im-online removal cleanup: remove off-chain storage (#2290)

This is a follow-up for `im-online` pallet removal that is cleaning up
its off-chain storage. Must be merged no earlier than #2265 is enacted.
Related: #1964

---------

Co-authored-by: Bastian Köcher <git@kchr.de>

polkadot/runtime/rococo/src/lib.rs

@@ -1824,6 +1824,12 @@ sp_api::impl_runtime_apis! {
 
 	impl offchain_primitives::OffchainWorkerApi<Block> for Runtime {
 		fn offchain_worker(header: &<Block as BlockT>::Header) {
+			use sp_runtime::{traits::Header, DigestItem};
+
+			if header.digest().logs().iter().any(|di| di == &DigestItem::RuntimeEnvironmentUpdated) {
+				pallet_im_online::migration::clear_offchain_storage(Session::validators().len() as u32);
+			}
+
 			Executive::offchain_worker(header)
 		}
 	}

polkadot/runtime/westend/src/lib.rs

@@ -1794,6 +1794,12 @@ sp_api::impl_runtime_apis! {
 
 	impl offchain_primitives::OffchainWorkerApi<Block> for Runtime {
 		fn offchain_worker(header: &<Block as BlockT>::Header) {
+			use sp_runtime::{traits::Header, DigestItem};
+
+			if header.digest().logs().iter().any(|di| di == &DigestItem::RuntimeEnvironmentUpdated) {
+				pallet_im_online::migration::clear_offchain_storage(Session::validators().len() as u32);
+			}
+
 			Executive::offchain_worker(header)
 		}
 	}

prdoc/pr_2290.prdoc

@@ -0,0 +1,10 @@
+title: im-online pallet offcain storage cleanup
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      Adds a function `clear_offchain_storage` to `pallet-im-online`. This function can be used 
+      after the pallet was removed to clear its offchain storage.
+
+crates:
+  - name: pallet-im-online

substrate/frame/im-online/src/migration.rs

@@ -116,6 +116,21 @@ pub mod v1 {
 	}
 }
 
+/// Clears the pallet's offchain storage.
+///
+/// Must be put in `OffchainWorkerApi::offchain_worker` after
+/// the pallet was removed.
+pub fn clear_offchain_storage(validator_set_size: u32) {
+	(0..validator_set_size).for_each(|idx| {
+		let key = {
+			let mut key = DB_PREFIX.to_vec();
+			key.extend(idx.encode());
+			key
+		};
+		sp_runtime::offchain::storage::StorageValueRef::persistent(&key).clear();
+	});
+}
+
 #[cfg(all(feature = "try-runtime", test))]
 mod test {
 	use super::*;