gate approval-checking logic (#2470)

Cargo.toml

@@ -91,6 +91,7 @@ panic = "unwind"
 [features]
 runtime-benchmarks=["cli/runtime-benchmarks"]
 real-overseer=["cli/real-overseer"]
+approval-checking=["real-overseer", "service/approval-checking"]
 
 # Configuration for building a .deb package - for use with `cargo-deb`
 [package.metadata.deb]

node/service/Cargo.toml

@@ -136,3 +136,7 @@ real-overseer = [
 	"polkadot-approval-distribution",
 	"polkadot-node-core-approval-voting",
 ]
+
+approval-checking = [
+	"real-overseer"
+]

node/service/src/grandpa_support.rs

@@ -18,17 +18,20 @@
 
 use std::sync::Arc;
 
-use polkadot_primitives::v1::{Block as PolkadotBlock, Header as PolkadotHeader, BlockNumber, Hash};
-use polkadot_subsystem::messages::ApprovalVotingMessage;
+use polkadot_primitives::v1::Hash;
 
 use sp_runtime::traits::{Block as BlockT, NumberFor};
 use sp_runtime::generic::BlockId;
 use sp_runtime::traits::Header as _;
 
-use prometheus_endpoint::{self, Registry};
-use polkadot_overseer::OverseerHandler;
-
-use futures::channel::oneshot;
+#[cfg(feature = "approval-checking")]
+use {
+	polkadot_primitives::v1::{Block as PolkadotBlock, Header as PolkadotHeader, BlockNumber},
+	polkadot_subsystem::messages::ApprovalVotingMessage,
+	prometheus_endpoint::{self, Registry},
+	polkadot_overseer::OverseerHandler,
+	futures::channel::oneshot,
+};
 
 /// A custom GRANDPA voting rule that acts as a diagnostic for the approval
 /// voting subsystem's desired votes.
@@ -36,14 +39,14 @@ use futures::channel::oneshot;
 /// The practical effect of this voting rule is to implement a fixed delay of
 /// blocks and to issue a prometheus metric on the lag behind the head that
 /// approval checking would indicate.
-#[cfg(feature = "full-node")]
+#[cfg(feature = "approval-checking")]
 #[derive(Clone)]
 pub(crate) struct ApprovalCheckingDiagnostic {
 	checking_lag: Option<prometheus_endpoint::Histogram>,
 	overseer: OverseerHandler,
 }
 
-#[cfg(feature = "full-node")]
+#[cfg(feature = "approval-checking")]
 impl ApprovalCheckingDiagnostic {
 	/// Create a new approval checking diagnostic voting rule.
 	pub fn new(overseer: OverseerHandler, registry: Option<&Registry>)
@@ -68,7 +71,7 @@ impl ApprovalCheckingDiagnostic {
 	}
 }
 
-#[cfg(feature = "full-node")]
+#[cfg(feature = "approval-checking")]
 impl<B> grandpa::VotingRule<PolkadotBlock, B> for ApprovalCheckingDiagnostic
 	where B: sp_blockchain::HeaderBackend<PolkadotBlock>
 {

node/service/src/lib.rs

@@ -412,8 +412,13 @@ where
 	use polkadot_statement_distribution::StatementDistribution as StatementDistributionSubsystem;
 	use polkadot_availability_recovery::AvailabilityRecoverySubsystem;
 	use polkadot_approval_distribution::ApprovalDistribution as ApprovalDistributionSubsystem;
+
+	#[cfg(feature = "approval-checking")]
 	use polkadot_node_core_approval_voting::ApprovalVotingSubsystem;
 
+	#[cfg(not(feature = "approval-checking"))]
+	let _ = slot_duration; // silence.
+
 	let all_subsystems = AllSubsystems {
 		availability_distribution: AvailabilityDistributionSubsystem::new(
 			keystore.clone(),
@@ -488,11 +493,14 @@ where
 		approval_distribution: ApprovalDistributionSubsystem::new(
 			Metrics::register(registry)?,
 		),
+		#[cfg(feature = "approval-checking")]
 		approval_voting: ApprovalVotingSubsystem::new(
 			keystore.clone(),
 			slot_duration,
 			runtime_client.clone(),
 		),
+		#[cfg(not(feature = "approval-checking"))]
+		approval_voting: polkadot_subsystem::DummySubsystem,
 	};
 
 	Overseer::new(
@@ -828,6 +836,7 @@ pub fn new_full<RuntimeApi, Executor>(
 		// given delay.
 		let builder = grandpa::VotingRulesBuilder::default();
 
+		#[cfg(feature = "approval-checking")]
 		let builder = if let Some(ref overseer) = overseer_handler {
 			builder.add(grandpa_support::ApprovalCheckingDiagnostic::new(
 				overseer.clone(),