babe: restore epoch changes migration code (#5487)

This was removed in https://github.com/paritytech/substrate/pull/5291
but is still too early to do it as it requires nodes still on Polkadot
v0.7.26 to resync from scratch.

substrate/client/consensus/epochs/src/lib.rs

@@ -16,6 +16,8 @@
 
 //! Generic utilities for epoch-based consensus engines.
 
+pub mod migration;
+
 use std::{sync::Arc, ops::Add, collections::BTreeMap, borrow::{Borrow, BorrowMut}};
 use parking_lot::Mutex;
 use codec::{Encode, Decode};

substrate/client/consensus/epochs/src/migration.rs

@@ -0,0 +1,55 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Migration types for epoch changes.
+
+use std::collections::BTreeMap;
+use codec::{Encode, Decode};
+use fork_tree::ForkTree;
+use sp_runtime::traits::{Block as BlockT, NumberFor};
+use crate::{Epoch, EpochChanges, PersistedEpoch, PersistedEpochHeader};
+
+/// Legacy definition of epoch changes.
+#[derive(Clone, Encode, Decode)]
+pub struct EpochChangesV0<Hash, Number, E: Epoch> {
+	inner: ForkTree<Hash, Number, PersistedEpoch<E>>,
+}
+
+/// Type alias for legacy definition of epoch changes.
+pub type EpochChangesForV0<Block, Epoch> = EpochChangesV0<<Block as BlockT>::Hash, NumberFor<Block>, Epoch>;
+
+impl<Hash, Number, E: Epoch> EpochChangesV0<Hash, Number, E> where
+	Hash: PartialEq + Ord + Copy,
+	Number: Ord + Copy,
+{
+	/// Create a new value of this type from raw.
+	pub fn from_raw(inner: ForkTree<Hash, Number, PersistedEpoch<E>>) -> Self {
+		Self { inner }
+	}
+
+	/// Migrate the type into current epoch changes definition.
+	pub fn migrate(self) -> EpochChanges<Hash, Number, E> {
+		let mut epochs = BTreeMap::new();
+
+		let inner = self.inner.map(&mut |hash, number, data| {
+			let header = PersistedEpochHeader::from(&data);
+			epochs.insert((*hash, *number), data);
+			header
+		});
+
+		EpochChanges { inner, epochs }
+	}
+}