fix: validate integers before encoding. (#15)

Integers in CBOR range from -2^64 to 2^64-1 inclusive. This means that we can have negative
integers that are bigger than the usual minimum. Internally we use i128 for the integers,
but we need to make sure that if integers encoded that they fall into the range CBOR
specifies. This commit introduces such a check.

Fixes #14.

src/ser.rs

@@ -302,12 +302,23 @@ impl<'a, W: enc::Write> serde::Serializer for &'a mut Serializer<W> {
 
     #[inline]
     fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
+        if !(u64::MAX as i128 >= v && -(u64::MAX as i128) <= v) {
+            return Err(EncodeError::Msg(
+                "Integer must be within [-u64::MAX, u64::MAX] range".into(),
+            ));
+        }
+
         v.encode(&mut self.writer)?;
         Ok(())
     }
 
     #[inline]
     fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
+        if (u64::MAX as u128) < v {
+            return Err(EncodeError::Msg(
+                "Unsigned integer must be within [0, u64::MAX] range".into(),
+            ));
+        }
         v.encode(&mut self.writer)?;
         Ok(())
     }

tests/ser.rs

@@ -74,6 +74,19 @@ fn test_integer() {
     // u64
     let vec = to_vec(&::std::u64::MAX).unwrap();
     assert_eq!(vec, b"\x1b\xff\xff\xff\xff\xff\xff\xff\xff");
+    // u128 within u64 range
+    let vec = to_vec(&(u64::MAX as u128)).unwrap();
+    assert_eq!(vec, b"\x1b\xff\xff\xff\xff\xff\xff\xff\xff");
+    // u128 out of range
+    assert!(to_vec(&(u64::MAX as u128 + 1)).is_err());
+    // i128 within u64 range
+    let vec = to_vec(&(u64::MAX as i128)).unwrap();
+    assert_eq!(vec, b"\x1b\xff\xff\xff\xff\xff\xff\xff\xff");
+    // i128 within -u64 range
+    let vec = to_vec(&(-(u64::MAX as i128))).unwrap();
+    assert_eq!(vec, b"\x3B\xff\xff\xff\xff\xff\xff\xff\xfe");
+    // i128 out of -u64 range
+    assert!(to_vec(&(-(u64::MAX as i128) - 1)).is_err());
 }
 
 #[test]

tests/std_types.rs

@@ -142,10 +142,4 @@ testcase!(
     "a165416c706861821a00039447183c"
 );
 testcase!(test_i128_a, i128, -1i128, "20");
-testcase!(
-    test_i128_b,
-    i128,
-    -18446744073709551616i128,
-    "3BFFFFFFFFFFFFFFFF"
-);
 testcase!(test_u128, u128, 17, "11");