frame-system: Add last_runtime_upgrade_spec_version (#2351)

Adds a function for querying the last runtime upgrade spec version. This
can be useful for when writing runtime level migrations to ensure that
they are not executed multiple times. An example would be a session key
migration.

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/frame/system/src/tests.rs

@@ -19,7 +19,7 @@ use crate::*;
 use frame_support::{
 	assert_noop, assert_ok,
 	dispatch::{Pays, PostDispatchInfo, WithPostDispatchInfo},
-	traits::WhitelistedStorageKeys,
+	traits::{OnRuntimeUpgrade, WhitelistedStorageKeys},
 };
 use std::collections::BTreeSet;
 
@@ -773,3 +773,26 @@ pub fn from_actual_ref_time(ref_time: Option<u64>) -> PostDispatchInfo {
 pub fn from_post_weight_info(ref_time: Option<u64>, pays_fee: Pays) -> PostDispatchInfo {
 	PostDispatchInfo { actual_weight: ref_time.map(|t| Weight::from_all(t)), pays_fee }
 }
+
+#[docify::export]
+#[test]
+fn last_runtime_upgrade_spec_version_usage() {
+	struct Migration;
+
+	impl OnRuntimeUpgrade for Migration {
+		fn on_runtime_upgrade() -> Weight {
+			// Ensure to compare the spec version against some static version to prevent applying
+			// the same migration multiple times.
+			//
+			// `1337` here is the spec version of the runtime running on chain. If there is maybe
+			// a runtime upgrade in the pipeline of being applied, you should use the spec version
+			// of this upgrade.
+			if System::last_runtime_upgrade_spec_version() > 1337 {
+				return Weight::zero();
+			}
+
+			// Do the migration.
+			Weight::zero()
+		}
+	}
+}