Adding try_state hook for Tips pallet (#1871)

Part of https://github.com/paritytech/polkadot-sdk/issues/239.

Invariant

1. The number of entries in `Tips` should be equal to `Reasons`.
2. If `OpenTip.finders_fee` is true, then `OpenTip.deposit` should be
greater than zero.
3. Reasons exists for each Tip[`OpenTip.reason`], implying equal length
of storage.

polkadot address: 12zsKEDVcHpKEWb99iFt3xrTCQQXZMu477nJQsTBBrof5k2h

---------

Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/tips/src/lib.rs

@@ -68,6 +68,7 @@ use sp_std::prelude::*;
 
 use codec::{Decode, Encode};
 use frame_support::{
+	ensure,
 	traits::{
 		ContainsLengthBound, Currency, EnsureOrigin, ExistenceRequirement::KeepAlive, Get,
 		OnUnbalanced, ReservableCurrency, SortedMembers,
@@ -76,6 +77,9 @@ use frame_support::{
 };
 use frame_system::pallet_prelude::BlockNumberFor;
 
+#[cfg(any(feature = "try-runtime", test))]
+use sp_runtime::TryRuntimeError;
+
 pub use pallet::*;
 pub use weights::WeightInfo;
 
@@ -150,7 +154,7 @@ pub mod pallet {
 		#[pallet::constant]
 		type TipFindersFee: Get<Percent>;
 
-		/// The amount held on deposit for placing a tip report.
+		/// The non-zero amount held on deposit for placing a tip report.
 		#[pallet::constant]
 		type TipReportDepositBase: Get<BalanceOf<Self, I>>;
 
@@ -465,6 +469,21 @@ pub mod pallet {
 			Ok(())
 		}
 	}
+
+	#[pallet::hooks]
+	impl<T: Config<I>, I: 'static> Hooks<BlockNumberFor<T>> for Pallet<T, I> {
+		fn integrity_test() {
+			assert!(
+				!T::TipReportDepositBase::get().is_zero(),
+				"`TipReportDepositBase` should not be zero",
+			);
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn try_state(_n: BlockNumberFor<T>) -> Result<(), TryRuntimeError> {
+			Self::do_try_state()
+		}
+	}
 }
 
 impl<T: Config<I>, I: 'static> Pallet<T, I> {
@@ -611,4 +630,42 @@ impl<T: Config<I>, I: 'static> Pallet<T, I> {
 			Tips::<T, I>::insert(hash, new_tip)
 		}
 	}
+
+	/// Ensure the correctness of the state of this pallet.
+	///
+	/// This should be valid before and after each state transition of this pallet.
+	///
+	/// ## Invariants:
+	/// 1. The number of entries in `Tips` should be equal to `Reasons`.
+	/// 2. Reasons exists for each Tip[`OpenTip.reason`].
+	/// 3. If `OpenTip.finders_fee` is true, then OpenTip.deposit should be greater than zero.
+	#[cfg(any(feature = "try-runtime", test))]
+	pub fn do_try_state() -> Result<(), TryRuntimeError> {
+		let reasons = Reasons::<T, I>::iter_keys().collect::<Vec<_>>();
+		let tips = Tips::<T, I>::iter_keys().collect::<Vec<_>>();
+
+		ensure!(
+			reasons.len() == tips.len(),
+			TryRuntimeError::Other("Equal length of entries in `Tips` and `Reasons` Storage")
+		);
+
+		for tip in Tips::<T, I>::iter_keys() {
+			let open_tip = Tips::<T, I>::get(&tip).expect("All map keys are valid; qed");
+
+			if open_tip.finders_fee {
+				ensure!(
+					!open_tip.deposit.is_zero(),
+					TryRuntimeError::Other(
+						"Tips with `finders_fee` should have non-zero `deposit`."
+					)
+				)
+			}
+
+			ensure!(
+				reasons.contains(&open_tip.reason),
+				TryRuntimeError::Other("no reason for this tip")
+			);
+		}
+		Ok(())
+	}
 }