Fix over/under flow risk in OnDilution (#2548)

* fix over/under flow risk in OnDilution

* bump impl

substrate/node/runtime/src/lib.rs

@@ -59,7 +59,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	impl_name: create_runtime_str!("substrate-node"),
 	authoring_version: 10,
 	spec_version: 78,
-	impl_version: 78,
+	impl_version: 79,
 	apis: RUNTIME_API_VERSIONS,
 };
 

substrate/srml/treasury/src/lib.rs

@@ -64,7 +64,9 @@ use serde::{Serialize, Deserialize};
 use rstd::prelude::*;
 use srml_support::{StorageValue, StorageMap, decl_module, decl_storage, decl_event, ensure};
 use srml_support::traits::{Currency, ReservableCurrency, OnDilution, OnUnbalanced, Imbalance};
-use runtime_primitives::{Permill, traits::{Zero, EnsureOrigin, StaticLookup}};
+use runtime_primitives::{Permill,
+	traits::{Zero, EnsureOrigin, StaticLookup, Saturating, CheckedSub, CheckedMul}
+};
 use parity_codec::{Encode, Decode};
 use system::ensure_signed;
 
@@ -291,8 +293,12 @@ impl<T: Trait> OnDilution<BalanceOf<T>> for Module<T> {
 		// pre dilution and post-dilution.
 		if !minted.is_zero() && !portion.is_zero() {
 			let total_issuance = T::Currency::total_issuance();
-			let funding = (total_issuance - portion) / portion * minted;
-			<Pot<T>>::mutate(|x| *x += funding);
+			if let Some(funding) = total_issuance.checked_sub(&portion) {
+				let funding = funding / portion;
+				if let Some(funding) = funding.checked_mul(&minted) {
+					<Pot<T>>::mutate(|x| *x = x.saturating_add(funding));
+				}
+			}
 		}
 	}
 }