Export Compare-And-Swap (CAS) id in Item

Even though the CAS id is a transparent token there are use cases where the
client of the module could be interested in storing the token from a retrieved
Item out-of-process for later use with a CAS operation. This is impossible if
the field is unexported.

memcache/memcache.go

@@ -171,8 +171,11 @@ type Item struct {
 	// Zero means the Item has no expiration time.
 	Expiration int32
 
-	// Compare and swap ID.
-	casid uint64
+	// CasID is the compare and swap ID.
+	//
+	// It's populated by get requests and then the same value is
+	// required for a CompareAndSwap request to succeed.
+	CasID uint64
 }
 
 // conn is a connection to a server.
@@ -535,7 +538,7 @@ func parseGetResponse(r *bufio.Reader, cb func(*Item)) error {
 // It does not read the bytes of the item.
 func scanGetResponseLine(line []byte, it *Item) (size int, err error) {
 	pattern := "VALUE %s %d %d %d\r\n"
-	dest := []interface{}{&it.Key, &it.Flags, &size, &it.casid}
+	dest := []interface{}{&it.Key, &it.Flags, &size, &it.CasID}
 	if bytes.Count(line, space) == 3 {
 		pattern = "VALUE %s %d %d\r\n"
 		dest = dest[:3]
@@ -618,7 +621,7 @@ func (c *Client) populateOne(rw *bufio.ReadWriter, verb string, item *Item) erro
 	var err error
 	if verb == "cas" {
 		_, err = fmt.Fprintf(rw, "%s %s %d %d %d %d\r\n",
-			verb, item.Key, item.Flags, item.Expiration, len(item.Value), item.casid)
+			verb, item.Key, item.Flags, item.Expiration, len(item.Value), item.CasID)
 	} else {
 		_, err = fmt.Fprintf(rw, "%s %s %d %d %d\r\n",
 			verb, item.Key, item.Flags, item.Expiration, len(item.Value))