Add Etag support

go/storage/crc.go

@@ -1,6 +1,8 @@
 package storage
 
 import (
+	"code.google.com/p/weed-fs/go/util"
+	"fmt"
 	"hash/crc32"
 )
 
@@ -19,3 +21,9 @@ func (c CRC) Update(b []byte) CRC {
 func (c CRC) Value() uint32 {
 	return uint32(c>>15|c<<17) + 0xa282ead8
 }
+
+func (n *Needle) Etag() string {
+	bits := make([]byte, 4)
+	util.Uint32toBytes(bits, uint32(n.Checksum))
+	return fmt.Sprintf("%x", bits)
+}

go/weed/weed_server/volume_server_handlers.go

@@ -90,6 +90,12 @@ func (vs *VolumeServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 			}
 		}
 	}
+	etag := n.Etag()
+	if inm := r.Header.Get("If-None-Match"); inm == etag {
+		w.WriteHeader(http.StatusNotModified)
+		return
+	}
+	w.Header().Set("Etag", etag)
 	if n.NameSize > 0 && filename == "" {
 		filename = string(n.Name)
 		dotIndex := strings.LastIndex(filename, ".")