Clears Old Storage for Session pallet (#7132)

* Fixes migration for Session pallet

* Moves migration to polkadot

* Minor change

* Fixes test

* ".git/.scripts/commands/fmt/fmt.sh"

* Allow dead_code for test

* removes test

* Minor change

* Fixes build

* Import vec for try-runtime

* Addresses review comment

* ".git/.scripts/commands/fmt/fmt.sh"

* Addresses review comment

---------

Co-authored-by: command-bot <>

runtime/common/src/lib.rs

@@ -27,9 +27,11 @@ pub mod impls;
 pub mod paras_registrar;
 pub mod paras_sudo_wrapper;
 pub mod purchase;
+pub mod session;
 pub mod slot_range;
 pub mod slots;
 pub mod traits;
+
 #[cfg(feature = "try-runtime")]
 pub mod try_runtime;
 pub mod xcm_sender;

runtime/common/src/session/migration.rs

@@ -0,0 +1,70 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+use frame_support::{
+	storage::storage_prefix,
+	traits::{Get, OnRuntimeUpgrade},
+	weights::Weight,
+};
+use pallet_session::Config;
+use sp_io::{storage::clear_prefix, KillStorageResult};
+#[cfg(feature = "try-runtime")]
+use sp_std::vec::Vec;
+
+/// This migration clears everything under `Session::HistoricalSessions`
+/// and `Session::StoredRange` that were not cleared when
+/// the pallet was moved to its new prefix (`Historical`)  
+pub struct ClearOldSessionStorage<T>(sp_std::marker::PhantomData<T>);
+impl<T: Config> OnRuntimeUpgrade for ClearOldSessionStorage<T> {
+	#[cfg(feature = "try-runtime")]
+	fn pre_upgrade() -> Result<Vec<u8>, &'static str> {
+		Ok(Vec::new())
+	}
+
+	fn on_runtime_upgrade() -> Weight {
+		let prefix = storage_prefix(b"Session", b"StoredRange");
+		let keys_removed_stored_range = match clear_prefix(&prefix, None) {
+			KillStorageResult::AllRemoved(value) => value,
+			KillStorageResult::SomeRemaining(value) => {
+				log::error!(
+					"`clear_prefix` failed to remove all keys. THIS SHOULD NEVER HAPPEN! 🚨",
+				);
+				value
+			},
+		} as u64;
+
+		let prefix = storage_prefix(b"Session", b"HistoricalSessions");
+		let keys_removed_historical_sessions = match clear_prefix(&prefix, None) {
+			KillStorageResult::AllRemoved(value) => value,
+			KillStorageResult::SomeRemaining(value) => {
+				log::error!(
+					"`clear_prefix` failed to remove all keys. THIS SHOULD NEVER HAPPEN! 🚨",
+				);
+				value
+			},
+		} as u64;
+
+		let keys_removed = keys_removed_stored_range + keys_removed_historical_sessions;
+		log::info!("Removed {} keys 🧹", keys_removed);
+
+		T::DbWeight::get().reads_writes(keys_removed, keys_removed)
+	}
+
+	#[cfg(feature = "try-runtime")]
+	fn post_upgrade(_state: Vec<u8>) -> Result<(), &'static str> {
+		Ok(())
+	}
+}

runtime/common/src/session/mod.rs

@@ -0,0 +1,17 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+pub mod migration;

runtime/kusama/src/lib.rs

@@ -1472,6 +1472,7 @@ pub type Migrations = (
 	// Unreleased - add new migrations here:
 	parachains_configuration::migration::v5::MigrateToV5<Runtime>,
 	pallet_offences::migration::v1::MigrateToV1<Runtime>,
+	runtime_common::session::migration::ClearOldSessionStorage<Runtime>,
 );
 
 /// Unchecked extrinsic type as expected by this runtime.

runtime/polkadot/src/lib.rs

@@ -1428,6 +1428,7 @@ pub type Migrations = (
 	// Unreleased - add new migrations here:
 	parachains_configuration::migration::v5::MigrateToV5<Runtime>,
 	pallet_offences::migration::v1::MigrateToV1<Runtime>,
+	runtime_common::session::migration::ClearOldSessionStorage<Runtime>,
 );
 
 /// Unchecked extrinsic type as expected by this runtime.