Uniform framework for bg variable management

src/block/manager.rs

@@ -23,10 +23,12 @@ use garage_rpc::rpc_helper::netapp::stream::{stream_asyncread, ByteStream};
 
 use garage_db as db;
 
-use garage_util::background::BackgroundRunner;
+use garage_util::background::{vars, BackgroundRunner};
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::metrics::RecordDuration;
+use garage_util::persister::PersisterShared;
+use garage_util::time::msec_to_rfc3339;
 
 use garage_rpc::rpc_helper::OrderTag;
 use garage_rpc::system::System;
@@ -89,6 +91,7 @@ pub struct BlockManager {
 
 	pub(crate) metrics: BlockManagerMetrics,
 
+	pub scrub_persister: PersisterShared<ScrubWorkerPersisted>,
 	tx_scrub_command: ArcSwapOption<mpsc::Sender<ScrubWorkerCommand>>,
 }
 
@@ -128,6 +131,8 @@ impl BlockManager {
 		let metrics =
 			BlockManagerMetrics::new(rc.rc.clone(), resync.queue.clone(), resync.errors.clone());
 
+		let scrub_persister = PersisterShared::new(&system.metadata_dir, "scrub_info");
+
 		let block_manager = Arc::new(Self {
 			replication,
 			data_dir,
@@ -138,6 +143,7 @@ impl BlockManager {
 			system,
 			endpoint,
 			metrics,
+			scrub_persister,
 			tx_scrub_command: ArcSwapOption::new(None),
 		});
 		block_manager.endpoint.set_handler(block_manager.clone());
@@ -155,7 +161,28 @@ impl BlockManager {
 		// Spawn scrub worker
 		let (scrub_tx, scrub_rx) = mpsc::channel(1);
 		self.tx_scrub_command.store(Some(Arc::new(scrub_tx)));
-		bg.spawn_worker(ScrubWorker::new(self.clone(), scrub_rx));
+		bg.spawn_worker(ScrubWorker::new(
+			self.clone(),
+			scrub_rx,
+			self.scrub_persister.clone(),
+		));
+	}
+
+	pub fn register_bg_vars(&self, vars: &mut vars::BgVars) {
+		self.resync.register_bg_vars(vars);
+
+		vars.register_rw(
+			&self.scrub_persister,
+			"scrub-tranquility",
+			|p| p.get_with(|x| x.tranquility),
+			|p, tranquility| p.set_with(|x| x.tranquility = tranquility),
+		);
+		vars.register_ro(&self.scrub_persister, "scrub-last-completed", |p| {
+			p.get_with(|x| msec_to_rfc3339(x.time_last_complete_scrub))
+		});
+		vars.register_ro(&self.scrub_persister, "scrub-corruptions_detected", |p| {
+			p.get_with(|x| x.corruptions_detected)
+		});
 	}
 
 	/// Ask nodes that might have a (possibly compressed) block for it

src/block/repair.rs

@@ -13,7 +13,7 @@ use tokio::sync::watch;
 use garage_util::background::*;
 use garage_util::data::*;
 use garage_util::error::*;
-use garage_util::persister::Persister;
+use garage_util::persister::PersisterShared;
 use garage_util::time::*;
 use garage_util::tranquilizer::Tranquilizer;
 
@@ -168,17 +168,25 @@ pub struct ScrubWorker {
 	work: ScrubWorkerState,
 	tranquilizer: Tranquilizer,
 
-	persister: Persister<ScrubWorkerPersisted>,
-	persisted: ScrubWorkerPersisted,
+	persister: PersisterShared<ScrubWorkerPersisted>,
 }
 
 #[derive(Serialize, Deserialize)]
-struct ScrubWorkerPersisted {
-	tranquility: u32,
-	time_last_complete_scrub: u64,
-	corruptions_detected: u64,
+pub struct ScrubWorkerPersisted {
+	pub tranquility: u32,
+	pub(crate) time_last_complete_scrub: u64,
+	pub(crate) corruptions_detected: u64,
 }
 impl garage_util::migrate::InitialFormat for ScrubWorkerPersisted {}
+impl Default for ScrubWorkerPersisted {
+	fn default() -> Self {
+		ScrubWorkerPersisted {
+			time_last_complete_scrub: 0,
+			tranquility: INITIAL_SCRUB_TRANQUILITY,
+			corruptions_detected: 0,
+		}
+	}
+}
 
 enum ScrubWorkerState {
 	Running(BlockStoreIterator),
@@ -198,27 +206,20 @@ pub enum ScrubWorkerCommand {
 	Pause(Duration),
 	Resume,
 	Cancel,
-	SetTranquility(u32),
 }
 
 impl ScrubWorker {
-	pub fn new(manager: Arc<BlockManager>, rx_cmd: mpsc::Receiver<ScrubWorkerCommand>) -> Self {
-		let persister = Persister::new(&manager.system.metadata_dir, "scrub_info");
-		let persisted = match persister.load() {
-			Ok(v) => v,
-			Err(_) => ScrubWorkerPersisted {
-				time_last_complete_scrub: 0,
-				tranquility: INITIAL_SCRUB_TRANQUILITY,
-				corruptions_detected: 0,
-			},
-		};
+	pub(crate) fn new(
+		manager: Arc<BlockManager>,
+		rx_cmd: mpsc::Receiver<ScrubWorkerCommand>,
+		persister: PersisterShared<ScrubWorkerPersisted>,
+	) -> Self {
 		Self {
 			manager,
 			rx_cmd,
 			work: ScrubWorkerState::Finished,
 			tranquilizer: Tranquilizer::new(30),
 			persister,
-			persisted,
 		}
 	}
 
@@ -267,12 +268,6 @@ impl ScrubWorker {
 					}
 				}
 			}
-			ScrubWorkerCommand::SetTranquility(t) => {
-				self.persisted.tranquility = t;
-				if let Err(e) = self.persister.save_async(&self.persisted).await {
-					error!("Could not save new tranquilitiy value: {}", e);
-				}
-			}
 		}
 	}
 }
@@ -284,9 +279,18 @@ impl Worker for ScrubWorker {
 	}
 
 	fn status(&self) -> WorkerStatus {
+		let (corruptions_detected, tranquility, time_last_complete_scrub) =
+			self.persister.get_with(|p| {
+				(
+					p.corruptions_detected,
+					p.tranquility,
+					p.time_last_complete_scrub,
+				)
+			});
+
 		let mut s = WorkerStatus {
-			persistent_errors: Some(self.persisted.corruptions_detected),
-			tranquility: Some(self.persisted.tranquility),
+			persistent_errors: Some(corruptions_detected),
+			tranquility: Some(tranquility),
 			..Default::default()
 		};
 		match &self.work {
@@ -300,7 +304,7 @@ impl Worker for ScrubWorker {
 			ScrubWorkerState::Finished => {
 				s.freeform = vec![format!(
 					"Last scrub completed at {}",
-					msec_to_rfc3339(self.persisted.time_last_complete_scrub)
+					msec_to_rfc3339(time_last_complete_scrub)
 				)];
 			}
 		}
@@ -321,18 +325,17 @@ impl Worker for ScrubWorker {
 					match self.manager.read_block(&hash).await {
 						Err(Error::CorruptData(_)) => {
 							error!("Found corrupt data block during scrub: {:?}", hash);
-							self.persisted.corruptions_detected += 1;
-							self.persister.save_async(&self.persisted).await?;
+							self.persister.set_with(|p| p.corruptions_detected += 1)?;
 						}
 						Err(e) => return Err(e),
 						_ => (),
 					};
 					Ok(self
 						.tranquilizer
-						.tranquilize_worker(self.persisted.tranquility))
+						.tranquilize_worker(self.persister.get_with(|p| p.tranquility)))
 				} else {
-					self.persisted.time_last_complete_scrub = now_msec();
-					self.persister.save_async(&self.persisted).await?;
+					self.persister
+						.set_with(|p| p.time_last_complete_scrub = now_msec())?;
 					self.work = ScrubWorkerState::Finished;
 					self.tranquilizer.clear();
 					Ok(WorkerState::Idle)
@@ -347,7 +350,8 @@ impl Worker for ScrubWorker {
 			ScrubWorkerState::Running(_) => return WorkerState::Busy,
 			ScrubWorkerState::Paused(_, resume_time) => (*resume_time, ScrubWorkerCommand::Resume),
 			ScrubWorkerState::Finished => (
-				self.persisted.time_last_complete_scrub + SCRUB_INTERVAL.as_millis() as u64,
+				self.persister.get_with(|p| p.time_last_complete_scrub)
+					+ SCRUB_INTERVAL.as_millis() as u64,
 				ScrubWorkerCommand::Start,
 			),
 		};

src/block/resync.rs

@@ -3,7 +3,6 @@ use std::convert::TryInto;
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use arc_swap::ArcSwap;
 use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 
@@ -22,7 +21,7 @@ use garage_util::background::*;
 use garage_util::data::*;
 use garage_util::error::*;
 use garage_util::metrics::RecordDuration;
-use garage_util::persister::Persister;
+use garage_util::persister::PersisterShared;
 use garage_util::time::*;
 use garage_util::tranquilizer::Tranquilizer;
 
@@ -49,13 +48,12 @@ const INITIAL_RESYNC_TRANQUILITY: u32 = 2;
 
 pub struct BlockResyncManager {
 	pub(crate) queue: CountedTree,
-	pub(crate) notify: Notify,
+	pub(crate) notify: Arc<Notify>,
 	pub(crate) errors: CountedTree,
 
 	busy_set: BusySet,
 
-	persister: Persister<ResyncPersistedConfig>,
-	persisted: ArcSwap<ResyncPersistedConfig>,
+	persister: PersisterShared<ResyncPersistedConfig>,
 }
 
 #[derive(Serialize, Deserialize, Clone, Copy)]
@@ -64,6 +62,14 @@ struct ResyncPersistedConfig {
 	tranquility: u32,
 }
 impl garage_util::migrate::InitialFormat for ResyncPersistedConfig {}
+impl Default for ResyncPersistedConfig {
+	fn default() -> Self {
+		ResyncPersistedConfig {
+			n_workers: 1,
+			tranquility: INITIAL_RESYNC_TRANQUILITY,
+		}
+	}
+}
 
 enum ResyncIterResult {
 	BusyDidSomething,
@@ -91,22 +97,14 @@ impl BlockResyncManager {
 			.expect("Unable to open block_local_resync_errors tree");
 		let errors = CountedTree::new(errors).expect("Could not count block_local_resync_errors");
 
-		let persister = Persister::new(&system.metadata_dir, "resync_cfg");
-		let persisted = match persister.load() {
-			Ok(v) => v,
-			Err(_) => ResyncPersistedConfig {
-				n_workers: 1,
-				tranquility: INITIAL_RESYNC_TRANQUILITY,
-			},
-		};
+		let persister = PersisterShared::new(&system.metadata_dir, "resync_cfg");
 
 		Self {
 			queue,
-			notify: Notify::new(),
+			notify: Arc::new(Notify::new()),
 			errors,
 			busy_set: Arc::new(Mutex::new(HashSet::new())),
 			persister,
-			persisted: ArcSwap::new(Arc::new(persisted)),
 		}
 	}
 
@@ -142,6 +140,38 @@ impl BlockResyncManager {
 		)))
 	}
 
+	pub fn register_bg_vars(&self, vars: &mut vars::BgVars) {
+		let notify = self.notify.clone();
+		vars.register_rw(
+			&self.persister,
+			"resync-worker-count",
+			|p| p.get_with(|x| x.n_workers),
+			move |p, n_workers| {
+				if !(1..=MAX_RESYNC_WORKERS).contains(&n_workers) {
+					return Err(Error::Message(format!(
+						"Invalid number of resync workers, must be between 1 and {}",
+						MAX_RESYNC_WORKERS
+					)));
+				}
+				p.set_with(|x| x.n_workers = n_workers)?;
+				notify.notify_waiters();
+				Ok(())
+			},
+		);
+
+		let notify = self.notify.clone();
+		vars.register_rw(
+			&self.persister,
+			"resync-tranquility",
+			|p| p.get_with(|x| x.tranquility),
+			move |p, tranquility| {
+				p.set_with(|x| x.tranquility = tranquility)?;
+				notify.notify_waiters();
+				Ok(())
+			},
+		);
+	}
+
 	// ---- Resync loop ----
 
 	// This part manages a queue of blocks that need to be
@@ -436,33 +466,6 @@ impl BlockResyncManager {
 
 		Ok(())
 	}
-
-	async fn update_persisted(
-		&self,
-		update: impl Fn(&mut ResyncPersistedConfig),
-	) -> Result<(), Error> {
-		let mut cfg: ResyncPersistedConfig = *self.persisted.load().as_ref();
-		update(&mut cfg);
-		self.persister.save_async(&cfg).await?;
-		self.persisted.store(Arc::new(cfg));
-		self.notify.notify_waiters();
-		Ok(())
-	}
-
-	pub async fn set_n_workers(&self, n_workers: usize) -> Result<(), Error> {
-		if !(1..=MAX_RESYNC_WORKERS).contains(&n_workers) {
-			return Err(Error::Message(format!(
-				"Invalid number of resync workers, must be between 1 and {}",
-				MAX_RESYNC_WORKERS
-			)));
-		}
-		self.update_persisted(|cfg| cfg.n_workers = n_workers).await
-	}
-
-	pub async fn set_tranquility(&self, tranquility: u32) -> Result<(), Error> {
-		self.update_persisted(|cfg| cfg.tranquility = tranquility)
-			.await
-	}
 }
 
 impl Drop for BusyBlock {
@@ -477,15 +480,18 @@ pub(crate) struct ResyncWorker {
 	manager: Arc<BlockManager>,
 	tranquilizer: Tranquilizer,
 	next_delay: Duration,
+	persister: PersisterShared<ResyncPersistedConfig>,
 }
 
 impl ResyncWorker {
 	pub(crate) fn new(index: usize, manager: Arc<BlockManager>) -> Self {
+		let persister = manager.resync.persister.clone();
 		Self {
 			index,
 			manager,
 			tranquilizer: Tranquilizer::new(30),
 			next_delay: Duration::from_secs(10),
+			persister,
 		}
 	}
 }
@@ -497,9 +503,9 @@ impl Worker for ResyncWorker {
 	}
 
 	fn status(&self) -> WorkerStatus {
-		let persisted = self.manager.resync.persisted.load();
+		let (n_workers, tranquility) = self.persister.get_with(|x| (x.n_workers, x.tranquility));
 
-		if self.index >= persisted.n_workers {
+		if self.index >= n_workers {
 			return WorkerStatus {
 				freeform: vec!["This worker is currently disabled".into()],
 				..Default::default()
@@ -508,22 +514,24 @@ impl Worker for ResyncWorker {
 
 		WorkerStatus {
 			queue_length: Some(self.manager.resync.queue_len().unwrap_or(0) as u64),
-			tranquility: Some(persisted.tranquility),
+			tranquility: Some(tranquility),
 			persistent_errors: Some(self.manager.resync.errors_len().unwrap_or(0) as u64),
 			..Default::default()
 		}
 	}
 
 	async fn work(&mut self, _must_exit: &mut watch::Receiver<bool>) -> Result<WorkerState, Error> {
-		if self.index >= self.manager.resync.persisted.load().n_workers {
+		let (n_workers, tranquility) = self.persister.get_with(|x| (x.n_workers, x.tranquility));
+
+		if self.index >= n_workers {
 			return Ok(WorkerState::Idle);
 		}
 
 		self.tranquilizer.reset();
 		match self.manager.resync.resync_iter(&self.manager).await {
-			Ok(ResyncIterResult::BusyDidSomething) => Ok(self
-				.tranquilizer
-				.tranquilize_worker(self.manager.resync.persisted.load().tranquility)),
+			Ok(ResyncIterResult::BusyDidSomething) => {
+				Ok(self.tranquilizer.tranquilize_worker(tranquility))
+			}
 			Ok(ResyncIterResult::BusyDidNothing) => Ok(WorkerState::Busy),
 			Ok(ResyncIterResult::IdleFor(delay)) => {
 				self.next_delay = delay;
@@ -542,7 +550,7 @@ impl Worker for ResyncWorker {
 	}
 
 	async fn wait_for_work(&mut self) -> WorkerState {
-		while self.index >= self.manager.resync.persisted.load().n_workers {
+		while self.index >= self.persister.get_with(|x| x.n_workers) {
 			self.manager.resync.notify.notified().await
 		}
 

src/garage/admin.rs

@@ -18,7 +18,6 @@ use garage_table::*;
 use garage_rpc::*;
 
 use garage_block::manager::BlockResyncErrorInfo;
-use garage_block::repair::ScrubWorkerCommand;
 
 use garage_model::bucket_alias_table::*;
 use garage_model::bucket_table::*;
@@ -60,6 +59,7 @@ pub enum AdminRpc {
 		HashMap<usize, garage_util::background::WorkerInfo>,
 		WorkerListOpt,
 	),
+	WorkerVars(Vec<(String, String)>),
 	WorkerInfo(usize, garage_util::background::WorkerInfo),
 	BlockErrorList(Vec<BlockResyncErrorInfo>),
 	BlockInfo {
@@ -943,32 +943,27 @@ impl AdminRpcHandler {
 					.clone();
 				Ok(AdminRpc::WorkerInfo(*tid, info))
 			}
-			WorkerOperation::Set { opt } => match opt {
-				WorkerSetCmd::ScrubTranquility { tranquility } => {
-					let scrub_command = ScrubWorkerCommand::SetTranquility(*tranquility);
+			WorkerOperation::Get { variable } => {
+				if let Some(v) = variable {
+					Ok(AdminRpc::WorkerVars(vec![(
+						v.clone(),
+						self.garage.bg_vars.get(&v)?,
+					)]))
+				} else {
+					Ok(AdminRpc::WorkerVars(
 						self.garage
-						.block_manager
-						.send_scrub_command(scrub_command)
-						.await?;
-					Ok(AdminRpc::Ok("Scrub tranquility updated".into()))
+							.bg_vars
+							.get_all()
+							.into_iter()
+							.map(|(k, v)| (k.to_string(), v))
+							.collect(),
+					))
 				}
-				WorkerSetCmd::ResyncWorkerCount { worker_count } => {
-					self.garage
-						.block_manager
-						.resync
-						.set_n_workers(*worker_count)
-						.await?;
-					Ok(AdminRpc::Ok("Number of resync workers updated".into()))
 			}
-				WorkerSetCmd::ResyncTranquility { tranquility } => {
-					self.garage
-						.block_manager
-						.resync
-						.set_tranquility(*tranquility)
-						.await?;
-					Ok(AdminRpc::Ok("Resync tranquility updated".into()))
+			WorkerOperation::Set { variable, value } => {
+				self.garage.bg_vars.set(&variable, &value)?;
+				Ok(AdminRpc::Ok(format!("{} was set to {}", variable, value)))
 			}
-			},
 		}
 	}
 

src/garage/cli/cmd.rs

@@ -191,6 +191,9 @@ pub async fn cmd_admin(
 		AdminRpc::WorkerList(wi, wlo) => {
 			print_worker_list(wi, wlo);
 		}
+		AdminRpc::WorkerVars(wv) => {
+			print_worker_vars(wv);
+		}
 		AdminRpc::WorkerInfo(tid, wi) => {
 			print_worker_info(tid, wi);
 		}

src/garage/cli/structs.rs

@@ -517,12 +517,12 @@ pub enum WorkerOperation {
 	/// Get detailed information about a worker
 	#[structopt(name = "info", version = garage_version())]
 	Info { tid: usize },
+	/// Get worker parameter
+	#[structopt(name = "get", version = garage_version())]
+	Get { variable: Option<String> },
 	/// Set worker parameter
 	#[structopt(name = "set", version = garage_version())]
-	Set {
-		#[structopt(subcommand)]
-		opt: WorkerSetCmd,
-	},
+	Set { variable: String, value: String },
 }
 
 #[derive(Serialize, Deserialize, StructOpt, Debug, Eq, PartialEq, Clone, Copy)]
@@ -535,19 +535,6 @@ pub struct WorkerListOpt {
 	pub errors: bool,
 }
 
-#[derive(Serialize, Deserialize, StructOpt, Debug, Eq, PartialEq, Clone)]
-pub enum WorkerSetCmd {
-	/// Set tranquility of scrub operations
-	#[structopt(name = "scrub-tranquility", version = garage_version())]
-	ScrubTranquility { tranquility: u32 },
-	/// Set number of concurrent block resync workers
-	#[structopt(name = "resync-worker-count", version = garage_version())]
-	ResyncWorkerCount { worker_count: usize },
-	/// Set tranquility of block resync operations
-	#[structopt(name = "resync-tranquility", version = garage_version())]
-	ResyncTranquility { tranquility: u32 },
-}
-
 #[derive(Serialize, Deserialize, StructOpt, Debug, Eq, PartialEq, Clone)]
 pub enum BlockOperation {
 	/// List all blocks that currently have a resync error

src/garage/cli/util.rs

@@ -357,6 +357,14 @@ pub fn print_worker_info(tid: usize, info: WorkerInfo) {
 	format_table(table);
 }
 
+pub fn print_worker_vars(wv: Vec<(String, String)>) {
+	let table = wv
+		.into_iter()
+		.map(|(k, v)| format!("{}\t{}", k, v))
+		.collect::<Vec<_>>();
+	format_table(table);
+}
+
 pub fn print_block_error_list(el: Vec<BlockResyncErrorInfo>) {
 	let now = now_msec();
 	let tf = timeago::Formatter::new();

src/garage/repair/online.rs

@@ -51,7 +51,11 @@ pub async fn launch_online_repair(
 				ScrubCmd::Resume => ScrubWorkerCommand::Resume,
 				ScrubCmd::Cancel => ScrubWorkerCommand::Cancel,
 				ScrubCmd::SetTranquility { tranquility } => {
-					ScrubWorkerCommand::SetTranquility(tranquility)
+					garage
+						.block_manager
+						.scrub_persister
+						.set_with(|x| x.tranquility = tranquility)?;
+					return Ok(());
 				}
 			};
 			info!("Sending command to scrub worker: {:?}", cmd);

src/model/garage.rs

@@ -33,6 +33,8 @@ use crate::k2v::{item_table::*, poll::*, rpc::*};
 pub struct Garage {
 	/// The parsed configuration Garage is running
 	pub config: Config,
+	/// The set of background variables that can be viewed/modified at runtime
+	pub bg_vars: vars::BgVars,
 
 	/// The replication mode of this cluster
 	pub replication_mode: ReplicationMode,
@@ -249,9 +251,14 @@ impl Garage {
 		#[cfg(feature = "k2v")]
 		let k2v = GarageK2V::new(system.clone(), &db, meta_rep_param);
 
+		// Initialize bg vars
+		let mut bg_vars = vars::BgVars::new();
+		block_manager.register_bg_vars(&mut bg_vars);
+
 		// -- done --
 		Ok(Arc::new(Self {
 			config,
+			bg_vars,
 			replication_mode,
 			db,
 			system,

src/util/background/mod.rs

@@ -1,5 +1,6 @@
 //! Job runner for futures and async functions
 
+pub mod vars;
 pub mod worker;
 
 use std::collections::HashMap;

src/util/background/vars.rs

@@ -0,0 +1,107 @@
+use std::collections::HashMap;
+use std::str::FromStr;
+
+use crate::error::{Error, OkOrMessage};
+use crate::migrate::Migrate;
+use crate::persister::PersisterShared;
+
+pub struct BgVars {
+	vars: HashMap<&'static str, Box<dyn BgVarTrait>>,
+}
+
+impl BgVars {
+	pub fn new() -> Self {
+		Self {
+			vars: HashMap::new(),
+		}
+	}
+
+	pub fn register_rw<V, T, GF, SF>(
+		&mut self,
+		p: &PersisterShared<V>,
+		name: &'static str,
+		get_fn: GF,
+		set_fn: SF,
+	) where
+		V: Migrate + Default + Send + Sync,
+		T: FromStr + ToString + Send + Sync + 'static,
+		GF: Fn(&PersisterShared<V>) -> T + Send + Sync + 'static,
+		SF: Fn(&PersisterShared<V>, T) -> Result<(), Error> + Send + Sync + 'static,
+	{
+		let p1 = p.clone();
+		let get_fn = move || get_fn(&p1);
+
+		let p2 = p.clone();
+		let set_fn = move |v| set_fn(&p2, v);
+
+		self.vars.insert(name, Box::new(BgVar { get_fn, set_fn }));
+	}
+
+	pub fn register_ro<V, T, GF>(&mut self, p: &PersisterShared<V>, name: &'static str, get_fn: GF)
+	where
+		V: Migrate + Default + Send + Sync,
+		T: FromStr + ToString + Send + Sync + 'static,
+		GF: Fn(&PersisterShared<V>) -> T + Send + Sync + 'static,
+	{
+		let p1 = p.clone();
+		let get_fn = move || get_fn(&p1);
+
+		let set_fn = move |_| Err(Error::Message(format!("Cannot set value of {}", name)));
+
+		self.vars.insert(name, Box::new(BgVar { get_fn, set_fn }));
+	}
+
+	pub fn get(&self, var: &str) -> Result<String, Error> {
+		Ok(self
+			.vars
+			.get(var)
+			.ok_or_message("variable does not exist")?
+			.get())
+	}
+
+	pub fn get_all(&self) -> Vec<(&'static str, String)> {
+		self.vars.iter().map(|(k, v)| (*k, v.get())).collect()
+	}
+
+	pub fn set(&self, var: &str, val: &str) -> Result<(), Error> {
+		self.vars
+			.get(var)
+			.ok_or_message("variable does not exist")?
+			.set(val)
+	}
+}
+
+// ----
+
+trait BgVarTrait: Send + Sync + 'static {
+	fn get(&self) -> String;
+	fn set(&self, v: &str) -> Result<(), Error>;
+}
+
+struct BgVar<T, GF, SF>
+where
+	T: FromStr + ToString + Send + Sync + 'static,
+	GF: Fn() -> T + Send + Sync + 'static,
+	SF: Fn(T) -> Result<(), Error> + Sync + Send + 'static,
+{
+	get_fn: GF,
+	set_fn: SF,
+}
+
+impl<T, GF, SF> BgVarTrait for BgVar<T, GF, SF>
+where
+	T: FromStr + ToString + Sync + Send + 'static,
+	GF: Fn() -> T + Sync + Send + 'static,
+	SF: Fn(T) -> Result<(), Error> + Sync + Send + 'static,
+{
+	fn get(&self) -> String {
+		(self.get_fn)().to_string()
+	}
+
+	fn set(&self, vstr: &str) -> Result<(), Error> {
+		let value = vstr
+			.parse()
+			.map_err(|_| Error::Message(format!("invalid value: {}", vstr)))?;
+		(self.set_fn)(value)
+	}
+}

src/util/persister.rs

@@ -1,5 +1,6 @@
 use std::io::{Read, Write};
 use std::path::{Path, PathBuf};
+use std::sync::{Arc, RwLock};
 
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
@@ -84,3 +85,36 @@ impl<T: Migrate> Persister<T> {
 		Ok(())
 	}
 }
+
+pub struct PersisterShared<V: Migrate + Default>(Arc<(Persister<V>, RwLock<V>)>);
+
+impl<V: Migrate + Default> Clone for PersisterShared<V> {
+	fn clone(&self) -> PersisterShared<V> {
+		PersisterShared(self.0.clone())
+	}
+}
+
+impl<V: Migrate + Default> PersisterShared<V> {
+	pub fn new(base_dir: &Path, file_name: &str) -> Self {
+		let persister = Persister::new(base_dir, file_name);
+		let value = persister.load().unwrap_or_default();
+		Self(Arc::new((persister, RwLock::new(value))))
+	}
+
+	pub fn get_with<F, R>(&self, f: F) -> R
+	where
+		F: FnOnce(&V) -> R,
+	{
+		let value = self.0 .1.read().unwrap();
+		f(&value)
+	}
+
+	pub fn set_with<F>(&self, f: F) -> Result<(), Error>
+	where
+		F: FnOnce(&mut V),
+	{
+		let mut value = self.0 .1.write().unwrap();
+		f(&mut value);
+		self.0 .0.save(&value)
+	}
+}