Asynchronous-backing compatible Aura, not plugged in (#2573)

* rough draft of potential parent search

* get things compiling

* fmt

* add new function to all RelayChainInterface implementations

* fix compilation

* set slot and timestamp based on relay parent, prepare for find-parent

* skeleton of new aura logic

* fmt

* introduce a collator module in the Aura crate

* extract different implementations into own modules

* make interface more convenient

* docs and todos for lookahead

* refactor basic collator to use new collator utility

* some more refactoring

* finish most of the control flow for new aura

* introduce backend as parameter

* fix compilation

* fix a couple more TODOs

* add an `announce_block` function to collator service

* announce with barrier

* rename block announcement validator to be more specific

* fmt

* clean up unused import errors

* update references to BlockAnnounceValidator

* rename unstable_reimpl

* add AuraUnincludedSegmentApi

* finish rename

* integrate AuraUnincludedSegmentApi

* add a new block announcement validator for backwards compatibility

* add some naive equivocation defenses

* rustfmt

* clean up remaining TODO [now]s

* fmt

* try to fix inprocess-interface

* actually fix compilation

* ignored -> rejected rephrase

* fix test compilation

* fmt

* clippy

cumulus/primitives/aura/src/lib.rs

@@ -0,0 +1,50 @@
+// Copyright 2023 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate 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.
+
+// Substrate 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/>.
+
+//! Core primitives for Aura in Cumulus.
+//!
+//! In particular, this exposes the [`AuraUnincludedSegmentApi`] which is used to regulate
+//! the behavior of Aura within a parachain context.
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+pub use sp_consensus_aura::Slot;
+
+sp_api::decl_runtime_apis! {
+	/// This runtime API is used to inform potential block authors whether they will
+	/// have the right to author at a slot, assuming they have claimed the slot.
+	///
+	/// In particular, this API allows Aura-based parachains to regulate their "unincluded segment",
+	/// which is the section of the head of the chain which has not yet been made available in the
+	/// relay chain.
+	///
+	/// When the unincluded segment is short, Aura chains will allow authors to create multiple
+	/// blocks per slot in order to build a backlog. When it is saturated, this API will limit
+	/// the amount of blocks that can be created.
+	pub trait AuraUnincludedSegmentApi {
+		/// Whether it is legal to extend the chain, assuming the given block is the most
+		/// recently included one as-of the relay parent that will be built against, and
+		/// the given slot.
+		///
+		/// This should be consistent with the logic the runtime uses when validating blocks to
+		/// avoid issues.
+		///
+		/// When the unincluded segment is empty, i.e. `included_hash == at`, where at is the block
+		/// whose state we are querying against, this must always return `true` as long as the slot
+		/// is more recent than the included block itself.
+		fn can_build_upon(included_hash: Block::Hash, slot: Slot) -> bool;
+	}
+}