seal: Prevent contracts from going below subsistence (#6623)

* seal: Do not allow transfers to bring total balance below subsistence deposit

This also reworks the rent system to take the total balance into account
when evaluating whether the account is above the subsistence deposit.

* Fix nits from review

* Fix typo

* Do not enforce subsistence when called from EOA

* Rename CallOrigin to TransactorKind

* Add debug asserts to check the invariants of a plain account transactor

* Fix typo

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Sergei Shulepov <sergei@parity.io>

substrate/frame/contracts/src/lib.rs

@@ -102,7 +102,7 @@ use sp_std::{prelude::*, marker::PhantomData, fmt::Debug};
 use codec::{Codec, Encode, Decode};
 use sp_runtime::{
 	traits::{
-		Hash, StaticLookup, Zero, MaybeSerializeDeserialize, Member, Convert,
+		Hash, StaticLookup, Zero, MaybeSerializeDeserialize, Member, Convert, Saturating,
 	},
 	RuntimeDebug,
 };
@@ -415,6 +415,11 @@ decl_error! {
 		OutOfGas,
 		/// The output buffer supplied to a contract API call was too small.
 		OutputBufferTooSmall,
+		/// Performing the requested transfer would have brought the contract below
+		/// the subsistence threshold. No transfer is allowed to do this in order to allow
+		/// for a tombstone to be created. Use `ext_terminate` to remove a contract without
+		/// leaving a tombstone behind.
+		InsufficientBalance,
 	}
 }
 
@@ -726,6 +731,25 @@ impl<T: Trait> Config<T> {
 			max_value_size: T::MaxValueSize::get(),
 		}
 	}
+
+	/// Subsistence threshold is the extension of the minimum balance (aka existential deposit) by the
+	/// tombstone deposit, required for leaving a tombstone.
+	///
+	/// Rent or any contract initiated balance transfer mechanism cannot make the balance lower
+	/// than the subsistence threshold in order to guarantee that a tombstone is created.
+	///
+	/// The only way to completely kill a contract without a tombstone is calling `ext_terminate`.
+	fn subsistence_threshold(&self) -> BalanceOf<T> {
+		self.existential_deposit.saturating_add(self.tombstone_deposit)
+	}
+
+	/// The same as `subsistence_threshold` but without the need for a preloaded instance.
+	///
+	/// This is for cases where this value is needed in rent calculation rather than
+	/// during contract execution.
+	fn subsistence_threshold_uncached() -> BalanceOf<T> {
+		T::Currency::minimum_balance().saturating_add(T::TombstoneDeposit::get())
+	}
 }
 
 /// Definition of the cost schedule and other parameterizations for wasm vm.