Adds support for returning a custom header from validate_block (#825)

* Adds support for returning a custom header from `validate_block`

This adds support for returning a custom header from `validate_block`. Before this, we always
returned the header of the block that was validated (and still do it by default). However, after
this pr it is possible to set a custom header or better custom head data that will be returned
instead from `validate_block`. This can be for example when a chain wants to fork.

* FMT

test/runtime/src/test_pallet.rs

@@ -0,0 +1,49 @@
+// Copyright 2021 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Cumulus is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Cumulus is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+/// A special pallet that exposes dispatchables that are only useful for testing.
+pub use pallet::*;
+
+#[frame_support::pallet]
+pub mod pallet {
+	use frame_support::pallet_prelude::*;
+	use frame_system::pallet_prelude::*;
+
+	#[pallet::pallet]
+	#[pallet::generate_store(pub(super) trait Store)]
+	pub struct Pallet<T>(_);
+
+	#[pallet::config]
+	pub trait Config: frame_system::Config + cumulus_pallet_parachain_system::Config {}
+
+	#[pallet::hooks]
+	impl<T: Config> Hooks<BlockNumberFor<T>> for Pallet<T> {}
+
+	#[pallet::call]
+	impl<T: Config> Pallet<T> {
+		/// A test dispatchable for setting a custom head data in `validate_block`.
+		#[pallet::weight(0)]
+		pub fn set_custom_validation_head_data(
+			_: OriginFor<T>,
+			custom_header: sp_std::vec::Vec<u8>,
+		) -> DispatchResult {
+			cumulus_pallet_parachain_system::Pallet::<T>::set_custom_validation_head_data(
+				custom_header,
+			);
+			Ok(())
+		}
+	}
+}