spec: Validate required divisor fields are not 0 (#7933)

* Add validate_non_zero function

It's used to validate that a Spec's uint field used as a divisor is not zero.

* Add deserialize_with to gas_limit_bound_divisor

Prevents panics due to divide-by-zero on the gas_limit_bound_divisor
field.

* Add deserialize_with to difficulty_bound_divisor

Prevents panics due to divide-by-zero on the difficulty_bound_divisor
field.

* Add validate_optional_non_zero function

Used to validate Option<Uint> divisor fields.

* Use deserialize_with on optional divisor fields.

* Add #[serde(default)] attribute to divisor fields

When using `#[serde(deserialize_with)]`, `#[serde(default)]` must be specified so that missing
fields can be deserialized with the deserializer for `None`.

json/src/spec/params.rs

@@ -16,7 +16,7 @@
 
 //! Spec params deserialization.
 
-use uint::Uint;
+use uint::{self, Uint};
 use hash::{H256, Address};
 use bytes::Bytes;
 
@@ -102,6 +102,7 @@ pub struct Params {
 	pub wasm: Option<bool>,
 	/// See `CommonParams` docs.
 	#[serde(rename="gasLimitBoundDivisor")]
+	#[serde(deserialize_with="uint::validate_non_zero")]
 	pub gas_limit_bound_divisor: Uint,
 	/// See `CommonParams` docs.
 	pub registrar: Option<Address>,
@@ -149,4 +150,21 @@ mod tests {
 		assert_eq!(deserialized.gas_limit_bound_divisor, Uint(U256::from(0x20)));
 		assert_eq!(deserialized.max_code_size, Some(Uint(U256::from(0x1000))));
 	}
+
+	#[test]
+	#[should_panic(expected = "a non-zero value")]
+	fn test_zero_value_divisor() {
+		let s = r#"{
+			"maximumExtraDataSize": "0x20",
+			"networkID" : "0x1",
+			"chainID" : "0x15",
+			"subprotocolName" : "exp",
+			"minGasLimit": "0x1388",
+			"accountStartNonce": "0x01",
+			"gasLimitBoundDivisor": "0x0",
+			"maxCodeSize": "0x1000"
+		}"#;
+
+		let _deserialized: Params = serde_json::from_str(s).unwrap();
+	}
 }