Migration for over locked accounts in phgragmen elections (#10649)

* use free balance rather than total balance

* Docs

* Migration for over-locked phrag voters

* New line

* comment

* Update frame/elections-phragmen/src/migrations/v5.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Only set lock, don't remove it

* delete commented out

* docs

* Update migration to just take a set of accounts

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

frame/elections-phragmen/src/migrations/mod.rs

@@ -21,3 +21,5 @@
 pub mod v3;
 /// Version 4.
 pub mod v4;
+/// Version 5.
+pub mod v5;

frame/elections-phragmen/src/migrations/v5.rs

@@ -0,0 +1,70 @@
+use super::super::*;
+
+/// Migrate the locks and vote stake on accounts (as specified with param `to_migrate`) that have
+/// more than their free balance locked.
+///
+/// This migration addresses a bug were a voter could lock up to their reserved balance + free
+/// balance. Since locks are only designed to operate on free balance, this put those affected in a
+/// situation where they could increase their free balance but still not be able to use their funds
+/// because they were less than the lock.
+pub fn migrate<T: Config>(to_migrate: Vec<T::AccountId>) -> Weight {
+	let mut weight = 0;
+
+	for who in to_migrate.iter() {
+		if let Ok(mut voter) = Voting::<T>::try_get(who) {
+			let free_balance = T::Currency::free_balance(&who);
+
+			weight = weight.saturating_add(T::DbWeight::get().reads(2));
+
+			if voter.stake > free_balance {
+				voter.stake = free_balance;
+				Voting::<T>::insert(&who, voter);
+
+				let pallet_id = T::PalletId::get();
+				T::Currency::set_lock(pallet_id, &who, free_balance, WithdrawReasons::all());
+
+				weight = weight.saturating_add(T::DbWeight::get().writes(2));
+			}
+		}
+	}
+
+	weight
+}
+
+/// Given the list of voters to migrate return a function that does some checks and information
+/// prior to migration. This can be linked to [`frame_support::traits::OnRuntimeUpgrade::
+/// pre_upgrade`] for further testing.
+pub fn pre_migrate_fn<T: Config>(to_migrate: Vec<T::AccountId>) -> Box<dyn Fn() -> ()> {
+	Box::new(move || {
+		for who in to_migrate.iter() {
+			if let Ok(voter) = Voting::<T>::try_get(who) {
+				let free_balance = T::Currency::free_balance(&who);
+
+				if voter.stake > free_balance {
+					// all good
+				} else {
+					log::warn!("pre-migrate elections-phragmen: voter={:?} has less stake then free balance", who);
+				}
+			} else {
+				log::warn!("pre-migrate elections-phragmen: cannot find voter={:?}", who);
+			}
+		}
+		log::info!("pre-migrate elections-phragmen complete");
+	})
+}
+
+/// Some checks for after migration. This can be linked to
+/// [`frame_support::traits::OnRuntimeUpgrade::post_upgrade`] for further testing.
+///
+/// Panics if anything goes wrong.
+pub fn post_migrate<T: crate::Config>() {
+	for (who, voter) in Voting::<T>::iter() {
+		let free_balance = T::Currency::free_balance(&who);
+
+		assert!(voter.stake <= free_balance, "migration should have made locked <= free_balance");
+		// Ideally we would also check that the locks and AccountData.misc_frozen where correctly
+		// updated, but since both of those are generic we can't do that without further bounding T.
+	}
+
+	log::info!("post-migrate elections-phragmen complete");
+}