0x00, 0x01, 0x02, 0x03, // crc32
+			0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, // compressed size
+			0x0c, 0x0d, 0x0e, 0x0f, 0x10, 0x11, // unexpected end
+		},
+		zip64:   true,
+		wantErr: io.ErrUnexpectedEOF,
+	}}
+
+	for _, test := range tests {
+		t.Run(test.desc, func(t *testing.T) {
+			r := bytes.NewReader(test.in)
+
+			desc, err := readDataDescriptor(r, test.zip64)
+			if err != test.wantErr {
+				t.Fatalf("got err %v; want nil", err)
+			}
+			if test.want == nil {
+				return
+			}
+			if desc == nil {
+				t.Fatalf("got nil DataDescriptor; want non-nil")
+			}
+			if desc.crc32 != test.want.crc32 {
+				t.Errorf("got CRC32 %#x; want %#x", desc.crc32, test.want.crc32)
+			}
+			if desc.compressedSize != test.want.compressedSize {
+				t.Errorf("got CompressedSize %#x; want %#x", desc.compressedSize, test.want.compressedSize)
+			}
+			if desc.uncompressedSize != test.want.uncompressedSize {
+				t.Errorf("got UncompressedSize %#x; want %#x", desc.uncompressedSize, test.want.uncompressedSize)
+			}
+		})
+	}
+}
diff --git a/src/archive/zip/struct.go b/src/archive/zip/struct.go
index 3dc0c50122..ff9f605eb6 100644
--- a/src/archive/zip/struct.go
+++ b/src/archive/zip/struct.go
@@ -42,7 +42,7 @@ const (
 	directoryHeaderLen       = 46         // + filename + extra + comment
 	directoryEndLen          = 22         // + comment
 	dataDescriptorLen        = 16         // four uint32: descriptor signature, crc32, compressed size, size
-	dataDescriptor64Len      = 24         // descriptor with 8 byte sizes
+	dataDescriptor64Len      = 24         // two uint32: signature, crc32 | two uint64: compressed size, size
 	directory64LocLen        = 20         //
 	directory64EndLen        = 56         // + extra
 
@@ -315,6 +315,10 @@ func (h *FileHeader) isZip64() bool {
 	return h.CompressedSize64 >= uint32max || h.UncompressedSize64 >= uint32max
 }
 
+func (f *FileHeader) hasDataDescriptor() bool {
+	return f.Flags&0x8 != 0
+}
+
 func msdosModeToFileMode(m uint32) (mode fs.FileMode) {
 	if m&msdosDir != 0 {
 		mode = fs.ModeDir | 0777
@@ -386,3 +390,11 @@ func unixModeToFileMode(m uint32) fs.FileMode {
 	}
 	return mode
 }
+
+// dataDescriptor holds the data descriptor that optionally follows the file
+// contents in the zip file.
+type dataDescriptor struct {
+	crc32            uint32
+	compressedSize   uint64
+	uncompressedSize uint64
+}
diff --git a/src/archive/zip/writer.go b/src/archive/zip/writer.go
index cdc534eaf0..3b23cc3391 100644
--- a/src/archive/zip/writer.go
+++ b/src/archive/zip/writer.go
@@ -37,6 +37,7 @@ type Writer struct {
 type header struct {
 	*FileHeader
 	offset uint64
+	raw    bool
 }
 
 // NewWriter returns a new Writer writing a zip file to w.
@@ -245,22 +246,31 @@ func detectUTF8(s string) (valid, require bool) {
 	return true, require
 }
 
+// prepare performs the bookkeeping operations required at the start of
+// CreateHeader and CreateRaw.
+func (w *Writer) prepare(fh *FileHeader) error {
+	if w.last != nil && !w.last.closed {
+		if err := w.last.close(); err != nil {
+			return err
+		}
+	}
+	if len(w.dir) > 0 && w.dir[len(w.dir)-1].FileHeader == fh {
+		// See https://golang.org/issue/11144 confusion.
+		return errors.New("archive/zip: invalid duplicate FileHeader")
+	}
+	return nil
+}
+
 // CreateHeader adds a file to the zip archive using the provided FileHeader
 // for the file metadata. Writer takes ownership of fh and may mutate
 // its fields. The caller must not modify fh after calling CreateHeader.
 //
 // This returns a Writer to which the file contents should be written.
 // The file's contents must be written to the io.Writer before the next
-// call to Create, CreateHeader, or Close.
+// call to Create, CreateHeader, CreateRaw, or Close.
 func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
-	if w.last != nil && !w.last.closed {
-		if err := w.last.close(); err != nil {
-			return nil, err
-		}
-	}
-	if len(w.dir) > 0 && w.dir[len(w.dir)-1].FileHeader == fh {
-		// See https://golang.org/issue/11144 confusion.
-		return nil, errors.New("archive/zip: invalid duplicate FileHeader")
+	if err := w.prepare(fh); err != nil {
+		return nil, err
 	}
 
 	// The ZIP format has a sad state of affairs regarding character encoding.
@@ -365,7 +375,7 @@ func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 		ow = fw
 	}
 	w.dir = append(w.dir, h)
-	if err := writeHeader(w.cw, fh); err != nil {
+	if err := writeHeader(w.cw, h); err != nil {
 		return nil, err
 	}
 	// If we're creating a directory, fw is nil.
@@ -373,7 +383,7 @@ func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 	return ow, nil
 }
 
-func writeHeader(w io.Writer, h *FileHeader) error {
+func writeHeader(w io.Writer, h *header) error {
 	const maxUint16 = 1<<16 - 1
 	if len(h.Name) > maxUint16 {
 		return errLongName
@@ -390,9 +400,20 @@ func writeHeader(w io.Writer, h *FileHeader) error {
 	b.uint16(h.Method)
 	b.uint16(h.ModifiedTime)
 	b.uint16(h.ModifiedDate)
-	b.uint32(0) // since we are writing a data descriptor crc32,
-	b.uint32(0) // compressed size,
-	b.uint32(0) // and uncompressed size should be zero
+	// In raw mode (caller does the compression), the values are either
+	// written here or in the trailing data descriptor based on the header
+	// flags.
+	if h.raw && !h.hasDataDescriptor() {
+		b.uint32(h.CRC32)
+		b.uint32(uint32(min64(h.CompressedSize64, uint32max)))
+		b.uint32(uint32(min64(h.UncompressedSize64, uint32max)))
+	} else {
+		// When this package handle the compression, these values are
+		// always written to the trailing data descriptor.
+		b.uint32(0) // crc32
+		b.uint32(0) // compressed size
+		b.uint32(0) // uncompressed size
+	}
 	b.uint16(uint16(len(h.Name)))
 	b.uint16(uint16(len(h.Extra)))
 	if _, err := w.Write(buf[:]); err != nil {
@@ -405,6 +426,65 @@ func writeHeader(w io.Writer, h *FileHeader) error {
 	return err
 }
 
+func min64(x, y uint64) uint64 {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+// CreateRaw adds a file to the zip archive using the provided FileHeader and
+// returns a Writer to which the file contents should be written. The file's
+// contents must be written to the io.Writer before the next call to Create,
+// CreateHeader, CreateRaw, or Close.
+//
+// In contrast to CreateHeader, the bytes passed to Writer are not compressed.
+func (w *Writer) CreateRaw(fh *FileHeader) (io.Writer, error) {
+	if err := w.prepare(fh); err != nil {
+		return nil, err
+	}
+
+	fh.CompressedSize = uint32(min64(fh.CompressedSize64, uint32max))
+	fh.UncompressedSize = uint32(min64(fh.UncompressedSize64, uint32max))
+
+	h := &header{
+		FileHeader: fh,
+		offset:     uint64(w.cw.count),
+		raw:        true,
+	}
+	w.dir = append(w.dir, h)
+	if err := writeHeader(w.cw, h); err != nil {
+		return nil, err
+	}
+
+	if strings.HasSuffix(fh.Name, "/") {
+		w.last = nil
+		return dirWriter{}, nil
+	}
+
+	fw := &fileWriter{
+		header: h,
+		zipw:   w.cw,
+	}
+	w.last = fw
+	return fw, nil
+}
+
+// Copy copies the file f (obtained from a Reader) into w. It copies the raw
+// form directly bypassing decompression, compression, and validation.
+func (w *Writer) Copy(f *File) error {
+	r, err := f.OpenRaw()
+	if err != nil {
+		return err
+	}
+	fw, err := w.CreateRaw(&f.FileHeader)
+	if err != nil {
+		return err
+	}
+	_, err = io.Copy(fw, r)
+	return err
+}
+
 // RegisterCompressor registers or overrides a custom compressor for a specific
 // method ID. If a compressor for a given method is not found, Writer will
 // default to looking up the compressor at the package level.
@@ -446,6 +526,9 @@ func (w *fileWriter) Write(p []byte) (int, error) {
 	if w.closed {
 		return 0, errors.New("zip: write to closed file")
 	}
+	if w.raw {
+		return w.zipw.Write(p)
+	}
 	w.crc32.Write(p)
 	return w.rawCount.Write(p)
 }
@@ -455,6 +538,9 @@ func (w *fileWriter) close() error {
 		return errors.New("zip: file closed twice")
 	}
 	w.closed = true
+	if w.raw {
+		return w.writeDataDescriptor()
+	}
 	if err := w.comp.Close(); err != nil {
 		return err
 	}
@@ -474,26 +560,33 @@ func (w *fileWriter) close() error {
 		fh.UncompressedSize = uint32(fh.UncompressedSize64)
 	}
 
+	return w.writeDataDescriptor()
+}
+
+func (w *fileWriter) writeDataDescriptor() error {
+	if !w.hasDataDescriptor() {
+		return nil
+	}
 	// Write data descriptor. This is more complicated than one would
 	// think, see e.g. comments in zipfile.c:putextended() and
 	// http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7073588.
 	// The approach here is to write 8 byte sizes if needed without
 	// adding a zip64 extra in the local header (too late anyway).
 	var buf []byte
-	if fh.isZip64() {
+	if w.isZip64() {
 		buf = make([]byte, dataDescriptor64Len)
 	} else {
 		buf = make([]byte, dataDescriptorLen)
 	}
 	b := writeBuf(buf)
 	b.uint32(dataDescriptorSignature) // de-facto standard, required by OS X
-	b.uint32(fh.CRC32)
-	if fh.isZip64() {
-		b.uint64(fh.CompressedSize64)
-		b.uint64(fh.UncompressedSize64)
+	b.uint32(w.CRC32)
+	if w.isZip64() {
+		b.uint64(w.CompressedSize64)
+		b.uint64(w.UncompressedSize64)
 	} else {
-		b.uint32(fh.CompressedSize)
-		b.uint32(fh.UncompressedSize)
+		b.uint32(w.CompressedSize)
+		b.uint32(w.UncompressedSize)
 	}
 	_, err := w.zipw.Write(buf)
 	return err
diff --git a/src/archive/zip/writer_test.go b/src/archive/zip/writer_test.go
index 3fa8bef055..97c6c52979 100644
--- a/src/archive/zip/writer_test.go
+++ b/src/archive/zip/writer_test.go
@@ -6,8 +6,10 @@ package zip
 
 import (
 	"bytes"
+	"compress/flate"
 	"encoding/binary"
 	"fmt"
+	"hash/crc32"
 	"io"
 	"io/fs"
 	"math/rand"
@@ -365,6 +367,171 @@ func TestWriterDirAttributes(t *testing.T) {
 	}
 }
 
+func TestWriterCopy(t *testing.T) {
+	// make a zip file
+	buf := new(bytes.Buffer)
+	w := NewWriter(buf)
+	for _, wt := range writeTests {
+		testCreate(t, w, &wt)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	src, err := NewReader(bytes.NewReader(buf.Bytes()), int64(buf.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, src.File[i], &wt)
+	}
+
+	// make a new zip file copying the old compressed data.
+	buf2 := new(bytes.Buffer)
+	dst := NewWriter(buf2)
+	for _, f := range src.File {
+		if err := dst.Copy(f); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := dst.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read the new one back
+	r, err := NewReader(bytes.NewReader(buf2.Bytes()), int64(buf2.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, wt := range writeTests {
+		testReadFile(t, r.File[i], &wt)
+	}
+}
+
+func TestWriterCreateRaw(t *testing.T) {
+	files := []struct {
+		name             string
+		content          []byte
+		method           uint16
+		flags            uint16
+		crc32            uint32
+		uncompressedSize uint64
+		compressedSize   uint64
+	}{
+		{
+			name:    "small store w desc",
+			content: []byte("gophers"),
+			method:  Store,
+			flags:   0x8,
+		},
+		{
+			name:    "small deflate wo desc",
+			content: bytes.Repeat([]byte("abcdefg"), 2048),
+			method:  Deflate,
+		},
+	}
+
+	// write a zip file
+	archive := new(bytes.Buffer)
+	w := NewWriter(archive)
+
+	for i := range files {
+		f := &files[i]
+		f.crc32 = crc32.ChecksumIEEE(f.content)
+		size := uint64(len(f.content))
+		f.uncompressedSize = size
+		f.compressedSize = size
+
+		var compressedContent []byte
+		if f.method == Deflate {
+			var buf bytes.Buffer
+			w, err := flate.NewWriter(&buf, flate.BestSpeed)
+			if err != nil {
+				t.Fatalf("flate.NewWriter err = %v", err)
+			}
+			_, err = w.Write(f.content)
+			if err != nil {
+				t.Fatalf("flate Write err = %v", err)
+			}
+			err = w.Close()
+			if err != nil {
+				t.Fatalf("flate Writer.Close err = %v", err)
+			}
+			compressedContent = buf.Bytes()
+			f.compressedSize = uint64(len(compressedContent))
+		}
+
+		h := &FileHeader{
+			Name:               f.name,
+			Method:             f.method,
+			Flags:              f.flags,
+			CRC32:              f.crc32,
+			CompressedSize64:   f.compressedSize,
+			UncompressedSize64: f.uncompressedSize,
+		}
+		w, err := w.CreateRaw(h)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if compressedContent != nil {
+			_, err = w.Write(compressedContent)
+		} else {
+			_, err = w.Write(f.content)
+		}
+		if err != nil {
+			t.Fatalf("%s Write got %v; want nil", f.name, err)
+		}
+	}
+
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// read it back
+	r, err := NewReader(bytes.NewReader(archive.Bytes()), int64(archive.Len()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, want := range files {
+		got := r.File[i]
+		if got.Name != want.name {
+			t.Errorf("got Name %s; want %s", got.Name, want.name)
+		}
+		if got.Method != want.method {
+			t.Errorf("%s: got Method %#x; want %#x", want.name, got.Method, want.method)
+		}
+		if got.Flags != want.flags {
+			t.Errorf("%s: got Flags %#x; want %#x", want.name, got.Flags, want.flags)
+		}
+		if got.CRC32 != want.crc32 {
+			t.Errorf("%s: got CRC32 %#x; want %#x", want.name, got.CRC32, want.crc32)
+		}
+		if got.CompressedSize64 != want.compressedSize {
+			t.Errorf("%s: got CompressedSize64 %d; want %d", want.name, got.CompressedSize64, want.compressedSize)
+		}
+		if got.UncompressedSize64 != want.uncompressedSize {
+			t.Errorf("%s: got UncompressedSize64 %d; want %d", want.name, got.UncompressedSize64, want.uncompressedSize)
+		}
+
+		r, err := got.Open()
+		if err != nil {
+			t.Errorf("%s: Open err = %v", got.Name, err)
+			continue
+		}
+
+		buf, err := io.ReadAll(r)
+		if err != nil {
+			t.Errorf("%s: ReadAll err = %v", got.Name, err)
+			continue
+		}
+
+		if !bytes.Equal(buf, want.content) {
+			t.Errorf("%v: ReadAll returned unexpected bytes", got.Name)
+		}
+	}
+}
+
 func testCreate(t *testing.T, w *Writer, wt *WriteTest) {
 	header := &FileHeader{
 		Name:   wt.Name,
@@ -390,15 +557,15 @@ func testReadFile(t *testing.T, f *File, wt *WriteTest) {
 	testFileMode(t, f, wt.Mode)
 	rc, err := f.Open()
 	if err != nil {
-		t.Fatal("opening:", err)
+		t.Fatalf("opening %s: %v", f.Name, err)
 	}
 	b, err := io.ReadAll(rc)
 	if err != nil {
-		t.Fatal("reading:", err)
+		t.Fatalf("reading %s: %v", f.Name, err)
 	}
 	err = rc.Close()
 	if err != nil {
-		t.Fatal("closing:", err)
+		t.Fatalf("closing %s: %v", f.Name, err)
 	}
 	if !bytes.Equal(b, wt.Data) {
 		t.Errorf("File contents %q, want %q", b, wt.Data)