Runtime worker threads (#7089)

* std variant

* principal work

* format and naming

* format and naming continued

* working nested fork

* add comment

* naming and tabs

* line width

* fix wording

* address review

* refactor dynamic dispatch

* update wasmtime

* some care

* move ext

* more refactor

* doc effort

* simplify

* doc effort

* tests and docs

* address review

* naming

* explain some args

* add example

* unwinding for native and tests

* rename stray

* fix refs

* fix tests

* fix warnings

* stray naming

* fixes and comments

* Update primitives/io/src/tasks.rs

Co-authored-by: cheme <emericchevalier.pro@gmail.com>

* make examples "compile"

* dyn_dispatch -> spawn_call

* fix impl

* address review

* Update primitives/io/src/lib.rs

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

* Update primitives/io/src/tasks.rs

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

* Update primitives/io/src/async_externalities.rs

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

* Update primitives/io/src/tasks.rs

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

* Update frame/example-parallel/src/lib.rs

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

* fix compilation

* Update client/executor/common/src/wasm_runtime.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* address review

* Update client/executor/wasmtime/src/instance_wrapper.rs

Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Update client/executor/src/native_executor.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update primitives/io/src/tasks.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update client/executor/src/native_executor.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update primitives/io/src/tasks.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update client/executor/wasmtime/src/instance_wrapper.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* address some issues

* address more issues

* wasm_only interface

* define sp_tasks

* avoid anyhow

* fix example

Co-authored-by: cheme <emericchevalier.pro@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Sergei Shulepov <sergei@parity.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/Cargo.lock

@@ -4594,6 +4594,20 @@ dependencies = [
  "sp-std",
 ]
 
+[[package]]
+name = "pallet-example-parallel"
+version = "2.0.0"
+dependencies = [
+ "frame-support",
+ "frame-system",
+ "parity-scale-codec",
+ "sp-core",
+ "sp-io",
+ "sp-runtime",
+ "sp-std",
+ "sp-tasks",
+]
+
 [[package]]
 name = "pallet-grandpa"
 version = "2.0.0"
@@ -6790,6 +6804,7 @@ dependencies = [
  "sp-runtime-interface",
  "sp-serializer",
  "sp-state-machine",
+ "sp-tasks",
  "sp-tracing",
  "sp-trie",
  "sp-version",
@@ -7229,6 +7244,7 @@ dependencies = [
  "sp-runtime",
  "sp-sandbox",
  "sp-std",
+ "sp-tasks",
  "substrate-wasm-builder-runner",
 ]
 
@@ -8494,6 +8510,19 @@ dependencies = [
  "sp-std",
 ]
 
+[[package]]
+name = "sp-tasks"
+version = "2.0.0"
+dependencies = [
+ "log",
+ "parity-scale-codec",
+ "sp-core",
+ "sp-externalities",
+ "sp-io",
+ "sp-runtime-interface",
+ "sp-std",
+]
+
 [[package]]
 name = "sp-test-primitives"
 version = "2.0.0"

substrate/Cargo.toml

@@ -77,6 +77,7 @@ members = [
 	"frame/evm",
 	"frame/example",
 	"frame/example-offchain-worker",
+	"frame/example-parallel",
 	"frame/executive",
 	"frame/grandpa",
 	"frame/identity",
@@ -162,6 +163,7 @@ members = [
 	"primitives/std",
 	"primitives/version",
 	"primitives/state-machine",
+	"primitives/tasks",
 	"primitives/timestamp",
 	"primitives/test-primitives",
 	"primitives/transaction-pool",

substrate/client/executor/Cargo.toml

@@ -18,6 +18,7 @@ derive_more = "0.99.2"
 codec = { package = "parity-scale-codec", version = "1.3.4" }
 sp-io = { version = "2.0.0", path = "../../primitives/io" }
 sp-core = { version = "2.0.0", path = "../../primitives/core" }
+sp-tasks = { version = "2.0.0", path = "../../primitives/tasks" }
 sp-trie = { version = "2.0.0", path = "../../primitives/trie" }
 sp-serializer = { version = "2.0.0", path = "../../primitives/serializer" }
 sp-version = { version = "2.0.0", path = "../../primitives/version" }

substrate/client/executor/common/src/error.rs

@@ -81,6 +81,25 @@ pub enum Error {
 	/// Execution of a host function failed.
 	#[display(fmt="Host function {} execution failed with: {}", _0, _1)]
 	FunctionExecution(String, String),
+	/// No table is present.
+	///
+	/// Call was requested that requires table but none was present in the instance.
+	#[display(fmt="No table exported by wasm blob")]
+	NoTable,
+	/// No table entry is present.
+	///
+	/// Call was requested that requires specific entry in the table to be present.
+	#[display(fmt="No table entry with index {} in wasm blob exported table", _0)]
+	#[from(ignore)]
+	NoTableEntryWithIndex(u32),
+	/// Table entry is not a function.
+	#[display(fmt="Table element with index {} is not a function in wasm blob exported table", _0)]
+	#[from(ignore)]
+	TableElementIsNotAFunction(u32),
+	/// Function in table is null and thus cannot be called.
+	#[display(fmt="Table entry with index {} in wasm blob is null", _0)]
+	#[from(ignore)]
+	FunctionRefIsNull(u32),
 }
 
 impl std::error::Error for Error {

substrate/client/executor/common/src/wasm_runtime.rs

@@ -19,6 +19,46 @@
 use crate::error::Error;
 use sp_wasm_interface::Value;
 
+/// A method to be used to find the entrypoint when calling into the runtime
+///
+/// Contains variants on how to resolve wasm function that will be invoked.
+pub enum InvokeMethod<'a> {
+	/// Call function exported with this name.
+	///
+	/// Located function should have (u32, u32) -> u64 signature.
+	Export(&'a str),
+	/// Call a function found in the exported table found under the given index.
+	///
+	/// Located function should have (u32, u32) -> u64 signature.
+	Table(u32),
+	/// Call function by reference from table through a wrapper.
+	///
+	/// Invoked function (`dispatcher_ref`) function
+	/// should have (u32, u32, u32) -> u64 signature.
+	///
+	/// `func` will be passed to the invoked function as a first argument.
+	TableWithWrapper {
+		/// Wrapper for the call.
+		///
+		/// Function pointer, index into runtime exported table.
+		dispatcher_ref: u32,
+		/// Extra argument for dispatch.
+		///
+		/// Common usage would be to use it as an actual wasm function pointer
+		/// that should be invoked, but can be used as any extra argument on the
+		/// callee side.
+		///
+		/// This is typically generated and invoked by the runtime itself.
+		func: u32,
+	},
+}
+
+impl<'a> From<&'a str> for InvokeMethod<'a> {
+	fn from(val: &'a str) -> InvokeMethod<'a> {
+		InvokeMethod::Export(val)
+	}
+}
+
 /// A trait that defines an abstract WASM runtime module.
 ///
 /// This can be implemented by an execution engine.
@@ -31,11 +71,24 @@ pub trait WasmModule: Sync + Send {
 ///
 /// This can be implemented by an execution engine.
 pub trait WasmInstance: Send {
-	/// Call a method on this WASM instance and reset it afterwards.
+	/// Call a method on this WASM instance.
+	///
+	/// Before execution, instance is reset.
+	///
 	/// Returns the encoded result on success.
-	fn call(&self, method: &str, data: &[u8]) -> Result<Vec<u8>, Error>;
+	fn call(&self, method: InvokeMethod, data: &[u8]) -> Result<Vec<u8>, Error>;
+
+	/// Call an exported method on this WASM instance.
+	///
+	/// Before execution, instance is reset.
+	///
+	/// Returns the encoded result on success.
+	fn call_export(&self, method: &str, data: &[u8]) -> Result<Vec<u8>, Error> {
+		self.call(method.into(), data)
+	}
 
 	/// Get the value from a global with the given `name`.
+	///
 	/// This method is only suitable for getting immutable globals.
 	fn get_global_const(&self, name: &str) -> Result<Option<Value>, Error>;
 }

substrate/client/executor/runtime-test/Cargo.toml

@@ -13,12 +13,13 @@ repository = "https://github.com/paritytech/substrate/"
 targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
-sp-std = { version = "2.0.0", default-features = false, path = "../../../primitives/std" }
-sp-io = { version = "2.0.0", default-features = false, path = "../../../primitives/io" }
-sp-sandbox = { version = "0.8.0", default-features = false, path = "../../../primitives/sandbox" }
-sp-core = { version = "2.0.0", default-features = false, path = "../../../primitives/core" }
-sp-runtime = { version = "2.0.0", default-features = false, path = "../../../primitives/runtime" }
 sp-allocator = { version = "2.0.0", default-features = false, path = "../../../primitives/allocator" }
+sp-core = { version = "2.0.0", default-features = false, path = "../../../primitives/core" }
+sp-io = { version = "2.0.0", default-features = false, path = "../../../primitives/io" }
+sp-runtime = { version = "2.0.0", default-features = false, path = "../../../primitives/runtime" }
+sp-sandbox = { version = "0.8.0", default-features = false, path = "../../../primitives/sandbox" }
+sp-std = { version = "2.0.0", default-features = false, path = "../../../primitives/std" }
+sp-tasks = { version = "2.0.0", default-features = false, path = "../../../primitives/tasks" }
 
 [build-dependencies]
 wasm-builder-runner = { version = "2.0.0", package = "substrate-wasm-builder-runner", path = "../../../utils/wasm-builder-runner" }
@@ -26,10 +27,11 @@ wasm-builder-runner = { version = "2.0.0", package = "substrate-wasm-builder-run
 [features]
 default = [ "std" ]
 std = [
+	"sp-allocator/std",
+	"sp-core/std",
 	"sp-io/std",
+	"sp-runtime/std",
 	"sp-sandbox/std",
 	"sp-std/std",
-	"sp-core/std",
-	"sp-runtime/std",
-	"sp-allocator/std",
+ 	"sp-tasks/std",
 ]

substrate/client/executor/runtime-test/src/lib.rs

@@ -309,6 +309,43 @@ sp_core::wasm_export_functions! {
 		assert_ne!(test_message, message_slice);
 		message_slice.copy_from_slice(test_message);
 	}
+
+	fn test_spawn() {
+		let data = vec![1u8, 2u8];
+		let data_new = sp_tasks::spawn(tasks::incrementer, data).join();
+
+		assert_eq!(data_new, vec![2u8, 3u8]);
+	}
+
+	fn test_nested_spawn() {
+		let data = vec![7u8, 13u8];
+		let data_new = sp_tasks::spawn(tasks::parallel_incrementer, data).join();
+
+		assert_eq!(data_new, vec![10u8, 16u8]);
+	}
+
+	fn test_panic_in_spawned() {
+		sp_tasks::spawn(tasks::panicker, vec![]).join();
+	}
+ }
+
+ #[cfg(not(feature = "std"))]
+ mod tasks {
+	use sp_std::prelude::*;
+
+	pub fn incrementer(data: Vec<u8>) -> Vec<u8> {
+	   data.into_iter().map(|v| v + 1).collect()
+	}
+
+	pub fn panicker(_: Vec<u8>) -> Vec<u8> {
+		panic!()
+	}
+
+	pub fn parallel_incrementer(data: Vec<u8>) -> Vec<u8> {
+	   let first = data.into_iter().map(|v| v + 2).collect::<Vec<_>>();
+	   let second = sp_tasks::spawn(incrementer, first).join();
+	   second
+	}
  }
 
 #[cfg(not(feature = "std"))]

substrate/client/executor/src/integration_tests/mod.rs

@@ -555,13 +555,13 @@ fn returns_mutable_static(wasm_method: WasmExecutionMethod) {
 	).expect("Creates runtime");
 
 	let instance = runtime.new_instance().unwrap();
-	let res = instance.call("returns_mutable_static", &[0]).unwrap();
+	let res = instance.call_export("returns_mutable_static", &[0]).unwrap();
 	assert_eq!(33, u64::decode(&mut &res[..]).unwrap());
 
 	// We expect that every invocation will need to return the initial
 	// value plus one. If the value increases more than that then it is
 	// a sign that the wasm runtime preserves the memory content.
-	let res = instance.call("returns_mutable_static", &[0]).unwrap();
+	let res = instance.call_export("returns_mutable_static", &[0]).unwrap();
 	assert_eq!(33, u64::decode(&mut &res[..]).unwrap());
 }
 
@@ -590,11 +590,11 @@ fn restoration_of_globals(wasm_method: WasmExecutionMethod) {
 	let instance = runtime.new_instance().unwrap();
 
 	// On the first invocation we allocate approx. 768KB (75%) of stack and then trap.
-	let res = instance.call("allocates_huge_stack_array", &true.encode());
+	let res = instance.call_export("allocates_huge_stack_array", &true.encode());
 	assert!(res.is_err());
 
 	// On the second invocation we allocate yet another 768KB (75%) of stack
-	let res = instance.call("allocates_huge_stack_array", &false.encode());
+	let res = instance.call_export("allocates_huge_stack_array", &false.encode());
 	assert!(res.is_ok());
 }
 
@@ -616,10 +616,10 @@ fn heap_is_reset_between_calls(wasm_method: WasmExecutionMethod) {
 		.expect("`__heap_base` is an `i32`");
 
 	let params = (heap_base as u32, 512u32 * 64 * 1024).encode();
-	instance.call("check_and_set_in_heap", &params).unwrap();
+	instance.call_export("check_and_set_in_heap", &params).unwrap();
 
 	// Cal it a second time to check that the heap was freed.
-	instance.call("check_and_set_in_heap", &params).unwrap();
+	instance.call_export("check_and_set_in_heap", &params).unwrap();
 }
 
 #[test_case(WasmExecutionMethod::Interpreted)]
@@ -720,3 +720,51 @@ fn wasm_tracing_should_work(wasm_method: WasmExecutionMethod) {
 	assert_eq!(span_datum.name, "");
 	assert_eq!(values.bool_values.get("wasm").unwrap(), &true);
 }
+
+#[test_case(WasmExecutionMethod::Interpreted)]
+#[cfg_attr(feature = "wasmtime", test_case(WasmExecutionMethod::Compiled))]
+fn spawning_runtime_instance_should_work(wasm_method: WasmExecutionMethod) {
+
+	let mut ext = TestExternalities::default();
+	let mut ext = ext.ext();
+
+	call_in_wasm(
+		"test_spawn",
+		&[],
+		wasm_method,
+		&mut ext,
+	).unwrap();
+}
+
+#[test_case(WasmExecutionMethod::Interpreted)]
+#[cfg_attr(feature = "wasmtime", test_case(WasmExecutionMethod::Compiled))]
+fn spawning_runtime_instance_nested_should_work(wasm_method: WasmExecutionMethod) {
+
+	let mut ext = TestExternalities::default();
+	let mut ext = ext.ext();
+
+	call_in_wasm(
+		"test_nested_spawn",
+		&[],
+		wasm_method,
+		&mut ext,
+	).unwrap();
+}
+
+#[test_case(WasmExecutionMethod::Interpreted)]
+#[cfg_attr(feature = "wasmtime", test_case(WasmExecutionMethod::Compiled))]
+fn panic_in_spawned_instance_panics_on_joining_its_result(wasm_method: WasmExecutionMethod) {
+
+	let mut ext = TestExternalities::default();
+	let mut ext = ext.ext();
+
+	let error_result = call_in_wasm(
+		"test_panic_in_spawned",
+		&[],
+		wasm_method,
+		&mut ext,
+	).unwrap_err();
+
+	dbg!(&error_result);
+	assert!(format!("{}", error_result).contains("Spawned task"));
+}

substrate/client/executor/src/native_executor.rs

@@ -20,15 +20,28 @@ use crate::{
 	RuntimeInfo, error::{Error, Result},
 	wasm_runtime::{RuntimeCache, WasmExecutionMethod},
 };
+
+use std::{
+	collections::HashMap,
+	panic::{UnwindSafe, AssertUnwindSafe},
+	result,
+	sync::{Arc, atomic::{AtomicU64, Ordering}, mpsc},
+};
+
 use sp_version::{NativeVersion, RuntimeVersion};
 use codec::{Decode, Encode};
 use sp_core::{
-	NativeOrEncoded, traits::{CodeExecutor, Externalities, RuntimeCode, MissingHostFunctions},
+	NativeOrEncoded,
+	traits::{
+		CodeExecutor, Externalities, RuntimeCode, MissingHostFunctions,
+		RuntimeSpawnExt, RuntimeSpawn,
+	},
 };
 use log::trace;
-use std::{result, panic::{UnwindSafe, AssertUnwindSafe}, sync::Arc};
 use sp_wasm_interface::{HostFunctions, Function};
-use sc_executor_common::wasm_runtime::WasmInstance;
+use sc_executor_common::wasm_runtime::{WasmInstance, WasmModule, InvokeMethod};
+use sp_externalities::ExternalitiesExt as _;
+use sp_tasks::new_async_externalities;
 
 /// Default num of pages for the heap
 const DEFAULT_HEAP_PAGES: u64 = 1024;
@@ -136,6 +149,7 @@ impl WasmExecutor {
 		f: F,
 	) -> Result<R>
 		where F: FnOnce(
+			AssertUnwindSafe<&Arc<dyn WasmModule>>,
 			AssertUnwindSafe<&dyn WasmInstance>,
 			Option<&RuntimeVersion>,
 			AssertUnwindSafe<&mut dyn Externalities>,
@@ -148,10 +162,11 @@ impl WasmExecutor {
 			self.default_heap_pages,
 			&*self.host_functions,
 			allow_missing_host_functions,
-			|instance, version, ext| {
+			|module, instance, version, ext| {
+				let module = AssertUnwindSafe(module);
 				let instance = AssertUnwindSafe(instance);
 				let ext = AssertUnwindSafe(ext);
-				f(instance, version, ext)
+				f(module, instance, version, ext)
 			}
 		)? {
 			Ok(r) => r,
@@ -179,10 +194,13 @@ impl sp_core::traits::CallInWasm for WasmExecutor {
 				heap_pages: None,
 			};
 
-			self.with_instance(&code, ext, allow_missing_host_functions, |instance, _, mut ext| {
+			self.with_instance(&code, ext, allow_missing_host_functions, |module, instance, _, mut ext| {
 				with_externalities_safe(
 					&mut **ext,
-					move || instance.call(method, call_data),
+					move || {
+						RuntimeInstanceSpawn::register_on_externalities(module.clone());
+						instance.call_export(method, call_data)
+					}
 				)
 			}).map_err(|e| e.to_string())
 		} else {
@@ -200,10 +218,14 @@ impl sp_core::traits::CallInWasm for WasmExecutor {
 
 			let instance = AssertUnwindSafe(instance);
 			let mut ext = AssertUnwindSafe(ext);
+			let module = AssertUnwindSafe(module);
 
 			with_externalities_safe(
 				&mut **ext,
-				move || instance.call(method, call_data),
+				move || {
+					RuntimeInstanceSpawn::register_on_externalities(module.clone());
+					instance.call_export(method, call_data)
+				}
 			)
 			.and_then(|r| r)
 			.map_err(|e| e.to_string())
@@ -269,12 +291,149 @@ impl<D: NativeExecutionDispatch> RuntimeInfo for NativeExecutor<D> {
 			runtime_code,
 			ext,
 			false,
-			|_instance, version, _ext|
+			|_module, _instance, version, _ext|
 				Ok(version.cloned().ok_or_else(|| Error::ApiError("Unknown version".into()))),
 		)
 	}
 }
 
+/// Helper inner struct to implement `RuntimeSpawn` extension.
+pub struct RuntimeInstanceSpawn {
+	module: Arc<dyn WasmModule>,
+	tasks: parking_lot::Mutex<HashMap<u64, mpsc::Receiver<Vec<u8>>>>,
+	counter: AtomicU64,
+	scheduler: Box<dyn sp_core::traits::SpawnNamed>,
+}
+
+impl RuntimeSpawn for RuntimeInstanceSpawn {
+	fn spawn_call(&self, dispatcher_ref: u32, func: u32, data: Vec<u8>) -> u64 {
+		let new_handle = self.counter.fetch_add(1, Ordering::Relaxed);
+
+		let (sender, receiver) = mpsc::channel();
+		self.tasks.lock().insert(new_handle, receiver);
+
+		let module = self.module.clone();
+		let scheduler = self.scheduler.clone();
+		self.scheduler.spawn("executor-extra-runtime-instance", Box::pin(async move {
+			let module = AssertUnwindSafe(module);
+
+			let async_ext = match new_async_externalities(scheduler.clone()) {
+				Ok(val) => val,
+				Err(e) => {
+					log::error!(
+						target: "executor",
+						"Failed to setup externalities for async context: {}",
+						e,
+					);
+
+					// This will drop sender and receiver end will panic
+					return;
+				}
+			};
+
+			let mut async_ext = match async_ext.with_runtime_spawn(
+				Box::new(RuntimeInstanceSpawn::new(module.clone(), scheduler))
+			) {
+				Ok(val) => val,
+				Err(e) => {
+					log::error!(
+						target: "executor",
+						"Failed to setup runtime extension for async externalities: {}",
+						e,
+					);
+
+					// This will drop sender and receiver end will panic
+					return;
+				}
+			};
+
+			let result = with_externalities_safe(
+				&mut async_ext,
+				move || {
+
+					// FIXME: Should be refactored to shared "instance factory".
+					// Instantiating wasm here every time is suboptimal at the moment, shared
+					// pool of instances should be used.
+					//
+					// https://github.com/paritytech/substrate/issues/7354
+					let instance = module.new_instance()
+						.expect("Failed to create new instance from module");
+
+					instance.call(
+						InvokeMethod::TableWithWrapper { dispatcher_ref, func },
+						&data[..],
+					).expect("Failed to invoke instance.")
+				}
+			);
+
+			match result {
+				Ok(output) => {
+					let _ = sender.send(output);
+				},
+				Err(error) => {
+					// If execution is panicked, the `join` in the original runtime code will panic as well,
+					// since the sender is dropped without sending anything.
+					log::error!("Call error in spawned task: {:?}", error);
+				},
+			}
+		}));
+
+
+		new_handle
+	}
+
+	fn join(&self, handle: u64) -> Vec<u8> {
+		let receiver = self.tasks.lock().remove(&handle).expect("No task for the handle");
+		let output = receiver.recv().expect("Spawned task panicked for the handle");
+		output
+	}
+}
+
+impl RuntimeInstanceSpawn {
+	pub fn new(
+		module: Arc<dyn WasmModule>,
+		scheduler: Box<dyn sp_core::traits::SpawnNamed>,
+	) -> Self {
+		Self {
+			module,
+			scheduler,
+			counter: 0.into(),
+			tasks: HashMap::new().into(),
+		}
+	}
+
+	fn with_externalities_and_module(
+		module: Arc<dyn WasmModule>,
+		mut ext: &mut dyn Externalities,
+	) -> Option<Self> {
+		ext.extension::<sp_core::traits::TaskExecutorExt>()
+			.map(move |task_ext| Self::new(module, task_ext.clone()))
+	}
+
+	/// Register new `RuntimeSpawnExt` on current externalities.
+	///
+	/// This extensions will spawn instances from provided `module`.
+	pub fn register_on_externalities(module: Arc<dyn WasmModule>) {
+		sp_externalities::with_externalities(
+			move |mut ext| {
+				if let Some(runtime_spawn) =
+					Self::with_externalities_and_module(module.clone(), ext)
+				{
+					if let Err(e) = ext.register_extension(
+						RuntimeSpawnExt(Box::new(runtime_spawn))
+					) {
+						trace!(
+							target: "executor",
+							"Failed to register `RuntimeSpawnExt` instance on externalities: {:?}",
+							e,
+						)
+					}
+				}
+			}
+		);
+	}
+}
+
 impl<D: NativeExecutionDispatch + 'static> CodeExecutor for NativeExecutor<D> {
 	type Error = Error;
 
@@ -295,32 +454,34 @@ impl<D: NativeExecutionDispatch + 'static> CodeExecutor for NativeExecutor<D> {
 			runtime_code,
 			ext,
 			false,
-			|instance, onchain_version, mut ext| {
+			|module, instance, onchain_version, mut ext| {
 				let onchain_version = onchain_version.ok_or_else(
 					|| Error::ApiError("Unknown version".into())
 				)?;
+
+				let can_call_with = onchain_version.can_call_with(&self.native_version.runtime_version);
+
 				match (
 					use_native,
-					onchain_version.can_call_with(&self.native_version.runtime_version),
+					can_call_with,
 					native_call,
 				) {
-					(_, false, _) => {
-						trace!(
-							target: "executor",
-							"Request for native execution failed (native: {}, chain: {})",
-							self.native_version.runtime_version,
-							onchain_version,
-						);
+					(_, false, _) | (false, _, _) => {
+						if !can_call_with {
+							trace!(
+								target: "executor",
+								"Request for native execution failed (native: {}, chain: {})",
+								self.native_version.runtime_version,
+								onchain_version,
+							);
+						}
 
 						with_externalities_safe(
 							&mut **ext,
-							move || instance.call(method, data).map(NativeOrEncoded::Encoded)
-						)
-					}
-					(false, _, _) => {
-						with_externalities_safe(
-							&mut **ext,
-							move || instance.call(method, data).map(NativeOrEncoded::Encoded)
+							move || {
+								RuntimeInstanceSpawn::register_on_externalities(module.clone());
+								instance.call_export(method, data).map(NativeOrEncoded::Encoded)
+							}
 						)
 					},
 					(true, true, Some(call)) => {

substrate/client/executor/src/wasm_runtime.rs

@@ -53,7 +53,7 @@ struct VersionedRuntime {
 	/// Wasm runtime type.
 	wasm_method: WasmExecutionMethod,
 	/// Shared runtime that can spawn instances.
-	module: Box<dyn WasmModule>,
+	module: Arc<dyn WasmModule>,
 	/// The number of WebAssembly heap pages this instance was created with.
 	heap_pages: u64,
 	/// Runtime version according to `Core_version` if any.
@@ -70,6 +70,7 @@ impl VersionedRuntime {
 		f: F,
 	) -> Result<R, Error>
 		where F: FnOnce(
+			&Arc<dyn WasmModule>,
 			&dyn WasmInstance,
 			Option<&RuntimeVersion>,
 			&mut dyn Externalities)
@@ -87,7 +88,7 @@ impl VersionedRuntime {
 					.map(|r| Ok((r, false)))
 					.unwrap_or_else(|| self.module.new_instance().map(|i| (i, true)))?;
 
-				let result = f(&*instance, self.version.as_ref(), ext);
+				let result = f(&self.module, &*instance, self.version.as_ref(), ext);
 				if let Err(e) = &result {
 					if new_inst {
 						log::warn!(
@@ -123,7 +124,7 @@ impl VersionedRuntime {
 				// Allocate a new instance
 				let instance = self.module.new_instance()?;
 
-				f(&*instance, self.version.as_ref(), ext)
+				f(&self.module, &*instance, self.version.as_ref(), ext)
 			}
 		}
 	}
@@ -199,6 +200,7 @@ impl RuntimeCache {
 		f: F,
 	) -> Result<Result<R, Error>, Error>
 		where F: FnOnce(
+			&Arc<dyn WasmModule>,
 			&dyn WasmInstance,
 			Option<&RuntimeVersion>,
 			&mut dyn Externalities)
@@ -267,7 +269,7 @@ pub fn create_wasm_runtime_with_code(
 	code: &[u8],
 	host_functions: Vec<&'static dyn Function>,
 	allow_missing_func_imports: bool,
-) -> Result<Box<dyn WasmModule>, WasmError> {
+) -> Result<Arc<dyn WasmModule>, WasmError> {
 	match wasm_method {
 		WasmExecutionMethod::Interpreted =>
 			sc_executor_wasmi::create_runtime(
@@ -275,7 +277,7 @@ pub fn create_wasm_runtime_with_code(
 				heap_pages,
 				host_functions,
 				allow_missing_func_imports
-			).map(|runtime| -> Box<dyn WasmModule> { Box::new(runtime) }),
+			).map(|runtime| -> Arc<dyn WasmModule> { Arc::new(runtime) }),
 		#[cfg(feature = "wasmtime")]
 		WasmExecutionMethod::Compiled =>
 			sc_executor_wasmtime::create_runtime(
@@ -283,7 +285,7 @@ pub fn create_wasm_runtime_with_code(
 				heap_pages,
 				host_functions,
 				allow_missing_func_imports
-			).map(|runtime| -> Box<dyn WasmModule> { Box::new(runtime) }),
+			).map(|runtime| -> Arc<dyn WasmModule> { Arc::new(runtime) }),
 	}
 }
 
@@ -318,7 +320,7 @@ fn create_versioned_wasm_runtime(
 ) -> Result<VersionedRuntime, WasmError> {
 	#[cfg(not(target_os = "unknown"))]
 	let time = std::time::Instant::now();
-	let mut runtime = create_wasm_runtime_with_code(
+	let runtime = create_wasm_runtime_with_code(
 		wasm_method,
 		heap_pages,
 		&code,
@@ -333,10 +335,10 @@ fn create_versioned_wasm_runtime(
 
 		// The following unwind safety assertion is OK because if the method call panics, the
 		// runtime will be dropped.
-		let runtime = AssertUnwindSafe(runtime.as_mut());
+		let runtime = AssertUnwindSafe(runtime.as_ref());
 		crate::native_executor::with_externalities_safe(
 			&mut **ext,
-			move || runtime.new_instance()?.call("Core_version", &[])
+			move || runtime.new_instance()?.call("Core_version".into(), &[])
 		).map_err(|_| WasmError::Instantiation("panic in call to get runtime version".into()))?
 	};
 	let version = match version_result {

substrate/client/executor/wasmi/src/lib.rs

@@ -19,7 +19,7 @@
 use std::{str, cell::RefCell, sync::Arc};
 use wasmi::{
 	Module, ModuleInstance, MemoryInstance, MemoryRef, TableRef, ImportsBuilder, ModuleRef,
-	memory_units::Pages,
+	FuncInstance, memory_units::Pages,
 	RuntimeValue::{I32, I64, self},
 };
 use codec::{Encode, Decode};
@@ -29,7 +29,7 @@ use sp_wasm_interface::{
 	FunctionContext, Pointer, WordSize, Sandbox, MemoryId, Result as WResult, Function,
 };
 use sp_runtime_interface::unpack_ptr_and_len;
-use sc_executor_common::wasm_runtime::{WasmModule, WasmInstance};
+use sc_executor_common::wasm_runtime::{WasmModule, WasmInstance, InvokeMethod};
 use sc_executor_common::{
 	error::{Error, WasmError},
 	sandbox,
@@ -434,7 +434,7 @@ fn get_heap_base(module: &ModuleRef) -> Result<u32, Error> {
 fn call_in_wasm_module(
 	module_instance: &ModuleRef,
 	memory: &MemoryRef,
-	method: &str,
+	method: InvokeMethod,
 	data: &[u8],
 	host_functions: &[&'static dyn Function],
 	allow_missing_func_imports: bool,
@@ -446,24 +446,49 @@ fn call_in_wasm_module(
 		.and_then(|e| e.as_table().cloned());
 	let heap_base = get_heap_base(module_instance)?;
 
-	let mut fec = FunctionExecutor::new(
+	let mut function_executor = FunctionExecutor::new(
 		memory.clone(),
 		heap_base,
-		table,
+		table.clone(),
 		host_functions,
 		allow_missing_func_imports,
 		missing_functions,
 	)?;
 
 	// Write the call data
-	let offset = fec.allocate_memory(data.len() as u32)?;
-	fec.write_memory(offset, data)?;
+	let offset = function_executor.allocate_memory(data.len() as u32)?;
+	function_executor.write_memory(offset, data)?;
 
-	let result = module_instance.invoke_export(
-		method,
-		&[I32(u32::from(offset) as i32), I32(data.len() as i32)],
-		&mut fec,
-	);
+	let result = match method {
+		InvokeMethod::Export(method) => {
+			module_instance.invoke_export(
+				method,
+				&[I32(u32::from(offset) as i32), I32(data.len() as i32)],
+				&mut function_executor,
+			)
+		},
+		InvokeMethod::Table(func_ref) => {
+			let func = table.ok_or(Error::NoTable)?
+				.get(func_ref)?
+				.ok_or(Error::NoTableEntryWithIndex(func_ref))?;
+			FuncInstance::invoke(
+				&func,
+				&[I32(u32::from(offset) as i32), I32(data.len() as i32)],
+				&mut function_executor,
+			).map_err(Into::into)
+		},
+		InvokeMethod::TableWithWrapper { dispatcher_ref, func } => {
+			let dispatcher = table.ok_or(Error::NoTable)?
+				.get(dispatcher_ref)?
+				.ok_or(Error::NoTableEntryWithIndex(dispatcher_ref))?;
+
+			FuncInstance::invoke(
+				&dispatcher,
+				&[I32(func as _), I32(u32::from(offset) as i32), I32(data.len() as i32)],
+				&mut function_executor,
+			).map_err(Into::into)
+		},
+	};
 
 	match result {
 		Ok(Some(I64(r))) => {
@@ -474,7 +499,7 @@ fn call_in_wasm_module(
 			trace!(
 				target: "wasm-executor",
 				"Failed to execute code with {} pages",
-				memory.current_size().0
+				memory.current_size().0,
 			);
 			Err(e.into())
 		},
@@ -677,7 +702,7 @@ pub struct WasmiInstance {
 unsafe impl Send for WasmiInstance {}
 
 impl WasmInstance for WasmiInstance {
-	fn call(&self, method: &str, data: &[u8]) -> Result<Vec<u8>, Error> {
+	fn call(&self, method: InvokeMethod, data: &[u8]) -> Result<Vec<u8>, Error> {
 		// We reuse a single wasm instance for multiple calls and a previous call (if any)
 		// altered the state. Therefore, we need to restore the instance to original state.
 

substrate/client/executor/wasmtime/src/instance_wrapper.rs

@@ -26,6 +26,7 @@ use std::{slice, marker};
 use sc_executor_common::{
 	error::{Error, Result},
 	util::{WasmModuleInfo, DataSegmentsSnapshot},
+	wasm_runtime::InvokeMethod,
 };
 use sp_wasm_interface::{Pointer, WordSize, Value};
 use wasmtime::{Engine, Instance, Module, Memory, Table, Val, Func, Extern, Global, Store};
@@ -72,6 +73,82 @@ impl ModuleWrapper {
 	}
 }
 
+/// Invoked entrypoint format.
+pub enum EntryPointType {
+	/// Direct call.
+	///
+	/// Call is made by providing only payload reference and length.
+	Direct,
+	/// Indirect call.
+	///
+	/// Call is made by providing payload reference and length, and extra argument
+	/// for advanced routing (typically extra WASM function pointer).
+	Wrapped(u32),
+}
+
+/// Wasm blob entry point.
+pub struct EntryPoint {
+	call_type: EntryPointType,
+	func: wasmtime::Func,
+}
+
+impl EntryPoint {
+	/// Call this entry point.
+	pub fn call(&self, data_ptr: Pointer<u8>, data_len: WordSize) -> Result<u64> {
+		let data_ptr = u32::from(data_ptr) as i32;
+		let data_len = u32::from(data_len) as i32;
+
+		(match self.call_type {
+			EntryPointType::Direct => {
+				self.func.call(&[
+					wasmtime::Val::I32(data_ptr),
+					wasmtime::Val::I32(data_len),
+				])
+			},
+			EntryPointType::Wrapped(func) => {
+				self.func.call(&[
+					wasmtime::Val::I32(func as _),
+					wasmtime::Val::I32(data_ptr),
+					wasmtime::Val::I32(data_len),
+				])
+			},
+		})
+			.map(|results| 
+				// the signature is checked to have i64 return type
+				results[0].unwrap_i64() as u64
+			)
+			.map_err(|err| Error::from(format!(
+				"Wasm execution trapped: {}",
+				err
+			)))
+	}
+
+	pub fn direct(func: wasmtime::Func) -> std::result::Result<Self, &'static str> {
+		match (func.ty().params(), func.ty().results()) {
+			(&[wasmtime::ValType::I32, wasmtime::ValType::I32], &[wasmtime::ValType::I64]) => {
+				Ok(Self { func, call_type: EntryPointType::Direct })
+			}
+			_ => {
+				Err("Invalid signature for direct entry point")
+			}
+		}
+	}
+
+	pub fn wrapped(dispatcher: wasmtime::Func, func: u32) -> std::result::Result<Self, &'static str> {
+		match (dispatcher.ty().params(), dispatcher.ty().results()) {
+			(
+				&[wasmtime::ValType::I32, wasmtime::ValType::I32, wasmtime::ValType::I32],
+				&[wasmtime::ValType::I64],
+			) => {
+				Ok(Self { func: dispatcher, call_type: EntryPointType::Wrapped(func) })
+			},
+			_ => {
+				Err("Invalid signature for wrapped entry point")
+			}
+		}
+	}
+}
+
 /// Wrap the given WebAssembly Instance of a wasm module with Substrate-runtime.
 ///
 /// This struct is a handy wrapper around a wasmtime `Instance` that provides substrate specific
@@ -150,24 +227,62 @@ impl InstanceWrapper {
 	///
 	/// An entrypoint must have a signature `(i32, i32) -> i64`, otherwise this function will return
 	/// an error.
-	pub fn resolve_entrypoint(&self, name: &str) -> Result<wasmtime::Func> {
-		// Resolve the requested method and verify that it has a proper signature.
-		let export = self
-			.instance
-			.get_export(name)
-			.ok_or_else(|| Error::from(format!("Exported method {} is not found", name)))?;
-		let entrypoint = extern_func(&export)
-			.ok_or_else(|| Error::from(format!("Export {} is not a function", name)))?;
-		match (entrypoint.ty().params(), entrypoint.ty().results()) {
-			(&[wasmtime::ValType::I32, wasmtime::ValType::I32], &[wasmtime::ValType::I64]) => {}
-			_ => {
-				return Err(Error::from(format!(
-					"method {} have an unsupported signature",
-					name
-				)))
-			}
-		}
-		Ok(entrypoint.clone())
+	pub fn resolve_entrypoint(&self, method: InvokeMethod) -> Result<EntryPoint> {
+		Ok(match method {
+			InvokeMethod::Export(method) => {
+				// Resolve the requested method and verify that it has a proper signature.
+				let export = self
+					.instance
+					.get_export(method)
+					.ok_or_else(|| Error::from(format!("Exported method {} is not found", method)))?;
+				let func = extern_func(&export)
+					.ok_or_else(|| Error::from(format!("Export {} is not a function", method)))?
+					.clone();
+				EntryPoint::direct(func)
+					.map_err(|_|
+						Error::from(format!(
+							"Exported function '{}' has invalid signature.",
+							method,
+						))
+					)?
+			},
+			InvokeMethod::Table(func_ref) => {
+				let table = self.instance.get_table("__indirect_function_table").ok_or(Error::NoTable)?;
+				let val = table.get(func_ref)
+					.ok_or(Error::NoTableEntryWithIndex(func_ref))?;
+				let func = val
+					.funcref()
+					.ok_or(Error::TableElementIsNotAFunction(func_ref))?
+					.ok_or(Error::FunctionRefIsNull(func_ref))?
+					.clone();
+
+				EntryPoint::direct(func)
+					.map_err(|_|
+						Error::from(format!(
+							"Function @{} in exported table has invalid signature for direct call.",
+							func_ref,
+						))
+					)?
+				},
+			InvokeMethod::TableWithWrapper { dispatcher_ref, func } => {
+				let table = self.instance.get_table("__indirect_function_table").ok_or(Error::NoTable)?;
+				let val = table.get(dispatcher_ref)
+					.ok_or(Error::NoTableEntryWithIndex(dispatcher_ref))?;
+				let dispatcher = val
+					.funcref()
+					.ok_or(Error::TableElementIsNotAFunction(dispatcher_ref))?
+					.ok_or(Error::FunctionRefIsNull(dispatcher_ref))?
+					.clone();
+
+				EntryPoint::wrapped(dispatcher, func)
+					.map_err(|_|
+						Error::from(format!(
+							"Function @{} in exported table has invalid signature for wrapped call.",
+							dispatcher_ref,
+						))
+					)?
+			},
+		})
 	}
 
 	/// Returns an indirect function table of this instance.

substrate/client/executor/wasmtime/src/runtime.rs

@@ -18,14 +18,14 @@
 
 use crate::host::HostState;
 use crate::imports::{Imports, resolve_imports};
-use crate::instance_wrapper::{ModuleWrapper, InstanceWrapper, GlobalsSnapshot};
+use crate::instance_wrapper::{ModuleWrapper, InstanceWrapper, GlobalsSnapshot, EntryPoint};
 use crate::state_holder;
 
 use std::rc::Rc;
 use std::sync::Arc;
 use sc_executor_common::{
-	error::{Error, Result, WasmError},
-	wasm_runtime::{WasmModule, WasmInstance},
+	error::{Result, WasmError},
+	wasm_runtime::{WasmModule, WasmInstance, InvokeMethod},
 };
 use sp_allocator::FreeingBumpHeapAllocator;
 use sp_runtime_interface::unpack_ptr_and_len;
@@ -90,7 +90,7 @@ pub struct WasmtimeInstance {
 unsafe impl Send for WasmtimeInstance {}
 
 impl WasmInstance for WasmtimeInstance {
-	fn call(&self, method: &str, data: &[u8]) -> Result<Vec<u8>> {
+	fn call(&self, method: InvokeMethod, data: &[u8]) -> Result<Vec<u8>> {
 		let entrypoint = self.instance_wrapper.resolve_entrypoint(method)?;
 		let allocator = FreeingBumpHeapAllocator::new(self.heap_base);
 
@@ -146,28 +146,14 @@ pub fn create_runtime(
 fn perform_call(
 	data: &[u8],
 	instance_wrapper: Rc<InstanceWrapper>,
-	entrypoint: wasmtime::Func,
+	entrypoint: EntryPoint,
 	mut allocator: FreeingBumpHeapAllocator,
 ) -> Result<Vec<u8>> {
 	let (data_ptr, data_len) = inject_input_data(&instance_wrapper, &mut allocator, data)?;
 
 	let host_state = HostState::new(allocator, instance_wrapper.clone());
-	let ret = state_holder::with_initialized_state(&host_state, || {
-		match entrypoint.call(&[
-			wasmtime::Val::I32(u32::from(data_ptr) as i32),
-			wasmtime::Val::I32(u32::from(data_len) as i32),
-		]) {
-			Ok(results) => {
-				let retval = results[0].unwrap_i64() as u64;
-				Ok(unpack_ptr_and_len(retval))
-			}
-			Err(trap) => {
-				return Err(Error::from(format!(
-					"Wasm execution trapped: {}",
-					trap
-				)));
-			}
-		}
+	let ret = state_holder::with_initialized_state(&host_state, || -> Result<_> {
+		Ok(unpack_ptr_and_len(entrypoint.call(data_ptr, data_len)?))
 	});
 	let (output_ptr, output_len) = ret?;
 	let output = extract_output_data(&instance_wrapper, output_ptr, output_len)?;

substrate/frame/example-parallel/Cargo.toml

@@ -0,0 +1,35 @@
+[package]
+name = "pallet-example-parallel"
+version = "2.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "Unlicense"
+homepage = "https://substrate.dev"
+repository = "https://github.com/paritytech/substrate/"
+description = "FRAME example pallet using runtime worker threads"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+codec = { package = "parity-scale-codec", version = "1.3.4", default-features = false }
+frame-support = { version = "2.0.0", default-features = false, path = "../support" }
+frame-system = { version = "2.0.0", default-features = false, path = "../system" }
+sp-core = { version = "2.0.0", default-features = false, path = "../../primitives/core" }
+sp-io = { version = "2.0.0", default-features = false, path = "../../primitives/io" }
+sp-runtime = { version = "2.0.0", default-features = false, path = "../../primitives/runtime" }
+sp-std = { version = "2.0.0", default-features = false, path = "../../primitives/std" }
+sp-tasks = { version = "2.0.0", default-features = false, path = "../../primitives/tasks" }
+
+[features]
+default = ["std"]
+std = [
+	"codec/std",
+	"frame-support/std",
+	"frame-system/std",
+	"sp-core/std",
+	"sp-io/std",
+	"sp-runtime/std",
+	"sp-std/std",
+	"sp-tasks/std",
+]

substrate/frame/example-parallel/src/lib.rs

@@ -0,0 +1,152 @@
+// This file is part of Substrate.
+
+// Copyright (C) 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.
+
+//! Parallel tasks example
+//!
+//! This example pallet parallelizes validation of the enlisted participants
+//! (see `enlist_participants` dispatch).
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+use frame_system::ensure_signed;
+use frame_support::{
+	dispatch::DispatchResult, decl_module, decl_storage, decl_event,
+};
+use sp_runtime::RuntimeDebug;
+
+use codec::{Encode, Decode};
+use sp_std::vec::Vec;
+
+#[cfg(test)]
+mod tests;
+
+pub trait Trait: frame_system::Trait {
+	/// The overarching event type.
+	type Event: From<Event> + Into<<Self as frame_system::Trait>::Event>;
+	/// The overarching dispatch call type.
+	type Call: From<Call<Self>>;
+}
+
+decl_storage! {
+	trait Store for Module<T: Trait> as ExampleOffchainWorker {
+		/// A vector of current participants
+		///
+		/// To enlist someone to participate, signed payload should be
+		/// sent to `enlist`.
+		Participants get(fn participants): Vec<Vec<u8>>;
+
+		/// Current event id to enlist participants to.
+		CurrentEventId get(fn get_current_event_id): Vec<u8>;
+	}
+}
+
+decl_event!(
+	/// Events generated by the module.
+	pub enum Event {
+		/// When new event is drafted.
+		NewEventDrafted(Vec<u8>),
+	}
+);
+
+/// Request to enlist participant.
+#[derive(Encode, Decode, Clone, PartialEq, Eq, RuntimeDebug)]
+pub struct EnlistedParticipant {
+	pub account: Vec<u8>,
+	pub signature: Vec<u8>,
+}
+
+impl EnlistedParticipant {
+	fn verify(&self, event_id: &[u8]) -> bool {
+		use sp_core::Public;
+		use std::convert::TryFrom;
+		use sp_runtime::traits::Verify;
+
+		match sp_core::sr25519::Signature::try_from(&self.signature[..]) {
+			Ok(signature) => {
+				let public = sp_core::sr25519::Public::from_slice(self.account.as_ref());
+				signature.verify(event_id, &public)
+			}
+			_ => false
+		}
+	}
+}
+
+decl_module! {
+	/// A public part of the pallet.
+	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+		fn deposit_event() = default;
+
+		/// Get the new event running.
+		#[weight = 0]
+		pub fn run_event(origin, id: Vec<u8>) -> DispatchResult {
+			let _ = ensure_signed(origin)?;
+			Participants::kill();
+			CurrentEventId::mutate(move |event_id| *event_id = id);
+			Ok(())
+		}
+
+		/// Submit list of participants to the current event.
+		///
+		/// The example utilizes parallel execution by checking half of the
+		/// signatures in spawned task.
+		#[weight = 0]
+		pub fn enlist_participants(origin, participants: Vec<EnlistedParticipant>)
+			-> DispatchResult
+		{
+			let _ = ensure_signed(origin)?;
+
+			if validate_participants_parallel(&CurrentEventId::get(), &participants[..]) {
+				for participant in participants {
+					Participants::append(participant.account);
+				}
+			}
+			Ok(())
+		}
+	}
+}
+
+fn validate_participants_parallel(event_id: &[u8], participants: &[EnlistedParticipant]) -> bool {
+
+	fn spawn_verify(data: Vec<u8>) -> Vec<u8> {
+		let stream = &mut &data[..];
+		let event_id = Vec::<u8>::decode(stream).expect("Failed to decode");
+		let participants = Vec::<EnlistedParticipant>::decode(stream).expect("Failed to decode");
+
+		for participant in participants {
+			if !participant.verify(&event_id) {
+				return false.encode()
+			}
+		}
+		true.encode()
+	}
+
+	let mut async_payload = Vec::new();
+	event_id.encode_to(&mut async_payload);
+	participants[..participants.len() / 2].encode_to(&mut async_payload);
+
+	let handle = sp_tasks::spawn(spawn_verify, async_payload);
+	let mut result = true;
+
+	for participant in &participants[participants.len()/2+1..] {
+		if !participant.verify(event_id) {
+			result = false;
+			break;
+		}
+	}
+
+	bool::decode(&mut &handle.join()[..]).expect("Failed to decode result") && result
+}

substrate/frame/example-parallel/src/tests.rs

@@ -0,0 +1,151 @@
+// This file is part of Substrate.
+
+// Copyright (C) 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.
+
+use crate::*;
+
+use codec::{Encode, Decode};
+use frame_support::{impl_outer_origin, parameter_types, weights::Weight};
+use sp_core::H256;
+use sp_runtime::{
+	Perbill,
+	testing::{Header},
+	traits::{BlakeTwo256, IdentityLookup},
+};
+
+impl_outer_origin! {
+	pub enum Origin for Test where system = frame_system {}
+}
+
+#[derive(Clone, Eq, PartialEq, Encode, Decode)]
+pub struct Test;
+parameter_types! {
+	pub const BlockHashCount: u64 = 250;
+	pub const MaximumBlockWeight: Weight = 1024;
+	pub const MaximumBlockLength: u32 = 2 * 1024;
+	pub const AvailableBlockRatio: Perbill = Perbill::one();
+}
+
+impl frame_system::Trait for Test {
+	type BaseCallFilter = ();
+	type Origin = Origin;
+	type Call = ();
+	type PalletInfo = ();
+	type Index = u64;
+	type BlockNumber = u64;
+	type Hash = H256;
+	type Hashing = BlakeTwo256;
+	type AccountId = sp_core::sr25519::Public;
+	type Lookup = IdentityLookup<Self::AccountId>;
+	type Header = Header;
+	type Event = ();
+	type BlockHashCount = BlockHashCount;
+	type MaximumBlockWeight = MaximumBlockWeight;
+	type DbWeight = ();
+	type BlockExecutionWeight = ();
+	type ExtrinsicBaseWeight = ();
+	type MaximumExtrinsicWeight = MaximumBlockWeight;
+	type MaximumBlockLength = MaximumBlockLength;
+	type AvailableBlockRatio = AvailableBlockRatio;
+	type Version = ();
+	type AccountData = ();
+	type OnNewAccount = ();
+	type OnKilledAccount = ();
+	type SystemWeightInfo = ();
+}
+
+parameter_types! {
+	pub const GracePeriod: u64 = 5;
+	pub const UnsignedInterval: u64 = 128;
+	pub const UnsignedPriority: u64 = 1 << 20;
+}
+
+impl Trait for Test {
+	type Event = ();
+	type Call = Call<Test>;
+}
+
+type Example = Module<Test>;
+
+#[test]
+fn it_can_enlist() {
+	use sp_core::Pair;
+
+	sp_io::TestExternalities::default().execute_with(|| {
+		let (pair1, _) = sp_core::sr25519::Pair::generate();
+		let (pair2, _) = sp_core::sr25519::Pair::generate();
+
+		let event_name = b"test";
+
+		Example::run_event(Origin::signed(Default::default()), event_name.to_vec())
+			.expect("Failed to enlist");
+
+		let participants = vec![
+			EnlistedParticipant {
+				account: pair1.public().to_vec(),
+				signature: AsRef::<[u8]>::as_ref(&pair1.sign(event_name)).to_vec(),
+			},
+			EnlistedParticipant {
+				account: pair2.public().to_vec(),
+				signature: AsRef::<[u8]>::as_ref(&pair2.sign(event_name)).to_vec(),
+			},
+		];
+
+		Example::enlist_participants(Origin::signed(Default::default()), participants)
+			.expect("Failed to enlist");
+
+		assert_eq!(Example::participants().len(), 2);
+	});
+
+}
+
+#[test]
+fn one_wrong_will_not_enlist_anyone() {
+	use sp_core::Pair;
+
+	sp_io::TestExternalities::default().execute_with(|| {
+		let (pair1, _) = sp_core::sr25519::Pair::generate();
+		let (pair2, _) = sp_core::sr25519::Pair::generate();
+		let (pair3, _) = sp_core::sr25519::Pair::generate();
+
+		let event_name = b"test";
+
+		Example::run_event(Origin::signed(Default::default()), event_name.to_vec())
+			.expect("Failed to enlist");
+
+		let participants = vec![
+			EnlistedParticipant {
+				account: pair1.public().to_vec(),
+				signature: AsRef::<[u8]>::as_ref(&pair1.sign(event_name)).to_vec(),
+			},
+			EnlistedParticipant {
+				account: pair2.public().to_vec(),
+				signature: AsRef::<[u8]>::as_ref(&pair2.sign(event_name)).to_vec(),
+			},
+			// signing wrong event
+			EnlistedParticipant {
+				account: pair3.public().to_vec(),
+				signature: AsRef::<[u8]>::as_ref(&pair3.sign(&[])).to_vec(),
+			},
+		];
+
+		Example::enlist_participants(Origin::signed(Default::default()), participants)
+			.expect("Failed to enlist");
+
+		assert_eq!(Example::participants().len(), 0);
+	});
+
+}

substrate/primitives/core/src/traits.rs

@@ -186,6 +186,25 @@ impl TaskExecutorExt {
 	}
 }
 
+/// Runtime spawn extension.
+pub trait RuntimeSpawn: Send {
+	/// Create new runtime instance and use dynamic dispatch to invoke with specified payload.
+	///
+	/// Returns handle of the spawned task.
+	///
+	/// Function pointers (`dispatcher_ref`, `func`) are WASM pointer types.
+	fn spawn_call(&self, dispatcher_ref: u32, func: u32, payload: Vec<u8>) -> u64;
+
+	/// Join the result of previously created runtime instance invocation.
+	fn join(&self, handle: u64) -> Vec<u8>;
+}
+
+#[cfg(feature = "std")]
+sp_externalities::decl_extension! {
+	/// Extension that supports spawning extra runtime instances in externalities.
+	pub struct RuntimeSpawnExt(Box<dyn RuntimeSpawn>);
+}
+
 /// Something that can spawn futures (blocking and non-blocking) with an assigned name.
 #[dyn_clonable::clonable]
 pub trait SpawnNamed: Clone + Send + Sync {

substrate/primitives/externalities/src/extensions.rs

@@ -79,6 +79,12 @@ macro_rules! decl_extension {
 				&mut self.0
 			}
 		}
+
+		impl From<$inner> for $ext_name {
+			fn from(inner: $inner) -> Self {
+				Self(inner)
+			}
+ 		}
 	}
 }
 

substrate/primitives/io/src/lib.rs

@@ -38,7 +38,7 @@ use tracing;
 #[cfg(feature = "std")]
 use sp_core::{
 	crypto::Pair,
-	traits::{CallInWasmExt, TaskExecutorExt},
+	traits::{CallInWasmExt, TaskExecutorExt, RuntimeSpawnExt},
 	offchain::{OffchainExt, TransactionPoolExt},
 	hexdisplay::HexDisplay,
 	storage::ChildInfo,
@@ -520,7 +520,6 @@ pub trait Crypto {
 	fn start_batch_verify(&mut self) {
 		let scheduler = self.extension::<TaskExecutorExt>()
 			.expect("No task executor associated with the current context!")
-			.0
 			.clone();
 
 		self.register_extension(VerificationExt(BatchVerifier::new(scheduler)))
@@ -755,7 +754,7 @@ pub trait OffchainIndex {
 
 #[cfg(feature = "std")]
 sp_externalities::decl_extension! {
-	/// The keystore extension to register/retrieve from the externalities.
+	/// Batch verification extension to register/retrieve from the externalities.
 	pub struct VerificationExt(BatchVerifier);
 }
 
@@ -1239,6 +1238,34 @@ pub trait Sandbox {
 	}
 }
 
+/// Wasm host functions for managing tasks.
+///
+/// This should not be used directly. Use `sp_tasks` for running parallel tasks instead.
+#[runtime_interface(wasm_only)]
+pub trait RuntimeTasks {
+	/// Wasm host function for spawning task.
+	///
+	/// This should not be used directly. Use `sp_tasks::spawn` instead.
+	fn spawn(dispatcher_ref: u32, entry: u32, payload: Vec<u8>) -> u64 {
+		sp_externalities::with_externalities(|mut ext|{
+			let runtime_spawn = ext.extension::<RuntimeSpawnExt>()
+				.expect("Cannot spawn without dynamic runtime dispatcher (RuntimeSpawnExt)");
+			runtime_spawn.spawn_call(dispatcher_ref, entry, payload)
+		}).expect("`RuntimeTasks::spawn`: called outside of externalities context")
+	}
+
+	/// Wasm host function for joining a task.
+	///
+	/// This should not be used directly. Use `join` of `sp_tasks::spawn` result instead.
+	fn join(handle: u64) -> Vec<u8> {
+		sp_externalities::with_externalities(|mut ext| {
+			let runtime_spawn = ext.extension::<RuntimeSpawnExt>()
+				.expect("Cannot join without dynamic runtime dispatcher (RuntimeSpawnExt)");
+			runtime_spawn.join(handle)
+		}).expect("`RuntimeTasks::join`: called outside of externalities context")
+	}
+ }
+
 /// Allocator used by Substrate when executing the Wasm runtime.
 #[cfg(not(feature = "std"))]
 struct WasmAllocator;
@@ -1306,6 +1333,7 @@ pub type SubstrateHostFunctions = (
 	sandbox::HostFunctions,
 	crate::trie::HostFunctions,
 	offchain_index::HostFunctions,
+	runtime_tasks::HostFunctions,
 );
 
 #[cfg(test)]

substrate/primitives/runtime-interface/src/lib.rs

@@ -312,7 +312,7 @@ pub mod pass_by;
 
 mod util;
 
-pub use util::unpack_ptr_and_len;
+pub use util::{unpack_ptr_and_len, pack_ptr_and_len};
 
 /// Something that can be used by the runtime interface as type to communicate between wasm and the
 /// host.

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

@@ -18,7 +18,7 @@
 //! Basic implementation for Externalities.
 
 use std::{
-	collections::BTreeMap, any::{TypeId, Any}, iter::FromIterator, ops::Bound
+	collections::BTreeMap, any::{TypeId, Any}, iter::FromIterator, ops::Bound,
 };
 use crate::{Backend, StorageKey, StorageValue};
 use hash_db::Hasher;

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

@@ -17,9 +17,8 @@
 
 //! Test implementation for Externalities.
 
-use std::any::{Any, TypeId};
-use codec::Decode;
-use hash_db::Hasher;
+use std::{any::{Any, TypeId}, panic::{AssertUnwindSafe, UnwindSafe}};
+
 use crate::{
 	backend::Backend, OverlayedChanges, StorageTransactionCache, ext::Ext, InMemoryBackend,
 	StorageKey, StorageValue,
@@ -30,6 +29,9 @@ use crate::{
 		State as ChangesTrieState,
 	},
 };
+
+use codec::{Decode, Encode};
+use hash_db::Hasher;
 use sp_core::{
 	offchain::{
 		testing::TestPersistentOffchainDB,
@@ -42,7 +44,6 @@ use sp_core::{
 	traits::TaskExecutorExt,
 	testing::TaskExecutor,
 };
-use codec::Encode;
 use sp_externalities::{Extensions, Extension};
 
 /// Simple HashMap-based Externalities impl.
@@ -178,6 +179,19 @@ impl<H: Hasher, N: ChangesTrieBlockNumber> TestExternalities<H, N>
 		let mut ext = self.ext();
 		sp_externalities::set_and_run_with_externalities(&mut ext, execute)
 	}
+
+	/// Execute the given closure while `self` is set as externalities.
+	///
+	/// Returns the result of the given closure, if no panics occured.
+	/// Otherwise, returns `Err`.
+	pub fn execute_with_safe<R>(&mut self, f: impl FnOnce() -> R + UnwindSafe) -> Result<R, String> {
+		let mut ext = AssertUnwindSafe(self.ext());
+		std::panic::catch_unwind(move ||
+			sp_externalities::set_and_run_with_externalities(&mut *ext, f)
+		).map_err(|e| {
+			format!("Closure panicked: {:?}", e)
+		})
+	}
 }
 
 impl<H: Hasher, N: ChangesTrieBlockNumber> std::fmt::Debug for TestExternalities<H, N>

substrate/primitives/tasks/Cargo.toml

@@ -0,0 +1,36 @@
+[package]
+name = "sp-tasks"
+version = "2.0.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+license = "Apache-2.0"
+homepage = "https://substrate.dev"
+repository = "https://github.com/paritytech/substrate/"
+description = "Runtime asynchronous, pure computational tasks"
+documentation = "https://docs.rs/sp-tasks"
+readme = "README.md"
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]
+
+[dependencies]
+log = { version = "0.4.8", optional = true }
+sp-core = { version = "2.0.0", default-features = false, path = "../core" }
+sp-externalities = { version = "0.8.0", optional = true, path = "../externalities" }
+sp-io = { version = "2.0.0", default-features = false, path = "../io" }
+sp-runtime-interface = { version = "2.0.0", default-features = false, path = "../runtime-interface" }
+sp-std = { version = "2.0.0", default-features = false, path = "../std" }
+
+[dev-dependencies]
+codec = { package = "parity-scale-codec", default-features = false, version = "1.3.1" }
+
+[features]
+default = ["std"]
+std = [
+	"log",
+	"sp-core/std",
+	"sp-externalities",
+	"sp-io/std",
+	"sp-runtime-interface/std",
+	"sp-std/std",
+]

substrate/primitives/tasks/README.md

@@ -0,0 +1,3 @@
+Runtime asynchronous, pure computational tasks. 
+
+License: Apache-2.0

substrate/primitives/tasks/src/async_externalities.rs

@@ -0,0 +1,215 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2020 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program 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.
+
+// This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Async externalities.
+
+use std::any::{TypeId, Any};
+use sp_core::{
+	storage::{ChildInfo, TrackedStorageKey},
+	traits::{Externalities, SpawnNamed, TaskExecutorExt, RuntimeSpawnExt, RuntimeSpawn},
+};
+use sp_externalities::{Extensions, ExternalitiesExt as _};
+
+/// Simple state-less externalities for use in async context.
+///
+/// Will panic if anything is accessing the storage.
+#[derive(Debug)]
+pub struct AsyncExternalities {
+	extensions: Extensions,
+}
+
+/// New Async externalities.
+pub fn new_async_externalities(scheduler: Box<dyn SpawnNamed>) -> Result<AsyncExternalities, &'static str> {
+	let mut res = AsyncExternalities { extensions: Default::default() };
+	let mut ext = &mut res as &mut dyn Externalities;
+	ext.register_extension::<TaskExecutorExt>(TaskExecutorExt(scheduler.clone()))
+		.map_err(|_| "Failed to register task executor extension.")?;
+
+	Ok(res)
+}
+
+impl AsyncExternalities {
+	/// Extend async externalities with the ability to spawn wasm instances.
+	pub fn with_runtime_spawn(
+		mut self,
+		runtime_ext: Box<dyn RuntimeSpawn>,
+	) -> Result<Self, &'static str> {
+		let mut ext = &mut self as &mut dyn Externalities;
+		ext.register_extension::<RuntimeSpawnExt>(RuntimeSpawnExt(runtime_ext))
+			.map_err(|_| "Failed to register task executor extension.")?;
+
+		Ok(self)
+	}
+}
+
+type StorageKey = Vec<u8>;
+
+type StorageValue = Vec<u8>;
+
+impl Externalities for AsyncExternalities {
+	fn set_offchain_storage(&mut self, _key: &[u8], _value: Option<&[u8]>) {
+		panic!("`set_offchain_storage`: should not be used in async externalities!")
+	}
+
+	fn storage(&self, _key: &[u8]) -> Option<StorageValue> {
+		panic!("`storage`: should not be used in async externalities!")
+	}
+
+	fn storage_hash(&self, _key: &[u8]) -> Option<Vec<u8>> {
+		panic!("`storage_hash`: should not be used in async externalities!")
+	}
+
+	fn child_storage(
+		&self,
+		_child_info: &ChildInfo,
+		_key: &[u8],
+	) -> Option<StorageValue> {
+		panic!("`child_storage`: should not be used in async externalities!")
+	}
+
+	fn child_storage_hash(
+		&self,
+		_child_info: &ChildInfo,
+		_key: &[u8],
+	) -> Option<Vec<u8>> {
+		panic!("`child_storage_hash`: should not be used in async externalities!")
+	}
+
+	fn next_storage_key(&self, _key: &[u8]) -> Option<StorageKey> {
+		panic!("`next_storage_key`: should not be used in async externalities!")
+	}
+
+	fn next_child_storage_key(
+		&self,
+		_child_info: &ChildInfo,
+		_key: &[u8],
+	) -> Option<StorageKey> {
+		panic!("`next_child_storage_key`: should not be used in async externalities!")
+	}
+
+	fn place_storage(&mut self, _key: StorageKey, _maybe_value: Option<StorageValue>) {
+		panic!("`place_storage`: should not be used in async externalities!")
+	}
+
+	fn place_child_storage(
+		&mut self,
+		_child_info: &ChildInfo,
+		_key: StorageKey,
+		_value: Option<StorageValue>,
+	) {
+		panic!("`place_child_storage`: should not be used in async externalities!")
+	}
+
+	fn kill_child_storage(
+		&mut self,
+		_child_info: &ChildInfo,
+	) {
+		panic!("`kill_child_storage`: should not be used in async externalities!")
+	}
+
+	fn clear_prefix(&mut self, _prefix: &[u8]) {
+		panic!("`clear_prefix`: should not be used in async externalities!")
+	}
+
+	fn clear_child_prefix(
+		&mut self,
+		_child_info: &ChildInfo,
+		_prefix: &[u8],
+	) {
+		panic!("`clear_child_prefix`: should not be used in async externalities!")
+	}
+
+	fn storage_append(
+		&mut self,
+		_key: Vec<u8>,
+		_value: Vec<u8>,
+	) {
+		panic!("`storage_append`: should not be used in async externalities!")
+	}
+
+	fn chain_id(&self) -> u64 { 42 }
+
+	fn storage_root(&mut self) -> Vec<u8> {
+		panic!("`storage_root`: should not be used in async externalities!")
+	}
+
+	fn child_storage_root(
+		&mut self,
+		_child_info: &ChildInfo,
+	) -> Vec<u8> {
+		panic!("`child_storage_root`: should not be used in async externalities!")
+	}
+
+	fn storage_changes_root(&mut self, _parent: &[u8]) -> Result<Option<Vec<u8>>, ()> {
+		panic!("`storage_changes_root`: should not be used in async externalities!")
+	}
+
+	fn storage_start_transaction(&mut self) {
+		unimplemented!("Transactions are not supported by AsyncExternalities");
+	}
+
+	fn storage_rollback_transaction(&mut self) -> Result<(), ()> {
+		unimplemented!("Transactions are not supported by AsyncExternalities");
+	}
+
+	fn storage_commit_transaction(&mut self) -> Result<(), ()> {
+		unimplemented!("Transactions are not supported by AsyncExternalities");
+	}
+
+	fn wipe(&mut self) {}
+
+	fn commit(&mut self) {}
+
+	fn read_write_count(&self) -> (u32, u32, u32, u32) {
+		unimplemented!("read_write_count is not supported in AsyncExternalities")
+	}
+
+	fn reset_read_write_count(&mut self) {
+		unimplemented!("reset_read_write_count is not supported in AsyncExternalities")
+	}
+
+	fn get_whitelist(&self) -> Vec<TrackedStorageKey> {
+		unimplemented!("get_whitelist is not supported in AsyncExternalities")
+	}
+
+	fn set_whitelist(&mut self, _: Vec<TrackedStorageKey>) {
+		unimplemented!("set_whitelist is not supported in AsyncExternalities")
+	}
+}
+
+impl sp_externalities::ExtensionStore for AsyncExternalities {
+	fn extension_by_type_id(&mut self, type_id: TypeId) -> Option<&mut dyn Any> {
+		self.extensions.get_mut(type_id)
+	}
+
+	fn register_extension_with_type_id(
+		&mut self,
+		type_id: TypeId,
+		extension: Box<dyn sp_externalities::Extension>,
+	) -> Result<(), sp_externalities::Error> {
+		self.extensions.register_with_type_id(type_id, extension)
+	}
+
+	fn deregister_extension_by_type_id(&mut self, type_id: TypeId) -> Result<(), sp_externalities::Error> {
+		if self.extensions.deregister(type_id) {
+			Ok(())
+		} else {
+			Err(sp_externalities::Error::ExtensionIsNotRegistered(type_id))
+		}
+	}
+}

substrate/primitives/tasks/src/lib.rs

@@ -0,0 +1,247 @@
+// This file is part of Substrate.
+
+// Copyright (C) 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.
+
+//! Runtime tasks.
+//!
+//! Contains runtime-usable functions for spawning parallel purely computational tasks.
+//!
+//! NOTE: This is experimental API.
+//! NOTE: When using in actual runtime, make sure you don't produce unbounded parallelism.
+//! So this is bad example to use it:
+//! ```rust
+//!    fn my_parallel_computator(data: Vec<u8>) -> Vec<u8> {
+//!        unimplemented!()
+//!    }
+//!    fn test(dynamic_variable: i32) {
+//!        for _ in 0..dynamic_variable { sp_tasks::spawn(my_parallel_computator, vec![]); }
+//!    }
+//! ```
+//!
+//! While this is a good example:
+//! ```rust
+//!    use codec::Encode;
+//!    static STATIC_VARIABLE: i32 = 4;
+//!
+//!    fn my_parallel_computator(data: Vec<u8>) -> Vec<u8> {
+//!        unimplemented!()
+//!    }
+//!
+//!    fn test(computation_payload: Vec<u8>) {
+//!        let parallel_tasks = (0..STATIC_VARIABLE).map(|idx|
+//!            sp_tasks::spawn(my_parallel_computator, computation_payload.chunks(10).nth(idx as _).encode())
+//!        );
+//!    }
+//! ```
+//!
+//! When allowing unbounded parallelism, malicious transactions can exploit it and partition
+//! network consensus based on how much resources nodes have.
+//!
+
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(feature = "std")]
+mod async_externalities;
+
+#[cfg(feature = "std")]
+pub use async_externalities::{new_async_externalities, AsyncExternalities};
+
+#[cfg(feature = "std")]
+mod inner {
+	use std::{panic::AssertUnwindSafe, sync::mpsc};
+	use sp_externalities::ExternalitiesExt as _;
+	use sp_core::traits::TaskExecutorExt;
+
+	/// Task handle (wasm).
+	///
+	/// This can be `join`-ed to get (blocking) the result of
+	/// the spawned task execution.
+	#[must_use]
+	pub struct DataJoinHandle {
+		receiver: mpsc::Receiver<Vec<u8>>,
+	}
+
+	impl DataJoinHandle {
+		/// Join handle returned by `spawn` function
+		pub fn join(self) -> Vec<u8> {
+			self.receiver.recv().expect("Spawned runtime task terminated before sending result.")
+		}
+	}
+
+	/// Spawn new runtime task (native).
+	pub fn spawn(entry_point: fn(Vec<u8>) -> Vec<u8>, data: Vec<u8>) -> DataJoinHandle {
+		let scheduler = sp_externalities::with_externalities(|mut ext| ext.extension::<TaskExecutorExt>()
+			.expect("No task executor associated with the current context!")
+			.clone()
+		).expect("Spawn called outside of externalities context!");
+
+		let (sender, receiver) = mpsc::channel();
+		let extra_scheduler = scheduler.clone();
+		scheduler.spawn("parallel-runtime-spawn", Box::pin(async move {
+			let result = match crate::new_async_externalities(extra_scheduler) {
+				Ok(mut ext) => {
+					let mut ext = AssertUnwindSafe(&mut ext);
+					match std::panic::catch_unwind(move || {
+						sp_externalities::set_and_run_with_externalities(
+							&mut **ext,
+							move || entry_point(data),
+						)
+					}) {
+						Ok(result) => result,
+						Err(panic) => {
+							log::error!(
+								target: "runtime",
+								"Spawned task panicked: {:?}",
+								panic,
+							);
+
+							// This will drop sender without sending anything.
+							return;
+						}
+					}
+				},
+				Err(e) => {
+					log::error!(
+						target: "runtime",
+						"Unable to run async task: {}",
+						e,
+					);
+
+					return;
+				},
+			};
+
+			let _ = sender.send(result);
+		}));
+
+		DataJoinHandle { receiver }
+	}
+}
+
+#[cfg(not(feature = "std"))]
+mod inner {
+	use core::mem;
+	use sp_std::prelude::*;
+
+	/// Dispatch wrapper for wasm blob.
+	///
+	/// Serves as trampoline to call any rust function with (Vec<u8>) -> Vec<u8> compiled
+	/// into the runtime.
+	///
+	/// Function item should be provided with `func_ref`. Argument for the call
+	/// will be generated from bytes at `payload_ptr` with `payload_len`.
+	///
+	/// NOTE: Since this dynamic dispatch function and the invoked function are compiled with
+	/// the same compiler, there should be no problem with ABI incompatibility.
+	extern "C" fn dispatch_wrapper(func_ref: *const u8, payload_ptr: *mut u8, payload_len: u32) -> u64 {
+		let payload_len = payload_len as usize;
+		let output = unsafe {
+			let payload = Vec::from_raw_parts(payload_ptr, payload_len, payload_len);
+			let ptr: fn(Vec<u8>) -> Vec<u8> = mem::transmute(func_ref);
+			(ptr)(payload)
+		};
+		sp_runtime_interface::pack_ptr_and_len(output.as_ptr() as usize as _, output.len() as _)
+	}
+
+	/// Spawn new runtime task (wasm).
+	pub fn spawn(entry_point: fn(Vec<u8>) -> Vec<u8>, payload: Vec<u8>) -> DataJoinHandle {
+		let func_ptr: usize = unsafe { mem::transmute(entry_point) };
+
+		let handle = sp_io::runtime_tasks::spawn(
+			dispatch_wrapper as usize as _,
+			func_ptr as u32,
+			payload,
+		);
+		DataJoinHandle { handle }
+	}
+
+	/// Task handle (wasm).
+	///
+	/// This can be `join`-ed to get (blocking) the result of
+	/// the spawned task execution.
+	#[must_use]
+	pub struct DataJoinHandle {
+		handle: u64,
+	}
+
+	impl DataJoinHandle {
+		/// Join handle returned by `spawn` function
+		pub fn join(self) -> Vec<u8> {
+			sp_io::runtime_tasks::join(self.handle)
+		}
+	}
+}
+
+pub use inner::{DataJoinHandle, spawn};
+
+#[cfg(test)]
+mod tests {
+
+	use super::*;
+
+	fn async_runner(mut data: Vec<u8>) -> Vec<u8> {
+		data.sort();
+		data
+	}
+
+	fn async_panicker(_data: Vec<u8>) -> Vec<u8> {
+		panic!("panic in async panicker!")
+	}
+
+	#[test]
+	fn basic() {
+		sp_io::TestExternalities::default().execute_with(|| {
+			let a1 = spawn(async_runner, vec![5, 2, 1]).join();
+			assert_eq!(a1, vec![1, 2, 5]);
+		})
+	}
+
+	#[test]
+	fn panicking() {
+		let res = sp_io::TestExternalities::default().execute_with_safe(||{
+			spawn(async_panicker, vec![5, 2, 1]).join();
+		});
+
+		assert!(res.unwrap_err().contains("Closure panicked"));
+	}
+
+	#[test]
+	fn many_joins() {
+		sp_io::TestExternalities::default().execute_with_safe(|| {
+			// converges to 1 only after 1000+ steps
+			let mut running_val = 9780657630u64;
+			let mut data = vec![];
+			let handles = (0..1024).map(
+				|_| {
+					running_val = if running_val % 2 == 0 {
+						running_val / 2
+					} else {
+						3 * running_val + 1
+					};
+					data.push(running_val as u8);
+					(spawn(async_runner, data.clone()), data.clone())
+				}
+			).collect::<Vec<_>>();
+
+			for (handle, mut data) in handles {
+				let result = handle.join();
+				data.sort();
+
+				assert_eq!(result, data);
+			}
+		}).expect("Failed to run with externalities");
+	}
+}