node: add sentry mode flag (#3959)

* node: add sentry mode flag

* cli: extend docs on validator and sentry modes

* service: add missing field in test Configuration

* node: Display instead of Debug when printing node role

substrate/node-template/src/service.rs

@@ -80,6 +80,11 @@ pub fn new_full<C: Send + Default + 'static>(config: Configuration<C, GenesisCon
 	let name = config.name.clone();
 	let disable_grandpa = config.disable_grandpa;
 
+	// sentry nodes announce themselves as authorities to the network
+	// and should run the same protocols authorities do, but it should
+	// never actively participate in any consensus process.
+	let participates_in_consensus = is_authority && !config.sentry_mode;
+
 	let (builder, mut import_setup, inherent_data_providers) = new_full_start!(config);
 
 	let (block_import, grandpa_link) =
@@ -92,7 +97,7 @@ pub fn new_full<C: Send + Default + 'static>(config: Configuration<C, GenesisCon
 		)?
 		.build()?;
 
-	if is_authority {
+	if participates_in_consensus {
 		let proposer = basic_authorship::ProposerFactory {
 			client: service.client(),
 			transaction_pool: service.transaction_pool(),
@@ -119,13 +124,21 @@ pub fn new_full<C: Send + Default + 'static>(config: Configuration<C, GenesisCon
 		service.spawn_essential_task(aura);
 	}
 
+	// if the node isn't actively participating in consensus then it doesn't
+	// need a keystore, regardless of which protocol we use below.
+	let keystore = if participates_in_consensus {
+		Some(service.keystore())
+	} else {
+		None
+	};
+
 	let grandpa_config = grandpa::Config {
 		// FIXME #1578 make this available through chainspec
 		gossip_duration: Duration::from_millis(333),
 		justification_period: 512,
 		name: Some(name),
-		keystore: Some(service.keystore()),
 		observer_enabled: true,
+		keystore,
 		is_authority,
 	};