[block-ref-repair] Block refcount recalculation and repair

- We always recalculate the reference count of a block before deleting
  it locally, to make sure that it is indeed zero.

- If we had to fetch a remote block but we were not able to get it,
  check that refcount is indeed > 0.

- Repair procedure that checks everything

src/block/lib.rs

@@ -11,3 +11,4 @@ mod metrics;
 mod rc;
 
 pub use block::zstd_encode;
+pub use rc::CalculateRefcount;

src/block/manager.rs

@@ -88,7 +88,7 @@ pub struct BlockManager {
 
 	mutation_lock: Vec<Mutex<BlockManagerLocked>>,
 
-	pub(crate) rc: BlockRc,
+	pub rc: BlockRc,
 	pub resync: BlockResyncManager,
 
 	pub(crate) system: Arc<System>,
@@ -229,6 +229,12 @@ impl BlockManager {
 		}
 	}
 
+	/// Initialization: set how block references are recalculated
+	/// for repair operations
+	pub fn set_recalc_rc(&self, recalc: Vec<CalculateRefcount>) {
+		self.rc.recalc_rc.store(Some(Arc::new(recalc)));
+	}
+
 	/// Ask nodes that might have a (possibly compressed) block for it
 	/// Return it as a stream with a header
 	async fn rpc_get_raw_block_streaming(
@@ -316,9 +322,9 @@ impl BlockManager {
 			};
 		}
 
-		let msg = format!("Get block {:?}: no node returned a valid block", hash);
-		debug!("{}", msg);
-		Err(Error::Message(msg))
+		let err = Error::MissingBlock(*hash);
+		debug!("{}", err);
+		Err(err)
 	}
 
 	// ---- Public interface ----

src/block/rc.rs

@@ -1,5 +1,7 @@
 use std::convert::TryInto;
 
+use arc_swap::ArcSwapOption;
+
 use garage_db as db;
 
 use garage_util::data::*;
@@ -8,13 +10,20 @@ use garage_util::time::*;
 
 use crate::manager::BLOCK_GC_DELAY;
 
+pub type CalculateRefcount =
+	Box<dyn Fn(&db::Transaction, &Hash) -> db::TxResult<usize, Error> + Send + Sync>;
+
 pub struct BlockRc {
-	pub(crate) rc: db::Tree,
+	pub rc: db::Tree,
+	pub(crate) recalc_rc: ArcSwapOption<Vec<CalculateRefcount>>,
 }
 
 impl BlockRc {
 	pub(crate) fn new(rc: db::Tree) -> Self {
-		Self { rc }
+		Self {
+			rc,
+			recalc_rc: ArcSwapOption::new(None),
+		}
 	}
 
 	/// Increment the reference counter associated to a hash.
@@ -68,6 +77,58 @@ impl BlockRc {
 		})?;
 		Ok(())
 	}
+
+	/// Recalculate the reference counter of a block
+	/// to fix potential inconsistencies
+	pub fn recalculate_rc(&self, hash: &Hash) -> Result<(usize, bool), Error> {
+		if let Some(recalc_fns) = self.recalc_rc.load().as_ref() {
+			trace!("Repair block RC for {:?}", hash);
+			let res = self
+				.rc
+				.db()
+				.transaction(|tx| {
+					let mut cnt = 0;
+					for f in recalc_fns.iter() {
+						cnt += f(&tx, hash)?;
+					}
+					let old_rc = RcEntry::parse_opt(tx.get(&self.rc, hash)?);
+					trace!(
+						"Block RC for {:?}: stored={}, calculated={}",
+						hash,
+						old_rc.as_u64(),
+						cnt
+					);
+					if cnt as u64 != old_rc.as_u64() {
+						warn!(
+							"Fixing inconsistent block RC for {:?}: was {}, should be {}",
+							hash,
+							old_rc.as_u64(),
+							cnt
+						);
+						let new_rc = if cnt > 0 {
+							RcEntry::Present { count: cnt as u64 }
+						} else {
+							RcEntry::Deletable {
+								at_time: now_msec() + BLOCK_GC_DELAY.as_millis() as u64,
+							}
+						};
+						tx.insert(&self.rc, hash, new_rc.serialize().unwrap())?;
+						Ok((cnt, true))
+					} else {
+						Ok((cnt, false))
+					}
+				})
+				.map_err(Error::from);
+			if let Err(e) = &res {
+				error!("Failed to fix RC for block {:?}: {}", hash, e);
+			}
+			res
+		} else {
+			Err(Error::Message(
+				"Block RC recalculation is not available at this point".into(),
+			))
+		}
+	}
 }
 
 /// Describes the state of the reference counter for a block

src/block/resync.rs

@@ -367,6 +367,13 @@ impl BlockResyncManager {
 		}
 
 		if exists && rc.is_deletable() {
+			if manager.rc.recalculate_rc(hash)?.0 > 0 {
+				return Err(Error::Message(format!(
+					"Refcount for block {:?} was inconsistent, retrying later",
+					hash
+				)));
+			}
+
 			info!("Resync block {:?}: offloading and deleting", hash);
 			let existing_path = existing_path.unwrap();
 
@@ -453,7 +460,15 @@ impl BlockResyncManager {
 				hash
 			);
 
-			let block_data = manager.rpc_get_raw_block(hash, None).await?;
+			let block_data = manager.rpc_get_raw_block(hash, None).await;
+			if matches!(block_data, Err(Error::MissingBlock(_))) {
+				warn!(
+					"Could not fetch needed block {:?}, no node returned valid data. Checking that refcount is correct.",
+					hash
+				);
+				manager.rc.recalculate_rc(hash)?;
+			}
+			let block_data = block_data?;
 
 			manager.metrics.resync_recv_counter.add(1);