Persistent Local Storage for offchain workers. (#2894)

* WiP.

* Implement offchain storage APIs.

* Change compare_and_set to return bool.

* Add offchain http test.

* Fix tests.

* Bump spec version.

* Fix warnings and test.

* Fix compilation.

* Remove unused code.

* Introduce Local (fork-aware) and Persistent storage.

* Fix borked merge.

* Prevent warning on depreacated client.backend

* Fix long lines.

* Clean up dependencies.

* Update core/primitives/src/offchain.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

* Update core/primitives/src/offchain.rs

Co-Authored-By: André Silva <andre.beat@gmail.com>

substrate/core/client/db/src/offchain.rs

@@ -0,0 +1,137 @@
+// Copyright 2017-2019 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate 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.
+
+// Substrate 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 Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! RocksDB-based offchain workers local storage.
+
+use std::{
+	collections::HashMap,
+	sync::Arc,
+};
+
+use crate::columns;
+use kvdb::KeyValueDB;
+use parking_lot::Mutex;
+
+/// Offchain local storage
+#[derive(Clone)]
+pub struct LocalStorage {
+	db: Arc<KeyValueDB>,
+	locks: Arc<Mutex<HashMap<Vec<u8>, Arc<Mutex<()>>>>>,
+}
+
+impl std::fmt::Debug for LocalStorage {
+	fn fmt(&self, fmt: &mut std::fmt::Formatter) -> std::fmt::Result {
+		fmt.debug_struct("LocalStorage")
+			.finish()
+	}
+}
+
+impl LocalStorage {
+	/// Create new offchain storage for tests (backed by memorydb)
+	#[cfg(any(test, feature = "test-helpers"))]
+	pub fn new_test() -> Self {
+		let db = Arc::new(::kvdb_memorydb::create(crate::utils::NUM_COLUMNS));
+		Self::new(db as _)
+	}
+
+	/// Create offchain local storage with given `KeyValueDB` backend.
+	pub fn new(db: Arc<KeyValueDB>) -> Self {
+		Self {
+			db,
+			locks: Default::default(),
+		}
+	}
+}
+
+impl client::backend::OffchainStorage for LocalStorage {
+	fn set(&mut self, prefix: &[u8], key: &[u8], value: &[u8]) {
+		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
+		let mut tx = self.db.transaction();
+		tx.put(columns::OFFCHAIN, &key, value);
+
+		if let Err(e) = self.db.write(tx) {
+			log::warn!("Error writing to the offchain DB: {:?}", e);
+		}
+	}
+
+	fn get(&self, prefix: &[u8], key: &[u8]) -> Option<Vec<u8>> {
+		let key: Vec<u8> = prefix.iter().chain(key).cloned().collect();
+		self.db.get(columns::OFFCHAIN, &key)
+			.ok()
+			.and_then(|x| x)
+			.map(|v| v.to_vec())
+	}
+
+	fn compare_and_set(
+		&mut self,
+		prefix: &[u8],
+		item_key: &[u8],
+		old_value: &[u8],
+		new_value: &[u8],
+	) -> bool {
+		let key: Vec<u8> = prefix.iter().chain(item_key).cloned().collect();
+		let key_lock = {
+			let mut locks = self.locks.lock();
+			locks.entry(key.clone()).or_default().clone()
+		};
+
+		let is_set;
+		{
+			let _key_guard = key_lock.lock();
+			is_set = self.db.get(columns::OFFCHAIN, &key)
+				.ok()
+				.and_then(|x| x)
+				.map(|v| &*v == old_value)
+				.unwrap_or(true);
+
+			if is_set {
+				self.set(prefix, item_key, new_value)
+			}
+		}
+
+		// clean the lock map if we're the only entry
+		let mut locks = self.locks.lock();
+		{
+			drop(key_lock);
+			let key_lock = locks.get_mut(&key);
+			if let Some(_) = key_lock.and_then(Arc::get_mut) {
+				locks.remove(&key);
+			}
+		}
+		is_set
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use client::backend::OffchainStorage;
+
+	#[test]
+	fn should_compare_and_set_and_clear_the_locks_map() {
+		let mut storage = LocalStorage::new_test();
+		let prefix = b"prefix";
+		let key = b"key";
+		let value = b"value";
+
+		storage.set(prefix, key, value);
+		assert_eq!(storage.get(prefix, key), Some(value.to_vec()));
+
+		assert_eq!(storage.compare_and_set(prefix, key, value, b"asd"), true);
+		assert_eq!(storage.get(prefix, key), Some(b"asd".to_vec()));
+		assert!(storage.locks.lock().is_empty(), "Locks map should be empty!");
+	}
+}