Add `defer` (#12013)

* Add defer

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Convert to macro + review fixes

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Move into new file and add panic unwind tests

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

* Fix doc and Clippy

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/core/src/defer.rs

+// This file is part of Substrate.
+
+// Copyright (C) 2022 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Contains the [`crate::defer!`] macro for *deferring* the execution
+//! of code until the current scope is dropped.
+//! This helps with *always* executing cleanup code.
+
+/// Executes the wrapped closure on drop.
+///
+/// Should be used together with the [`crate::defer!`] macro.
+#[must_use]
+pub struct DeferGuard<F: FnOnce()>(pub Option<F>);
+
+impl<F: FnOnce()> Drop for DeferGuard<F> {
+	fn drop(&mut self) {
+		self.0.take().map(|f| f());
+	}
+}
+
+/// Executes the given code when the current scope is dropped.
+///
+/// Multiple calls to [`crate::defer!`] will execute the passed codes in reverse order.
+/// This also applies to panic stack unwinding.
+///
+/// # Example
+///
+/// ```rust
+/// use sp_core::defer;
+///
+/// let message = std::cell::RefCell::new("".to_string());
+/// {
+/// 	defer!(
+/// 		message.borrow_mut().push_str("world!");
+/// 	);
+/// 	defer!(
+/// 		message.borrow_mut().push_str("Hello ");
+/// 	);
+/// }
+/// assert_eq!(*message.borrow(), "Hello world!");
+/// ```
+#[macro_export]
+macro_rules! defer(
+	( $( $code:tt )* ) => {
+		let _guard = $crate::defer::DeferGuard(Some(|| { $( $code )* }));
+	};
+);
+
+#[cfg(test)]
+mod test {
+	#[test]
+	fn defer_guard_works() {
+		let mut called = false;
+		{
+			defer!(
+				called = true;
+			);
+		}
+		assert!(called, "DeferGuard should have executed the closure");
+	}
+
+	#[test]
+	/// `defer` executes the code in reverse order of being called.
+	fn defer_guard_order_works() {
+		let called = std::cell::RefCell::new(1);
+
+		defer!(
+			assert_eq!(*called.borrow(), 3);
+		);
+		defer!(
+			assert_eq!(*called.borrow(), 2);
+			*called.borrow_mut() = 3;
+		);
+		defer!({
+			assert_eq!(*called.borrow(), 1);
+			*called.borrow_mut() = 2;
+		});
+	}
+
+	#[test]
+	#[allow(unused_braces)]
+	#[allow(clippy::unnecessary_operation)]
+	fn defer_guard_syntax_works() {
+		let called = std::cell::RefCell::new(0);
+		{
+			defer!(*called.borrow_mut() += 1);
+			defer!(*called.borrow_mut() += 1;); // With ;
+			defer!({ *called.borrow_mut() += 1 });
+			defer!({ *called.borrow_mut() += 1 };); // With ;
+		}
+		assert_eq!(*called.borrow(), 4);
+	}
+
+	#[test]
+	/// `defer` executes the code even in case of a panic.
+	fn defer_guard_panic_unwind_works() {
+		use std::panic::{catch_unwind, AssertUnwindSafe};
+		let mut called = false;
+
+		let should_panic = catch_unwind(AssertUnwindSafe(|| {
+			defer!(called = true);
+			panic!();
+		}));
+
+		assert!(should_panic.is_err(), "DeferGuard should have panicked");
+		assert!(called, "DeferGuard should have executed the closure");
+	}
+
+	#[test]
+	/// `defer` executes the code even in case another `defer` panics.
+	fn defer_guard_defer_panics_unwind_works() {
+		use std::panic::{catch_unwind, AssertUnwindSafe};
+		let counter = std::cell::RefCell::new(0);
+
+		let should_panic = catch_unwind(AssertUnwindSafe(|| {
+			defer!(*counter.borrow_mut() += 1);
+			defer!(
+				*counter.borrow_mut() += 1;
+				panic!();
+			);
+			defer!(*counter.borrow_mut() += 1);
+		}));
+
+		assert!(should_panic.is_err(), "DeferGuard should have panicked");
+		assert_eq!(*counter.borrow(), 3, "DeferGuard should have executed the closure");
+	}
+}

substrate/primitives/core/src/lib.rs

@@ -56,6 +56,7 @@ pub use hashing::{blake2_128, blake2_256, keccak_256, twox_128, twox_256, twox_6
 pub mod crypto;
 pub mod hexdisplay;
 
+pub mod defer;
 pub mod ecdsa;
 pub mod ed25519;
 pub mod hash;