Handle sparse files (fixes #245)

lib/protocol/message.go

@@ -5,7 +5,13 @@
 
 package protocol
 
-import "fmt"
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+)
+
+var sha256OfEmptyBlock = sha256.Sum256(make([]byte, BlockSize))
 
 type IndexMessage struct {
 	Folder  string     // max:256
@@ -98,6 +104,11 @@ func (b BlockInfo) String() string {
 	return fmt.Sprintf("Block{%d/%d/%x}", b.Offset, b.Size, b.Hash)
 }
 
+// IsEmpty returns true if the block is a full block of zeroes.
+func (b BlockInfo) IsEmpty() bool {
+	return b.Size == BlockSize && bytes.Equal(b.Hash, sha256OfEmptyBlock[:])
+}
+
 type RequestMessage struct {
 	Folder  string // max:256
 	Name    string // max:8192