tape: Safely cast when dynamically encoding/decoding

tape/limits.go

@@ -11,3 +11,16 @@ package tape
 // You shouldn't need to change this. If you do, it should only be set once at
 // the start of the program.
 var MaxStructureLength = 1024 * 1024
+
+// MaxInt is the maximum value an int can hold. This varies depending on the
+// system.
+const MaxInt int = int(^uint(0) >> 1)
+
+// Uint64ToIntSafe casts the input to an int if it can be done without overflow,
+// or returns an error otherwise.
+func Uint64ToIntSafe(input uint64) (int, error) {
+	if input > uint64(MaxInt) {
+		return 0, ErrTooLarge
+	}
+	return int(input), nil
+}