refactor: Use ReplicationFactor type in more places

- Remove the replication_factor.replication_factor() in favor of
  usize::from(replication_factor) to make the conversion more explicit.

- Implement Display on ReplicationFactor so that it can be formatted
  without converting to usize

- Use ReplicationFactor in the constructor of LayoutVersion and add a
  method to get a ReplicationFactor from a LayoutVersion, despite
  LayoutVersion still storing it as usize internally.

src/rpc/layout/manager.rs

@@ -46,11 +46,11 @@ impl LayoutManager {
 
 		let cluster_layout = match persist_cluster_layout.load() {
 			Ok(x) => {
-				if x.current().replication_factor != replication_factor.replication_factor() {
+				if x.current().replication_factor() != replication_factor {
 					return Err(Error::Message(format!(
 						"Previous cluster layout has replication factor {}, which is different than the one specified in the config file ({}). The previous cluster layout can be purged, if you know what you are doing, simply by deleting the `cluster_layout` file in your metadata directory.",
-						x.current().replication_factor,
-						replication_factor.replication_factor()
+						x.current().replication_factor(),
+						replication_factor,
 					)));
 				}
 				x
@@ -301,11 +301,11 @@ impl LayoutManager {
 			adv.update_trackers
 		);
 
-		if adv.current().replication_factor != self.replication_factor.replication_factor() {
+		if adv.current().replication_factor() != self.replication_factor {
 			let msg = format!(
 				"Received a cluster layout from another node with replication factor {}, which is different from what we have in our configuration ({}). Discarding the cluster layout we received.",
-				adv.current().replication_factor,
-				self.replication_factor.replication_factor()
+				adv.current().replication_factor(),
+				self.replication_factor,
 			);
 			error!("{}", msg);
 			return Err(Error::Message(msg));

src/rpc/layout/version.rs

@@ -11,12 +11,13 @@ use garage_util::error::*;
 
 use super::graph_algo::*;
 use super::*;
+use crate::replication_mode::*;
 
 // The Message type will be used to collect information on the algorithm.
 pub type Message = Vec<String>;
 
 impl LayoutVersion {
-	pub fn new(replication_factor: usize) -> Self {
+	pub fn new(replication_factor: ReplicationFactor) -> Self {
 		// We set the default zone redundancy to be Maximum, meaning that the maximum
 		// possible value will be used depending on the cluster topology
 		let parameters = LayoutParameters {
@@ -25,7 +26,7 @@ impl LayoutVersion {
 
 		LayoutVersion {
 			version: 0,
-			replication_factor,
+			replication_factor: usize::from(replication_factor),
 			partition_size: 0,
 			roles: LwwMap::new(),
 			node_id_vec: Vec::new(),
@@ -132,6 +133,10 @@ impl LayoutVersion {
 			.map(move |i| self.node_id_vec[*i as usize])
 	}
 
+	pub fn replication_factor(&self) -> ReplicationFactor {
+		ReplicationFactor::new(self.replication_factor).unwrap()
+	}
+
 	// ===================== internal information extractors ======================
 
 	pub(crate) fn expect_get_node_capacity(&self, uuid: &Uuid) -> u64 {

src/rpc/replication_mode.rs

@@ -38,14 +38,10 @@ impl ReplicationFactor {
 		}
 	}
 
-	pub fn replication_factor(&self) -> usize {
-		self.0
-	}
-
 	pub fn read_quorum(&self, consistency_mode: ConsistencyMode) -> usize {
 		match consistency_mode {
 			ConsistencyMode::Dangerous | ConsistencyMode::Degraded => 1,
-			ConsistencyMode::Consistent => self.replication_factor().div_ceil(2),
+			ConsistencyMode::Consistent => usize::from(*self).div_ceil(2),
 		}
 	}
 
@@ -53,7 +49,7 @@ impl ReplicationFactor {
 		match consistency_mode {
 			ConsistencyMode::Dangerous => 1,
 			ConsistencyMode::Degraded | ConsistencyMode::Consistent => {
-				(self.replication_factor() + 1) - self.read_quorum(ConsistencyMode::Consistent)
+				(usize::from(*self) + 1) - self.read_quorum(ConsistencyMode::Consistent)
 			}
 		}
 	}
@@ -65,6 +61,12 @@ impl std::convert::From<ReplicationFactor> for usize {
 	}
 }
 
+impl std::fmt::Display for ReplicationFactor {
+	fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+		self.0.fmt(f)
+	}
+}
+
 pub fn parse_replication_mode(
 	config: &Config,
 ) -> Result<(ReplicationFactor, ConsistencyMode), Error> {

src/rpc/system_metrics.rs

@@ -68,7 +68,7 @@ impl SystemMetrics {
 				let replication_factor = system.replication_factor;
 				meter
 					.u64_value_observer("garage_replication_factor", move |observer| {
-						observer.observe(replication_factor.replication_factor() as u64, &[])
+						observer.observe(usize::from(replication_factor) as u64, &[])
 					})
 					.with_description("Garage replication factor setting")
 					.init()