bytes: add Buffer.Peek

Fixes #73794

Change-Id: I0a57db05aacfa805213fe8278fc727e76eb8a65e
GitHub-Last-Rev: 3494d93f80
GitHub-Pull-Request: golang/go#73795
Reviewed-on: https://go-review.googlesource.com/c/go/+/674415
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/bytes/buffer.go

@@ -77,6 +77,18 @@ func (b *Buffer) String() string {
 	return string(b.buf[b.off:])
 }
 
+// Peek returns the next n bytes without advancing the buffer.
+// If Peek returns fewer than n bytes, it also returns [io.EOF].
+// The slice is only valid until the next call to a read or write method.
+// The slice aliases the buffer content at least until the next buffer modification,
+// so immediate changes to the slice will affect the result of future reads.
+func (b *Buffer) Peek(n int) ([]byte, error) {
+	if b.Len() < n {
+		return b.buf[b.off:], io.EOF
+	}
+	return b.buf[b.off:n], nil
+}
+
 // empty reports whether the unread portion of the buffer is empty.
 func (b *Buffer) empty() bool { return len(b.buf) <= b.off }