compress/gzip: add Reset method to Reader

Fixes #6364.

LGTM=rsc
R=golang-codereviews, bradfitz, rsc, gobot
CC=golang-codereviews
https://golang.org/cl/13512052

src/pkg/compress/gzip/gunzip.go

@@ -89,6 +89,17 @@ func NewReader(r io.Reader) (*Reader, error) {
 	return z, nil
 }
 
+// Reset discards the Reader z's state and makes it equivalent to the
+// 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.Reset()
+	z.size = 0
+	z.err = nil
+	return z.readHeader(true)
+}
+
 // GZIP (RFC 1952) is little-endian, unlike ZLIB (RFC 1950).
 func get4(p []byte) uint32 {
 	return uint32(p[0]) | uint32(p[1])<<8 | uint32(p[2])<<16 | uint32(p[3])<<24

src/pkg/compress/gzip/gunzip_test.go

@@ -303,6 +303,26 @@ func TestDecompressor(t *testing.T) {
 		if s != tt.raw {
 			t.Errorf("%s: got %d-byte %q want %d-byte %q", tt.name, n, s, len(tt.raw), tt.raw)
 		}
+
+		// Test Reader Reset.
+		in = bytes.NewReader(tt.gzip)
+		err = gzip.Reset(in)
+		if err != nil {
+			t.Errorf("%s: Reset: %s", tt.name, err)
+			continue
+		}
+		if tt.name != gzip.Name {
+			t.Errorf("%s: got name %s", tt.name, gzip.Name)
+		}
+		b.Reset()
+		n, err = io.Copy(b, gzip)
+		if err != tt.err {
+			t.Errorf("%s: io.Copy: %v want %v", tt.name, 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)
+		}
 	}
 }