compress: make flate, gzip and zlib's NewWriterXxx functions all return

(*Writer, error) if they take a compression level, and *Writer otherwise.
Rename gzip's Compressor and Decompressor to Writer and Reader, similar to
flate and zlib.

Clarify commentary when writing gzip metadata that is not representable
as Latin-1, and fix io.EOF comment bug.

Also refactor gzip_test to be more straightforward.

Fixes #2839.

R=rsc, r, rsc, bradfitz
CC=golang-dev
https://golang.org/cl/5639057

doc/go1.html

@@ -878,6 +878,20 @@ If the argument size is too small or invalid, it is adjusted.
 What little code is affected will be caught by the compiler and must be updated by hand.
 </p>
 
+<h3 id="bufio">The compress/flate, compress/gzip and compress/zlib packages</h3>
+
+<p>
+In Go 1, the NewWriterXxx functions in compress/flate, compress/gzip and
+compress/zlib all return (*Writer, error) if they take a compression level,
+and *Writer otherwise. Package gzip's Compressor and Decompressor types have
+been renamed to Writer and Reader.
+</p>
+
+<p>
+<em>Updating</em>:
+What little code is affected will be caught by the compiler and must be updated by hand.
+</p>
+
 <h3 id="crypto_elliptic">The crypto/elliptic package</h3>
 
 <p>

doc/go1.tmpl

@@ -782,6 +782,20 @@ If the argument size is too small or invalid, it is adjusted.
 What little code is affected will be caught by the compiler and must be updated by hand.
 </p>
 
+<h3 id="bufio">The compress/flate, compress/gzip and compress/zlib packages</h3>
+
+<p>
+In Go 1, the NewWriterXxx functions in compress/flate, compress/gzip and
+compress/zlib all return (*Writer, error) if they take a compression level,
+and *Writer otherwise. Package gzip's Compressor and Decompressor types have
+been renamed to Writer and Reader.
+</p>
+
+<p>
+<em>Updating</em>:
+What little code is affected will be caught by the compiler and must be updated by hand.
+</p>
+
 <h3 id="crypto_elliptic">The crypto/elliptic package</h3>
 
 <p>

src/pkg/archive/zip/writer.go

@@ -127,7 +127,11 @@ func (w *Writer) CreateHeader(fh *FileHeader) (io.Writer, error) {
 	case Store:
 		fw.comp = nopCloser{fw.compCount}
 	case Deflate:
-		fw.comp = flate.NewWriter(fw.compCount, 5)
+		var err error
+		fw.comp, err = flate.NewWriter(fw.compCount, 5)
+		if err != nil {
+			return nil, err
+		}
 	default:
 		return nil, ErrAlgorithm
 	}

src/pkg/compress/flate/deflate.go

@@ -408,17 +408,22 @@ func (d *compressor) close() error {
 	return d.w.err
 }
 
-// NewWriter returns a new Writer compressing
-// data at the given level.  Following zlib, levels
-// range from 1 (BestSpeed) to 9 (BestCompression);
-// higher levels typically run slower but compress more.
-// Level 0 (NoCompression) does not attempt any
-// compression; it only adds the necessary DEFLATE framing.
-func NewWriter(w io.Writer, level int) *Writer {
+// NewWriter returns a new Writer compressing data at the given level.
+// Following zlib, levels range from 1 (BestSpeed) to 9 (BestCompression);
+// higher levels typically run slower but compress more. Level 0
+// (NoCompression) does not attempt any compression; it only adds the
+// necessary DEFLATE framing. Level -1 (DefaultCompression) uses the default
+// compression level.
+//
+// If level is in the range [-1, 9] then the error returned will be nil.
+// Otherwise the error returned will be non-nil.
+func NewWriter(w io.Writer, level int) (*Writer, error) {
 	const logWindowSize = logMaxOffsetSize
 	var dw Writer
-	dw.d.init(w, level)
-	return &dw
+	if err := dw.d.init(w, level); err != nil {
+		return nil, err
+	}
+	return &dw, nil
 }
 
 // NewWriterDict is like NewWriter but initializes the new
@@ -427,13 +432,16 @@ func NewWriter(w io.Writer, level int) *Writer {
 // any compressed output.  The compressed data written to w
 // can only be decompressed by a Reader initialized with the
 // same dictionary.
-func NewWriterDict(w io.Writer, level int, dict []byte) *Writer {
+func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
 	dw := &dictWriter{w, false}
-	zw := NewWriter(dw, level)
+	zw, err := NewWriter(dw, level)
+	if err != nil {
+		return nil, err
+	}
 	zw.Write(dict)
 	zw.Flush()
 	dw.enabled = true
-	return zw
+	return zw, err
 }
 
 type dictWriter struct {

src/pkg/compress/flate/deflate_test.go

@@ -81,7 +81,11 @@ func largeDataChunk() []byte {
 func TestDeflate(t *testing.T) {
 	for _, h := range deflateTests {
 		var buf bytes.Buffer
-		w := NewWriter(&buf, h.level)
+		w, err := NewWriter(&buf, h.level)
+		if err != nil {
+			t.Errorf("NewWriter: %v", err)
+			continue
+		}
 		w.Write(h.in)
 		w.Close()
 		if !bytes.Equal(buf.Bytes(), h.out) {
@@ -151,7 +155,11 @@ func testSync(t *testing.T, level int, input []byte, name string) {
 	buf := newSyncBuffer()
 	buf1 := new(bytes.Buffer)
 	buf.WriteMode()
-	w := NewWriter(io.MultiWriter(buf, buf1), level)
+	w, err := NewWriter(io.MultiWriter(buf, buf1), level)
+	if err != nil {
+		t.Errorf("NewWriter: %v", err)
+		return
+	}
 	r := NewReader(buf)
 
 	// Write half the input and read back.
@@ -213,7 +221,7 @@ func testSync(t *testing.T, level int, input []byte, name string) {
 
 	// stream should work for ordinary reader too
 	r = NewReader(buf1)
-	out, err := ioutil.ReadAll(r)
+	out, err = ioutil.ReadAll(r)
 	if err != nil {
 		t.Errorf("testSync: read: %s", err)
 		return
@@ -224,31 +232,31 @@ func testSync(t *testing.T, level int, input []byte, name string) {
 	}
 }
 
-func testToFromWithLevel(t *testing.T, level int, input []byte, name string) error {
-	return testToFromWithLevelAndLimit(t, level, input, name, -1)
-}
-
-func testToFromWithLevelAndLimit(t *testing.T, level int, input []byte, name string, limit int) error {
+func testToFromWithLevelAndLimit(t *testing.T, level int, input []byte, name string, limit int) {
 	var buffer bytes.Buffer
-	w := NewWriter(&buffer, level)
+	w, err := NewWriter(&buffer, level)
+	if err != nil {
+		t.Errorf("NewWriter: %v", err)
+		return
+	}
 	w.Write(input)
 	w.Close()
 	if limit > 0 && buffer.Len() > limit {
 		t.Errorf("level: %d, len(compress(data)) = %d > limit = %d", level, buffer.Len(), limit)
+		return
 	}
 	r := NewReader(&buffer)
 	out, err := ioutil.ReadAll(r)
 	if err != nil {
 		t.Errorf("read: %s", err)
-		return err
+		return
 	}
 	r.Close()
 	if !bytes.Equal(input, out) {
 		t.Errorf("decompress(compress(data)) != data: level=%d input=%s", level, name)
+		return
 	}
-
 	testSync(t, level, input, name)
-	return nil
 }
 
 func testToFromWithLimit(t *testing.T, input []byte, name string, limit [10]int) {
@@ -257,13 +265,9 @@ func testToFromWithLimit(t *testing.T, input []byte, name string, limit [10]int)
 	}
 }
 
-func testToFrom(t *testing.T, input []byte, name string) {
-	testToFromWithLimit(t, input, name, [10]int{})
-}
-
 func TestDeflateInflate(t *testing.T) {
 	for i, h := range deflateInflateTests {
-		testToFrom(t, h.in, fmt.Sprintf("#%d", i))
+		testToFromWithLimit(t, h.in, fmt.Sprintf("#%d", i), [10]int{})
 	}
 }
 
@@ -311,7 +315,10 @@ func TestReaderDict(t *testing.T) {
 		text = "hello again world"
 	)
 	var b bytes.Buffer
-	w := NewWriter(&b, 5)
+	w, err := NewWriter(&b, 5)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
 	w.Write([]byte(dict))
 	w.Flush()
 	b.Reset()
@@ -334,7 +341,10 @@ func TestWriterDict(t *testing.T) {
 		text = "hello again world"
 	)
 	var b bytes.Buffer
-	w := NewWriter(&b, 5)
+	w, err := NewWriter(&b, 5)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
 	w.Write([]byte(dict))
 	w.Flush()
 	b.Reset()
@@ -342,7 +352,7 @@ func TestWriterDict(t *testing.T) {
 	w.Close()
 
 	var b1 bytes.Buffer
-	w = NewWriterDict(&b1, 5, []byte(dict))
+	w, _ = NewWriterDict(&b1, 5, []byte(dict))
 	w.Write([]byte(text))
 	w.Close()
 
@@ -353,7 +363,10 @@ func TestWriterDict(t *testing.T) {
 
 // See http://code.google.com/p/go/issues/detail?id=2508
 func TestRegression2508(t *testing.T) {
-	w := NewWriter(ioutil.Discard, 1)
+	w, err := NewWriter(ioutil.Discard, 1)
+	if err != nil {
+		t.Fatalf("NewWriter: %v", err)
+	}
 	buf := make([]byte, 1024)
 	for i := 0; i < 131072; i++ {
 		if _, err := w.Write(buf); err != nil {

src/pkg/compress/gzip/gunzip.go

@@ -16,9 +16,6 @@ import (
 	"time"
 )
 
-// BUG(nigeltao): Comments and Names don't properly map UTF-8 character codes outside of
-// the 0x00-0x7f range to ISO 8859-1 (Latin-1).
-
 const (
 	gzipID1     = 0x1f
 	gzipID2     = 0x8b
@@ -41,7 +38,7 @@ var ErrHeader = errors.New("invalid gzip header")
 var ErrChecksum = errors.New("gzip checksum error")
 
 // The gzip file stores a header giving metadata about the compressed file.
-// That header is exposed as the fields of the Compressor and Decompressor structs.
+// That header is exposed as the fields of the Writer and Reader structs.
 type Header struct {
 	Comment string    // comment
 	Extra   []byte    // "extra data"
@@ -50,21 +47,21 @@ type Header struct {
 	OS      byte      // operating system type
 }
 
-// An Decompressor is an io.Reader that can be read to retrieve
+// A Reader is an io.Reader that can be read to retrieve
 // uncompressed data from a gzip-format compressed file.
 //
 // In general, a gzip file can be a concatenation of gzip files,
-// each with its own header.  Reads from the Decompressor
+// each with its own header.  Reads from the Reader
 // return the concatenation of the uncompressed data of each.
-// Only the first header is recorded in the Decompressor fields.
+// Only the first header is recorded in the Reader fields.
 //
 // Gzip files store a length and checksum of the uncompressed data.
-// The Decompressor will return a ErrChecksum when Read
+// The Reader will return a ErrChecksum when Read
 // reaches the end of the uncompressed data if it does not
 // have the expected length or checksum.  Clients should treat data
-// returned by Read as tentative until they receive the successful
-// (zero length, nil error) Read marking the end of the data.
-type Decompressor struct {
+// returned by Read as tentative until they receive the io.EOF
+// marking the end of the data.
+type Reader struct {
 	Header
 	r            flate.Reader
 	decompressor io.ReadCloser
@@ -75,11 +72,11 @@ type Decompressor struct {
 	err          error
 }
 
-// NewReader creates a new Decompressor reading the given reader.
+// NewReader creates a new Reader reading the given reader.
 // The implementation buffers input and may read more data than necessary from r.
-// It is the caller's responsibility to call Close on the Decompressor when done.
-func NewReader(r io.Reader) (*Decompressor, error) {
-	z := new(Decompressor)
+// It is the caller's responsibility to call Close on the Reader when done.
+func NewReader(r io.Reader) (*Reader, error) {
+	z := new(Reader)
 	z.r = makeReader(r)
 	z.digest = crc32.NewIEEE()
 	if err := z.readHeader(true); err != nil {
@@ -93,7 +90,7 @@ func get4(p []byte) uint32 {
 	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24
 }
 
-func (z *Decompressor) readString() (string, error) {
+func (z *Reader) readString() (string, error) {
 	var err error
 	needconv := false
 	for i := 0; ; i++ {
@@ -122,7 +119,7 @@ func (z *Decompressor) readString() (string, error) {
 	panic("not reached")
 }
 
-func (z *Decompressor) read2() (uint32, error) {
+func (z *Reader) read2() (uint32, error) {
 	_, err := io.ReadFull(z.r, z.buf[0:2])
 	if err != nil {
 		return 0, err
@@ -130,7 +127,7 @@ func (z *Decompressor) read2() (uint32, error) {
 	return uint32(z.buf[0]) | uint32(z.buf[1])<<8, nil
 }
 
-func (z *Decompressor) readHeader(save bool) error {
+func (z *Reader) readHeader(save bool) error {
 	_, err := io.ReadFull(z.r, z.buf[0:10])
 	if err != nil {
 		return err
@@ -196,7 +193,7 @@ func (z *Decompressor) readHeader(save bool) error {
 	return nil
 }
 
-func (z *Decompressor) Read(p []byte) (n int, err error) {
+func (z *Reader) Read(p []byte) (n int, err error) {
 	if z.err != nil {
 		return 0, z.err
 	}
@@ -236,5 +233,5 @@ func (z *Decompressor) Read(p []byte) (n int, err error) {
 	return z.Read(p)
 }
 
-// Calling Close does not close the wrapped io.Reader originally passed to NewReader.
-func (z *Decompressor) Close() error { return z.decompressor.Close() }
+// Close closes the Reader. It does not close the underlying io.Reader.
+func (z *Reader) Close() error { return z.decompressor.Close() }

src/pkg/compress/gzip/gzip.go

@@ -7,6 +7,7 @@ package gzip
 import (
 	"compress/flate"
 	"errors"
+	"fmt"
 	"hash"
 	"hash/crc32"
 	"io"
@@ -21,9 +22,9 @@ const (
 	DefaultCompression = flate.DefaultCompression
 )
 
-// A Compressor is an io.WriteCloser that satisfies writes by compressing data written
+// A Writer is an io.WriteCloser that satisfies writes by compressing data written
 // to its wrapped io.Writer.
-type Compressor struct {
+type Writer struct {
 	Header
 	w          io.Writer
 	level      int
@@ -35,25 +36,40 @@ type Compressor struct {
 	err        error
 }
 
-// NewWriter calls NewWriterLevel with the default compression level.
-func NewWriter(w io.Writer) (*Compressor, error) {
-	return NewWriterLevel(w, DefaultCompression)
+// NewWriter creates a new Writer that satisfies writes by compressing data
+// written to w.
+//
+// It is the caller's responsibility to call Close on the WriteCloser when done.
+// Writes may be buffered and not flushed until Close.
+//
+// Callers that wish to set the fields in Writer.Header must do so before
+// the first call to Write or Close. The Comment and Name header fields are
+// UTF-8 strings in Go, but the underlying format requires NUL-terminated ISO
+// 8859-1 (Latin-1). NUL or non-Latin-1 runes in those strings will lead to an
+// error on Write.
+func NewWriter(w io.Writer) *Writer {
+	z, _ := NewWriterLevel(w, DefaultCompression)
+	return z
 }
 
-// NewWriterLevel creates a new Compressor writing to the given writer.
-// Writes may be buffered and not flushed until Close.
-// Callers that wish to set the fields in Compressor.Header must
-// do so before the first call to Write or Close.
-// It is the caller's responsibility to call Close on the WriteCloser when done.
-// level is the compression level, which can be DefaultCompression, NoCompression,
-// or any integer value between BestSpeed and BestCompression (inclusive).
-func NewWriterLevel(w io.Writer, level int) (*Compressor, error) {
-	z := new(Compressor)
-	z.OS = 255 // unknown
-	z.w = w
-	z.level = level
-	z.digest = crc32.NewIEEE()
-	return z, nil
+// NewWriterLevel is like NewWriter but specifies the compression level instead
+// of assuming DefaultCompression.
+//
+// The compression level can be DefaultCompression, NoCompression, or any
+// integer value between BestSpeed and BestCompression inclusive. The error
+// returned will be nil if the level is valid.
+func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
+	if level < DefaultCompression || level > BestCompression {
+		return nil, fmt.Errorf("gzip: invalid compression level: %d", level)
+	}
+	return &Writer{
+		Header: Header{
+			OS: 255, // unknown
+		},
+		w:      w,
+		level:  level,
+		digest: crc32.NewIEEE(),
+	}, nil
 }
 
 // GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).
@@ -70,7 +86,7 @@ func put4(p []byte, v uint32) {
 }
 
 // writeBytes writes a length-prefixed byte slice to z.w.
-func (z *Compressor) writeBytes(b []byte) error {
+func (z *Writer) writeBytes(b []byte) error {
 	if len(b) > 0xffff {
 		return errors.New("gzip.Write: Extra data is too large")
 	}
@@ -83,10 +99,10 @@ func (z *Compressor) writeBytes(b []byte) error {
 	return err
 }
 
-// writeString writes a string (in ISO 8859-1 (Latin-1) format) to z.w.
-func (z *Compressor) writeString(s string) error {
-	// GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).
-	var err error
+// writeString writes a UTF-8 string s in GZIP's format to z.w.
+// GZIP (RFC 1952) specifies that strings are NUL-terminated ISO 8859-1 (Latin-1).
+func (z *Writer) writeString(s string) (err error) {
+	// GZIP stores Latin-1 strings; error if non-Latin-1; convert if non-ASCII.
 	needconv := false
 	for _, v := range s {
 		if v == 0 || v > 0xff {
@@ -114,7 +130,7 @@ func (z *Compressor) writeString(s string) error {
 	return err
 }
 
-func (z *Compressor) Write(p []byte) (int, error) {
+func (z *Writer) Write(p []byte) (int, error) {
 	if z.err != nil {
 		return 0, z.err
 	}
@@ -165,7 +181,7 @@ func (z *Compressor) Write(p []byte) (int, error) {
 				return n, z.err
 			}
 		}
-		z.compressor = flate.NewWriter(z.w, z.level)
+		z.compressor, _ = flate.NewWriter(z.w, z.level)
 	}
 	z.size += uint32(len(p))
 	z.digest.Write(p)
@@ -173,8 +189,8 @@ func (z *Compressor) Write(p []byte) (int, error) {
 	return n, z.err
 }
 
-// Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
-func (z *Compressor) Close() error {
+// Close closes the Writer. It does not close the underlying io.Writer.
+func (z *Writer) Close() error {
 	if z.err != nil {
 		return z.err
 	}

src/pkg/compress/gzip/gzip_test.go

@@ -7,108 +7,153 @@ package gzip
 import (
 	"bufio"
 	"bytes"
-	"io"
 	"io/ioutil"
 	"testing"
 	"time"
 )
 
-// pipe creates two ends of a pipe that gzip and gunzip, and runs dfunc at the
-// writer end and cfunc at the reader end.
-func pipe(t *testing.T, dfunc func(*Compressor), cfunc func(*Decompressor)) {
-	piper, pipew := io.Pipe()
-	defer piper.Close()
-	go func() {
-		defer pipew.Close()
-		compressor, err := NewWriter(pipew)
-		if err != nil {
-			t.Fatalf("%v", err)
-		}
-		defer compressor.Close()
-		dfunc(compressor)
-	}()
-	decompressor, err := NewReader(piper)
+// TestEmpty tests that an empty payload still forms a valid GZIP stream.
+func TestEmpty(t *testing.T) {
+	buf := new(bytes.Buffer)
+
+	if err := NewWriter(buf).Close(); err != nil {
+		t.Fatalf("Writer.Close: %v", err)
+	}
+
+	r, err := NewReader(buf)
+	if err != nil {
+		t.Fatalf("NewReader: %v", err)
+	}
+	b, err := ioutil.ReadAll(r)
 	if err != nil {
-		t.Fatalf("%v", err)
+		t.Fatalf("ReadAll: %v", err)
+	}
+	if len(b) != 0 {
+		t.Fatalf("got %d bytes, want 0", len(b))
+	}
+	if err := r.Close(); err != nil {
+		t.Fatalf("Reader.Close: %v", err)
 	}
-	defer decompressor.Close()
-	cfunc(decompressor)
 }
 
-// Tests that an empty payload still forms a valid GZIP stream.
-func TestEmpty(t *testing.T) {
-	pipe(t,
-		func(compressor *Compressor) {},
-		func(decompressor *Decompressor) {
-			b, err := ioutil.ReadAll(decompressor)
-			if err != nil {
-				t.Fatalf("%v", err)
-			}
-			if len(b) != 0 {
-				t.Fatalf("did not read an empty slice")
-			}
-		})
-}
+// TestRoundTrip tests that gzipping and then gunzipping is the identity
+// function.
+func TestRoundTrip(t *testing.T) {
+	buf := new(bytes.Buffer)
+
+	w := NewWriter(buf)
+	w.Comment = "comment"
+	w.Extra = []byte("extra")
+	w.ModTime = time.Unix(1e8, 0)
+	w.Name = "name"
+	if _, err := w.Write([]byte("payload")); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	if err := w.Close(); err != nil {
+		t.Fatalf("Writer.Close: %v", err)
+	}
 
-// Tests that gzipping and then gunzipping is the identity function.
-func TestWriter(t *testing.T) {
-	pipe(t,
-		func(compressor *Compressor) {
-			compressor.Comment = "Äußerung"
-			//compressor.Comment = "comment"
-			compressor.Extra = []byte("extra")
-			compressor.ModTime = time.Unix(1e8, 0)
-			compressor.Name = "name"
-			_, err := compressor.Write([]byte("payload"))
-			if err != nil {
-				t.Fatalf("%v", err)
-			}
-		},
-		func(decompressor *Decompressor) {
-			b, err := ioutil.ReadAll(decompressor)
-			if err != nil {
-				t.Fatalf("%v", err)
-			}
-			if string(b) != "payload" {
-				t.Fatalf("payload is %q, want %q", string(b), "payload")
-			}
-			if decompressor.Comment != "Äußerung" {
-				t.Fatalf("comment is %q, want %q", decompressor.Comment, "Äußerung")
-			}
-			if string(decompressor.Extra) != "extra" {
-				t.Fatalf("extra is %q, want %q", decompressor.Extra, "extra")
-			}
-			if decompressor.ModTime.Unix() != 1e8 {
-				t.Fatalf("mtime is %d, want %d", decompressor.ModTime.Unix(), uint32(1e8))
-			}
-			if decompressor.Name != "name" {
-				t.Fatalf("name is %q, want %q", decompressor.Name, "name")
-			}
-		})
+	r, err := NewReader(buf)
+	if err != nil {
+		t.Fatalf("NewReader: %v", err)
+	}
+	b, err := ioutil.ReadAll(r)
+	if err != nil {
+		t.Fatalf("ReadAll: %v", err)
+	}
+	if string(b) != "payload" {
+		t.Fatalf("payload is %q, want %q", string(b), "payload")
+	}
+	if r.Comment != "comment" {
+		t.Fatalf("comment is %q, want %q", r.Comment, "comment")
+	}
+	if string(r.Extra) != "extra" {
+		t.Fatalf("extra is %q, want %q", r.Extra, "extra")
+	}
+	if r.ModTime.Unix() != 1e8 {
+		t.Fatalf("mtime is %d, want %d", r.ModTime.Unix(), uint32(1e8))
+	}
+	if r.Name != "name" {
+		t.Fatalf("name is %q, want %q", r.Name, "name")
+	}
+	if err := r.Close(); err != nil {
+		t.Fatalf("Reader.Close: %v", err)
+	}
 }
 
+// TestLatin1 tests the internal functions for converting to and from Latin-1.
 func TestLatin1(t *testing.T) {
 	latin1 := []byte{0xc4, 'u', 0xdf, 'e', 'r', 'u', 'n', 'g', 0}
 	utf8 := "Äußerung"
-	z := Decompressor{r: bufio.NewReader(bytes.NewBuffer(latin1))}
+	z := Reader{r: bufio.NewReader(bytes.NewBuffer(latin1))}
 	s, err := z.readString()
 	if err != nil {
-		t.Fatalf("%v", err)
+		t.Fatalf("readString: %v", err)
 	}
 	if s != utf8 {
-		t.Fatalf("string is %q, want %q", s, utf8)
+		t.Fatalf("read latin-1: got %q, want %q", s, utf8)
 	}
 
 	buf := bytes.NewBuffer(make([]byte, 0, len(latin1)))
-	c := Compressor{w: buf}
+	c := Writer{w: buf}
 	if err = c.writeString(utf8); err != nil {
-		t.Fatalf("%v", err)
+		t.Fatalf("writeString: %v", err)
 	}
 	s = buf.String()
 	if s != string(latin1) {
-		t.Fatalf("string is %v, want %v", s, latin1)
+		t.Fatalf("write utf-8: got %q, want %q", s, string(latin1))
+	}
+}
+
+// TestLatin1RoundTrip tests that metadata that is representable in Latin-1
+// survives a round trip.
+func TestLatin1RoundTrip(t *testing.T) {
+	testCases := []struct {
+		name string
+		ok   bool
+	}{
+		{"", true},
+		{"ASCII is OK", true},
+		{"unless it contains a NUL\x00", false},
+		{"no matter where \x00 occurs", false},
+		{"\x00\x00\x00", false},
+		{"Látin-1 also passes (U+00E1)", true},
+		{"but LĀtin Extended-A (U+0100) does not", false},
+		{"neither does 日本語", false},
+		{"invalid UTF-8 also \xffails", false},
+		{"\x00 as does Látin-1 with NUL", false},
+	}
+	for _, tc := range testCases {
+		buf := new(bytes.Buffer)
+
+		w := NewWriter(buf)
+		w.Name = tc.name
+		err := w.Close()
+		if (err == nil) != tc.ok {
+			t.Errorf("Writer.Close: name = %q, err = %v", tc.name, err)
+			continue
+		}
+		if !tc.ok {
+			continue
+		}
+
+		r, err := NewReader(buf)
+		if err != nil {
+			t.Errorf("NewReader: %v", err)
+			continue
+		}
+		_, err = ioutil.ReadAll(r)
+		if err != nil {
+			t.Errorf("ReadAll: %v", err)
+			continue
+		}
+		if r.Name != tc.name {
+			t.Errorf("name is %q, want %q", r.Name, tc.name)
+			continue
+		}
+		if err := r.Close(); err != nil {
+			t.Errorf("Reader.Close: %v", err)
+			continue
+		}
 	}
-	//if s, err = buf.ReadString(0); err != nil {
-	//t.Fatalf("%v", err)
-	//}
 }

src/pkg/compress/zlib/writer.go

@@ -6,7 +6,7 @@ package zlib
 
 import (
 	"compress/flate"
-	"errors"
+	"fmt"
 	"hash"
 	"hash/adler32"
 	"io"
@@ -24,30 +24,55 @@ const (
 // A Writer takes data written to it and writes the compressed
 // form of that data to an underlying writer (see NewWriter).
 type Writer struct {
-	w          io.Writer
-	compressor *flate.Writer
-	digest     hash.Hash32
-	err        error
-	scratch    [4]byte
+	w           io.Writer
+	level       int
+	dict        []byte
+	compressor  *flate.Writer
+	digest      hash.Hash32
+	err         error
+	scratch     [4]byte
+	wroteHeader bool
 }
 
-// NewWriter calls NewWriterLevel with the default compression level.
-func NewWriter(w io.Writer) (*Writer, error) {
-	return NewWriterLevel(w, DefaultCompression)
+// NewWriter creates a new Writer that satisfies writes by compressing data
+// written to w.
+//
+// It is the caller's responsibility to call Close on the WriteCloser when done.
+// Writes may be buffered and not flushed until Close.
+func NewWriter(w io.Writer) *Writer {
+	z, _ := NewWriterLevelDict(w, DefaultCompression, nil)
+	return z
 }
 
-// NewWriterLevel calls NewWriterDict with no dictionary.
+// NewWriterLevel is like NewWriter but specifies the compression level instead
+// of assuming DefaultCompression.
+//
+// The compression level can be DefaultCompression, NoCompression, or any
+// integer value between BestSpeed and BestCompression inclusive. The error
+// returned will be nil if the level is valid.
 func NewWriterLevel(w io.Writer, level int) (*Writer, error) {
-	return NewWriterDict(w, level, nil)
+	return NewWriterLevelDict(w, level, nil)
 }
 
-// NewWriterDict creates a new io.WriteCloser that satisfies writes by compressing data written to w.
-// It is the caller's responsibility to call Close on the WriteCloser when done.
-// level is the compression level, which can be DefaultCompression, NoCompression,
-// or any integer value between BestSpeed and BestCompression (inclusive).
-// dict is the preset dictionary to compress with, or nil to use no dictionary.
-func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
-	z := new(Writer)
+// NewWriterLevelDict is like NewWriterLevel but specifies a dictionary to
+// compress with.
+//
+// The dictionary may be nil. If not, its contents should not be modified until
+// the Writer is closed.
+func NewWriterLevelDict(w io.Writer, level int, dict []byte) (*Writer, error) {
+	if level < DefaultCompression || level > BestCompression {
+		return nil, fmt.Errorf("zlib: invalid compression level: %d", level)
+	}
+	return &Writer{
+		w:     w,
+		level: level,
+		dict:  dict,
+	}, nil
+}
+
+// writeHeader writes the ZLIB header.
+func (z *Writer) writeHeader() (err error) {
+	z.wroteHeader = true
 	// ZLIB has a two-byte header (as documented in RFC 1950).
 	// The first four bits is the CINFO (compression info), which is 7 for the default deflate window size.
 	// The next four bits is the CM (compression method), which is 8 for deflate.
@@ -56,7 +81,7 @@ func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
 	// 0=fastest, 1=fast, 2=default, 3=best.
 	// The next bit, FDICT, is set if a dictionary is given.
 	// The final five FCHECK bits form a mod-31 checksum.
-	switch level {
+	switch z.level {
 	case 0, 1:
 		z.scratch[1] = 0 << 6
 	case 2, 3, 4, 5:
@@ -66,35 +91,38 @@ func NewWriterDict(w io.Writer, level int, dict []byte) (*Writer, error) {
 	case 7, 8, 9:
 		z.scratch[1] = 3 << 6
 	default:
-		return nil, errors.New("level out of range")
+		panic("unreachable")
 	}
-	if dict != nil {
+	if z.dict != nil {
 		z.scratch[1] |= 1 << 5
 	}
 	z.scratch[1] += uint8(31 - (uint16(z.scratch[0])<<8+uint16(z.scratch[1]))%31)
-	_, err := w.Write(z.scratch[0:2])
-	if err != nil {
-		return nil, err
+	if _, err = z.w.Write(z.scratch[0:2]); err != nil {
+		return err
 	}
-	if dict != nil {
+	if z.dict != nil {
 		// The next four bytes are the Adler-32 checksum of the dictionary.
-		checksum := adler32.Checksum(dict)
+		checksum := adler32.Checksum(z.dict)
 		z.scratch[0] = uint8(checksum >> 24)
 		z.scratch[1] = uint8(checksum >> 16)
 		z.scratch[2] = uint8(checksum >> 8)
 		z.scratch[3] = uint8(checksum >> 0)
-		_, err = w.Write(z.scratch[0:4])
-		if err != nil {
-			return nil, err
+		if _, err = z.w.Write(z.scratch[0:4]); err != nil {
+			return err
 		}
 	}
-	z.w = w
-	z.compressor = flate.NewWriterDict(w, level, dict)
+	z.compressor, err = flate.NewWriterDict(z.w, z.level, z.dict)
+	if err != nil {
+		return err
+	}
 	z.digest = adler32.New()
-	return z, nil
+	return nil
 }
 
 func (z *Writer) Write(p []byte) (n int, err error) {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
 	if z.err != nil {
 		return 0, z.err
 	}
@@ -112,6 +140,9 @@ func (z *Writer) Write(p []byte) (n int, err error) {
 
 // Flush flushes the underlying compressor.
 func (z *Writer) Flush() error {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
 	if z.err != nil {
 		return z.err
 	}
@@ -121,6 +152,9 @@ func (z *Writer) Flush() error {
 
 // Calling Close does not close the wrapped io.Writer originally passed to NewWriter.
 func (z *Writer) Close() error {
+	if !z.wroteHeader {
+		z.err = z.writeHeader()
+	}
 	if z.err != nil {
 		return z.err
 	}

src/pkg/compress/zlib/writer_test.go

@@ -52,7 +52,7 @@ func testLevelDict(t *testing.T, fn string, b0 []byte, level int, d string) {
 	defer piper.Close()
 	go func() {
 		defer pipew.Close()
-		zlibw, err := NewWriterDict(pipew, level, dict)
+		zlibw, err := NewWriterLevelDict(pipew, level, dict)
 		if err != nil {
 			t.Errorf("%s (level=%d, dict=%q): %v", fn, level, d, err)
 			return
@@ -125,9 +125,9 @@ func TestWriterDict(t *testing.T) {
 func TestWriterDictIsUsed(t *testing.T) {
 	var input = []byte("Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.")
 	var buf bytes.Buffer
-	compressor, err := NewWriterDict(&buf, BestCompression, input)
+	compressor, err := NewWriterLevelDict(&buf, BestCompression, input)
 	if err != nil {
-		t.Errorf("error in NewWriterDict: %s", err)
+		t.Errorf("error in NewWriterLevelDict: %s", err)
 		return
 	}
 	compressor.Write(input)

src/pkg/crypto/rand/rand_test.go

@@ -22,7 +22,7 @@ func TestRead(t *testing.T) {
 	}
 
 	var z bytes.Buffer
-	f := flate.NewWriter(&z, 5)
+	f, _ := flate.NewWriter(&z, 5)
 	f.Write(b)
 	f.Close()
 	if z.Len() < len(b)*99/100 {

src/pkg/image/png/writer.go

@@ -263,10 +263,7 @@ func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
 }
 
 func writeImage(w io.Writer, m image.Image, cb int) error {
-	zw, err := zlib.NewWriter(w)
-	if err != nil {
-		return err
-	}
+	zw := zlib.NewWriter(w)
 	defer zw.Close()
 
 	bpp := 0 // Bytes per pixel.
@@ -391,8 +388,7 @@ func writeImage(w io.Writer, m image.Image, cb int) error {
 		f := filter(&cr, pr, bpp)
 
 		// Write the compressed bytes.
-		_, err = zw.Write(cr[f])
-		if err != nil {
+		if _, err := zw.Write(cr[f]); err != nil {
 			return err
 		}
 

src/pkg/net/http/response_test.go

@@ -321,9 +321,7 @@ func TestReadResponseCloseInMiddle(t *testing.T) {
 		}
 		if test.compressed {
 			buf.WriteString("Content-Encoding: gzip\r\n")
-			var err error
-			wr, err = gzip.NewWriter(wr)
-			checkErr(err, "gzip.NewWriter")
+			wr = gzip.NewWriter(wr)
 		}
 		buf.WriteString("\r\n")
 
@@ -337,7 +335,7 @@ func TestReadResponseCloseInMiddle(t *testing.T) {
 			wr.Write(chunk)
 		}
 		if test.compressed {
-			err := wr.(*gzip.Compressor).Close()
+			err := wr.(*gzip.Writer).Close()
 			checkErr(err, "compressor close")
 		}
 		if test.chunked {

src/pkg/net/http/transport_test.go

@@ -441,11 +441,7 @@ func TestRoundTripGzip(t *testing.T) {
 		}
 		if accept == "gzip" {
 			rw.Header().Set("Content-Encoding", "gzip")
-			gz, err := gzip.NewWriter(rw)
-			if err != nil {
-				t.Errorf("gzip NewWriter: %v", err)
-				return
-			}
+			gz := gzip.NewWriter(rw)
 			gz.Write([]byte(responseBody))
 			gz.Close()
 		} else {
@@ -512,7 +508,7 @@ func TestTransportGzip(t *testing.T) {
 				rw.Header().Set("Content-Length", strconv.Itoa(buf.Len()))
 			}()
 		}
-		gz, _ := gzip.NewWriter(w)
+		gz := gzip.NewWriter(w)
 		gz.Write([]byte(testString))
 		if req.FormValue("body") == "large" {
 			io.CopyN(gz, rand.Reader, nRandBytes)