Contracts: seal0::balance should return the free balance

substrate/frame/contracts/fixtures/balance.wat

@@ -0,0 +1,42 @@
+(module
+	(import "seal0" "seal_balance" (func $seal_balance (param i32 i32)))
+	(import "env" "memory" (memory 1 1))
+
+	;; [0, 8) reserved for $seal_balance output
+
+	;; [8, 16) length of the buffer for $seal_balance
+	(data (i32.const 8) "\08")
+
+	;; [16, inf) zero initialized
+
+	(func $assert (param i32)
+		(block $ok
+			(br_if $ok
+				(get_local 0)
+			)
+			(unreachable)
+		)
+	)
+
+	(func (export "deploy"))
+
+	(func (export "call")
+		(call $seal_balance (i32.const 0) (i32.const 8))
+
+		;; Balance should be encoded as a u64.
+		(call $assert
+			(i32.eq
+				(i32.load (i32.const 8))
+				(i32.const 8)
+			)
+		)
+
+		;; Assert the free balance to be zero.
+		(call $assert
+			(i64.eq
+				(i64.load (i32.const 0))
+				(i64.const 0)
+			)
+		)
+	)
+)

substrate/frame/contracts/fixtures/drain.wat

@@ -1,14 +1,20 @@
 (module
 	(import "seal0" "seal_balance" (func $seal_balance (param i32 i32)))
+	(import "seal0" "seal_minimum_balance" (func $seal_minimum_balance (param i32 i32)))
 	(import "seal0" "seal_transfer" (func $seal_transfer (param i32 i32 i32 i32) (result i32)))
 	(import "env" "memory" (memory 1 1))
 
 	;; [0, 8) reserved for $seal_balance output
 
-	;; [8, 16) length of the buffer
+	;; [8, 16) length of the buffer for $seal_balance
 	(data (i32.const 8) "\08")
 
-	;; [16, inf) zero initialized
+	;; [16, 24) reserved for $seal_minimum_balance
+
+	;; [24, 32) length of the buffer for $seal_minimum_balance
+	(data (i32.const 24) "\08")
+
+	;; [32, inf) zero initialized
 
 	(func $assert (param i32)
 		(block $ok
@@ -33,13 +39,32 @@
 			)
 		)
 
-		;; Try to self-destruct by sending full balance to the 0 address.
+		;; Get the minimum balance.
+		(call $seal_minimum_balance (i32.const 16) (i32.const 24))
+
+		;; Minimum balance should be encoded as a u64.
+		(call $assert
+			(i32.eq
+				(i32.load (i32.const 24))
+				(i32.const 8)
+			)
+		)
+
+		;; Make the transferred value exceed the balance by adding the minimum balance.
+		(i64.store (i32.const 0)
+			(i64.add
+				(i64.load (i32.const 0))
+				(i64.load (i32.const 16))
+			)
+		)
+
+		;; Try to self-destruct by sending more balance to the 0 address.
 		;; The call will fail because a contract transfer has a keep alive requirement
 		(call $assert
 			(i32.eq
 				(call $seal_transfer
-					(i32.const 16)	;; Pointer to destination address
-					(i32.const 32)	;; Length of destination address
+					(i32.const 32)	;; Pointer to destination address
+					(i32.const 48)	;; Length of destination address
 					(i32.const 0)	;; Pointer to the buffer with value to transfer
 					(i32.const 8)	;; Length of the buffer with value to transfer
 				)

substrate/frame/contracts/src/exec.rs

@@ -30,7 +30,7 @@ use frame_support::{
 	storage::{with_transaction, TransactionOutcome},
 	traits::{
 		fungible::{Inspect, Mutate},
-		tokens::Preservation,
+		tokens::{Fortitude, Preservation},
 		Contains, OriginTrait, Randomness, Time,
 	},
 	weights::Weight,
@@ -1368,7 +1368,11 @@ where
 	}
 
 	fn balance(&self) -> BalanceOf<T> {
-		T::Currency::balance(&self.top_frame().account_id)
+		T::Currency::reducible_balance(
+			&self.top_frame().account_id,
+			Preservation::Preserve,
+			Fortitude::Polite,
+		)
 	}
 
 	fn value_transferred(&self) -> BalanceOf<T> {

substrate/frame/contracts/src/tests.rs

@@ -5891,3 +5891,53 @@ fn root_cannot_instantiate() {
 		);
 	});
 }
+
+#[test]
+fn balance_api_returns_free_balance() {
+	let (wasm, _code_hash) = compile_module::<Test>("balance").unwrap();
+	ExtBuilder::default().existential_deposit(200).build().execute_with(|| {
+		let _ = <Test as Config>::Currency::set_balance(&ALICE, 1_000_000);
+
+		// Instantiate the BOB contract without any extra balance.
+		let addr = Contracts::bare_instantiate(
+			ALICE,
+			0,
+			GAS_LIMIT,
+			None,
+			Code::Upload(wasm.to_vec()),
+			vec![],
+			vec![],
+			DebugInfo::Skip,
+			CollectEvents::Skip,
+		)
+		.result
+		.unwrap()
+		.account_id;
+
+		let value = 0;
+		// Call BOB which makes it call the balance runtime API.
+		// The contract code asserts that the returned balance is 0.
+		assert_ok!(Contracts::call(
+			RuntimeOrigin::signed(ALICE),
+			addr.clone(),
+			value,
+			GAS_LIMIT,
+			None,
+			vec![]
+		));
+
+		let value = 1;
+		// Calling with value will trap the contract.
+		assert_err_ignore_postinfo!(
+			Contracts::call(
+				RuntimeOrigin::signed(ALICE),
+				addr.clone(),
+				value,
+				GAS_LIMIT,
+				None,
+				vec![]
+			),
+			<Error<Test>>::ContractTrapped
+		);
+	});
+}