hash: allow runtime configuration of supported hashes for testing

fs/hash/hash.go

@@ -55,6 +55,16 @@ func RegisterHash(name, alias string, width int, newFunc func() hash.Hash) Type
 	return hashType
 }
 
+// SupportOnly makes the hash package only support the types passed
+// in. Used for testing.
+//
+// It returns the previously supported types.
+func SupportOnly(new []Type) (old []Type) {
+	old = supported
+	supported = new
+	return old
+}
+
 // ErrUnsupported should be returned by filesystem,
 // if it is requested to deliver an unsupported hash type.
 var ErrUnsupported = errors.New("hash type not supported")