block manager: refactor: split resync into separate file

1 files changed, 48 insertions, 550 deletions

diff --git a/src/block/manager.rs b/src/block/manager.rs
index ef48107f..efb5349c 100644
--- a/src/block/manager.rs
+++ b/src/block/manager.rs
@@ -1,33 +1,19 @@
-use std::convert::TryInto;
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::time::Duration;
 
-use arc_swap::{ArcSwap, ArcSwapOption};
 use async_trait::async_trait;
 use serde::{Deserialize, Serialize};
 
-use futures::future::*;
 use tokio::fs;
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
-use tokio::select;
-use tokio::sync::{mpsc, watch, Mutex, Notify};
-
-use opentelemetry::{
-	trace::{FutureExt as OtelFutureExt, TraceContextExt, Tracer},
-	Context, KeyValue,
-};
+use tokio::sync::{mpsc, Mutex};
 
 use garage_db as db;
-use garage_db::counted_tree_hack::CountedTree;
 
-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::time::*;
-use garage_util::tranquilizer::Tranquilizer;
 
 use garage_rpc::system::System;
 use garage_rpc::*;
@@ -38,27 +24,13 @@ use crate::block::*;
 use crate::metrics::*;
 use crate::rc::*;
 use crate::repair::*;
+use crate::resync::*;
 
 /// Size under which data will be stored inlined in database instead of as files
 pub const INLINE_THRESHOLD: usize = 3072;
 
 // Timeout for RPCs that read and write blocks to remote nodes
-const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
-// Timeout for RPCs that ask other nodes whether they need a copy
-// of a given block before we delete it locally
-const NEED_BLOCK_QUERY_TIMEOUT: Duration = Duration::from_secs(5);
-
-// The delay between the time where a resync operation fails
-// and the time when it is retried, with exponential backoff
-// (multiplied by 2, 4, 8, 16, etc. for every consecutive failure).
-const RESYNC_RETRY_DELAY: Duration = Duration::from_secs(60);
-// The minimum retry delay is 60 seconds = 1 minute
-// The maximum retry delay is 60 seconds * 2^6 = 60 seconds << 6 = 64 minutes (~1 hour)
-const RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER: u64 = 6;
-
-// Resync tranquility is initially set to 2, but can be changed in the CLI
-// and the updated version is persisted over Garage restarts
-const INITIAL_RESYNC_TRANQUILITY: u32 = 2;
+pub(crate) const BLOCK_RW_TIMEOUT: Duration = Duration::from_secs(30);
 
 // The delay between the moment when the reference counter
 // drops to zero, and the moment where we allow ourselves
@@ -96,35 +68,23 @@ pub struct BlockManager {
 
 	compression_level: Option<i32>,
 
-	mutation_lock: Mutex<BlockManagerLocked>,
+	pub(crate) mutation_lock: Mutex<BlockManagerLocked>,
 
 	pub(crate) rc: BlockRc,
-
-	resync_queue: CountedTree,
-	resync_notify: Notify,
-	resync_errors: CountedTree,
-
-	resync_persister: Persister<ResyncPersistedConfig>,
-	resync_persisted: ArcSwap<ResyncPersistedConfig>,
+	pub resync: BlockResyncManager,
 
 	pub(crate) system: Arc<System>,
-	endpoint: Arc<Endpoint<BlockRpc, Self>>,
+	pub(crate) endpoint: Arc<Endpoint<BlockRpc, Self>>,
 
-	metrics: BlockManagerMetrics,
+	pub(crate) metrics: BlockManagerMetrics,
 
-	tx_scrub_command: ArcSwapOption<mpsc::Sender<ScrubWorkerCommand>>,
+	tx_scrub_command: mpsc::Sender<ScrubWorkerCommand>,
 }
 
 // This custom struct contains functions that must only be ran
 // when the lock is held. We ensure that it is the case by storing
 // it INSIDE a Mutex.
-struct BlockManagerLocked();
-
-enum ResyncIterResult {
-	BusyDidSomething,
-	BusyDidNothing,
-	IdleFor(Duration),
-}
+pub(crate) struct BlockManagerLocked();
 
 impl BlockManager {
 	pub fn new(
@@ -139,25 +99,7 @@ impl BlockManager {
 			.expect("Unable to open block_local_rc tree");
 		let rc = BlockRc::new(rc);
 
-		let resync_queue = db
-			.open_tree("block_local_resync_queue")
-			.expect("Unable to open block_local_resync_queue tree");
-		let resync_queue =
-			CountedTree::new(resync_queue).expect("Could not count block_local_resync_queue");
-
-		let resync_errors = db
-			.open_tree("block_local_resync_errors")
-			.expect("Unable to open block_local_resync_errors tree");
-		let resync_errors =
-			CountedTree::new(resync_errors).expect("Could not count block_local_resync_errors");
-
-		let resync_persister = Persister::new(&system.metadata_dir, "resync_cfg");
-		let resync_persisted = match resync_persister.load() {
-			Ok(v) => v,
-			Err(_) => ResyncPersistedConfig {
-				tranquility: INITIAL_RESYNC_TRANQUILITY,
-			},
-		};
+		let resync = BlockResyncManager::new(db, &system);
 
 		let endpoint = system
 			.netapp
@@ -165,7 +107,9 @@ impl BlockManager {
 
 		let manager_locked = BlockManagerLocked();
 
-		let metrics = BlockManagerMetrics::new(resync_queue.clone(), resync_errors.clone());
+		let metrics = BlockManagerMetrics::new(resync.queue.clone(), resync.errors.clone());
+
+		let (scrub_tx, scrub_rx) = mpsc::channel(1);
 
 		let block_manager = Arc::new(Self {
 			replication,
@@ -173,25 +117,31 @@ impl BlockManager {
 			compression_level,
 			mutation_lock: Mutex::new(manager_locked),
 			rc,
-			resync_queue,
-			resync_notify: Notify::new(),
-			resync_errors,
-			resync_persister,
-			resync_persisted: ArcSwap::new(Arc::new(resync_persisted)),
+			resync,
 			system,
 			endpoint,
 			metrics,
-			tx_scrub_command: ArcSwapOption::new(None),
+			tx_scrub_command: scrub_tx,
 		});
 		block_manager.endpoint.set_handler(block_manager.clone());
 
-		block_manager.clone().spawn_background_workers();
+		// Spawn one resync worker
+		let background = block_manager.system.background.clone();
+		let worker = ResyncWorker::new(block_manager.clone());
+		tokio::spawn(async move {
+			tokio::time::sleep(Duration::from_secs(10)).await;
+			background.spawn_worker(worker);
+		});
+
+		// Spawn scrub worker
+		let scrub_worker = ScrubWorker::new(block_manager.clone(), scrub_rx);
+		block_manager.system.background.spawn_worker(scrub_worker);
 
 		block_manager
 	}
 
 	/// Ask nodes that might have a (possibly compressed) block for it
-	async fn rpc_get_raw_block(&self, hash: &Hash) -> Result<DataBlock, Error> {
+	pub(crate) async fn rpc_get_raw_block(&self, hash: &Hash) -> Result<DataBlock, Error> {
 		let who = self.replication.read_nodes(hash);
 		let resps = self
 			.system
@@ -243,20 +193,6 @@ impl BlockManager {
 		Ok(())
 	}
 
-	/// Get lenght of resync queue
-	pub fn resync_queue_len(&self) -> Result<usize, Error> {
-		// This currently can't return an error because the CountedTree hack
-		// doesn't error on .len(), but this will change when we remove the hack
-		// (hopefully someday!)
-		Ok(self.resync_queue.len())
-	}
-
-	/// Get number of blocks that have an error
-	pub fn resync_errors_len(&self) -> Result<usize, Error> {
-		// (see resync_queue_len comment)
-		Ok(self.resync_errors.len())
-	}
-
 	/// Get number of items in the refcount table
 	pub fn rc_len(&self) -> Result<usize, Error> {
 		Ok(self.rc.rc.len()?)
@@ -264,13 +200,7 @@ impl BlockManager {
 
 	/// Send command to start/stop/manager scrub worker
 	pub async fn send_scrub_command(&self, cmd: ScrubWorkerCommand) {
-		let _ = self
-			.tx_scrub_command
-			.load()
-			.as_ref()
-			.unwrap()
-			.send(cmd)
-			.await;
+		let _ = self.tx_scrub_command.send(cmd).await;
 	}
 
 	//// ----- Managing the reference counter ----
@@ -291,7 +221,7 @@ impl BlockManager {
 			// we will fecth it from someone.
 			let this = self.clone();
 			tokio::spawn(async move {
-				if let Err(e) = this.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
+				if let Err(e) = this.resync.put_to_resync(&hash, 2 * BLOCK_RW_TIMEOUT) {
 					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
 				}
 			});
@@ -313,7 +243,9 @@ impl BlockManager {
 			// after that delay has passed.
 			let this = self.clone();
 			tokio::spawn(async move {
-				if let Err(e) = this.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
+				if let Err(e) = this
+					.resync
+					.put_to_resync(&hash, BLOCK_GC_DELAY + Duration::from_secs(10))
 				{
 					error!("Block {:?} could not be put in resync queue: {}.", hash, e);
 				}
@@ -325,7 +257,11 @@ impl BlockManager {
 	// ---- Reading and writing blocks locally ----
 
 	/// Write a block to disk
-	async fn write_block(&self, hash: &Hash, data: &DataBlock) -> Result<BlockRpc, Error> {
+	pub(crate) async fn write_block(
+		&self,
+		hash: &Hash,
+		data: &DataBlock,
+	) -> Result<BlockRpc, Error> {
 		let write_size = data.inner_buffer().len() as u64;
 
 		let res = self
@@ -361,7 +297,7 @@ impl BlockManager {
 			Ok(c) => c,
 			Err(e) => {
 				// Not found but maybe we should have had it ??
-				self.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
+				self.resync.put_to_resync(hash, 2 * BLOCK_RW_TIMEOUT)?;
 				return Err(Into::into(e));
 			}
 		};
@@ -388,7 +324,7 @@ impl BlockManager {
 				.await
 				.move_block_to_corrupted(hash, self)
 				.await?;
-			self.put_to_resync(hash, Duration::from_millis(0))?;
+			self.resync.put_to_resync(hash, Duration::from_millis(0))?;
 			return Err(Error::CorruptData(*hash));
 		}
 
@@ -432,322 +368,6 @@ impl BlockManager {
 		path.set_extension("");
 		fs::metadata(&path).await.map(|_| false).map_err(Into::into)
 	}
-
-	// ---- Resync loop ----
-
-	// This part manages a queue of blocks that need to be
-	// "resynchronized", i.e. that need to have a check that
-	// they are at present if we need them, or that they are
-	// deleted once the garbage collection delay has passed.
-	//
-	// Here are some explanations on how the resync queue works.
-	// There are two Sled trees that are used to have information
-	// about the status of blocks that need to be resynchronized:
-	//
-	// - resync_queue: a tree that is ordered first by a timestamp
-	//   (in milliseconds since Unix epoch) that is the time at which
-	//   the resync must be done, and second by block hash.
-	//   The key in this tree is just:
-	//       concat(timestamp (8 bytes), hash (32 bytes))
-	//   The value is the same 32-byte hash.
-	//
-	// - resync_errors: a tree that indicates for each block
-	//   if the last resync resulted in an error, and if so,
-	//   the following two informations (see the ErrorCounter struct):
-	//   - how many consecutive resync errors for this block?
-	//   - when was the last try?
-	//   These two informations are used to implement an
-	//   exponential backoff retry strategy.
-	//   The key in this tree is the 32-byte hash of the block,
-	//   and the value is the encoded ErrorCounter value.
-	//
-	// We need to have these two trees, because the resync queue
-	// is not just a queue of items to process, but a set of items
-	// that are waiting a specific delay until we can process them
-	// (the delay being necessary both internally for the exponential
-	// backoff strategy, and exposed as a parameter when adding items
-	// to the queue, e.g. to wait until the GC delay has passed).
-	// This is why we need one tree ordered by time, and one
-	// ordered by identifier of item to be processed (block hash).
-	//
-	// When the worker wants to process an item it takes from
-	// resync_queue, it checks in resync_errors that if there is an
-	// exponential back-off delay to await, it has passed before we
-	// process the item. If not, the item in the queue is skipped
-	// (but added back for later processing after the time of the
-	// delay).
-	//
-	// An alternative that would have seemed natural is to
-	// only add items to resync_queue with a processing time that is
-	// after the delay, but there are several issues with this:
-	// - This requires to synchronize updates to resync_queue and
-	//   resync_errors (with the current model, there is only one thread,
-	//   the worker thread, that accesses resync_errors,
-	//   so no need to synchronize) by putting them both in a lock.
-	//   This would mean that block_incref might need to take a lock
-	//   before doing its thing, meaning it has much more chances of
-	//   not completing successfully if something bad happens to Garage.
-	//   Currently Garage is not able to recover from block_incref that
-	//   doesn't complete successfully, because it is necessary to ensure
-	//   the consistency between the state of the block manager and
-	//   information in the BlockRef table.
-	// - If a resync fails, we put that block in the resync_errors table,
-	//   and also add it back to resync_queue to be processed after
-	//   the exponential back-off delay,
-	//   but maybe the block is already scheduled to be resynced again
-	//   at another time that is before the exponential back-off delay,
-	//   and we have no way to check that easily. This means that
-	//   in all cases, we need to check the resync_errors table
-	//   in the resync loop at the time when a block is popped from
-	//   the resync_queue.
-	// Overall, the current design is therefore simpler and more robust
-	// because it tolerates inconsistencies between the resync_queue
-	// and resync_errors table (items being scheduled in resync_queue
-	// for times that are earlier than the exponential back-off delay
-	// is a natural condition that is handled properly).
-
-	fn spawn_background_workers(self: Arc<Self>) {
-		// Launch a background workers for background resync loop processing
-		let background = self.system.background.clone();
-		let worker = ResyncWorker::new(self.clone());
-		tokio::spawn(async move {
-			tokio::time::sleep(Duration::from_secs(10)).await;
-			background.spawn_worker(worker);
-		});
-
-		// Launch a background worker for data store scrubs
-		let (scrub_tx, scrub_rx) = mpsc::channel(1);
-		self.tx_scrub_command.store(Some(Arc::new(scrub_tx)));
-		let scrub_worker = ScrubWorker::new(self.clone(), scrub_rx);
-		self.system.background.spawn_worker(scrub_worker);
-	}
-
-	pub(crate) fn put_to_resync(&self, hash: &Hash, delay: Duration) -> db::Result<()> {
-		let when = now_msec() + delay.as_millis() as u64;
-		self.put_to_resync_at(hash, when)
-	}
-
-	fn put_to_resync_at(&self, hash: &Hash, when: u64) -> db::Result<()> {
-		trace!("Put resync_queue: {} {:?}", when, hash);
-		let mut key = u64::to_be_bytes(when).to_vec();
-		key.extend(hash.as_ref());
-		self.resync_queue.insert(key, hash.as_ref())?;
-		self.resync_notify.notify_waiters();
-		Ok(())
-	}
-
-	async fn resync_iter(&self) -> Result<ResyncIterResult, db::Error> {
-		if let Some((time_bytes, hash_bytes)) = self.resync_queue.first()? {
-			let time_msec = u64::from_be_bytes(time_bytes[0..8].try_into().unwrap());
-			let now = now_msec();
-
-			if now >= time_msec {
-				let hash = Hash::try_from(&hash_bytes[..]).unwrap();
-
-				if let Some(ec) = self.resync_errors.get(hash.as_slice())? {
-					let ec = ErrorCounter::decode(&ec);
-					if now < ec.next_try() {
-						// if next retry after an error is not yet,
-						// don't do resync and return early, but still
-						// make sure the item is still in queue at expected time
-						self.put_to_resync_at(&hash, ec.next_try())?;
-						// ec.next_try() > now >= time_msec, so this remove
-						// is not removing the one we added just above
-						// (we want to do the remove after the insert to ensure
-						// that the item is not lost if we crash in-between)
-						self.resync_queue.remove(time_bytes)?;
-						return Ok(ResyncIterResult::BusyDidNothing);
-					}
-				}
-
-				let tracer = opentelemetry::global::tracer("garage");
-				let trace_id = gen_uuid();
-				let span = tracer
-					.span_builder("Resync block")
-					.with_trace_id(
-						opentelemetry::trace::TraceId::from_hex(&hex::encode(
-							&trace_id.as_slice()[..16],
-						))
-						.unwrap(),
-					)
-					.with_attributes(vec![KeyValue::new("block", format!("{:?}", hash))])
-					.start(&tracer);
-
-				let res = self
-					.resync_block(&hash)
-					.with_context(Context::current_with_span(span))
-					.bound_record_duration(&self.metrics.resync_duration)
-					.await;
-
-				self.metrics.resync_counter.add(1);
-
-				if let Err(e) = &res {
-					self.metrics.resync_error_counter.add(1);
-					warn!("Error when resyncing {:?}: {}", hash, e);
-
-					let err_counter = match self.resync_errors.get(hash.as_slice())? {
-						Some(ec) => ErrorCounter::decode(&ec).add1(now + 1),
-						None => ErrorCounter::new(now + 1),
-					};
-
-					self.resync_errors
-						.insert(hash.as_slice(), err_counter.encode())?;
-
-					self.put_to_resync_at(&hash, err_counter.next_try())?;
-					// err_counter.next_try() >= now + 1 > now,
-					// the entry we remove from the queue is not
-					// the entry we inserted with put_to_resync_at
-					self.resync_queue.remove(time_bytes)?;
-				} else {
-					self.resync_errors.remove(hash.as_slice())?;
-					self.resync_queue.remove(time_bytes)?;
-				}
-
-				Ok(ResyncIterResult::BusyDidSomething)
-			} else {
-				Ok(ResyncIterResult::IdleFor(Duration::from_millis(
-					time_msec - now,
-				)))
-			}
-		} else {
-			// Here we wait either for a notification that an item has been
-			// added to the queue, or for a constant delay of 10 secs to expire.
-			// The delay avoids a race condition where the notification happens
-			// between the time we checked the queue and the first poll
-			// to resync_notify.notified(): if that happens, we'll just loop
-			// back 10 seconds later, which is fine.
-			Ok(ResyncIterResult::IdleFor(Duration::from_secs(10)))
-		}
-	}
-
-	async fn resync_block(&self, hash: &Hash) -> Result<(), Error> {
-		let BlockStatus { exists, needed } = self
-			.mutation_lock
-			.lock()
-			.await
-			.check_block_status(hash, self)
-			.await?;
-
-		if exists != needed.is_needed() || exists != needed.is_nonzero() {
-			debug!(
-				"Resync block {:?}: exists {}, nonzero rc {}, deletable {}",
-				hash,
-				exists,
-				needed.is_nonzero(),
-				needed.is_deletable(),
-			);
-		}
-
-		if exists && needed.is_deletable() {
-			info!("Resync block {:?}: offloading and deleting", hash);
-
-			let mut who = self.replication.write_nodes(hash);
-			if who.len() < self.replication.write_quorum() {
-				return Err(Error::Message("Not trying to offload block because we don't have a quorum of nodes to write to".to_string()));
-			}
-			who.retain(|id| *id != self.system.id);
-
-			let msg = Arc::new(BlockRpc::NeedBlockQuery(*hash));
-			let who_needs_fut = who.iter().map(|to| {
-				self.system.rpc.call_arc(
-					&self.endpoint,
-					*to,
-					msg.clone(),
-					RequestStrategy::with_priority(PRIO_BACKGROUND)
-						.with_timeout(NEED_BLOCK_QUERY_TIMEOUT),
-				)
-			});
-			let who_needs_resps = join_all(who_needs_fut).await;
-
-			let mut need_nodes = vec![];
-			for (node, needed) in who.iter().zip(who_needs_resps.into_iter()) {
-				match needed.err_context("NeedBlockQuery RPC")? {
-					BlockRpc::NeedBlockReply(needed) => {
-						if needed {
-							need_nodes.push(*node);
-						}
-					}
-					m => {
-						return Err(Error::unexpected_rpc_message(m));
-					}
-				}
-			}
-
-			if !need_nodes.is_empty() {
-				trace!(
-					"Block {:?} needed by {} nodes, sending",
-					hash,
-					need_nodes.len()
-				);
-
-				for node in need_nodes.iter() {
-					self.metrics
-						.resync_send_counter
-						.add(1, &[KeyValue::new("to", format!("{:?}", node))]);
-				}
-
-				let put_block_message = self.read_block(hash).await?;
-				self.system
-					.rpc
-					.try_call_many(
-						&self.endpoint,
-						&need_nodes[..],
-						put_block_message,
-						RequestStrategy::with_priority(PRIO_BACKGROUND)
-							.with_quorum(need_nodes.len())
-							.with_timeout(BLOCK_RW_TIMEOUT),
-					)
-					.await
-					.err_context("PutBlock RPC")?;
-			}
-			info!(
-				"Deleting unneeded block {:?}, offload finished ({} / {})",
-				hash,
-				need_nodes.len(),
-				who.len()
-			);
-
-			self.mutation_lock
-				.lock()
-				.await
-				.delete_if_unneeded(hash, self)
-				.await?;
-
-			self.rc.clear_deleted_block_rc(hash)?;
-		}
-
-		if needed.is_nonzero() && !exists {
-			info!(
-				"Resync block {:?}: fetching absent but needed block (refcount > 0)",
-				hash
-			);
-
-			let block_data = self.rpc_get_raw_block(hash).await?;
-
-			self.metrics.resync_recv_counter.add(1);
-
-			self.write_block(hash, &block_data).await?;
-		}
-
-		Ok(())
-	}
-
-	async fn update_resync_persisted(
-		&self,
-		update: impl Fn(&mut ResyncPersistedConfig),
-	) -> Result<(), Error> {
-		let mut cfg: ResyncPersistedConfig = *self.resync_persisted.load().as_ref();
-		update(&mut cfg);
-		self.resync_persister.save_async(&cfg).await?;
-		self.resync_persisted.store(Arc::new(cfg));
-		self.resync_notify.notify_one();
-		Ok(())
-	}
-
-	pub async fn set_resync_tranquility(&self, tranquility: u32) -> Result<(), Error> {
-		self.update_resync_persisted(|cfg| cfg.tranquility = tranquility)
-			.await
-	}
 }
 
 #[async_trait]
@@ -766,92 +386,13 @@ impl EndpointHandler<BlockRpc> for BlockManager {
 	}
 }
 
-#[derive(Serialize, Deserialize, Clone, Copy)]
-struct ResyncPersistedConfig {
-	tranquility: u32,
-}
-
-struct ResyncWorker {
-	manager: Arc<BlockManager>,
-	tranquilizer: Tranquilizer,
-	next_delay: Duration,
-}
-
-impl ResyncWorker {
-	fn new(manager: Arc<BlockManager>) -> Self {
-		Self {
-			manager,
-			tranquilizer: Tranquilizer::new(30),
-			next_delay: Duration::from_secs(10),
-		}
-	}
-}
-
-#[async_trait]
-impl Worker for ResyncWorker {
-	fn name(&self) -> String {
-		"Block resync worker".into()
-	}
-
-	fn info(&self) -> Option<String> {
-		let mut ret = vec![];
-		ret.push(format!(
-			"tranquility = {}",
-			self.manager.resync_persisted.load().tranquility
-		));
-
-		let qlen = self.manager.resync_queue_len().unwrap_or(0);
-		if qlen > 0 {
-			ret.push(format!("{} blocks in queue", qlen));
-		}
-
-		let elen = self.manager.resync_errors_len().unwrap_or(0);
-		if elen > 0 {
-			ret.push(format!("{} blocks in error state", elen));
-		}
-
-		Some(ret.join(", "))
-	}
-
-	async fn work(&mut self, _must_exit: &mut watch::Receiver<bool>) -> Result<WorkerState, Error> {
-		self.tranquilizer.reset();
-		match self.manager.resync_iter().await {
-			Ok(ResyncIterResult::BusyDidSomething) => Ok(self
-				.tranquilizer
-				.tranquilize_worker(self.manager.resync_persisted.load().tranquility)),
-			Ok(ResyncIterResult::BusyDidNothing) => Ok(WorkerState::Busy),
-			Ok(ResyncIterResult::IdleFor(delay)) => {
-				self.next_delay = delay;
-				Ok(WorkerState::Idle)
-			}
-			Err(e) => {
-				// The errors that we have here are only Sled errors
-				// We don't really know how to handle them so just ¯\_(ツ)_/¯
-				// (there is kind of an assumption that Sled won't error on us,
-				// if it does there is not much we can do -- TODO should we just panic?)
-				// Here we just give the error to the worker manager,
-				// it will print it to the logs and increment a counter
-				Err(e.into())
-			}
-		}
-	}
-
-	async fn wait_for_work(&mut self, _must_exit: &watch::Receiver<bool>) -> WorkerState {
-		select! {
-			_ = tokio::time::sleep(self.next_delay) => (),
-			_ = self.manager.resync_notify.notified() => (),
-		};
-		WorkerState::Busy
-	}
-}
-
-struct BlockStatus {
-	exists: bool,
-	needed: RcEntry,
+pub(crate) struct BlockStatus {
+	pub(crate) exists: bool,
+	pub(crate) needed: RcEntry,
 }
 
 impl BlockManagerLocked {
-	async fn check_block_status(
+	pub(crate) async fn check_block_status(
 		&self,
 		hash: &Hash,
 		mgr: &BlockManager,
@@ -938,7 +479,11 @@ impl BlockManagerLocked {
 		Ok(())
 	}
 
-	async fn delete_if_unneeded(&self, hash: &Hash, mgr: &BlockManager) -> Result<(), Error> {
+	pub(crate) async fn delete_if_unneeded(
+		&self,
+		hash: &Hash,
+		mgr: &BlockManager,
+	) -> Result<(), Error> {
 		let BlockStatus { exists, needed } = self.check_block_status(hash, mgr).await?;
 
 		if exists && needed.is_deletable() {
@@ -952,50 +497,3 @@ impl BlockManagerLocked {
 		Ok(())
 	}
 }
-
-/// Counts the number of errors when resyncing a block,
-/// and the time of the last try.
-/// Used to implement exponential backoff.
-#[derive(Clone, Copy, Debug)]
-struct ErrorCounter {
-	errors: u64,
-	last_try: u64,
-}
-
-impl ErrorCounter {
-	fn new(now: u64) -> Self {
-		Self {
-			errors: 1,
-			last_try: now,
-		}
-	}
-
-	fn decode(data: &[u8]) -> Self {
-		Self {
-			errors: u64::from_be_bytes(data[0..8].try_into().unwrap()),
-			last_try: u64::from_be_bytes(data[8..16].try_into().unwrap()),
-		}
-	}
-	fn encode(&self) -> Vec<u8> {
-		[
-			u64::to_be_bytes(self.errors),
-			u64::to_be_bytes(self.last_try),
-		]
-		.concat()
-	}
-
-	fn add1(self, now: u64) -> Self {
-		Self {
-			errors: self.errors + 1,
-			last_try: now,
-		}
-	}
-
-	fn delay_msec(&self) -> u64 {
-		(RESYNC_RETRY_DELAY.as_millis() as u64)
-			<< std::cmp::min(self.errors - 1, RESYNC_RETRY_DELAY_MAX_BACKOFF_POWER)
-	}
-	fn next_try(&self) -> u64 {
-		self.last_try + self.delay_msec()
-	}
-}