From 44a8aa23d5c4419608a8c685a5e2bcaa0a2acaf2 Mon Sep 17 00:00:00 2001
From: Bernhard Schuster <bernhard@ahoi.io>
Date: Wed, 16 Jun 2021 12:45:21 +0200
Subject: [PATCH] malus - mockable overseer mvp (#3224)

---
 polkadot/Cargo.lock                           |  19 +-
 polkadot/Cargo.toml                           |   1 +
 polkadot/cli/Cargo.toml                       |   2 +
 polkadot/cli/src/command.rs                   |  95 +++---
 polkadot/cli/src/lib.rs                       |   5 +-
 .../node/core/candidate-validation/src/lib.rs |   1 +
 polkadot/node/malus/Cargo.toml                |  28 ++
 polkadot/node/malus/src/lib.rs                | 184 ++++++++++++
 polkadot/node/malus/src/variant-a.rs          | 110 +++++++
 polkadot/node/service/Cargo.toml              |   1 +
 polkadot/node/service/src/grandpa_support.rs  |  11 +-
 polkadot/node/service/src/lib.rs              | 228 ++++----------
 polkadot/node/service/src/overseer.rs         | 280 ++++++++++++++++++
 .../node/service/src/parachains_db/mod.rs     |   3 +
 polkadot/node/test/service/src/lib.rs         |   3 +-
 .../adder/collator/src/main.rs                |   1 +
 16 files changed, 753 insertions(+), 219 deletions(-)
 create mode 100644 polkadot/node/malus/Cargo.toml
 create mode 100644 polkadot/node/malus/src/lib.rs
 create mode 100644 polkadot/node/malus/src/variant-a.rs
 create mode 100644 polkadot/node/service/src/overseer.rs

diff --git a/polkadot/Cargo.lock b/polkadot/Cargo.lock
index 7f6f2395c28..e8ed4e235e3 100644
--- a/polkadot/Cargo.lock
+++ b/polkadot/Cargo.lock
@@ -356,9 +356,9 @@ checksum = "e91831deabf0d6d7ec49552e489aed63b7456a7a3c46cff62adad428110b0af0"
 
 [[package]]
 name = "async-trait"
-version = "0.1.48"
+version = "0.1.50"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "36ea56748e10732c49404c153638a15ec3d6211ec5ff35d9bb20e13b93576adf"
+checksum = "0b98e84bbb4cbcdd97da190ba0c58a1bb0de2c1fdf67d159e192ed766aeca722"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -6819,6 +6819,21 @@ dependencies = [
  "substrate-test-client",
 ]
 
+[[package]]
+name = "polkadot-test-malus"
+version = "0.9.4"
+dependencies = [
+ "assert_matches",
+ "async-trait",
+ "color-eyre",
+ "parity-util-mem",
+ "polkadot-cli",
+ "polkadot-node-core-candidate-validation",
+ "polkadot-node-subsystem",
+ "polkadot-node-subsystem-util",
+ "structopt",
+]
+
 [[package]]
 name = "polkadot-test-runtime"
 version = "0.9.5"
diff --git a/polkadot/Cargo.toml b/polkadot/Cargo.toml
index f7c341a99aa..be022f7b877 100644
--- a/polkadot/Cargo.toml
+++ b/polkadot/Cargo.toml
@@ -64,6 +64,7 @@ members = [
 	"node/network/collator-protocol",
 	"node/network/gossip-support",
 	"node/overseer",
+	"node/malus",
 	"node/primitives",
 	"node/service",
 	"node/subsystem",
diff --git a/polkadot/cli/Cargo.toml b/polkadot/cli/Cargo.toml
index fd7125115b4..65a9267e511 100644
--- a/polkadot/cli/Cargo.toml
+++ b/polkadot/cli/Cargo.toml
@@ -67,3 +67,5 @@ try-runtime = [ "service/try-runtime" ]
 kusama-native = [ "service/kusama-native" ]
 westend-native = [ "service/westend-native" ]
 rococo-native = [ "service/rococo-native" ]
+
+malus = [ "full-node", "service/malus" ]
diff --git a/polkadot/cli/src/command.rs b/polkadot/cli/src/command.rs
index 6c3575b55e9..3a245580de7 100644
--- a/polkadot/cli/src/command.rs
+++ b/polkadot/cli/src/command.rs
@@ -181,53 +181,64 @@ fn ensure_dev(spec: &Box<dyn service::ChainSpec>) -> std::result::Result<(), Str
 	}
 }
 
-/// Parses polkadot specific CLI arguments and run the service.
-pub fn run() -> Result<()> {
-	let cli = Cli::from_args();
+/// Launch a node, accepting arguments just like a regular node,
+/// accepts an alternative overseer generator, to adjust behavior
+/// for integration tests as needed.
+#[cfg(feature = "malus")]
+pub fn run_node(cli: Cli, overseer_gen: impl service::OverseerGen) -> Result<()> {
+	run_node_inner(cli, overseer_gen)
+}
 
-	match &cli.subcommand {
-		None => {
-			let runner = cli.create_runner(&cli.run.base)
-				.map_err(Error::from)?;
-			let chain_spec = &runner.config().chain_spec;
+fn run_node_inner(cli: Cli, overseer_gen: impl service::OverseerGen) -> Result<()> {
+	let runner = cli.create_runner(&cli.run.base)
+		.map_err(Error::from)?;
+	let chain_spec = &runner.config().chain_spec;
 
-			set_default_ss58_version(chain_spec);
+	set_default_ss58_version(chain_spec);
 
-			let grandpa_pause = if cli.run.grandpa_pause.is_empty() {
-				None
-			} else {
-				Some((cli.run.grandpa_pause[0], cli.run.grandpa_pause[1]))
-			};
+	let grandpa_pause = if cli.run.grandpa_pause.is_empty() {
+		None
+	} else {
+		Some((cli.run.grandpa_pause[0], cli.run.grandpa_pause[1]))
+	};
 
-			if chain_spec.is_kusama() {
-				info!("----------------------------");
-				info!("This chain is not in any way");
-				info!("      endorsed by the       ");
-				info!("     KUSAMA FOUNDATION      ");
-				info!("----------------------------");
-			}
+	if chain_spec.is_kusama() {
+		info!("----------------------------");
+		info!("This chain is not in any way");
+		info!("      endorsed by the       ");
+		info!("     KUSAMA FOUNDATION      ");
+		info!("----------------------------");
+	}
 
-			let jaeger_agent = cli.run.jaeger_agent;
-
-			runner.run_node_until_exit(move |config| async move {
-				let role = config.role.clone();
-
-				match role {
-					#[cfg(feature = "browser")]
-					Role::Light => service::build_light(config).map(|(task_manager, _)| task_manager).map_err(Into::into),
-					#[cfg(not(feature = "browser"))]
-					Role::Light => Err(Error::Other("Light client not enabled".into())),
-					_ => service::build_full(
-						config,
-						service::IsCollator::No,
-						grandpa_pause,
-						cli.run.no_beefy,
-						jaeger_agent,
-						None,
-					).map(|full| full.task_manager).map_err(Into::into)
-				}
-			})
-		},
+	let jaeger_agent = cli.run.jaeger_agent;
+
+	runner.run_node_until_exit(move |config| async move {
+		let role = config.role.clone();
+
+		match role {
+			#[cfg(feature = "browser")]
+			Role::Light => service::build_light(config).map(|(task_manager, _)| task_manager).map_err(Into::into),
+			#[cfg(not(feature = "browser"))]
+			Role::Light => Err(Error::Other("Light client not enabled".into())),
+			_ => service::build_full(
+				config,
+				service::IsCollator::No,
+				grandpa_pause,
+				cli.run.no_beefy,
+				jaeger_agent,
+				None,
+				overseer_gen,
+			).map(|full| full.task_manager).map_err(Into::into)
+		}
+	})
+}
+
+/// Parses polkadot specific CLI arguments and run the service.
+pub fn run() -> Result<()> {
+	let cli = Cli::from_args();
+
+	match &cli.subcommand {
+		None => run_node_inner(cli, service::RealOverseerGen),
 		Some(Subcommand::BuildSpec(cmd)) => {
 			let runner = cli.create_runner(cmd)?;
 			Ok(runner.sync_run(|config| {
diff --git a/polkadot/cli/src/lib.rs b/polkadot/cli/src/lib.rs
index 43b8da81f96..02120081ee5 100644
--- a/polkadot/cli/src/lib.rs
+++ b/polkadot/cli/src/lib.rs
@@ -28,9 +28,12 @@ mod command;
 pub use service::{
 	self,
 	ProvideRuntimeApi, CoreApi, IdentifyVariant,
-	Block, RuntimeApiCollection, TFullClient
+	Block, RuntimeApiCollection, TFullClient,
 };
 
+#[cfg(feature = "malus")]
+pub use service::create_default_subsystems;
+
 #[cfg(feature = "cli")]
 pub use cli::*;
 
diff --git a/polkadot/node/core/candidate-validation/src/lib.rs b/polkadot/node/core/candidate-validation/src/lib.rs
index 4efec16db1c..b05ff9cabc3 100644
--- a/polkadot/node/core/candidate-validation/src/lib.rs
+++ b/polkadot/node/core/candidate-validation/src/lib.rs
@@ -56,6 +56,7 @@ use async_trait::async_trait;
 const LOG_TARGET: &'static str = "parachain::candidate-validation";
 
 /// Configuration for the candidate validation subsystem
+#[derive(Clone)]
 pub struct Config {
 	/// The path where candidate validation can store compiled artifacts for PVFs.
 	pub artifacts_cache_path: PathBuf,
diff --git a/polkadot/node/malus/Cargo.toml b/polkadot/node/malus/Cargo.toml
new file mode 100644
index 00000000000..30c1da1d405
--- /dev/null
+++ b/polkadot/node/malus/Cargo.toml
@@ -0,0 +1,28 @@
+[lib]
+name = "malus"
+path = "src/lib.rs"
+
+[[bin]]
+name = "malus-variant-a"
+path = "src/variant-a.rs"
+
+[package]
+name = "polkadot-test-malus"
+description = "Misbehaving nodes for local testnets, system and simnet tests."
+license = "GPL-3.0-only"
+version = "0.9.4"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+readme = "README.md"
+publish = false
+
+[dependencies]
+polkadot-cli = { path = "../../cli", default-features = false, features = [ "cli", "malus" ]  }
+polkadot-node-subsystem = { path = "../subsystem" }
+polkadot-node-subsystem-util = { path = "../subsystem-util" }
+polkadot-node-core-candidate-validation = { path = "../core/candidate-validation" }
+parity-util-mem = { version = "*", default-features = false, features = ["jemalloc-global"] }
+color-eyre = { version = "0.5.11", default-features = false }
+assert_matches = "1.5"
+structopt = "0.3.21"
+async-trait = "0.1.50"
diff --git a/polkadot/node/malus/src/lib.rs b/polkadot/node/malus/src/lib.rs
new file mode 100644
index 00000000000..6cd45a3c4de
--- /dev/null
+++ b/polkadot/node/malus/src/lib.rs
@@ -0,0 +1,184 @@
+// Copyright 2017-2021 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A small set of wrapping types to cover most of our adversary test cases.
+//!
+//! This allows types with internal mutability to synchronize across
+//! multiple subsystems and intercept or replace incoming and outgoing
+//! messages on the overseer level.
+
+use polkadot_node_subsystem::*;
+pub use polkadot_node_subsystem::{messages::AllMessages, FromOverseer};
+use std::future::Future;
+use std::pin::Pin;
+
+/// Filter incoming and outgoing messages.
+pub trait MsgFilter: Send + Sync + Clone + 'static {
+	/// The message type the original subsystm handles incoming.
+	type Message: Send + 'static;
+
+	/// Filter messages that are to be received by
+	/// the subsystem.
+	fn filter_in(&self, msg: FromOverseer<Self::Message>) -> Option<FromOverseer<Self::Message>> {
+		Some(msg)
+	}
+
+	/// Modify outgoing messages.
+	fn filter_out(&self, msg: AllMessages) -> Option<AllMessages> {
+		Some(msg)
+	}
+}
+
+/// A sender with the outgoing messages filtered.
+#[derive(Clone)]
+pub struct FilteredSender<Sender, Fil> {
+	inner: Sender,
+	message_filter: Fil,
+}
+
+#[async_trait::async_trait]
+impl<Sender, Fil> SubsystemSender for FilteredSender<Sender, Fil>
+where
+	Sender: SubsystemSender,
+	Fil: MsgFilter,
+{
+	async fn send_message(&mut self, msg: AllMessages) {
+		if let Some(msg) = self.message_filter.filter_out(msg) {
+			self.inner.send_message(msg).await;
+		}
+	}
+
+	async fn send_messages<T>(&mut self, msgs: T)
+	where
+		T: IntoIterator<Item = AllMessages> + Send,
+		T::IntoIter: Send,
+	{
+		for msg in msgs {
+			self.send_message(msg).await;
+		}
+	}
+
+	fn send_unbounded_message(&mut self, msg: AllMessages) {
+		if let Some(msg) = self.message_filter.filter_out(msg) {
+			self.inner.send_unbounded_message(msg);
+		}
+	}
+}
+
+/// A subsystem context, that filters the outgoing messages.
+pub struct FilteredContext<Context: SubsystemContext, Fil: MsgFilter> {
+	inner: Context,
+	message_filter: Fil,
+	sender: FilteredSender<<Context as SubsystemContext>::Sender, Fil>,
+}
+
+impl<Context, Fil> FilteredContext<Context, Fil>
+where
+	Context: SubsystemContext,
+	Fil: MsgFilter<Message = <Context as SubsystemContext>::Message>,
+{
+	pub fn new(mut inner: Context, message_filter: Fil) -> Self {
+		let sender = FilteredSender::<<Context as SubsystemContext>::Sender, Fil> {
+			inner: inner.sender().clone(),
+			message_filter: message_filter.clone(),
+		};
+		Self {
+			inner,
+			message_filter,
+			sender,
+		}
+	}
+}
+
+#[async_trait::async_trait]
+impl<Context, Fil> SubsystemContext for FilteredContext<Context, Fil>
+where
+	Context: SubsystemContext,
+	Fil: MsgFilter<Message = <Context as SubsystemContext>::Message>,
+{
+	type Message = <Context as SubsystemContext>::Message;
+	type Sender = FilteredSender<<Context as SubsystemContext>::Sender, Fil>;
+
+	async fn try_recv(&mut self) -> Result<Option<FromOverseer<Self::Message>>, ()> {
+		loop {
+			match self.inner.try_recv().await? {
+				None => return Ok(None),
+				Some(msg) => {
+					if let Some(msg) = self.message_filter.filter_in(msg) {
+						return Ok(Some(msg));
+					}
+				}
+			}
+		}
+	}
+
+	async fn recv(&mut self) -> SubsystemResult<FromOverseer<Self::Message>> {
+		loop {
+			let msg = self.inner.recv().await?;
+			if let Some(msg) = self.message_filter.filter_in(msg) {
+				return Ok(msg);
+			}
+		}
+	}
+
+	async fn spawn(
+		&mut self,
+		name: &'static str,
+		s: Pin<Box<dyn Future<Output = ()> + Send>>,
+	) -> SubsystemResult<()> {
+		self.inner.spawn(name, s).await
+	}
+
+	async fn spawn_blocking(
+		&mut self,
+		name: &'static str,
+		s: Pin<Box<dyn Future<Output = ()> + Send>>,
+	) -> SubsystemResult<()> {
+		self.inner.spawn_blocking(name, s).await
+	}
+
+	fn sender(&mut self) -> &mut Self::Sender {
+		&mut self.sender
+	}
+}
+
+/// A subsystem to which incoming and outgoing filters are applied.
+pub struct FilteredSubsystem<Sub, Fil> {
+	subsystem: Sub,
+	message_filter: Fil,
+}
+
+impl<Sub, Fil> FilteredSubsystem<Sub, Fil> {
+	pub fn new(subsystem: Sub, message_filter: Fil) -> Self {
+		Self {
+			subsystem,
+			message_filter,
+		}
+	}
+}
+
+impl<Context, Sub, Fil> Subsystem<Context> for FilteredSubsystem<Sub, Fil>
+where
+	Context: SubsystemContext + Sync + Send,
+	Sub: Subsystem<FilteredContext<Context, Fil>>,
+	FilteredContext<Context, Fil>: SubsystemContext,
+	Fil: MsgFilter<Message = <Context as SubsystemContext>::Message>,
+{
+	fn start(self, ctx: Context) -> SpawnedSubsystem {
+		let ctx = FilteredContext::new(ctx, self.message_filter);
+		Subsystem::<FilteredContext<Context, Fil>>::start(self.subsystem, ctx)
+	}
+}
diff --git a/polkadot/node/malus/src/variant-a.rs b/polkadot/node/malus/src/variant-a.rs
new file mode 100644
index 00000000000..1e9cb7928cb
--- /dev/null
+++ b/polkadot/node/malus/src/variant-a.rs
@@ -0,0 +1,110 @@
+// Copyright 2017-2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! A malicious overseer.
+//!
+//! An example on how to use the `OverseerGen` pattern to
+//! instantiate a modified subsystem implementation
+//! for usage with simnet/gurke.
+
+#![allow(missing_docs)]
+
+use color_eyre::eyre;
+use polkadot_cli::{
+	create_default_subsystems,
+	service::{
+		AuthorityDiscoveryApi, AuxStore, BabeApi, Block, Error, HeaderBackend, Overseer,
+		OverseerGen, OverseerGenArgs, OverseerHandler, ParachainHost, ProvideRuntimeApi,
+		SpawnNamed,
+	},
+	Cli,
+};
+
+// Import extra types relevant to the particular
+// subsystem.
+use polkadot_node_core_candidate_validation::{CandidateValidationSubsystem, Metrics};
+use polkadot_node_subsystem::messages::CandidateValidationMessage;
+use polkadot_node_subsystem_util::metrics::Metrics as _;
+
+// Filter wrapping related types.
+use malus::*;
+
+use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::Arc;
+
+use structopt::StructOpt;
+
+/// Silly example, just drop every second outgoing message.
+#[derive(Clone, Default, Debug)]
+struct Skippy(Arc<AtomicUsize>);
+
+impl MsgFilter for Skippy {
+	type Message = CandidateValidationMessage;
+
+	fn filter_in(&self, msg: FromOverseer<Self::Message>) -> Option<FromOverseer<Self::Message>> {
+		if self.0.fetch_add(1, Ordering::Relaxed) % 2 == 0 {
+			Some(msg)
+		} else {
+			None
+		}
+	}
+	fn filter_out(&self, msg: AllMessages) -> Option<AllMessages> {
+		Some(msg)
+	}
+}
+
+/// Generates an overseer that exposes bad behavior.
+struct BehaveMaleficient;
+
+impl OverseerGen for BehaveMaleficient {
+	fn generate<'a, Spawner, RuntimeClient>(
+		&self,
+		args: OverseerGenArgs<'a, Spawner, RuntimeClient>,
+	) -> Result<(Overseer<Spawner, Arc<RuntimeClient>>, OverseerHandler), Error>
+	where
+		RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
+		RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
+		Spawner: 'static + SpawnNamed + Clone + Unpin,
+	{
+		let spawner = args.spawner.clone();
+		let leaves = args.leaves.clone();
+		let runtime_client = args.runtime_client.clone();
+		let registry = args.registry.clone();
+		let candidate_validation_config = args.candidate_validation_config.clone();
+		// modify the subsystem(s) as needed:
+		let all_subsystems = create_default_subsystems(args)?.replace_candidate_validation(
+			// create the filtered subsystem
+			FilteredSubsystem::new(
+				CandidateValidationSubsystem::with_config(
+					candidate_validation_config,
+					Metrics::register(registry)?,
+				),
+				Skippy::default(),
+			),
+		);
+
+		Overseer::new(leaves, all_subsystems, registry, runtime_client, spawner)
+			.map_err(|e| e.into())
+	}
+}
+
+fn main() -> eyre::Result<()> {
+	color_eyre::install()?;
+	let cli = Cli::from_args();
+	assert_matches::assert_matches!(cli.subcommand, None);
+	polkadot_cli::run_node(cli, BehaveMaleficient)?;
+	Ok(())
+}
diff --git a/polkadot/node/service/Cargo.toml b/polkadot/node/service/Cargo.toml
index a35e6cdd686..29ba03c5322 100644
--- a/polkadot/node/service/Cargo.toml
+++ b/polkadot/node/service/Cargo.toml
@@ -159,3 +159,4 @@ try-runtime = [
 	"westend-runtime/try-runtime",
 	"rococo-runtime/try-runtime",
 ]
+malus = ["full-node"]
diff --git a/polkadot/node/service/src/grandpa_support.rs b/polkadot/node/service/src/grandpa_support.rs
index f2d87a8d563..f2c1145cf3a 100644
--- a/polkadot/node/service/src/grandpa_support.rs
+++ b/polkadot/node/service/src/grandpa_support.rs
@@ -18,15 +18,13 @@
 
 use std::sync::Arc;
 
-use polkadot_primitives::v1::{BlockNumber, Hash};
-
 use sp_runtime::traits::{Block as BlockT, NumberFor};
 use sp_runtime::generic::BlockId;
 use sp_runtime::traits::Header as _;
 
 #[cfg(feature = "full-node")]
 use {
-	polkadot_primitives::v1::{Block as PolkadotBlock, Header as PolkadotHeader},
+	polkadot_primitives::v1::{Hash, Block as PolkadotBlock, Header as PolkadotHeader},
 	polkadot_subsystem::messages::ApprovalVotingMessage,
 	prometheus_endpoint::{self, Registry},
 	polkadot_overseer::OverseerHandler,
@@ -71,9 +69,10 @@ impl ApprovalCheckingVotingRule {
 	}
 }
 
+#[cfg(feature = "full-node")]
 #[derive(Debug, PartialEq)]
+/// Vote explicitly on the given hash.
 enum ParachainVotingRuleTarget<H, N> {
-	/// Vote explicitly on the given hash.
 	Explicit((H, N)),
 	/// Vote on the current target.
 	Current,
@@ -81,6 +80,7 @@ enum ParachainVotingRuleTarget<H, N> {
 	Base,
 }
 
+#[cfg(feature = "full-node")]
 fn approval_checking_vote_to_grandpa_vote<H, N: PartialOrd>(
 	approval_checking_vote: Option<(H, N)>,
 	current_number: N,
@@ -99,7 +99,8 @@ fn approval_checking_vote_to_grandpa_vote<H, N: PartialOrd>(
 
 /// The maximum amount of unfinalized blocks we are willing to allow due to approval checking lag.
 /// This is a safety net that should be removed at some point in the future.
-const MAX_APPROVAL_CHECKING_FINALITY_LAG: BlockNumber = 50;
+#[cfg(feature = "full-node")]
+const MAX_APPROVAL_CHECKING_FINALITY_LAG: polkadot_primitives::v1::BlockNumber = 50;
 
 #[cfg(feature = "full-node")]
 impl<B> grandpa::VotingRule<PolkadotBlock, B> for ApprovalCheckingVotingRule
diff --git a/polkadot/node/service/src/lib.rs b/polkadot/node/service/src/lib.rs
index f3074ac863f..be78081ba1d 100644
--- a/polkadot/node/service/src/lib.rs
+++ b/polkadot/node/service/src/lib.rs
@@ -22,6 +22,17 @@ pub mod chain_spec;
 mod grandpa_support;
 mod parachains_db;
 
+#[cfg(feature = "full-node")]
+mod overseer;
+
+#[cfg(feature = "full-node")]
+pub use self::overseer::{
+	OverseerGen,
+	OverseerGenArgs,
+	RealOverseerGen,
+	create_default_subsystems,
+};
+
 #[cfg(feature = "full-node")]
 use {
 	tracing::info,
@@ -30,22 +41,26 @@ use {
 	polkadot_node_core_av_store::Error as AvailabilityError,
 	polkadot_node_core_approval_voting::Config as ApprovalVotingConfig,
 	polkadot_node_core_candidate_validation::Config as CandidateValidationConfig,
-	polkadot_overseer::{AllSubsystems, BlockInfo, Overseer, OverseerHandler},
-	polkadot_primitives::v1::ParachainHost,
-	sc_authority_discovery::Service as AuthorityDiscoveryService,
-	sp_authority_discovery::AuthorityDiscoveryApi,
-	sp_blockchain::HeaderBackend,
+	polkadot_overseer::BlockInfo,
 	sp_trie::PrefixedMemoryDB,
-	sc_client_api::{AuxStore, ExecutorProvider},
-	sc_keystore::LocalKeystore,
-	sp_consensus_babe::BabeApi,
+	sc_client_api::ExecutorProvider,
 	grandpa::{self, FinalityProofProvider as GrandpaFinalityProofProvider},
 	beefy_primitives::ecdsa::AuthoritySignature as BeefySignature,
 	sp_runtime::traits::Header as HeaderT,
 };
 
-use sp_core::traits::SpawnNamed;
+#[cfg(feature = "full-node")]
+pub use {
+	sp_blockchain::HeaderBackend,
+	sp_consensus_babe::BabeApi,
+	sp_authority_discovery::AuthorityDiscoveryApi,
+	sc_client_api::AuxStore,
+	polkadot_primitives::v1::ParachainHost,
+	polkadot_overseer::{Overseer, OverseerHandler},
+};
+pub use sp_core::traits::SpawnNamed;
 
+#[cfg(feature = "full-node")]
 use polkadot_subsystem::jaeger;
 
 use std::sync::Arc;
@@ -53,7 +68,9 @@ use std::time::Duration;
 
 use prometheus_endpoint::Registry;
 use service::RpcHandlers;
-use telemetry::{Telemetry, TelemetryWorker, TelemetryWorkerHandle};
+#[cfg(feature = "full-node")]
+use telemetry::{Telemetry, TelemetryWorkerHandle};
+use telemetry::TelemetryWorker;
 
 #[cfg(feature = "rococo-native")]
 pub use polkadot_client::RococoExecutor;
@@ -92,6 +109,7 @@ pub use rococo_runtime;
 pub use westend_runtime;
 
 /// The maximum number of active leaves we forward to the [`Overseer`] on startup.
+#[cfg(any(test,feature = "full-node"))]
 const MAX_ACTIVE_LEAVES: usize = 4;
 
 #[derive(thiserror::Error, Debug)]
@@ -182,6 +200,7 @@ fn set_prometheus_registry(config: &mut Configuration) -> Result<(), Error> {
 
 /// Initialize the `Jeager` collector. The destination must listen
 /// on the given address and port for `UDP` packets.
+#[cfg(any(test,feature = "full-node"))]
 fn jaeger_launch_collector_with_agent(spawner: impl SpawnNamed, config: &Configuration, agent: Option<std::net::SocketAddr>) -> Result<(), Error> {
 	if let Some(agent) = agent {
 		let cfg = jaeger::JaegerConfig::builder()
@@ -404,145 +423,6 @@ fn new_partial<RuntimeApi, Executor>(
 	})
 }
 
-#[cfg(feature = "full-node")]
-fn real_overseer<Spawner, RuntimeClient>(
-	leaves: impl IntoIterator<Item = BlockInfo>,
-	keystore: Arc<LocalKeystore>,
-	runtime_client: Arc<RuntimeClient>,
-	parachains_db: Arc<dyn kvdb::KeyValueDB>,
-	availability_config: AvailabilityConfig,
-	approval_voting_config: ApprovalVotingConfig,
-	network_service: Arc<sc_network::NetworkService<Block, Hash>>,
-	authority_discovery: AuthorityDiscoveryService,
-	request_multiplexer: RequestMultiplexer,
-	registry: Option<&Registry>,
-	spawner: Spawner,
-	is_collator: IsCollator,
-	candidate_validation_config: CandidateValidationConfig,
-) -> Result<(Overseer<Spawner, Arc<RuntimeClient>>, OverseerHandler), Error>
-where
-	RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
-	RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
-	Spawner: 'static + SpawnNamed + Clone + Unpin,
-{
-	use polkadot_node_subsystem_util::metrics::Metrics;
-
-	use polkadot_availability_distribution::AvailabilityDistributionSubsystem;
-	use polkadot_node_core_av_store::AvailabilityStoreSubsystem;
-	use polkadot_availability_bitfield_distribution::BitfieldDistribution as BitfieldDistributionSubsystem;
-	use polkadot_node_core_bitfield_signing::BitfieldSigningSubsystem;
-	use polkadot_node_core_backing::CandidateBackingSubsystem;
-	use polkadot_node_core_candidate_validation::CandidateValidationSubsystem;
-	use polkadot_node_core_chain_api::ChainApiSubsystem;
-	use polkadot_node_collation_generation::CollationGenerationSubsystem;
-	use polkadot_collator_protocol::{CollatorProtocolSubsystem, ProtocolSide};
-	use polkadot_network_bridge::NetworkBridge as NetworkBridgeSubsystem;
-	use polkadot_node_core_provisioner::ProvisioningSubsystem as ProvisionerSubsystem;
-	use polkadot_node_core_runtime_api::RuntimeApiSubsystem;
-	use polkadot_statement_distribution::StatementDistribution as StatementDistributionSubsystem;
-	use polkadot_availability_recovery::AvailabilityRecoverySubsystem;
-	use polkadot_approval_distribution::ApprovalDistribution as ApprovalDistributionSubsystem;
-	use polkadot_node_core_approval_voting::ApprovalVotingSubsystem;
-	use polkadot_gossip_support::GossipSupport as GossipSupportSubsystem;
-
-	let all_subsystems = AllSubsystems {
-		availability_distribution: AvailabilityDistributionSubsystem::new(
-			keystore.clone(),
-			Metrics::register(registry)?,
-		),
-		availability_recovery: AvailabilityRecoverySubsystem::with_chunks_only(
-		),
-		availability_store: AvailabilityStoreSubsystem::new(
-			parachains_db.clone(),
-			availability_config,
-			Metrics::register(registry)?,
-		),
-		bitfield_distribution: BitfieldDistributionSubsystem::new(
-			Metrics::register(registry)?,
-		),
-		bitfield_signing: BitfieldSigningSubsystem::new(
-			spawner.clone(),
-			keystore.clone(),
-			Metrics::register(registry)?,
-		),
-		candidate_backing: CandidateBackingSubsystem::new(
-			spawner.clone(),
-			keystore.clone(),
-			Metrics::register(registry)?,
-		),
-		candidate_validation: CandidateValidationSubsystem::with_config(
-			candidate_validation_config,
-			Metrics::register(registry)?,
-		),
-		chain_api: ChainApiSubsystem::new(
-			runtime_client.clone(),
-			Metrics::register(registry)?,
-		),
-		collation_generation: CollationGenerationSubsystem::new(
-			Metrics::register(registry)?,
-		),
-		collator_protocol: {
-			let side = match is_collator {
-				IsCollator::Yes(collator_pair) => ProtocolSide::Collator(
-					network_service.local_peer_id().clone(),
-					collator_pair,
-					Metrics::register(registry)?,
-				),
-				IsCollator::No => ProtocolSide::Validator {
-					keystore: keystore.clone(),
-					eviction_policy: Default::default(),
-					metrics: Metrics::register(registry)?,
-				},
-			};
-			CollatorProtocolSubsystem::new(
-				side,
-			)
-		},
-		network_bridge: NetworkBridgeSubsystem::new(
-			network_service.clone(),
-			authority_discovery,
-			request_multiplexer,
-			Box::new(network_service.clone()),
-			Metrics::register(registry)?,
-		),
-		provisioner: ProvisionerSubsystem::new(
-			spawner.clone(),
-			(),
-			Metrics::register(registry)?,
-		),
-		runtime_api: RuntimeApiSubsystem::new(
-			runtime_client.clone(),
-			Metrics::register(registry)?,
-			spawner.clone(),
-		),
-		statement_distribution: StatementDistributionSubsystem::new(
-			keystore.clone(),
-			Metrics::register(registry)?,
-		),
-		approval_distribution: ApprovalDistributionSubsystem::new(
-			Metrics::register(registry)?,
-		),
-		approval_voting: ApprovalVotingSubsystem::with_config(
-			approval_voting_config,
-			parachains_db,
-			keystore.clone(),
-			Box::new(network_service.clone()),
-			Metrics::register(registry)?,
-		),
-		gossip_support: GossipSupportSubsystem::new(
-			keystore.clone(),
-		),
-	};
-
-	Overseer::new(
-		leaves,
-		all_subsystems,
-		registry,
-		runtime_client.clone(),
-		spawner,
-	).map_err(|e| e.into())
-}
-
 #[cfg(feature = "full-node")]
 pub struct NewFull<C> {
 	pub task_manager: TaskManager,
@@ -652,7 +532,7 @@ where
 /// This is an advanced feature and not recommended for general use. Generally, `build_full` is
 /// a better choice.
 #[cfg(feature = "full-node")]
-pub fn new_full<RuntimeApi, Executor>(
+pub fn new_full<RuntimeApi, Executor, OverseerGenerator>(
 	mut config: Configuration,
 	is_collator: IsCollator,
 	grandpa_pause: Option<(u32, u32)>,
@@ -660,12 +540,14 @@ pub fn new_full<RuntimeApi, Executor>(
 	jaeger_agent: Option<std::net::SocketAddr>,
 	telemetry_worker_handle: Option<TelemetryWorkerHandle>,
 	program_path: Option<std::path::PathBuf>,
+	overseer_gen: OverseerGenerator,
 ) -> Result<NewFull<Arc<FullClient<RuntimeApi, Executor>>>, Error>
 	where
 		RuntimeApi: ConstructRuntimeApi<Block, FullClient<RuntimeApi, Executor>> + Send + Sync + 'static,
 		RuntimeApi::RuntimeApi:
 		RuntimeApiCollection<StateBackend = sc_client_api::StateBackendFor<FullBackend, Block>>,
 		Executor: NativeExecutionDispatch + 'static,
+		OverseerGenerator: OverseerGen,
 {
 	let role = config.role.clone();
 	let force_authoring = config.force_authoring;
@@ -840,20 +722,25 @@ pub fn new_full<RuntimeApi, Executor>(
 		.and_then(move |k| authority_discovery_service.map(|a| (a, k)));
 
 	let overseer_handler = if let Some((authority_discovery_service, keystore)) = maybe_params {
-		let (overseer, overseer_handler) = real_overseer(
-			active_leaves,
-			keystore,
-			overseer_client.clone(),
-			parachains_db,
-			availability_config,
-			approval_voting_config,
-			network.clone(),
-			authority_discovery_service,
-			request_multiplexer,
-			prometheus_registry.as_ref(),
-			spawner,
-			is_collator,
-			candidate_validation_config,
+		let (overseer, overseer_handler) = overseer_gen.generate::<
+			service::SpawnTaskHandle,
+			FullClient<RuntimeApi, Executor>,
+		>(
+			OverseerGenArgs {
+				leaves: active_leaves,
+				keystore,
+				runtime_client: overseer_client.clone(),
+				parachains_db,
+				availability_config,
+				approval_voting_config,
+				network_service: network.clone(),
+				authority_discovery_service,
+				request_multiplexer,
+				registry: prometheus_registry.as_ref(),
+				spawner,
+				is_collator,
+				candidate_validation_config,
+			}
 		)?;
 		let overseer_handler_clone = overseer_handler.clone();
 
@@ -1285,10 +1172,11 @@ pub fn build_full(
 	disable_beefy: bool,
 	jaeger_agent: Option<std::net::SocketAddr>,
 	telemetry_worker_handle: Option<TelemetryWorkerHandle>,
+	overseer_gen: impl OverseerGen,
 ) -> Result<NewFull<Client>, Error> {
 	#[cfg(feature = "rococo-native")]
 	if config.chain_spec.is_rococo() || config.chain_spec.is_wococo() {
-		return new_full::<rococo_runtime::RuntimeApi, RococoExecutor>(
+		return new_full::<rococo_runtime::RuntimeApi, RococoExecutor, _>(
 			config,
 			is_collator,
 			grandpa_pause,
@@ -1296,12 +1184,13 @@ pub fn build_full(
 			jaeger_agent,
 			telemetry_worker_handle,
 			None,
+			overseer_gen,
 		).map(|full| full.with_client(Client::Rococo))
 	}
 
 	#[cfg(feature = "kusama-native")]
 	if config.chain_spec.is_kusama() {
-		return new_full::<kusama_runtime::RuntimeApi, KusamaExecutor>(
+		return new_full::<kusama_runtime::RuntimeApi, KusamaExecutor, _>(
 			config,
 			is_collator,
 			grandpa_pause,
@@ -1309,12 +1198,13 @@ pub fn build_full(
 			jaeger_agent,
 			telemetry_worker_handle,
 			None,
+			overseer_gen,
 		).map(|full| full.with_client(Client::Kusama))
 	}
 
 	#[cfg(feature = "westend-native")]
 	if config.chain_spec.is_westend() {
-		return new_full::<westend_runtime::RuntimeApi, WestendExecutor>(
+		return new_full::<westend_runtime::RuntimeApi, WestendExecutor, _>(
 			config,
 			is_collator,
 			grandpa_pause,
@@ -1322,10 +1212,11 @@ pub fn build_full(
 			jaeger_agent,
 			telemetry_worker_handle,
 			None,
+			overseer_gen,
 		).map(|full| full.with_client(Client::Westend))
 	}
 
-	new_full::<polkadot_runtime::RuntimeApi, PolkadotExecutor>(
+	new_full::<polkadot_runtime::RuntimeApi, PolkadotExecutor, _>(
 		config,
 		is_collator,
 		grandpa_pause,
@@ -1333,5 +1224,6 @@ pub fn build_full(
 		jaeger_agent,
 		telemetry_worker_handle,
 		None,
+		overseer_gen,
 	).map(|full| full.with_client(Client::Polkadot))
 }
diff --git a/polkadot/node/service/src/overseer.rs b/polkadot/node/service/src/overseer.rs
new file mode 100644
index 00000000000..968240074b8
--- /dev/null
+++ b/polkadot/node/service/src/overseer.rs
@@ -0,0 +1,280 @@
+// Copyright 2017-2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+use super::{
+	Error,
+	Registry,
+	IsCollator,
+	Block,
+	SpawnNamed,
+	Hash,
+	AuthorityDiscoveryApi,
+};
+use std::sync::Arc;
+use polkadot_network_bridge::RequestMultiplexer;
+use polkadot_node_core_av_store::Config as AvailabilityConfig;
+use polkadot_node_core_approval_voting::Config as ApprovalVotingConfig;
+use polkadot_node_core_candidate_validation::Config as CandidateValidationConfig;
+use polkadot_overseer::{AllSubsystems, BlockInfo, Overseer, OverseerHandler};
+use polkadot_primitives::v1::ParachainHost;
+use sc_authority_discovery::Service as AuthorityDiscoveryService;
+use sp_api::ProvideRuntimeApi;
+use sp_blockchain::HeaderBackend;
+use sc_client_api::AuxStore;
+use sc_keystore::LocalKeystore;
+use sp_consensus_babe::BabeApi;
+
+pub use polkadot_availability_distribution::AvailabilityDistributionSubsystem;
+pub use polkadot_node_core_av_store::AvailabilityStoreSubsystem;
+pub use polkadot_availability_bitfield_distribution::BitfieldDistribution as BitfieldDistributionSubsystem;
+pub use polkadot_node_core_bitfield_signing::BitfieldSigningSubsystem;
+pub use polkadot_node_core_backing::CandidateBackingSubsystem;
+pub use polkadot_node_core_candidate_validation::CandidateValidationSubsystem;
+pub use polkadot_node_core_chain_api::ChainApiSubsystem;
+pub use polkadot_node_collation_generation::CollationGenerationSubsystem;
+pub use polkadot_collator_protocol::{CollatorProtocolSubsystem, ProtocolSide};
+pub use polkadot_network_bridge::NetworkBridge as NetworkBridgeSubsystem;
+pub use polkadot_node_core_provisioner::ProvisioningSubsystem as ProvisionerSubsystem;
+pub use polkadot_node_core_runtime_api::RuntimeApiSubsystem;
+pub use polkadot_statement_distribution::StatementDistribution as StatementDistributionSubsystem;
+pub use polkadot_availability_recovery::AvailabilityRecoverySubsystem;
+pub use polkadot_approval_distribution::ApprovalDistribution as ApprovalDistributionSubsystem;
+pub use polkadot_node_core_approval_voting::ApprovalVotingSubsystem;
+pub use polkadot_gossip_support::GossipSupport as GossipSupportSubsystem;
+
+/// Arguments passed for overseer construction.
+pub struct OverseerGenArgs<'a, Spawner, RuntimeClient> where
+	RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
+	RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
+	Spawner: 'static + SpawnNamed + Clone + Unpin,
+{
+	/// Set of initial relay chain leaves to track.
+	pub leaves: Vec<BlockInfo>,
+	/// The keystore to use for i.e. validator keys.
+	pub keystore: Arc<LocalKeystore>,
+	/// Runtime client generic, providing the `ProvieRuntimeApi` trait besides others.
+	pub runtime_client: Arc<RuntimeClient>,
+	/// The underlying key value store for the parachains.
+	pub parachains_db: Arc<dyn kvdb::KeyValueDB>,
+	/// Configuration for the availability store subsystem.
+	pub availability_config: AvailabilityConfig,
+	/// Configuration for the approval voting subsystem.
+	pub approval_voting_config: ApprovalVotingConfig,
+	/// Underlying network service implementation.
+	pub network_service: Arc<sc_network::NetworkService<Block, Hash>>,
+	/// Underlying authority discovery service.
+	pub authority_discovery_service: AuthorityDiscoveryService,
+	/// A multiplexer to arbitrate incoming `IncomingRequest`s from the network.
+	pub request_multiplexer: RequestMultiplexer,
+	/// Prometheus registry, commonly used for production systems, less so for test.
+	pub registry: Option<&'a Registry>,
+	/// Task spawner to be used throughout the overseer and the APIs it provides.
+	pub spawner: Spawner,
+	/// Determines the behavior of the collator.
+	pub is_collator: IsCollator,
+	/// Configuration for the candidate validation subsystem.
+	pub candidate_validation_config: CandidateValidationConfig,
+}
+
+/// Create a default, unaltered set of subsystems.
+///
+/// A convenience for usage with malus, to avoid
+/// repetitive code across multiple behavior strain implementations.
+pub fn create_default_subsystems<'a, Spawner, RuntimeClient>
+(
+	OverseerGenArgs {
+		keystore,
+		runtime_client,
+		parachains_db,
+		availability_config,
+		approval_voting_config,
+		network_service,
+		authority_discovery_service,
+		request_multiplexer,
+		registry,
+		spawner,
+		is_collator,
+		candidate_validation_config,
+		..
+	} : OverseerGenArgs<'a, Spawner, RuntimeClient>
+) -> Result<
+	AllSubsystems<
+	CandidateValidationSubsystem,
+	CandidateBackingSubsystem<Spawner>,
+	StatementDistributionSubsystem,
+	AvailabilityDistributionSubsystem,
+	AvailabilityRecoverySubsystem,
+	BitfieldSigningSubsystem<Spawner>,
+	BitfieldDistributionSubsystem,
+	ProvisionerSubsystem<Spawner>,
+	RuntimeApiSubsystem<RuntimeClient>,
+	AvailabilityStoreSubsystem,
+	NetworkBridgeSubsystem<Arc<sc_network::NetworkService<Block, Hash>>, AuthorityDiscoveryService>,
+	ChainApiSubsystem<RuntimeClient>,
+	CollationGenerationSubsystem,
+	CollatorProtocolSubsystem,
+	ApprovalDistributionSubsystem,
+	ApprovalVotingSubsystem,
+	GossipSupportSubsystem,
+>,
+	Error
+>
+where
+	RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
+	RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
+	Spawner: 'static + SpawnNamed + Clone + Unpin
+{
+	use polkadot_node_subsystem_util::metrics::Metrics;
+
+	let all_subsystems = AllSubsystems {
+		availability_distribution: AvailabilityDistributionSubsystem::new(
+			keystore.clone(),
+			Metrics::register(registry)?,
+		),
+		availability_recovery: AvailabilityRecoverySubsystem::with_chunks_only(
+		),
+		availability_store: AvailabilityStoreSubsystem::new(
+			parachains_db.clone(),
+			availability_config,
+			Metrics::register(registry)?,
+		),
+		bitfield_distribution: BitfieldDistributionSubsystem::new(
+			Metrics::register(registry)?,
+		),
+		bitfield_signing: BitfieldSigningSubsystem::new(
+			spawner.clone(),
+			keystore.clone(),
+			Metrics::register(registry)?,
+		),
+		candidate_backing: CandidateBackingSubsystem::new(
+			spawner.clone(),
+			keystore.clone(),
+			Metrics::register(registry)?,
+		),
+		candidate_validation: CandidateValidationSubsystem::with_config(
+			candidate_validation_config,
+			Metrics::register(registry)?,
+		),
+		chain_api: ChainApiSubsystem::new(
+			runtime_client.clone(),
+			Metrics::register(registry)?,
+		),
+		collation_generation: CollationGenerationSubsystem::new(
+			Metrics::register(registry)?,
+		),
+		collator_protocol: {
+			let side = match is_collator {
+				IsCollator::Yes(collator_pair) => ProtocolSide::Collator(
+					network_service.local_peer_id().clone(),
+					collator_pair,
+					Metrics::register(registry)?,
+				),
+				IsCollator::No => ProtocolSide::Validator {
+					keystore: keystore.clone(),
+					eviction_policy: Default::default(),
+					metrics: Metrics::register(registry)?,
+				},
+			};
+			CollatorProtocolSubsystem::new(
+				side,
+			)
+		},
+		network_bridge: NetworkBridgeSubsystem::new(
+			network_service.clone(),
+			authority_discovery_service,
+			request_multiplexer,
+			Box::new(network_service.clone()),
+			Metrics::register(registry)?,
+		),
+		provisioner: ProvisionerSubsystem::new(
+			spawner.clone(),
+			(),
+			Metrics::register(registry)?,
+		),
+		runtime_api: RuntimeApiSubsystem::new(
+			runtime_client.clone(),
+			Metrics::register(registry)?,
+			spawner.clone(),
+		),
+		statement_distribution: StatementDistributionSubsystem::new(
+			keystore.clone(),
+			Metrics::register(registry)?,
+		),
+		approval_distribution: ApprovalDistributionSubsystem::new(
+			Metrics::register(registry)?,
+		),
+		approval_voting: ApprovalVotingSubsystem::with_config(
+			approval_voting_config,
+			parachains_db,
+			keystore.clone(),
+			Box::new(network_service.clone()),
+			Metrics::register(registry)?,
+		),
+		gossip_support: GossipSupportSubsystem::new(
+			keystore.clone(),
+		),
+	};
+	Ok(all_subsystems)
+}
+
+
+/// Trait for the `fn` generating the overseer.
+///
+/// Default behavior is to create an unmodified overseer, as `RealOverseerGen`
+/// would do.
+pub trait OverseerGen {
+	/// Overwrite the full generation of the overseer, including the subsystems.
+	fn generate<'a, Spawner, RuntimeClient>(&self, args: OverseerGenArgs<'a, Spawner, RuntimeClient>) -> Result<(Overseer<Spawner, Arc<RuntimeClient>>, OverseerHandler), Error>
+	where
+		RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
+		RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
+		Spawner: 'static + SpawnNamed + Clone + Unpin {
+		let gen = RealOverseerGen;
+		RealOverseerGen::generate::<Spawner, RuntimeClient>(&gen, args)
+	}
+	// It would be nice to make `create_subsystems` part of this trait,
+	// but the amount of generic arguments that would be required as
+	// as consequence make this rather annoying to implement and use.
+}
+
+/// The regular set of subsystems.
+pub struct RealOverseerGen;
+
+impl OverseerGen for RealOverseerGen {
+	fn generate<'a, Spawner, RuntimeClient>(&self,
+		args : OverseerGenArgs<'a, Spawner, RuntimeClient>
+	) -> Result<(Overseer<Spawner, Arc<RuntimeClient>>, OverseerHandler), Error>
+	where
+		RuntimeClient: 'static + ProvideRuntimeApi<Block> + HeaderBackend<Block> + AuxStore,
+		RuntimeClient::Api: ParachainHost<Block> + BabeApi<Block> + AuthorityDiscoveryApi<Block>,
+		Spawner: 'static + SpawnNamed + Clone + Unpin
+	{
+		let spawner = args.spawner.clone();
+		let leaves = args.leaves.clone();
+		let runtime_client = args.runtime_client.clone();
+		let registry = args.registry.clone();
+
+		let all_subsystems = create_default_subsystems::<Spawner, RuntimeClient>(args)?;
+
+		Overseer::new(
+			leaves,
+			all_subsystems,
+			registry,
+			runtime_client,
+			spawner,
+		).map_err(|e| e.into())
+	}
+}
diff --git a/polkadot/node/service/src/parachains_db/mod.rs b/polkadot/node/service/src/parachains_db/mod.rs
index 8853a6aebad..c9c86fad964 100644
--- a/polkadot/node/service/src/parachains_db/mod.rs
+++ b/polkadot/node/service/src/parachains_db/mod.rs
@@ -24,6 +24,7 @@ use {
 
 mod upgrade;
 
+#[cfg(any(test,feature = "full-node"))]
 mod columns {
 	pub const NUM_COLUMNS: u32 = 3;
 
@@ -34,6 +35,7 @@ mod columns {
 }
 
 /// Columns used by different subsystems.
+#[cfg(any(test,feature = "full-node"))]
 #[derive(Debug, Clone)]
 pub struct ColumnsConfig {
 	/// The column used by the av-store for data.
@@ -45,6 +47,7 @@ pub struct ColumnsConfig {
 }
 
 /// The real columns used by the parachains DB.
+#[cfg(any(test,feature = "full-node"))]
 pub const REAL_COLUMNS: ColumnsConfig = ColumnsConfig {
 	col_availability_data: columns::COL_AVAILABILITY_DATA,
 	col_availability_meta: columns::COL_AVAILABILITY_META,
diff --git a/polkadot/node/test/service/src/lib.rs b/polkadot/node/test/service/src/lib.rs
index c345c96875d..bb3bff46a46 100644
--- a/polkadot/node/test/service/src/lib.rs
+++ b/polkadot/node/test/service/src/lib.rs
@@ -78,7 +78,7 @@ pub fn new_full(
 	NewFull<Arc<Client>>,
 	Error,
 > {
-	polkadot_service::new_full::<polkadot_test_runtime::RuntimeApi, PolkadotTestExecutor>(
+	polkadot_service::new_full::<polkadot_test_runtime::RuntimeApi, PolkadotTestExecutor, _>(
 		config,
 		is_collator,
 		None,
@@ -86,6 +86,7 @@ pub fn new_full(
 		None,
 		None,
 		worker_program_path,
+		polkadot_service::RealOverseerGen,
 	)
 }
 
diff --git a/polkadot/parachain/test-parachains/adder/collator/src/main.rs b/polkadot/parachain/test-parachains/adder/collator/src/main.rs
index 68a89dc3f40..2b6b219f0d3 100644
--- a/polkadot/parachain/test-parachains/adder/collator/src/main.rs
+++ b/polkadot/parachain/test-parachains/adder/collator/src/main.rs
@@ -65,6 +65,7 @@ fn main() -> Result<()> {
 							true,
 							None,
 							None,
+							polkadot_service::RealOverseerGen,
 						).map_err(|e| e.to_string())?;
 						let mut overseer_handler = full_node
 							.overseer_handler
-- 
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