Detect nonstandard hash algo and stop folder (ref #2314)

lib/protocol/hashalgorithm.go

@@ -0,0 +1,54 @@
+// Copyright (C) 2016 The Protocol Authors.
+
+package protocol
+
+import "fmt"
+
+type HashAlgorithm int
+
+const (
+	SHA256 HashAlgorithm = iota
+)
+
+func (h HashAlgorithm) String() string {
+	switch h {
+	case SHA256:
+		return "sha256"
+	default:
+		return "unknown"
+	}
+}
+
+// FlagBits returns the bits that we should or into the folder flag field to
+// indicate the hash algorithm.
+func (h HashAlgorithm) FlagBits() uint32 {
+	switch h {
+	case SHA256:
+		return FolderHashSHA256 << FolderHashShiftBits
+	default:
+		panic("unknown hash algorithm")
+	}
+}
+
+func (h *HashAlgorithm) UnmarshalText(bs []byte) error {
+	switch string(bs) {
+	case "sha256":
+		*h = SHA256
+		return nil
+	}
+	return fmt.Errorf("Unknown hash algorithm %q", string(bs))
+}
+
+func (h *HashAlgorithm) MarshalText() ([]byte, error) {
+	return []byte(h.String()), nil
+}
+
+func HashAlgorithmFromFlagBits(flags uint32) (HashAlgorithm, error) {
+	algo := flags >> FolderHashShiftBits & FolderHashMask
+	switch algo {
+	case FolderHashSHA256:
+		return SHA256, nil
+	default:
+		return 0, fmt.Errorf("Unknown hash algorithm %d", algo)
+	}
+}