historical slashing w ocw w adhoc tree creation (#6220)

* draft

* steps

* chore: fmt

* step by step

* more details

* make test public

* refactor: split into on and offchain

* test stab

* tabs my friend

* offchain overlay: split key into prefix and true key

Simplifies inspection and makes key actually unique.

* test: share state

* fix & test

* docs improv

* address review comments

* cleanup test chore

* refactor, abbrev link text

* chore: linewidth

* fix prefix key split fallout

* minor fallout

* minor changes

* addresses review comments

* rename historical.rs -> historical/mod.rs

* avoid shared::* wildcard import

* fix: add missing call to store_session_validator_set_to_offchain

* fix/compile: missing shared:: prefix

* fix/test: flow

* fix/review: Apply suggestions from code review

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* fix/review: more review comment fixes

* fix/review: make ValidatorSet private

* fix/include: core -> sp_core

* fix/review: fallout

* fix/visbility: make them public API

Ref #6358

* fix/review: review changes fallout - again

Co-authored-by: Bernhard Schuster <bernhard@parity.io>
Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

substrate/client/db/src/lib.rs

@@ -544,7 +544,12 @@ pub struct BlockImportOperation<Block: BlockT> {
 
 impl<Block: BlockT> BlockImportOperation<Block> {
 	fn apply_offchain(&mut self, transaction: &mut Transaction<DbHash>) {
-		for (key, value_operation) in self.offchain_storage_updates.drain() {
+		for ((prefix, key), value_operation) in self.offchain_storage_updates.drain() {
+			let key: Vec<u8> = prefix
+				.into_iter()
+				.chain(sp_core::sp_std::iter::once(b'/'))
+				.chain(key.into_iter())
+				.collect();
 			match value_operation {
 				OffchainOverlayedChange::SetValue(val) => transaction.set_from_vec(columns::OFFCHAIN, &key, val),
 				OffchainOverlayedChange::Remove => transaction.remove(columns::OFFCHAIN, &key),

substrate/frame/session/Cargo.toml

@@ -14,7 +14,9 @@ targets = ["x86_64-unknown-linux-gnu"]
 [dependencies]
 serde = { version = "1.0.101", optional = true }
 codec = { package = "parity-scale-codec", version = "1.3.0", default-features = false, features = ["derive"] }
+sp-core = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/core" }
 sp-std = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/std" }
+sp-io = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/io" }
 sp-runtime = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/runtime" }
 sp-session = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/session" }
 sp-staking = { version = "2.0.0-rc3", default-features = false, path = "../../primitives/staking" }
@@ -25,8 +27,6 @@ sp-trie = { version = "2.0.0-rc3", optional = true, default-features = false, pa
 impl-trait-for-tuples = "0.1.3"
 
 [dev-dependencies]
-sp-core = { version = "2.0.0-rc3", path = "../../primitives/core" }
-sp-io ={ version = "2.0.0-rc3", path = "../../primitives/io" }
 sp-application-crypto = { version = "2.0.0-rc3", path = "../../primitives/application-crypto" }
 lazy_static = "1.4.0"
 
@@ -37,7 +37,9 @@ std = [
 	"serde",
 	"codec/std",
 	"sp-std/std",
+	"sp-io/std",
 	"frame-support/std",
+	"sp-core/std",
 	"sp-runtime/std",
 	"sp-session/std",
 	"sp-staking/std",

substrate/frame/session/src/historical/mod.rs

@@ -37,6 +37,10 @@ use sp_trie::{MemoryDB, Trie, TrieMut, Recorder, EMPTY_PREFIX};
 use sp_trie::trie_types::{TrieDBMut, TrieDB};
 use super::{SessionIndex, Module as SessionModule};
 
+mod shared;
+pub mod offchain;
+pub mod onchain;
+
 /// Trait necessary for the historical module.
 pub trait Trait: super::Trait {
 	/// Full identification of the validator.
@@ -116,6 +120,7 @@ impl<T: Trait, I> crate::SessionManager<T::ValidatorId> for NoteHistoricalRoot<T
 	where I: SessionManager<T::ValidatorId, T::FullIdentification>
 {
 	fn new_session(new_index: SessionIndex) -> Option<Vec<T::ValidatorId>> {
+
 		StoredRange::mutate(|range| {
 			range.get_or_insert_with(|| (new_index, new_index)).1 = new_index + 1;
 		});
@@ -143,10 +148,13 @@ impl<T: Trait, I> crate::SessionManager<T::ValidatorId> for NoteHistoricalRoot<T
 
 		new_validators
 	}
+
 	fn start_session(start_index: SessionIndex) {
 		<I as SessionManager<_, _>>::start_session(start_index)
 	}
+
 	fn end_session(end_index: SessionIndex) {
+		onchain::store_session_validator_set_to_offchain::<T>(end_index);
 		<I as SessionManager<_, _>>::end_session(end_index)
 	}
 }
@@ -154,7 +162,7 @@ impl<T: Trait, I> crate::SessionManager<T::ValidatorId> for NoteHistoricalRoot<T
 /// A tuple of the validator's ID and their full identification.
 pub type IdentificationTuple<T> = (<T as crate::Trait>::ValidatorId, <T as Trait>::FullIdentification);
 
-/// a trie instance for checking and generating proofs.
+/// A trie instance for checking and generating proofs.
 pub struct ProvingTrie<T: Trait> {
 	db: MemoryDB<T::Hashing>,
 	root: T::Hash,
@@ -250,7 +258,6 @@ impl<T: Trait> ProvingTrie<T> {
 			.ok()?
 			.and_then(|raw| <IdentificationTuple<T>>::decode(&mut &*raw).ok())
 	}
-
 }
 
 impl<T: Trait, D: AsRef<[u8]>> frame_support::traits::KeyOwnerProofSystem<(KeyTypeId, D)>
@@ -311,9 +318,9 @@ impl<T: Trait, D: AsRef<[u8]>> frame_support::traits::KeyOwnerProofSystem<(KeyTy
 }
 
 #[cfg(test)]
-mod tests {
+pub(crate) mod tests {
 	use super::*;
-	use sp_core::crypto::key_types::DUMMY;
+	use sp_runtime::key_types::DUMMY;
 	use sp_runtime::testing::UintAuthorityId;
 	use crate::mock::{
 		NEXT_VALIDATORS, force_new_session,
@@ -323,7 +330,7 @@ mod tests {
 
 	type Historical = Module<Test>;
 
-	fn new_test_ext() -> sp_io::TestExternalities {
+	pub(crate) fn new_test_ext() -> sp_io::TestExternalities {
 		let mut t = frame_system::GenesisConfig::default().build_storage::<Test>().unwrap();
 		crate::GenesisConfig::<Test> {
 			keys: NEXT_VALIDATORS.with(|l|

substrate/frame/session/src/historical/offchain.rs

@@ -0,0 +1,263 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Off-chain logic for creating a proof based data provided by on-chain logic.
+//!
+//! Validator-set extracting an iterator from an off-chain worker stored list containing
+//! historical validator-sets.
+//! Based on the logic of historical slashing, but the validation is done off-chain.
+//! Use [`fn store_current_session_validator_set_to_offchain()`](super::onchain) to store the
+//! required data to the offchain validator set.
+//! This is used in conjunction with [`ProvingTrie`](super::ProvingTrie) and
+//! the off-chain indexing API.
+
+use sp_runtime::{offchain::storage::StorageValueRef, KeyTypeId};
+use sp_session::MembershipProof;
+
+use super::super::{Module as SessionModule, SessionIndex};
+use super::{IdentificationTuple, ProvingTrie, Trait};
+
+use super::shared;
+use sp_std::prelude::*;
+
+
+/// A set of validators, which was used for a fixed session index.
+struct ValidatorSet<T: Trait> {
+	validator_set: Vec<IdentificationTuple<T>>,
+}
+
+impl<T: Trait> ValidatorSet<T> {
+	/// Load the set of validators for a particular session index from the off-chain storage.
+	///
+	/// If none is found or decodable given `prefix` and `session`, it will return `None`.
+	/// Empty validator sets should only ever exist for genesis blocks.
+	pub fn load_from_offchain_db(session_index: SessionIndex) -> Option<Self> {
+		let derived_key = shared::derive_key(shared::PREFIX, session_index);
+		StorageValueRef::persistent(derived_key.as_ref())
+			.get::<Vec<(T::ValidatorId, T::FullIdentification)>>()
+			.flatten()
+			.map(|validator_set| Self { validator_set })
+	}
+
+	#[inline]
+	fn len(&self) -> usize {
+		self.validator_set.len()
+	}
+}
+
+/// Implement conversion into iterator for usage
+/// with [ProvingTrie](super::ProvingTrie::generate_for).
+impl<T: Trait> sp_std::iter::IntoIterator for ValidatorSet<T> {
+	type Item = (T::ValidatorId, T::FullIdentification);
+	type IntoIter = sp_std::vec::IntoIter<Self::Item>;
+	fn into_iter(self) -> Self::IntoIter {
+		self.validator_set.into_iter()
+	}
+}
+
+/// Create a proof based on the data available in the off-chain database.
+///
+/// Based on the yielded `MembershipProof` the implementer may decide what
+/// to do, i.e. in case of a failed proof, enqueue a transaction back on
+/// chain reflecting that, with all its consequences such as i.e. slashing.
+pub fn prove_session_membership<T: Trait, D: AsRef<[u8]>>(
+	session_index: SessionIndex,
+	session_key: (KeyTypeId, D),
+) -> Option<MembershipProof> {
+	let validators = ValidatorSet::<T>::load_from_offchain_db(session_index)?;
+	let count = validators.len() as u32;
+	let trie = ProvingTrie::<T>::generate_for(validators.into_iter()).ok()?;
+
+	let (id, data) = session_key;
+	trie.prove(id, data.as_ref())
+		.map(|trie_nodes| MembershipProof {
+			session: session_index,
+			trie_nodes,
+			validator_count: count,
+		})
+}
+
+
+/// Attempt to prune anything that is older than `first_to_keep` session index.
+///
+/// Due to re-organisation it could be that the `first_to_keep` might be less
+/// than the stored one, in which case the conservative choice is made to keep records
+/// up to the one that is the lesser.
+pub fn prune_older_than<T: Trait>(first_to_keep: SessionIndex) {
+	let derived_key = shared::LAST_PRUNE.to_vec();
+	let entry = StorageValueRef::persistent(derived_key.as_ref());
+	match entry.mutate(|current: Option<Option<SessionIndex>>| -> Result<_, ()> {
+		match current {
+			Some(Some(current)) if current < first_to_keep => Ok(first_to_keep),
+			// do not move the cursor, if the new one would be behind ours
+			Some(Some(current)) => Ok(current),
+			None => Ok(first_to_keep),
+			// if the storage contains undecodable data, overwrite with current anyways
+			// which might leak some entries being never purged, but that is acceptable
+			// in this context
+			Some(None) => Ok(first_to_keep),
+		}
+	}) {
+		Ok(Ok(new_value)) => {
+			// on a re-org this is not necessarily true, with the above they might be equal
+			if new_value < first_to_keep {
+				for session_index in new_value..first_to_keep {
+					let derived_key = shared::derive_key(shared::PREFIX, session_index);
+					let _ = StorageValueRef::persistent(derived_key.as_ref()).clear();
+				}
+			}
+		}
+		Ok(Err(_)) => {} // failed to store the value calculated with the given closure
+		Err(_) => {}     // failed to calculate the value to store with the given closure
+	}
+}
+
+/// Keep the newest `n` items, and prune all items older than that.
+pub fn keep_newest<T: Trait>(n_to_keep: usize) {
+	let session_index = <SessionModule<T>>::current_index();
+	let n_to_keep = n_to_keep as SessionIndex;
+	if n_to_keep < session_index {
+		prune_older_than::<T>(session_index - n_to_keep)
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::super::{onchain, Module};
+	use super::*;
+	use crate::mock::{
+		force_new_session, set_next_validators, Session, System, Test, NEXT_VALIDATORS,
+	};
+	use codec::Encode;
+	use frame_support::traits::{KeyOwnerProofSystem, OnInitialize};
+	use sp_core::crypto::key_types::DUMMY;
+	use sp_core::offchain::{
+		testing::TestOffchainExt,
+		OffchainExt,
+		StorageKind,
+	};
+
+	use sp_runtime::testing::UintAuthorityId;
+
+	type Historical = Module<Test>;
+
+	pub fn new_test_ext() -> sp_io::TestExternalities {
+		let mut ext = frame_system::GenesisConfig::default()
+			.build_storage::<Test>()
+			.expect("Failed to create test externalities.");
+
+		crate::GenesisConfig::<Test> {
+			keys: NEXT_VALIDATORS.with(|l| {
+				l.borrow()
+					.iter()
+					.cloned()
+					.map(|i| (i, i, UintAuthorityId(i).into()))
+					.collect()
+			}),
+		}
+		.assimilate_storage(&mut ext)
+		.unwrap();
+
+
+		let mut ext = sp_io::TestExternalities::new(ext);
+
+		let (offchain, offchain_state) = TestOffchainExt::with_offchain_db(ext.offchain_db());
+
+		const ITERATIONS: u32 = 5u32;
+		let mut seed = [0u8; 32];
+		seed[0..4].copy_from_slice(&ITERATIONS.to_le_bytes());
+		offchain_state.write().seed = seed;
+
+		ext.register_extension(OffchainExt::new(offchain));
+		ext
+	}
+
+	#[test]
+	fn encode_decode_roundtrip() {
+		use codec::{Decode, Encode};
+		use super::super::super::Trait as SessionTrait;
+		use super::super::Trait as HistoricalTrait;
+
+		let sample = (
+				22u32 as <Test as SessionTrait>::ValidatorId,
+				7_777_777 as <Test as HistoricalTrait>::FullIdentification);
+
+		let encoded = sample.encode();
+		let decoded = Decode::decode(&mut encoded.as_slice()).expect("Must decode");
+		assert_eq!(sample, decoded);
+	}
+
+	#[test]
+	fn onchain_to_offchain() {
+		let mut ext = new_test_ext();
+
+		const DATA: &[u8] = &[7,8,9,10,11];
+		ext.execute_with(|| {
+			b"alphaomega"[..].using_encoded(|key| sp_io::offchain_index::set(key, DATA));
+		});
+
+		ext.persist_offchain_overlay();
+
+		ext.execute_with(|| {
+			let data =
+			b"alphaomega"[..].using_encoded(|key| {
+				sp_io::offchain::local_storage_get(StorageKind::PERSISTENT, key)
+			});
+			assert_eq!(data, Some(DATA.to_vec()));
+		});
+	}
+
+
+	#[test]
+	fn historical_proof_offchain() {
+		let mut ext = new_test_ext();
+		let encoded_key_1 = UintAuthorityId(1).encode();
+
+		ext.execute_with(|| {
+			set_next_validators(vec![1, 2]);
+			force_new_session();
+
+			System::set_block_number(1);
+			Session::on_initialize(1);
+
+			// "on-chain"
+			onchain::store_current_session_validator_set_to_offchain::<Test>();
+			assert_eq!(<SessionModule<Test>>::current_index(), 1);
+
+			set_next_validators(vec![7, 8]);
+
+			force_new_session();
+		});
+
+		ext.persist_offchain_overlay();
+
+		ext.execute_with(|| {
+
+
+			System::set_block_number(2);
+			Session::on_initialize(2);
+			assert_eq!(<SessionModule<Test>>::current_index(), 2);
+
+			// "off-chain"
+			let proof = prove_session_membership::<Test, _>(1, (DUMMY, &encoded_key_1));
+			assert!(proof.is_some());
+			let proof = proof.expect("Must be Some(Proof)");
+
+			assert!(Historical::check_proof((DUMMY, &encoded_key_1[..]), proof.clone()).is_some());
+		});
+	}
+}

substrate/frame/session/src/historical/onchain.rs

@@ -0,0 +1,62 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! On-chain logic to store a validator-set for deferred validation using an off-chain worker.
+
+use codec::Encode;
+use sp_runtime::traits::Convert;
+
+use super::super::Trait as SessionTrait;
+use super::super::{Module as SessionModule, SessionIndex};
+use super::Trait as HistoricalTrait;
+
+use super::shared;
+use sp_std::prelude::*;
+
+/// Store the validator-set associated to the `session_index` to the off-chain database.
+///
+/// Further processing is then done [`off-chain side`](super::offchain).
+///
+/// **Must** be called from on-chain, i.e. a call that originates from
+/// `on_initialize(..)` or `on_finalization(..)`.
+/// **Must** be called during the session, which validator-set is to be stored for further
+/// off-chain processing. Otherwise the `FullIdentification` might not be available.
+pub fn store_session_validator_set_to_offchain<T: HistoricalTrait + SessionTrait>(
+	session_index: SessionIndex,
+) {
+	let encoded_validator_list = <SessionModule<T>>::validators()
+		.into_iter()
+		.filter_map(|validator_id: <T as SessionTrait>::ValidatorId| {
+			let full_identification =
+				<<T as HistoricalTrait>::FullIdentificationOf>::convert(validator_id.clone());
+			full_identification.map(|full_identification| (validator_id, full_identification))
+		})
+		.collect::<Vec<_>>();
+
+	encoded_validator_list.using_encoded(|encoded_validator_list| {
+		let derived_key = shared::derive_key(shared::PREFIX, session_index);
+		sp_io::offchain_index::set(derived_key.as_slice(), encoded_validator_list);
+	});
+}
+
+/// Store the validator set associated to the _current_ session index to the off-chain database.
+///
+/// See [`fn store_session_validator_set_...(..)`](Self::store_session_validator_set_to_offchain)
+/// for further information and restrictions.
+pub fn store_current_session_validator_set_to_offchain<T: HistoricalTrait + SessionTrait>() {
+	store_session_validator_set_to_offchain::<T>(<SessionModule<T>>::current_index());
+}

substrate/frame/session/src/historical/shared.rs

@@ -0,0 +1,39 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2019-2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Shared logic between on-chain and off-chain components used for slashing using an off-chain
+//! worker.
+
+
+use super::SessionIndex;
+use sp_std::prelude::*;
+use codec::Encode;
+
+pub(super) const PREFIX: &[u8] = b"session_historical";
+pub(super) const LAST_PRUNE: &[u8] = b"session_historical_last_prune";
+
+/// Derive the key used to store the list of validators
+pub(super) fn derive_key<P: AsRef<[u8]>>(prefix: P, session_index: SessionIndex) -> Vec<u8> {
+	let prefix: &[u8] = prefix.as_ref();
+	session_index.using_encoded(|encoded_session_index| {
+		prefix.into_iter()
+			.chain(b"/".into_iter())
+			.chain(encoded_session_index.into_iter())
+			.copied()
+			.collect::<Vec<u8>>()
+	})
+}

substrate/primitives/core/src/offchain/storage.rs

@@ -101,8 +101,9 @@ pub enum OffchainOverlayedChange {
 pub enum OffchainOverlayedChanges {
 	/// Writing overlay changes to the offchain worker database is disabled by configuration.
 	Disabled,
-	/// Overlay changes can be recorded using the inner collection of this variant.
-	Enabled(HashMap<Vec<u8>, OffchainOverlayedChange>),
+	/// Overlay changes can be recorded using the inner collection of this variant,
+	/// where the identifier is the tuple of `(prefix, key)`.
+	Enabled(HashMap<(Vec<u8>, Vec<u8>), OffchainOverlayedChange>),
 }
 
 impl Default for OffchainOverlayedChanges {
@@ -140,23 +141,21 @@ impl OffchainOverlayedChanges {
 	/// Remove a key and its associated value from the offchain database.
 	pub fn remove(&mut self, prefix: &[u8], key: &[u8]) {
 		if let Self::Enabled(ref mut storage) = self {
-			let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
-			let _ = storage.insert(key, OffchainOverlayedChange::Remove);
+			let _ = storage.insert((prefix.to_vec(), key.to_vec()), OffchainOverlayedChange::Remove);
 		}
 	}
 
 	/// Set the value associated with a key under a prefix to the value provided.
 	pub fn set(&mut self, prefix: &[u8], key: &[u8], value: &[u8]) {
 		if let Self::Enabled(ref mut storage) = self {
-			let key = prefix.iter().chain(key).cloned().collect();
-			let _ = storage.insert(key, OffchainOverlayedChange::SetValue(value.to_vec()));
+			let _ = storage.insert((prefix.to_vec(), key.to_vec()), OffchainOverlayedChange::SetValue(value.to_vec()));
 		}
 	}
 
 	/// Obtain a associated value to the given key in storage with prefix.
 	pub fn get(&self, prefix: &[u8], key: &[u8]) -> Option<OffchainOverlayedChange> {
 		if let Self::Enabled(ref storage) = self {
-			let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
+			let key = (prefix.to_vec(), key.to_vec());
 			storage.get(&key).cloned()
 		} else {
 			None
@@ -168,11 +167,11 @@ use std::collections::hash_map;
 
 /// Iterate by reference over the prepared offchain worker storage changes.
 pub struct OffchainOverlayedChangesIter<'i> {
-	inner: Option<hash_map::Iter<'i, Vec<u8>, OffchainOverlayedChange>>,
+	inner: Option<hash_map::Iter<'i, (Vec<u8>, Vec<u8>), OffchainOverlayedChange>>,
 }
 
 impl<'i> Iterator for OffchainOverlayedChangesIter<'i> {
-	type Item = (&'i Vec<u8>, &'i OffchainOverlayedChange);
+	type Item = (&'i (Vec<u8>, Vec<u8>), &'i OffchainOverlayedChange);
 	fn next(&mut self) -> Option<Self::Item> {
 		if let Some(ref mut iter) = self.inner {
 			iter.next()
@@ -197,11 +196,11 @@ impl<'i> OffchainOverlayedChangesIter<'i> {
 
 /// Iterate by value over the prepared offchain worker storage changes.
 pub struct OffchainOverlayedChangesIntoIter {
-	inner: Option<hash_map::IntoIter<Vec<u8>,OffchainOverlayedChange>>,
+	inner: Option<hash_map::IntoIter<(Vec<u8>,Vec<u8>),OffchainOverlayedChange>>,
 }
 
 impl Iterator for OffchainOverlayedChangesIntoIter {
-	type Item = (Vec<u8>, OffchainOverlayedChange);
+	type Item = ((Vec<u8>, Vec<u8>), OffchainOverlayedChange);
 	fn next(&mut self) -> Option<Self::Item> {
 		if let Some(ref mut iter) = self.inner {
 			iter.next()
@@ -225,11 +224,11 @@ impl OffchainOverlayedChangesIntoIter {
 
 /// Iterate over all items while draining them from the collection.
 pub struct OffchainOverlayedChangesDrain<'d> {
-	inner: Option<hash_map::Drain<'d, Vec<u8>,OffchainOverlayedChange>>,
+	inner: Option<hash_map::Drain<'d, (Vec<u8>, Vec<u8>), OffchainOverlayedChange>>,
 }
 
 impl<'d> Iterator for OffchainOverlayedChangesDrain<'d> {
-	type Item = (Vec<u8>, OffchainOverlayedChange);
+	type Item = ((Vec<u8>, Vec<u8>), OffchainOverlayedChange);
 	fn next(&mut self) -> Option<Self::Item> {
 		if let Some(ref mut iter) = self.inner {
 			iter.next()
@@ -286,9 +285,13 @@ mod test {
 
 		ooc.set(STORAGE_PREFIX, b"ppp", b"rrr");
 		let mut iter = ooc.into_iter();
-		let mut k = STORAGE_PREFIX.to_vec();
-		k.extend_from_slice(&b"ppp"[..]);
-		assert_eq!(iter.next(), Some((k, OffchainOverlayedChange::SetValue(b"rrr".to_vec()))));
+		assert_eq!(
+			iter.next(),
+			Some(
+				((STORAGE_PREFIX.to_vec(), b"ppp".to_vec()),
+				OffchainOverlayedChange::SetValue(b"rrr".to_vec()))
+			)
+		);
 		assert_eq!(iter.next(), None);
 	}
 }

substrate/primitives/core/src/offchain/testing.rs

@@ -26,7 +26,7 @@ use std::{
 };
 use crate::offchain::{
 	self,
-	storage::InMemOffchainStorage,
+	storage::{InMemOffchainStorage, OffchainOverlayedChange, OffchainOverlayedChanges},
 	HttpError,
 	HttpRequestId as RequestId,
 	HttpRequestStatus as RequestStatus,
@@ -36,6 +36,7 @@ use crate::offchain::{
 	TransactionPool,
 	OffchainStorage,
 };
+
 use parking_lot::RwLock;
 
 /// Pending request.
@@ -61,6 +62,57 @@ pub struct PendingRequest {
 	pub response_headers: Vec<(String, String)>,
 }
 
+/// Sharable "persistent" offchain storage for test.
+#[derive(Debug, Clone, Default)]
+pub struct TestPersistentOffchainDB {
+	persistent: Arc<RwLock<InMemOffchainStorage>>,
+}
+
+impl TestPersistentOffchainDB {
+	/// Create a new and empty offchain storage db for persistent items
+	pub fn new() -> Self {
+		Self {
+			persistent: Arc::new(RwLock::new(InMemOffchainStorage::default()))
+		}
+	}
+
+	/// Apply a set of off-chain changes directly to the test backend
+	pub fn apply_offchain_changes(&mut self, changes: &mut OffchainOverlayedChanges) {
+		let mut me = self.persistent.write();
+		for ((_prefix, key), value_operation) in changes.drain() {
+			match value_operation {
+				OffchainOverlayedChange::SetValue(val) => me.set(b"", key.as_slice(), val.as_slice()),
+				OffchainOverlayedChange::Remove => me.remove(b"", key.as_slice()),
+			}
+		}
+	}
+}
+
+impl OffchainStorage for TestPersistentOffchainDB {
+	fn set(&mut self, prefix: &[u8], key: &[u8], value: &[u8]) {
+		self.persistent.write().set(prefix, key, value);
+	}
+
+	fn remove(&mut self, prefix: &[u8], key: &[u8]) {
+		self.persistent.write().remove(prefix, key);
+	}
+
+	fn get(&self, prefix: &[u8], key: &[u8]) -> Option<Vec<u8>> {
+		self.persistent.read().get(prefix, key)
+	}
+
+	fn compare_and_set(
+		&mut self,
+		prefix: &[u8],
+		key: &[u8],
+		old_value: Option<&[u8]>,
+		new_value: &[u8],
+	) -> bool {
+		self.persistent.write().compare_and_set(prefix, key, old_value, new_value)
+	}
+}
+
+
 /// Internal state of the externalities.
 ///
 /// This can be used in tests to respond or assert stuff about interactions.
@@ -70,7 +122,7 @@ pub struct OffchainState {
 	pub requests: BTreeMap<RequestId, PendingRequest>,
 	expected_requests: BTreeMap<RequestId, PendingRequest>,
 	/// Persistent local storage
-	pub persistent_storage: InMemOffchainStorage,
+	pub persistent_storage: TestPersistentOffchainDB,
 	/// Local storage
 	pub local_storage: InMemOffchainStorage,
 	/// A supposedly random seed.
@@ -145,6 +197,13 @@ impl TestOffchainExt {
 		let state = ext.0.clone();
 		(ext, state)
 	}
+
+	/// Create new `TestOffchainExt` and a reference to the internal state.
+	pub fn with_offchain_db(offchain_db: TestPersistentOffchainDB) -> (Self, Arc<RwLock<OffchainState>>) {
+		let (ext, state) = Self::new();
+		ext.0.write().persistent_storage = offchain_db;
+		(ext, state)
+	}
 }
 
 impl offchain::Externalities for TestOffchainExt {
@@ -174,17 +233,17 @@ impl offchain::Externalities for TestOffchainExt {
 	fn local_storage_set(&mut self, kind: StorageKind, key: &[u8], value: &[u8]) {
 		let mut state = self.0.write();
 		match kind {
-			StorageKind::LOCAL => &mut state.local_storage,
-			StorageKind::PERSISTENT => &mut state.persistent_storage,
-		}.set(b"", key, value);
+			StorageKind::LOCAL => state.local_storage.set(b"", key, value),
+			StorageKind::PERSISTENT => state.persistent_storage.set(b"", key, value),
+		};
 	}
 
 	fn local_storage_clear(&mut self, kind: StorageKind, key: &[u8]) {
 		let mut state = self.0.write();
 		match kind {
-			StorageKind::LOCAL => &mut state.local_storage,
-			StorageKind::PERSISTENT => &mut state.persistent_storage,
-		}.remove(b"", key);
+			StorageKind::LOCAL => state.local_storage.remove(b"", key),
+			StorageKind::PERSISTENT => state.persistent_storage.remove(b"", key),
+		};
 	}
 
 	fn local_storage_compare_and_set(
@@ -196,17 +255,17 @@ impl offchain::Externalities for TestOffchainExt {
 	) -> bool {
 		let mut state = self.0.write();
 		match kind {
-			StorageKind::LOCAL => &mut state.local_storage,
-			StorageKind::PERSISTENT => &mut state.persistent_storage,
-		}.compare_and_set(b"", key, old_value, new_value)
+			StorageKind::LOCAL => state.local_storage.compare_and_set(b"", key, old_value, new_value),
+			StorageKind::PERSISTENT => state.persistent_storage.compare_and_set(b"", key, old_value, new_value),
+		}
 	}
 
 	fn local_storage_get(&mut self, kind: StorageKind, key: &[u8]) -> Option<Vec<u8>> {
 		let state = self.0.read();
 		match kind {
-			StorageKind::LOCAL => &state.local_storage,
-			StorageKind::PERSISTENT => &state.persistent_storage,
-		}.get(b"", key)
+			StorageKind::LOCAL => state.local_storage.get(b"", key),
+			StorageKind::PERSISTENT => state.persistent_storage.get(b"", key),
+		}
 	}
 
 	fn http_request_start(&mut self, method: &str, uri: &str, meta: &[u8]) -> Result<RequestId, ()> {

substrate/primitives/state-machine/src/testing.rs

@@ -31,7 +31,10 @@ use crate::{
 	},
 };
 use sp_core::{
-	offchain::storage::OffchainOverlayedChanges,
+	offchain::{
+		testing::TestPersistentOffchainDB,
+		storage::OffchainOverlayedChanges
+	},
 	storage::{
 		well_known_keys::{CHANGES_TRIE_CONFIG, CODE, HEAP_PAGES, is_child_storage_key},
 		Storage,
@@ -47,6 +50,7 @@ where
 {
 	overlay: OverlayedChanges,
 	offchain_overlay: OffchainOverlayedChanges,
+	offchain_db: TestPersistentOffchainDB,
 	storage_transaction_cache: StorageTransactionCache<
 		<InMemoryBackend<H> as Backend<H>>::Transaction, H, N
 	>,
@@ -108,9 +112,12 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N>
 		extensions.register(sp_core::traits::TaskExecutorExt(sp_core::tasks::executor()));
 
 
+		let offchain_db = TestPersistentOffchainDB::new();
+
 		TestExternalities {
 			overlay,
 			offchain_overlay,
+			offchain_db,
 			changes_trie_config,
 			extensions,
 			changes_trie_storage: ChangesTrieInMemoryStorage::new(),
@@ -119,6 +126,16 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N>
 		}
 	}
 
+	/// Move offchain changes from overlay to the persistent store.
+	pub fn persist_offchain_overlay(&mut self) {
+		self.offchain_db.apply_offchain_changes(&mut self.offchain_overlay);
+	}
+
+	/// A shared reference type around the offchain worker storage.
+	pub fn offchain_db(&self) -> TestPersistentOffchainDB {
+		self.offchain_db.clone()
+	}
+
 	/// Insert key/value into backend
 	pub fn insert(&mut self, k: StorageKey, v: StorageValue) {
 		self.backend.insert(vec![(None, vec![(k, Some(v))])]);