compress/gzip: fix Reader.Reset

Rather than specifying every field that should be cleared in Reset,
it is better to just zero the entire struct and only preserve or set the
fields that we actually care about. This ensures that the Header field
is reset for the next use.

Fixes #15077

Change-Id: I41832e506d2d64c62b700aa1986e7de24a577511
Reviewed-on: https://go-review.googlesource.com/21465
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2 files changed, 43 insertions, 45 deletions

diff --git a/src/compress/gzip/gunzip.go b/src/compress/gzip/gunzip.go
index 1bd8769867..5d072878ee 100644
--- a/src/compress/gzip/gunzip.go
+++ b/src/compress/gzip/gunzip.go
@@ -26,13 +26,6 @@ const (
 	flagComment = 1 << 4
 )
 
-func makeReader(r io.Reader) flate.Reader {
-	if rr, ok := r.(flate.Reader); ok {
-		return rr
-	}
-	return bufio.NewReader(r)
-}
-
 var (
 	// ErrChecksum is returned when reading GZIP data that has an invalid checksum.
 	ErrChecksum = errors.New("gzip: invalid checksum")
@@ -87,9 +80,7 @@ type Reader struct {
 // The Reader.Header fields will be valid in the Reader returned.
 func NewReader(r io.Reader) (*Reader, error) {
 	z := new(Reader)
-	z.r = makeReader(r)
-	z.multistream = true
-	if err := z.readHeader(true); err != nil {
+	if err := z.Reset(r); err != nil {
 		return nil, err
 	}
 	return z, nil
@@ -99,11 +90,15 @@ func NewReader(r io.Reader) (*Reader, error) {
 // result of its original state from NewReader, but reading from r instead.
 // This permits reusing a Reader rather than allocating a new one.
 func (z *Reader) Reset(r io.Reader) error {
-	z.r = makeReader(r)
-	z.digest = 0
-	z.size = 0
-	z.err = nil
-	z.multistream = true
+	*z = Reader{
+		decompressor: z.decompressor,
+		multistream:  true,
+	}
+	if rr, ok := r.(flate.Reader); ok {
+		z.r = rr
+	} else {
+		z.r = bufio.NewReader(r)
+	}
 	return z.readHeader(true)
 }
 
diff --git a/src/compress/gzip/gunzip_test.go b/src/compress/gzip/gunzip_test.go
index 593644ac1b..e1f79e3db3 100644
--- a/src/compress/gzip/gunzip_test.go
+++ b/src/compress/gzip/gunzip_test.go
@@ -36,6 +36,17 @@ var gunzipTests = []gunzipTest{
 		},
 		nil,
 	},
+	{
+		"",
+		"empty - with no file name",
+		"",
+		[]byte{
+			0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88,
+			0x00, 0xff, 0x01, 0x00, 0x00, 0xff, 0xff, 0x00,
+			0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+		},
+		nil,
+	},
 	{ // has 1 non-empty fixed huffman block
 		"hello.txt",
 		"hello.txt",
@@ -284,46 +295,52 @@ var gunzipTests = []gunzipTest{
 }
 
 func TestDecompressor(t *testing.T) {
+	// Keep resetting this reader.
+	// It is intended behavior that Reader.Reset can be called on a zero-value
+	// Reader and be the equivalent as if NewReader was used instead.
+	r1 := new(Reader)
+
 	b := new(bytes.Buffer)
 	for _, tt := range gunzipTests {
+		// Test NewReader.
 		in := bytes.NewReader(tt.gzip)
-		gzip, err := NewReader(in)
+		r2, err := NewReader(in)
 		if err != nil {
-			t.Errorf("%s: NewReader: %s", tt.name, err)
+			t.Errorf("%s: NewReader: %s", tt.desc, err)
 			continue
 		}
-		defer gzip.Close()
-		if tt.name != gzip.Name {
-			t.Errorf("%s: got name %s", tt.name, gzip.Name)
+		defer r2.Close()
+		if tt.name != r2.Name {
+			t.Errorf("%s: got name %s", tt.desc, r2.Name)
 		}
 		b.Reset()
-		n, err := io.Copy(b, gzip)
+		n, err := io.Copy(b, r2)
 		if err != tt.err {
-			t.Errorf("%s: io.Copy: %v want %v", tt.name, err, tt.err)
+			t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
 		}
 		s := b.String()
 		if s != tt.raw {
-			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.name, n, s, len(tt.raw), tt.raw)
+			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
 		}
 
-		// Test Reader Reset.
+		// Test Reader.Reset.
 		in = bytes.NewReader(tt.gzip)
-		err = gzip.Reset(in)
+		err = r1.Reset(in)
 		if err != nil {
-			t.Errorf("%s: Reset: %s", tt.name, err)
+			t.Errorf("%s: Reset: %s", tt.desc, err)
 			continue
 		}
-		if tt.name != gzip.Name {
-			t.Errorf("%s: got name %s", tt.name, gzip.Name)
+		if tt.name != r1.Name {
+			t.Errorf("%s: got name %s", tt.desc, r1.Name)
 		}
 		b.Reset()
-		n, err = io.Copy(b, gzip)
+		n, err = io.Copy(b, r1)
 		if err != tt.err {
-			t.Errorf("%s: io.Copy: %v want %v", tt.name, err, tt.err)
+			t.Errorf("%s: io.Copy: %v want %v", tt.desc, err, tt.err)
 		}
 		s = b.String()
 		if s != tt.raw {
-			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.name, n, s, len(tt.raw), tt.raw)
+			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.desc, n, s, len(tt.raw), tt.raw)
 		}
 	}
 }
@@ -356,20 +373,6 @@ func TestIssue6550(t *testing.T) {
 	}
 }
 
-func TestInitialReset(t *testing.T) {
-	var r Reader
-	if err := r.Reset(bytes.NewReader(gunzipTests[1].gzip)); err != nil {
-		t.Error(err)
-	}
-	var buf bytes.Buffer
-	if _, err := io.Copy(&buf, &r); err != nil {
-		t.Error(err)
-	}
-	if s := buf.String(); s != gunzipTests[1].raw {
-		t.Errorf("got %q want %q", s, gunzipTests[1].raw)
-	}
-}
-
 func TestMultistreamFalse(t *testing.T) {
 	// Find concatenation test.
 	var tt gunzipTest