gzip: Fix WriterTo after initial read (#804)

Fix checksum calculation in `WriteTo` after something has been read from the stream.

Fixes #803

gzip/gunzip.go

@@ -288,10 +288,35 @@ func (z *Reader) Read(p []byte) (n int, err error) {
 	return n, nil
 }
 
-// Support the io.WriteTo interface for io.Copy and friends.
+type crcer interface {
+	io.Writer
+	Sum32() uint32
+	Reset()
+}
+type crcUpdater struct {
+	z *Reader
+}
+
+func (c *crcUpdater) Write(p []byte) (int, error) {
+	c.z.digest = crc32.Update(c.z.digest, crc32.IEEETable, p)
+	return len(p), nil
+}
+
+func (c *crcUpdater) Sum32() uint32 {
+	return c.z.digest
+}
+
+func (c *crcUpdater) Reset() {
+	c.z.digest = 0
+}
+
+// WriteTo support the io.WriteTo interface for io.Copy and friends.
 func (z *Reader) WriteTo(w io.Writer) (int64, error) {
 	total := int64(0)
-	crcWriter := crc32.NewIEEE()
+	crcWriter := crcer(crc32.NewIEEE())
+	if z.digest != 0 {
+		crcWriter = &crcUpdater{z: z}
+	}
 	for {
 		if z.err != nil {
 			if z.err == io.EOF {

gzip/gunzip_test.go

@@ -5,6 +5,7 @@
 package gzip
 
 import (
+	"bufio"
 	"bytes"
 	oldgz "compress/gzip"
 	"crypto/rand"
@@ -537,6 +538,7 @@ func TestWriteTo(t *testing.T) {
 		t.Fatal(err)
 	}
 	wtbuf := &bytes.Buffer{}
+
 	written, err := dec.WriteTo(wtbuf)
 	if err != nil {
 		t.Fatal(err)
@@ -708,3 +710,43 @@ func TestTruncatedGunzip(t *testing.T) {
 		}
 	}
 }
+
+func TestBufferedPartialCopyGzip(t *testing.T) {
+	var (
+		in           = []byte("hello\nworld")
+		compressedIn []byte
+	)
+
+	var buf bytes.Buffer
+	gzw := NewWriter(&buf)
+	if _, err := gzw.Write(in); err != nil {
+		panic(err)
+	}
+	if err := gzw.Flush(); err != nil {
+		panic(err)
+	}
+	if err := gzw.Close(); err != nil {
+		panic(err)
+	}
+
+	compressedIn = buf.Bytes()
+
+	gz, err := NewReader(bytes.NewReader(compressedIn))
+	if err != nil {
+		t.Errorf("constructing a reader: %v", err)
+	}
+
+	br := bufio.NewReader(gz)
+	if _, err := br.ReadBytes('\n'); err != nil {
+		t.Errorf("reading to the first newline: %v", err)
+	}
+
+	var out bytes.Buffer
+	_, err = io.Copy(&out, br)
+	if !bytes.Equal(out.Bytes(), []byte("world")) {
+		t.Errorf("unexpected output when reading the remainder")
+	}
+	if err != nil {
+		t.Errorf("reading the remainder: %v", err)
+	}
+}