bytes, string: add Reset method to Reader

Currently, there is no easy allocation-free way to turn a
[]byte or string into an io.Reader. Thus, we add a Reset method
to bytes.Reader and strings.Reader to allow the reuse of these
Readers with another []byte or string.

This is consistent with the fact that many standard library io.Readers
already support a Reset method of some type:
	bufio.Reader
	flate.Reader
	gzip.Reader
	zlib.Reader
	debug/dwarf.LineReader
	bytes.Buffer
	crypto/rc4.Cipher

Fixes #15033

Change-Id: I456fd1af77af6ef0b4ac6228b058ac1458ff3d19
Reviewed-on: https://go-review.googlesource.com/21386
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/bytes/reader.go b/src/bytes/reader.go
index 5941ebdab4..7aa30578b3 100644
--- a/src/bytes/reader.go
+++ b/src/bytes/reader.go
@@ -146,5 +146,8 @@ func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
+// Reset resets the Reader to be reading from b.
+func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
+
 // NewReader returns a new Reader reading from b.
 func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }